Developments in prompt gamma-ray neutron activation analysis and cold neutron tomography and their application in non-destructive testing

Transcription

1 Developments in prompt gamma-ray neutron activation analysis and cold neutron tomography and their application in non-destructive testing Inauguraldissertation der Philosophisch-naturwissenschaftlichen Fakultät der Universität Bern vorgelegt von Stefan Söllradl von Österreich Leiter der Arbeit Prof. Dr. A. Türler Departement für Chemie und Biochemie der Universität Bern Labor für Radio- und Umweltchemie Von der Philosophisch-naturwissenschaftlichen Fakultät angenommen Bern, den Der Dekan Prof. Dr. S. Decurtins

2 2

3 3 This work is dedicated to my grandmother.

4 4

5 Summary 5 Summary Prompt gamma-ray activation analysis (PGAA) is a non-destructive, multielemental, and bulk sensitive nuclear analytical technique. At the PGAA facility at the Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), a sample is activated in a cold neutron beam and the emitted gamma radiation is detected during this irradiation process. Based on the acquired prompt gammaray spectra, the average elemental composition of the irradiated sample volume can be determined. While PGAA gives the elemental composition of a sample, information about its inner structure cannot be obtained. In contrast to this, such information can be derived using neutron imaging techniques which do not give the sample s chemical composition. A combination of the two enables a non-destructive investigation of the structure in addition to the chemical composition, and is called prompt gamma-ray neutron activation imaging (PGAI). For this reason, a neutron tomography setup was developed, installed, and characterized at the PGAA instrument, located at FRM-II, which was also used for the first steps towards PGAI. At the same time, the PGAA technique was also further developed to meet the special conditions of the facility at FRM-II. The aim of this work is to present the developments, installation works, tests, and application in the frame of these projects. The content of this thesis is structured into a general introduction, describing the interaction of gamma radiation and neutrons with matter, followed by the principles of PGAA and neutron imaging. The two chapters Experimental and Results and Discussion are split into three parts. In the first part, the performed development and application of PGAA is reported for three experiments. The second part is focused on the characterization of the neutron beam at the PGAA instrument which was necessary for the installation of the new neutron tomography setup. The performance of this setup was justified with test objects. In the last part of each chapter the developments towards PGAI are shown. In the first part of the experimental section, the developments in the PGAA technique are presented. First, the standard PGAA instrument is described and the necessary modifications are explained. The analytical sensitivities of the elements in PGAA are matrix-independent, as they depend on the neutron-capture cross sections. However, the detection limit of an element depends on the spectral properties of the matrix components, and that defines the concentration range of the analyzed elements (i.e. the dynamic range). A method is introduced, how to override this inherent constraint of the technique in the case of the analysis of a high-cross-section element, boron. In addition, it was successfully tested for the analysis of copper-containing alloys, whose analysis is always challenging in PGAA because of the high signal intensity. In the case of low-cross-section samples, the problem is different: even the sample holder contributes to the spectra significantly. Teflon, typically used for sample packing, may sometimes have a larger mass than the sample itself. In the presented study, different sample-holder materials were investigated.

6 6 Summary In the last part of the PGAA section, an investigation of a new semiconductor material is presented where PGAA played a major role, where most of the above mentioned improvements were applied. The second part of the experimental section is dedicated to the development of a neutron imaging setup. In the frame of this study, the neutron beam and different arrangements were characterized. Later, a permanent and wellshielded setup was constructed. The most important property, the resolution of this setup was determined and two tomography experiments were made to test the performance. In the last part, a first experiment towards PGAI is reported. In a series of experiments with standard PGAA setup, the effects of the gamma collimation were investigated. The conclusions of the results are also summarized with a special regard to the future developments of the PGAI technique.

7 Table of Contents 7 Table of Contents Summary 5 Table of Contents 7 List of Abbreviations 9 1 Introduction The neutron and its basic properties Interaction of radiation with matter Neutron Radiation Gamma radiation Principles of Prompt Gamma Activation Analysis The activation of a sample Characteristics of a PGAA spectrum Quantitative gamma spectroscopy Principles of neutron imaging Principle of neutron radiography Principle of computed tomography Principles of prompt gamma activation imaging (PGAI) Overview 33 2 Experimental The PGAA instrument at FRM II Standard PGAA Modified PGAA setup with a gamma attenuator Increasing the dynamic range of PGAA measurements Boron-containing samples Copper alloys Investigation of different sample holder materials with PGAA Determination of nitrogen in ZnO powders with PGAA and other modern characterization techniques Introduction Synthesis Characterization techniques Neutron imaging at the PGAA instrument Introduction Characterization of the detector system Characterization of the neutron beam Neutron radiography and tomography First developments towards PGAI 61 3 Results and Discussion PGAA Increasing the dynamic range of PGAA Application of PGAA with gamma-ray attenuator for copper containing samples Investigation of different sample holder materials Chemical analysis of nitrogen in ZnO powders Development of an imaging system at the PGAA instrument Characterization of the detector system 83

8 8 Table of Contents Characterization of the neutron beam Neutron Radiography and Tomography Characterization of the field of view of the gamma collimator Conclusion and Outlook Literature Acknowledgement List of Publications Refereed articles Non-Refereed articles and reports Contribution to Conferences, Workshops, Seminars Appendix Drawings of neutron attenuators at the PGAA instrument Nitrogen determination in ZnO powders Crystallographic data of isocyanuric acid NMR Data 127 Erklärung 129 Curriculum Vitae 131

9 List of Abbreviations 9 List of Abbreviations Abbreviation FRM II PGAA PGAI FEP HPGe-Detector BGO GUI XPS SCM Meaning Forschungsneutronenquelle Heinz Maier-Leibnitz Prompt gamma-ray activation analysis Prompt gamma-ray activation imaging Fluorinated ethylene propylene copolymer High purity germanium detector Bismuth-germanate (scintillation material for Compton suppression) Graphical user interface X-ray photoelectron spectroscopy Solution combustion method

10 10 List of Abbreviations

11 Introduction Introduction In 1897 J. J. Thomson discovered the electron by experimenting with cathode rays and characterized the properties of the observed radiation in electric and magnetic fields[1]. A few years later, he postulated the theory of an atomic model based on the performed experiments, where the atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification [2] in order to result in a neutral, overall charge. H. Geiger and E. Marsden observed in 1909, that thin foils of different elements irradiated with alpha particles scatter the alpha radiation[3]. The yield of the scattered particles differed depending on the atomic mass of the used elements and the intensity of the scattered particles increased with increasing atomic mass. It was E. Rutherford, who interpreted the results of the experiment correctly in 1911 and postulated that atoms consist of a nucleus with concentrated positive charge and mass in the center of the atom[4] and a large shell, containing electrons to explain the observations by Geiger and Marsden. The α-particles are scattered due to the interaction of this strong charge concentrated on an extremely small volume of the nucleus. In addition, he concluded that the atomic mass is approximately proportional to the atomic charge but it was not possible to recommend a model for the offset in the case of heavier elements. Two years later, N. Bohr proposed an atomic model with selection rules, assuming the positive charged nucleus in the center of the atom and electrons on discrete orbits, which explained the spectral lines he observed experimentally [5,6]. In the following years Rutherford was still puzzled with the offset and the dependence of atomic mass and charge. To solve this problem, he postulated in 1920 the existence of the neutron, a second, neutral particle in the nucleus to allow a better description of the observed offset for heavy elements[7]. Twelve years later in 1932, J. Chadwick observed a deeply penetrating radiation during irradiation experiments of a 9 Be target with α-particles[8,9]. He investigated the effect of different attenuators in the form of paraffin wax and aluminum layers on the newly discovered, neutral particle radiation and attributed it to the postulated neutron by Rutherford. Nowadays the deep penetration of neutrons into condensed matter is widely utilized in science and they are common probes for non-destructive analysis and medical therapy. Neutrons are used in medical treatment of cancer[10], structure analysis[11], absorption radiography[12-14], determination of chemical composition[15-18], and fundamental research to determine the properties of the neutron[19,20], just to mention a few examples. 1.1 The neutron and its basic properties The free neutron is an unstable particle with a net charge of 0, a spin of ½ and a half-life of 10.24(2) minutes[5]. It decays to a proton with the emission of a β - particle and an anti-neutrino. The neutron s mass is m = x10-27 kg and its magnetic moment is µ m = nuclear magnetons.

12 12 Introduction Due to their instability, neutrons need to be produced when needed and cannot be stored. The most common neutron sources for modern applications are research reactors and spallation sources. In research reactors, neutrons are produced by the fission of 235 U in contrast to spallation sources, where heavy nuclei for example in the form of lead-bismuth alloys are irradiated with an high-energy proton beam. Both reactions produce neutrons with high energies, which have to be moderated for most applications. The emitted neutrons of fission reactions have energies in the range of 2 MeV, which results in an equivalent temperature of 2x10 10 K according to (1.1) where E is the energy in Joule, k B is the Boltzmann constant and T the temperature in Kelvin. These fission or spallation neutrons are then moderated to thermal neutrons with temperatures around 300 K or cold neutrons to about 30 K, corresponding to the temperature of the moderator. Common moderators are for example heavy water (D 2 O) for thermal neutrons and liquid deuterium (D 2 ) for cold neutrons. Based on this kinetic energy, the velocity can be calculated according to (1.2) where E is the kinetic energy, m is the mass of the particle and v is its velocity. According to debroglie s wave particle dualism, also a wavelength can be assigned to neutrons. The wavelength λ can be calculated from the mass and the energy of a neutron according to (1.3) where h is Planck s constant, v is the velocity of the particle, m the mass of the neutron and E its kinetic energy. In contrast to electro-magnetic radiation (e.g. γ-rays or X-rays) in form of photons, neutrons do not interact with the surrounding electrons of an atom but only with the short range nuclear forces of nuclei in the order of a few femtometers ( m)[21]. In that context, condensed matter is not very dense for neutrons and nuclei as scattering centers are typically 100,000 times smaller than the distance between these centers. A comparison of the interaction cross sections of selected elements, for neutrons and X-rays is shown in Fig. 1. Fig. 1: Interaction probability of selected elements with X-rays and neutrons [22].

13 Introduction 13 While the interaction cross-section for X-rays increases with increasing Z- number, the interaction cross section varies from element to element for neutrons. While hydrogen is mostly transparent for X-rays, its interaction with neutrons is significantly higher. In contrast, the element lead is relatively transparent for neutron radiation, but strongly attenuates an incident beam of X-rays. With that reason, both, neutron and X-ray techniques are in most cases complementary methods for bulk analysis and have their strengths and weaknesses.

14 14 Introduction 1.2 Interaction of radiation with matter In the following the interaction of radiation with matter is described in general and then, specifically, the two important types of radiation and their interactions used in this work. The attenuation of an incident beam with the intensity I 0 through a layer of material with the thickness x, can be split into two main processes: scattering and absorption. Fig. 2: Attenuation processes for radiation with an incident intensity I 0. In general, the incident radiation can be transmitted, scattered or is absorbed in a material with the thickness x. Depending on the type of incident radiation, also secondary radiation can be observed after the absorption or scattering[23]. Radiation, passing through an infinitely short section of matter along a path dx and an initial intensity I, will be attenuated by the factor di. This attenuation depends on the total attenuation coefficient µ and can be described with. (1.4) Thus the transmitted intensity of an incident radiation through a homogeneous material with the total attenuation coefficient µ is called the exponential attenuation law (Lambert-Beer law) and can be expressed as, (1.5) where I is the transmitted intensity, I 0 is the initial intensity, µ is the total attenuation coefficient of the material, and x is the thickness of the absorber according to the schematic sketch in Fig. 2. The total attenuation coefficient of the absorbing material is the sum of both modes of attenuation according to, (1.6) where µ s is the attenuation coefficient for scattering and µ a is the attenuation coefficient for absorption. The cross section for the interaction of radiation with matter is usually given in the unit barn and 1 barn equals an area of

15 Introduction m 2. It can be regarded as a probability value, at which the incident radiation interacts with the scattering center or absorbing nucleus. To keep independence from the physical form, a special coefficient µ m, which is called the total mass attenuation coefficient, was introduced. It is defined by the ratio of µ/ρ and takes into account the attenuation properties µ of an element or isotope (in the case of neutrons) as a function of material density. Therefore the attenuation law can be rearranged to, (1.7) where ρ is the density of the material, A is the area of the cross section and ρ f = m/a is the mass per area. If the absorbing material contains more than one element or isotope, the total mass attenuation coefficient is expressed as the sum of the mass attenuation coefficients according to, (1.8) where N is the number of elements or isotopes and each of them contributes to the total µ m by its mass fraction α i and the respective mass attenuation coefficient µ m. If all mass fractions along the trajectory are considered correctly, the sum of the fractions has to be unity Neutron Radiation Neutron scattering Though the process of neutron scattering can best be discussed within the frame of quantum mechanics, it will be described simplified and mainly based on the introduction to neutron scattering by R. Pynn[21]. In the case of scattering, the description of the neutron as a wave simplifies the model to explain the process. Based on Eq. (1.3) the magnitude of the neutron s wave vector 2π/λ can be given as, (1.9) where h is Planck s constant, m is the mass of the neutron, and v is the velocity. Typical wavelengths for scattering experiments are in the range of 0.1 to 1 nm and the scattering nucleus can be regarded as a fixed point. Two main modes of scattering can be distinguished: elastic and inelastic scattering. Elastic scattering is the process shown in Fig. 3, where the magnitude of the incident neutron and the scattered neutron wave equals and only the direction of the wave vector changes. Fig. 3 illustrates the scattering of a single neutron with the wave vector k on a single scattering center. The incident neutron is assumed as a plane wave, travelling along the x-axis and then scattered isotropically at a single, fixed scattering center (nucleus). The scattered neutron has then to be assumed as a spherical wave with its origin at the scattering center. The amplitude of these wave fronts is b/r. With increasing distance from the scattering center the intensity of this wave decreases according to r -2. Since the scattering center is fixed, and no energy transfer takes place between the neutron and the nucleus, the magnitude of

16 16 Introduction the incident and scattered wave vector is equal and thus, called elastic scattering[21]. Fig. 3: Model of elastic scattering[21]. During inelastic scattering the magnitude and direction of the incident and scattered neutron vector changes. The scattered neutron can either loose energy or gain energy due to the scattering process. Scattering takes place on all nuclei of a material and the scattered waves can interact with each other as well. Depending on the interaction of the neutron waves among each other, they can give information on the structure of the irradiated material. The terms elastic and inelastic scattering can be further split into coherent and incoherent scattering. Coherent scattering describes the interference of neutron waves scattered at different nuclei. They depend on interatomic distances and give structural information about the material. Elastic coherent scattering provides information about the equilibrium state of a material, while inelastic coherent scattering gives information about collective motions of the atoms. At incoherent scattering, no interference can be observed from the scattered neutrons from different nuclei and the intensities of the scattered waves sum up independently. Thus, incoherent elastic scattering leads to unwanted background in neutron scattering experiments, whereas incoherent inelastic scattering gives information of the same atom at different positions and times, providing information about diffusion processes[21] Neutron capture The other interaction of neutrons with matter is the neutron-capture process. The neutron-capture cross section contributes to µ a in (1.6) and is typically tabulated as σ 0 for thermal neutrons (v=2200 m/s, T=293 K, E=25 mev). At cold neutron energies, the probability of neutron capture is inversely proportional to the velocity and is called the 1/v law. For different neutron energies it can be calculated according to ( ), (1.10) where σ(v) is the neutron capture cross section at the velocity v, σ 0 is the thermal neutron-capture cross section and v 0 is the velocity of a thermal neu-

17 Introduction 17 tron. Most nuclides follow the 1/v law at thermal, sometimes even at higher energies. After neutron capture the binding energy of the neutron is released. Typical neutron binding energies are in the range of 6-10 MeV for about 80% of the stable nuclei. They are released subsequently to the neutron capture reaction in form of gamma radiation. Within a time interval of approximately s the excited nucleus decays and emits the binding energy as a cascade of gamma radiation[24]. Because the time resolution of present detection systems is in the range of a few ns to µs, the sequence of the emitted gamma cascade cannot be detected separately. The radiation emitted directly after the neutron capture process is called prompt gamma radiation as illustrated in Fig. 4. The emitted γ-rays are characteristic for each isotope and thus, they can be utilized for chemical analysis. The analytical technique based on the detection of this radiation is called prompt gamma activation analysis (PGAA). If the ground state of the produced daughter nuclide is stable, the nuclear reaction ends like in the case of 1 H(n, ) 2 D. If not, the most prominent reaction, which follows the neutron capture, is the β - decay, as in the example of the 55 Mn(n, ) 56 Mn reaction. In the following reaction, the formed 56 Mn decays to 56 Fe with a half-life of 2.58 h[26] according to. (1.11) Fig. 4: Illustration of the A X(n, ) A+1 X reaction and the later decay reaction [25] The β - decay is based on the decay of a nuclear neutron to a proton, an electron and an anti-neutrino. Half-lives of β - decay following neutron capture on stable nuclides may vary from milliseconds to billions of years. The β - decay is followed in most cases by the emission of gamma radiation. This can also be used for chemical analysis, and is called neutron activation analysis (NAA). The neutron-capture cross sections vary widely from nuclide to nuclide. To illustrate this, the case of hydrogen is taken where 1 H has a neutron-capture

18 18 Introduction cross section of mbarn in contrast to 2 D with a neutron-capture cross section of mbarn[5]. Another important reaction following neutron capture is the emission of a charged particle. Examples for these reactions are: 3 He(n,p) 3 H (used in neutron detectors), 6 Li(n,t) 4 He (utilized in scintillation screens), and 10 B(n,αγ) 7 Li (used in boron based neutron shielding materials) Gamma radiation Gamma radiation is the most energetic, electromagnetic radiation known. It interacts relatively weakly with matter and for example, a gamma ray with an energy of 1 MeV produces only one ion pair per centimeter of air[27]. The interaction of gamma radiation with matter takes place in the electron shell of an atom. In general, the attenuation of gamma radiation also follows the exponential law of attenuation shown in Eq. (1.5). The mass attenuation coefficient µ m depends in the case of gamma radiation on the number of electrons surrounding the absorbing atom and is expressed as (1.12) where ρ is the density, M is the molar mass, N A is the Avogadro constant and Z is the average atomic number of the absorber. σ a and σ e stand for the atomic reaction cross section and the electron reaction cross section, respectively. They are measures for the probability of a photon, to interact either with the electron cloud of an atom in general (σ a ), or with a single electron as part of the electron cloud (σ e ). To calculate the shielding power of a material for gamma radiation, halfthickness values are commonly used. They give the thickness x 1/2 of a specific material, at which the incident radiation is attenuated to a factor of 0.5. The half-thickness values of a material are depending on the material and can thus be expressed directly as a function of the attenuation coefficient µ with ( ) (1.13) Depending on the energy of the gamma photons, four modes of interaction are observed for gamma radiation with matter: Coherent scattering, the photo effect, Compton scattering and Pair production Coherent scattering In coherent scattering, the incident gamma ray is absorbed and immediately re-emitted (Fig. 5). Compared to the other attenuation processes. The gamma ray changes its direction but not its energy. The effect of coherent scattering increases with increasing Z and its effect is about 20% of the total attenuation for 0.1 MeV gamma rays[28]. It s contribution to the attenuation of gamma rays with different energies is illustrated in Fig. 9.

19 Introduction 19 Fig. 5: Schematic sketch of coherent scattering Photoelectric effect The photoelectric effect is the strongest absorption process for low-energy gamma rays. An incident gamma ray is completely absorbed by an atom and an orbital electron is excited above its binding energy. This electron is subsequently ejected from the atom with the creation of an electron-ion pair. Fig. 6: Schematic sketch of the photo effect. The energy of the emitted photoelectron is equivalent to the energy difference of the incident gamma ray and the binding energy of the orbital electron. Depending on the electron shell of the emitted photoelectron, secondary reactions, like the emission of characteristic X-rays, also occur when the vacancy is re-occupied or an Auger electron is emitted. The effect of the different electron shells is strongest for tightly bound electrons and decreases for the outer shells (K>L>M). The photo effect is the reason for the steps, indicated with the respective shell label in the total mass attenuation coefficient, shown in Fig Compton effect Fig. 7 illustrates the Compton effect. At higher gamma energies, the incident gamma rays do not interact with the field of the whole atom anymore as described in the photoelectric effect, but interact with the electrons directly. The incident gamma ray transfers a part of its initial energy to the interacting electron, which is then ejected from the atom. Its remaining energy is the difference of the initial energy and the kinetic energy of the ejected electron. Compton scattering can be observed only for loosely bound electrons and happens thus, mostly in the outer shells of the atom. Its probability increases with increasing Z, and decreases with increasing gamma-ray energy. The energy of the scattered gamma ray can be still sufficient for further interactions.

20 20 Introduction Fig. 7: Schematic sketch of the Compton effect Pair production Pair production is the fourth process, how gamma radiation can interact with matter. The name pair production originates from the type of interaction, producing a positron and electron pair from the energy of the incident gamma ray in the coulomb field of a nucleus. It is the inverse process to positron annihilation, where gamma radiation with a total energy of MeV is emitted after the annihilation of a positron and an electron. Fig. 8: Schematic sketch of the pair production process. For pair production, an energy of at least MeV is necessary, which corresponds to the rest mass of the two emitted particles (511 kev each). The remaining energy of the incident gamma is transferred to the two emitted particles in the form of kinetic energy. For this reason pair production can only be observed at gamma energies higher than MeV and is the main absorption process for gamma radiation with higher energies as illustrated in Fig. 9. The produced positron will annihilate with an electron in the absorber material followed by the emission of two 511 kev gamma rays, which are absorbed by the other processes described earlier Contribution of the different interaction modes The contribution of the different processes to the total attenuation of gamma radiation in matter is illustrated in Fig. 9 as a function of energy, based on data downloaded from the XCOM database[29]. For a better visualization, the downloaded, values corresponding to discrete gamma energies are plotted as a line diagram.

21 Introduction 21 Fig. 9: Partial attenuation coefficients of different interaction processes and the total attenuation coefficient for lead. The plot is based on values downloaded from the XCOM database[29] and the effect of the different electron shells is indicated with K, L, and M.

22 22 Introduction 1.3 Principles of Prompt Gamma Activation Analysis The activation of a sample Prompt gamma activation analysis is a nuclear analytical technique, which records the emitted gamma rays in the form of a spectrum during the irradiation with neutrons. It is a multi-elemental technique, which allows the detection of a broad range of elements simultaneously and is independent of the shape of the sample due to the deep penetration of neutrons. Preferentially, low-energy, cold neutrons are used for PGAA measurements since the neutron capture cross section increases with decreasing energy according to the 1/v-law given in Eq. (1.10). In general, all neutron-capture cross section values are tabulated (e.g. in [30]) for thermal neutrons with a velocity of 2200 m/s and can be calculated in most cases from the tabulated values for the specific experiment. The number of interactions in an ideally thin sample irradiated with a monochromatic neutron beam can be expressed with the reaction rate R expressed as, (1.14) where n is the number of atoms in the neutron beam, σ is the neutron-capture cross section and Φ is the neutron flux. From the reaction rate, the number of the gamma rays emitted during the irradiation can be derived. It is expressed with the activation equation ( ) ( ), (1.15) where A( ) is the net peak area at the energy, t is the live time of the measurement, m is the mass of the emitting nuclide, M is its molar mass, N A is the Avogadro constant, σ γ is the partial gamma-ray production cross section and Φ is the neutron flux[24]. Equation (1.15) holds for the case, when the neutron flux is homogeneous in the sample and the sample material does not absorb the emitted gamma radiation. For typical applications of PGAA this approximation is valid, otherwise corrections have to be applied Characteristics of a PGAA spectrum In the following, a few characteristics of prompt gamma spectra will be explained to illustrate a few differences to decay gamma spectra. Fig. 10 shows a prompt gamma spectrum over its full energy range from 0 kev to 11,000 kev and a decay spectrum of a radioactive 152 Eu source for comparison, recorded for the calibration of the detection system. Compared to typical delayed-gamma spectra with an energy range from 0 kev to 2,500 kev, PGAA spectra cover a significantly wider energy range due to the high binding energies released in the neutron capture. The shown PGAA spectrum was recorded with an empty sample holder using a Teflon frame, containing 0.28 mm thick fluorinated ethylene propylene copolymer (FEP) strings and can thus be regarded as the background of standard PGAA experiments. The sample chamber was evacuated to a pressure of 0.3 mbar to reduce the

23 Introduction 23 gamma signal from nitrogen in the air. In a background spectrum, still 150 to 200 gamma peaks can be identified due to the elements hydrogen, boron, carbon, fluorine, aluminum, silicon, sodium and lead. They are emitted from construction and shielding materials (aluminum, boron, carbon, fluorine, lead), the sample holder (carbon and fluorine) or the neutron guide (sodium and silicon). Traces of nitrogen can be observed due to residual nitrogen from air leaking into the sample chamber and are then activated in the high neutron flux, available at FRM-II. A detailed analysis background introduced with different packing materials will be discussed later in the experimental section. Fig. 10: Characteristic parts of a full PGAA spectrum of an empty sample holder. The baseline of the spectrum is low at very high energies and increases towards low energies with increasing number of peaks and intensity due to Compton scattering. On the high-energy end the characteristic peak of nitrogen at 10,829 kev is visible in an almost background-free region. Another important section of the PGAA spectrum is illustrated in Fig. 11. It shows the section from 460 kev to 530 kev with the Doppler-broadened boron peak and the annihilation peak due to pair production of energetic gamma rays. Boron is represented in nature with the two stable isotopes 10 B and 11 B. The natural abundance of 10 B is 20%, and of 11 B 80%[26]. Both isotopes have relatively low (n, ) cross sections with 100 mb in the case of 10 B and 11 B. However, the 10 B has a much higher cross section (763 b) for a neutroninduced alpha emission according to 10 B(n,αγ) 7 Li (1.16) In this reaction, a gamma ray with an energy of 478 kev[30] is also emitted. The 7 Li nucleus experiences strong recoil forces due to the emission of the α- particle and is still in motion when the gamma radiation is emitted. The resulting energy shift of the gamma ray due to the Doppler effect leads to the characteristic boron peak, shown in Fig. 11.

24 24 Introduction Fig. 11: Characteristic parts around the boron peak in an energy range of 460 to 530 kev Also the annihilation peak at 511 kev is slightly broadened compared to the regular gamma peak of chlorine at 517 kev. Also this broadening can be attributed to the kinetic energy of the positrons formed during pair production Quantitative gamma spectroscopy To apply prompt gamma activation analysis for quantitative analysis, the efficiency of the gamma spectrometer has to be determined. Therefore, the gamma spectra of two certified radioactive sources, in the present case 152 Eu and 133 Ba are recorded. In addition, the prompt gamma spectra of nitrogen in the form of an urea pellet and chlorine in form of a sheet of PVC foil are acquired. All peaks of the measured spectra are evaluated. After the evaluation, the spectrum of the 152 Eu source is used for absolute calibration and the peaks of the other spectra are normalized to the spectrum of the certified source based on the overlapping regions. A 6 th -order polynomial is used to describe the efficiency function from 50 to 11,000 kev, as presented in [31]. For the determination of the elemental composition of a sample with PGAA, every peak is assigned to an element based on energy match. With rearranging Eq. (1.15), a mass value m(e ) can be determined from the corresponding peak area at the energy E, according to ( ) ( ) (1.17) All determined masses m(e ) are then averaged to obtain the mass of the elements in the sample. From these average masses final concentrations are calculated for all visible elements[31]. Even if PGAA is a matrix independent technique, this approach assumes a homogeneous activation of the sample. Though the intensity of the signal is still matrix-independent, the detection limit of a given element depends on the matrix[32]. The minimum detectable area of a given peak is determined by the dominant peaks in the spectrum. Thus the area ratios of two close-lying characteristic peaks typically lie in a range of These six orders of magnitude can be regarded as a broad dynamic

25 Introduction 25 range. It can also be expressed in the unit of molar ratios (n 1 /n 2 ), when the peak area ratio (A 1 /A 2 ) is divided by the neutron-capture cross section ratio (σ 1 /σ 2 ), and the efficiency ratio (ε 1 /ε 2 ), as can be seen from Eq. (1.18), derived from the Eq. (1.15). ( ) (1.18)

26 26 Introduction 1.4 Principles of neutron imaging Radiography describes a technique, where the attenuation of an incident radiation after passing through an object is recorded. One of the mostcommonly known radiography methods is X-ray radiography in medicine. It provides information about the inner parts of the human body. The use of X-rays is advantageous for samples containing light elements since the absorption of X-rays depends on the atomic number. In contrast to X-rays, neutrons are not affected by the relatively large electron cloud of an atom and only interact with the nucleus. They penetrate much deeper in samples that contain heavy elements, and provide, in most cases, complementary information to the X-ray technique. Neutron radiography is a useful, nondestructive tool, especially for research performed in industry or in archeology, where bulky or valuable samples need to be investigated. In this chapter the principles of neutron radiography and tomography will be explained and follow the descriptions of A. Kak[33] and related works in the field of neutron radiography and tomography[34-36] Principle of neutron radiography When an incident beam of neutrons passes through a sample, the transmitted intensity of the beam will be reduced because of neutron scattering and absorption as discussed earlier. For simplicity, in the following description only the process of absorption will be taken into account and scattered neutrons are assumed to leave the field of view of the camera system. To explain the principle of radiography, a monochromatic, parallel neutron beam is assumed to pass through a sample and reach a 2-dimensional detector screen as illustrated in Fig. 12. Fig. 12: Concept of radiography with a parallel neutron beam and detection on a 2-dimensional detector screen[35]. A neutron beam, which passes through a sample is attenuated according to the Lambert-Beer law, given in Eq. (1.5). If the trajectory follows a straight path x, and the sample is made of a homogeneous material, a constant mass attenuation coefficient µ can be used to calculate the attenuation. However, real samples are often inhomogeneous and contain more than one material

27 Introduction 27 with different mass attenuation coefficients as illustrated in Fig. 12 with different colors. In such cases, the total mass attenuation coefficient along a straight path through the sample depends on the contribution of all mass attenuation coefficients along the trajectory of the beam and can be calculated according to (1.19) where N is the number of nuclides and each nuclide contributes to the total µ m by its mass fraction α i and the respective mass attenuation coefficient µ m. If all coefficients along the path are considered correctly, the sum of all fractions is unity. In conclusion, radiography is the recording of the integral of attenuation coefficients along the flight path through an inhomogeneous object. For the described case, a perfect, parallel neutron beam is assumed to map the attenuation of a sample on the scintillation screen. In real experiments, neutrons are produced in a reactor or a spallation source and are then guided towards the instruments. These beams are often inhomogeneous and divergent, and may need a collimation to achieve the proper resolution. Common imaging setups use therefore a pinhole collimation close to the neutron source, which produces a quasi-parallel neutron beam thanks to a long flight path[37]. Because the neutron beam is not perfectly parallel, a blurring of the image can be observed with increasing distance of the sample to the scintillation screen. The magnitude of the blurring can be characterized by the L/D value. The L/D ratio describes how well a point P of the sample can be mapped on a position sensitive detector screen as illustrated in Fig. 13. Fig. 13: A point P in the sample will be mapped on an area with the diameter d depending on the size of the pinhole and the distance of the sample to the detector. In the case of a perfect, parallel beam, a point P will be imaged perfectly on the scintillation screen. However, in the case of real experiments a point P of the sample will be mapped on an area with the diameter d on the scintillator screen. The size of the mapped area from an initial point, can be calculated according to

28 28 Introduction (1.20) and is usually expressed with the L/D value of the instrument. Following Eq. (1.20) an increase of the pinhole diameter D will increase the blurring proportionally. On the other hand, a smaller pinhole diameter will increase the resolution at the cost of a decreased neutron flux, which will result in longer exposure times. To achieve the highest resolution, the distance between the sample and the detector should be minimized, but usually depends on the proportions of the sample. Typical L/D ratios of imaging facilities are in the range of 200 to 1,000[37] Principle of computed tomography As neutron radiography provides two-dimensional information, computed tomography helps to reveal inner structures of the sample. Fig. 14: The concept of computed tomography[35]. Fig. 14 illustrates the concept of computed tomography. During an experiment, radiographs of the object are recorded under different angles. The acquired information is then computed with a reconstruction algorithm and the result is an image stack of slices, showing the spatial distribution of the different attenuation coefficients within each slice. By assembling these slices three-dimensionally, the inner structure of the object is shown and software tools allow a visualization of the results. Depending on the software, special parts of the inner structure may be colored and clipping of the image along surfaces and planes is possible to illustrate the inner parts. In the following, a brief introduction to the principles of the reconstruction process is given and more detailed information can be found in [33-35,37]. When using a parallel and monochromatic neutron beam, magnification effects due to beam divergence and energy dependence of the attenuation coefficient vanish. In the following Fig. 15 the relation between the image space, the Radon space and the Fourier space is shown for the following discussion.

29 Introduction 29 Fig. 15: Relation between image space, the Radon space, and the Fourier space according to the Fourier slice theorem. The object is defined as the distribution of mass attenuation coefficients µ m in a single plane, represented by the two dimensional function of attenuation coefficients µ(x,y). If parallel beams pass through the object, the total attenuation can then be described with a function of line integrals with parameters (,t) along the trajectory s, which reads ( ) ( ) ( ) The application of the delta function on p θ (t), it is rewritten as (1.21) ( ) ( ( )) ( ) ( ) (1.22) also known as the Radon transform of the function µ(x,y)[33]. To reconstruct an image from its set of projections recorded at angles θ(0 θ π), the inverse of the Radon transform has to be computed according to the Fourier Slice Theorem which reads: The 1D Fourier transform of a projection taken at an angle θ equals the central radial slice at an angle θ of the 2D Fourier transform of the original object. [35] The Fourier Slice Theorem is illustrated in Fig. 15 on the right. It states, that if all values in the two-dimensional Fourier space are known, also all attenuation properties of the original object can be determined from the reconstruction. The different steps can be expressed with

30 30 Introduction ( } ( ) ( )} ( ) ( ), (1.23) meaning that the spatial distribution of µ(x,y) is obtained by performing the inverse 2D-Fourier transform of P θ (ω). ( ) ( ( ( ))) (1.24) For the attenuation of a single ray of neutrons along the flight path s, the total attenuation is expressed by the line integral of attenuation coefficients distributed in the surface. In reality, it is not possible to acquire an infinite number of data within one experiment to cover all possible angles in the two dimensional Fourier space, which is illustrated schematically in Fig. 16. Real measurements lead only to a discrete number of recorded angles, which result in a decrease of the information density towards higher frequencies. Also the information density of each projection is limited due to the resolution of the detection system and thus only a few data points on the radial lines in Fig. 16 are known from the experimental data. Fig. 16: Measured values in the frequency domain. For the case that the experiment is performed with the acquisition of a sufficient number of pixels and angles, the Fourier transform in Eq. (1.24) can be replaced with a discrete Fourier transform. However, for the case of performing a discrete Fourier transform one has to consider the increase of information density towards the rotation axis as illustrated in Fig. 16. The procedure to do so is called filtered back projection and more detailed information about the reconstruction and computer tomography in general can be found in [33] Principles of prompt gamma activation imaging (PGAI) In 2006, an international collaboration to evaluate the use of different neutron techniques for the investigation of objects of cultural heritage was started,

31 Introduction 31 called Ancient Charm (Analysis by Neutron Resonant Capture Imaging and other Emerging Neutron Techniques: new Cultural Heritage and Archaeological Research Methods)[38]. The project lasted for 48 months and was carried out at different neutron research facilities like the Institute of Isotopes in Budapest, Hungary, the neutron source ISIS in the UK, the Joint Research Centre at the Institute for Reference Materials and Measurements of the European Comission in Geel, Belgium, and at the Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II) in Garching, Germany. Especially at FRM II in Garching first experiments were performed to combine PGAA with neutron radiography and neutron tomography. The goal of the study was the collection of structural information (neutron imaging) and elemental information (PGAA) of valuable artifacts with both techniques. The concept of the developed method called PGAI[25-28] is illustrated in Fig. 17. Fig. 17: Concept of PGAI (taken from [27]) Initially computed tomography is performed on the object to gain information on its homogeneity and inner structure. In the following step, positions for the elemental analysis are selected from the reconstructed tomography and the coordinates determined. Subsequently, PGAA measurements are performed on the sample with a strong, collimated neutron beam to limit the activation volume of the sample. The HPGe detector to record the emitted gamma rays is located behind a gamma collimator. The crossing of both collimator axes defines the volume, from where the gamma photons are detected, called isovolume. The inner diameter of the two collimators defines the resolution of the technique. With better resolution, the count-rate of the recorded gamma radiation decreases. Since the sample is positioned for the spatial analysis of

32 32 Introduction its chemical composition and the coordinates of the recorded spectrum are known, in the end, both information can be combined in one image[37,38].

33 Introduction Overview The reported experiments in the following chapters are split into two main parts: Prompt gamma-ray activation analysis and neutron radiography/tomography. In the first part, developments and applications of PGAA are presented, which were performed in the frame of this work. This chapter is followed by a chapter, which describes the characterization of the neutron beam at the PGAA instrument and the subsequent development of the neutron tomography setup. In the first part of the PGAA section the concept of increasing the dynamic range in PGAA is explained, followed by a validation of the method with appropriate samples. It is then applied for the characterization of samples with high-boron content and copper containing alloys. The second part of the PGAA section describes the evaluation of different sample holder materials. Goal of the study was the search for a material, which contributes negligible to the background of the gamma spectra. Such a sample holder can then be used especially for the characterization of samples with low neutron-capture cross sections or extremely low sample masses, smaller or equal to 10 mg containing of e.g. zirconium or carbon. In the last part of the PGAA section describes an experiment, which involves PGAA as a characterization method. It was an interdisciplinary study about the characterization of synthesized, orange-colored ZnO powders. PGAA measurements were performed to determine the concentration of nitrogen in the bulk of the synthesized ZnO powders. In the frame of this work, various samples were synthesized and compared to the values reported. In addition, different modern characterization techniques were applied for a refined investigation of the synthesized powders. PGAA played in this case an important role in the performed study since the penetration depth of neutrons in ZnO allowed a homogeneous activation of the complete sample material and nitrogen emits a very characteristic gamma peak in an almost background-free region of the gamma spectrum. The second main part of this work is about the development of a neutron tomography setup at the PGAA instrument. First, the different possible setups were evaluated and the resulting neutron beam of each setup was characterized. In the next part, the investigation of different scintillation screens for the camera system is reported and the resolution of these systems evaluated. The last part reports the development of the final setup for neutron tomography and how it was tested with the tomography measurements of an electric amplifier tube and an USB stick. The last part of this thesis is dedicated to first experiments towards PGAI, a combination of PGAA and neutron imaging. Due to the lack of time, during the first steps the field of view of the present PGAA gamma collimator was evaluated. This was achieved by moving a radioactive 152 Eu source along an axis perpendicular to the axis of the gamma-ray collimator. The field of view was then characterized as a function of the gamma-ray energy.

34 34 Introduction

35 Experimental Experimental 2.1 The PGAA instrument at FRM II The PGAA instrument used at FRM II was originally designed and operated at the Swiss Spallation Source SINQ by the Paul Scherrer Institute in 1997[39,40]. The plans for a new high-flux research reactor in Garching[41,42] offered a perfect opportunity to relocate the instrument and increase the cold neutron flux from 1.4 x 10 8 n cm -2 s -1 [43] to a cold neutron flux of n cm -2 s -1 (thermal equivalent of 6.07 x n cm -2 s -1 ), an increase by two orders of magnitude[44]. In August 2004 the reactor at the Forschungsneutronenquelle Heinz Maier Leibnitz (FRM II) went critical for the first time and in the time from 2004 to 2006 the PGAA instrument was carefully redesigned before it was ready for operation at FRM II. The instrument is located approximately 51 m from the reactor core at the end of a curved neutron guide in the neutron guide hall of FRM II shown in red in Fig. 18. Fig. 18: Schematic drawing of the reactor buildings and experimental halls, showing the position of the PGAA instrument indicated with red color. (taken and slightly modified from [45]) The neutron guide starts from the cold neutron source of FRM II[42], a tank filled with liquid deuterium. It is operated at a temperature of 25 K, which corresponds to a mean neutron wavelength at the PGAA instrument of 6.7 Å or 1.83 mev in terms of energy[15,46]. The first 41 m of the super-mirror based neutron guide (m = 2) at the reactor side has a strong curvature with a radius of 390 m [46]. The following 6.8 m (m = 3) are kept straight to homogenize the neutron beam, since the curvature introduces an inhomogeneous beam profile. To further increase the neutron flux, the straight section is elliptically tapered in horizontal and vertical directions with a focus of the ellipse in a distance of 35 cm in front of the exit of the neutron guide[46].

36 36 Experimental In addition, to the installed neutron guide, an elliptical extension can be introduced at the end of the neutron guide to further increase the neutron flux. It is a 1,100 mm long, elliptical extension to the neutron guide, leading to an even higher neutron flux of 6 x n cm -2 s -1 (thermal equivalent) in the focal spot at a distance of 90 mm in front of the end of the elliptical nose. Due to constructional issues with the present sample chamber, the actual sample position is located at a distance of 110 mm from the end of the neutron guide, which leads to a maximum flux of 4 x n cm -2 s -1 (thermal equivalent) at the sample position. With the neutron collimator setup a maximum flux of 2 x 10 9 n cm -2 s -1 (thermal equivalent) could be achieved. In addition, the neutron flux can be reduced with neutron attenuators made from a composite material called Boral [47]. Three attenuators are available, which allow the attenuations to 18% (Attenuator A1), 50% (Attenuator A2) and 6% (Attenuator A3) of the initial beam intensity. Combinations thereof are also possible. More detailed information about the characteristics of the PGAA instrument at FRM II can be found in [15,17,43,44,46].

37 Experimental Standard PGAA Standard PGAA measurements are performed either with the collimator or the elliptical guide in the neutron beam. Depending on the sample mass and material, the neutron attenuators may also be used to prevent a significant distortion of the gamma peaks in the spectrometer due to very high count rates. The main construction materials for the parts close to the neutron beam are fluorinated hydrocarbons like Teflon. The reason for that is the 34 times smaller neutron-capture cross section of fluorine compared to hydrogen. Thus, the introduced background is reduced significantly. During the sample preparation, the samples are packed in fluorinated ethylene propylene copolymer (FEP) foils with a thickness of 0.25 mm and sealed to prevent a contamination of the instrument with sample material. The packed samples are then placed between FEP strings with a diameter of 0.28 mm in the Teflon sample holder frames. For automated measurements, six frames are fixed on a mobile holder with an angle of 45 to the neutron beam direction. They can be positioned using a stepper motor, controlled with software that was developed in the frame of this work. A schematic concept of the PGAA instrument is shown in Fig. 19. Fig. 19: Schematic view of the PGAA setup with its shielding materials in the standard setup. The inner part of the sample chamber is lined with LiF-containing plastic to shield against scattered neutrons and prevent the activation of the outer aluminum housing. LiF as shielding material is advantageous over boroncontaining neutron shielding materials since no gamma radiation is emitted after absorbing neutrons. Especially the inner part of the sample chamber, which is in the field of view of the gamma collimator, has to be carefully shielded with Li-containing absorbers to reduce the spectral background. In front of the high-purity germanium (HPGe) detector, additional ceramic sheets made of LiF are mounted to protect the detector from scattered neutrons. Other areas around the sample chamber, which are not in the field of view of the HPGe detector, are shielded with sheets of rubber containing 50 % boron

38 38 Experimental carbide mounted to the inner part of the lead shielding. To improve the spectral background and prevent the activation of shielding material from neutron exposure, the first 5 cm of the lead shielding that face the sample chamber are antimony-free. Lead bricks usually contain 1-4% of antimony to increase the hardness of the material. The rest of the instrument is constructed using normal lead bricks. To allow a better view on the important parts of the PGAA instrument, two images of the disassembled instrument are shown in Fig. 20. A photograph, showing the bismuth germanate (BGO) scintillator, surrounding the HPGe detector is shown on the left. This is the alignment of detectors, used during PGAA measurements. The BGO is covered with sheets of Li-containing plastic (yellow in the left image) to prevent neutron activation of the detectors. Also the inner part of the sample chamber is lined with Li-containing plastic (pink), as shown on the photograph on the right in Fig. 20. All other parts are covered with sheets of boron-containing rubber. Fig. 20: Disassembled PGAA instrument in its standard configuration For a better understanding, a CAD model of the assembled gamma spectrometer is shown in Fig. 21. Some parts are shown transparent to allow the view of the inner parts of the spectrometer, usually hidden by thick layers of lead. Fig. 21: Schematic model of the present PGAA setup and its main components

39 Experimental 39 The axis of the gamma collimator is aligned perpendicular to the neutron beam and is shielded with a 20-cm-thick lead wall. The gamma collimator is a straight, cylindrical hole in the lead shielding with a diameter of 2 cm. It allows a direct view of the HPGe detector towards the sample position as illustrated in Fig. 19 and Fig. 21. The detection system contains a 60-% Ortec (n-type) Pop-Top detector, surrounded by a bismuth germanate (BGO) scintillator. The detector and scintillator are connected to an Ortec DSpec-50 digital spectrometer and operated in anti-coincidence mode to filter all signals, which are registered in the detector and the scintillator simultaneously. The spectrum is read out from the DSpec-50 spectrometer with the Maestro-32[48] software, also used for the control and setup of the spectrometer. New Python based[49] software was developed with a graphical user interface (GUI) during this work, to perform automatic sample changes controlled by the DSpec-50 spectrometer. An image of the user-interface and a brief introduction how to perform automated measurements is given the appendix. To determine the efficiency of the gamma spectrometer, the spectra of two radioactive sources were measured ( 133 Ba and 152 Eu)[32]. In addition, two neutron-induced prompt gamma reactions (activation of chlorine in the form of a PVC foil and of nitrogen in the form of a deuterated urea pellet) were used to compile the efficiency over the energy range from 50 kev to 11 MeV. The spectra were evaluated and the efficiency function determined using Hypermet-PC[50]. The efficiency is then introduced into the data evaluation program ProSpeRo[31], which is used for chemical analysis at the Garching PGAA laboratory. ProSpeRo assigns the fitted peaks to the elements based on the energy match. Then mass values for all the assignments are calculated according to Eq. (1.17). For every identified element, the mass values are then averaged[31] and a total composition is calculated.

40 40 Experimental Modified PGAA setup with a gamma attenuator For the determination of samples with e.g. high boron contents (see 2.2.1) or copper alloys (see 2.2.2), a special setup was developed and tested. Based on the Lambert Beer law, the gamma-ray transmission through lead sheets with different thicknesses was calculated for different energies according to Eq. (1.5) and plotted in for different energies as shown in Fig. 22. Fig. 22: Transmission of gamma rays through lead sheets with different thicknesses as a function of the energy. The energy of the boron gamma ray at 478 kev is indicated with a vertical line[32] The values for the mass absorption coefficients of lead at different energies were taken from the XCOM database[29]. The goal was to modify the efficiency of the spectrometer in such a way, that the intensity of the boron peak is suppressed by at least a factor of 5 and the suppression at higher energies is less than a factor of 2. A layer thickness of 10 mm lead meets both criteria, as can be read from the above plots. In front of the gamma collimator a 10-mm-thick lead attenuator was introduced as shown in Fig. 23. Fig. 23: Lead attenuator with 4 mm LiF polymer for neutron shielding between sample chamber and gamma collimator.

41 Experimental 41 On the left side of Fig. 23, the PGAA sample chamber is shown. It shows the entrance window from the direction of the neutron guide, covered with a zirconium foil. Perpendicular to the beam axis, the attenuator is aligned in front of the gamma collimator and the lead shielding. To protect the HPGe detector from scattered neutrons, the front side of the attenuator is covered with additional 4 mm of Li-containing plastic. The attenuation was tested with the prompt gamma spectra of a chlorinecontaining sample in the form of a PVC foil. It was measured with and without attenuator and the transmission was determined from the ratio of the peak areas at different energies[32]. Chlorine emits well-distributed gamma rays from 292 to 8578 kev and has a relatively high thermal neutron-capture cross section of 33 barn[30]. For the evaluation of the chlorine peaks in the two spectra, the acquisition software Maestro-32[48] was used. In preparation of quantitative measurements with the attenuator, the efficiency curves with and without attenuator were determined according to the standard procedure described in section and evaluated using the software Hypermet-PC[50]. Finally the method was benchmarked with boron-containing samples of known compositions and copper containing alloys like bronze and brass. Further details of the tested substances are described in the following sections.

42 42 Experimental 2.2 Increasing the dynamic range of PGAA measurements Boron-containing samples Boron analysis is one of the major fields of PGAA[32]. If boron is present in high concentrations the neutron self-shielding will be strong in the sample. It has a cross section of 716 barn[30], which allows the detection of trace amounts of boron down to ng/g. The activation of samples with relatively high boron concentrations may sometimes even saturate the spectrometer. If the neutron flux in such a case is reduced to lower the overall count rate to a reasonable level, the necessary counting statistics for other elements can be reached only with very long measurements. In contrast to the characteristic, Doppler broadened gamma peak of boron at an energy of 478 kev, all other elements have most of their prompt gamma lines above this energy. If boron becomes a major component in the sample, the counts of boron do not just dominate the first 500 kev of the spectrum, but due to the appearance of random-coincidence sum peaks, up to 1,000 kev or even 1,500 kev. This makes the analysis of the higher-energy peaks rather complicated. To reduce the misbalance between the elevated low-energy part from boron and the rest of the spectrum, a lowering of the counting efficiency in this energy range can help. This is achieved by introducing a 10 mm made of a heavy metal like lead as described in section Before applying the tested method to characterize chemically synthesized samples, it was tested with and without gamma attenuator it was tested with the following stoichiometric compounds: Alborite (9Al 2 O 3 2B 2 O 3, Alborite PF03, Shikoku Chemicals), Alborex (9Al 2 O 3 2B 2 O 3, Alborex Y, Shikoku Chemicals), NiB (15.6 wt% boron content, Sigma-Aldrich), and TiB 2 (31.1 wt% boron content Sigma-Aldrich) in the form of homogeneous powders. All other, synthesized samples were measured with attenuator and the obtained results were compared to the expected values Copper alloys Another typical application of PGAA as non-destructive technique is the analysis of archeological objects. One example is the investigation of copper alloys of artifacts made of bronze or brass. Besides copper, such samples contain up to 24 wt.% of tin in the case of bronze and up to 45 wt.% of zinc in the case of brass. Other minor elements in these alloys can be Al, Si, P, Mn, Pb. In addition, elements like chlorine can also be detected, known to cause corrosion of the artifacts and allow the decision about appropriate conservation methods. Copper containing samples are challenging, since copper alone emits already 655 prompt gamma lines with the relative intensity of more than 1%. Depending, if the other main alloying element is tin or zinc, additionally 485 or 382 prompt gamma lines appear respectively[51].

43 Experimental 43 To illustrate the high amount of prompt gamma lines emitted from the irradiation of copper, Fig. 24 illustrates a full spectrum and a zoomed-in section of it from 0 to 1500 kev. Fig. 24: On the left side the spectrum of the element copper is plotted. To illustrate the high density of gamma-ray peaks the region from kev is shown on the right side separately. Up to an energy of 1,500 kev a total of 227 prompt gamma rays are known and hosted in a database of the International Atomic Energy Agency[51]. Even if not all of these gamma lines can be observed in a recorded PGAA spectrum, they still contribute to the background and increase the count rate seen by the detector. This results in an increase of dead-time and may even lead to saturation depending on the sample size and neutron flux. Most of the minor elements with the exception of As and Sn emit prompt gamma lines at higher energies than 4,000 kev which can be used for analysis as shown in Tab. 1. Tab. 1: Main prompt gamma energies emitted from different elements. All energies are given in kev P Al Si Mn Cu Zn As Sn Pb 636 1,179 1, ,171-3,900 3,034 3,539 5,527 7,306 1, ,230-6,786 4,733 4,933 7,058 7,637 7, ,294-7,422 7,724 6,379 7,244 7,916 6,958 6,810 2,112 7,368 Also in this case, the application of a gamma attenuator was found to be beneficial for the measurement of copper-containing samples to reduce the count rate in the low-energy part of the spectrum and improve the statistics in the high-energy range. To evaluate the effect of the 10-mm attenuator for the case of copper alloys, 3 samples were prepared and tested. Two of the samples were the modern, homogeneous alloys CuSn6 and CuZn36. The third sample was prepared from metal sheets as shown in Fig. 25c.

44 44 Experimental a) Modern bronze CuSn6, mg, 20 x20 mm, thickness 1 mm b) Modern brass, CuZn36, mg, 20 x 20 mm, thickness 1 mm c) Sample with the elements S, Fe, Cu, Sn, Pb. Total mass: mg Fig. 25: Samples for testing the effect of a 10-mm lead attenuator. All measurements with copper alloys were performed using the collimated neutron beam. The samples were irradiated in a cold neutron beam with a flux of 2.5 x 10 8 n cm -2 s -1 (thermal equivalent). The 10-mm lead attenuator was introduced in front of the HPGe detector as described in section The bronze sample was irradiated with and without attenuator to illustrate the effect of the attenuator. The evaluation followed the procedure for standard PGAA using Hypermet-PC[50] and the ProSpeRo[31] program.

45 Experimental Investigation of different sample holder materials with PGAA The high neutron flux available at the PGAA instrument at FRM II allows the measurement of samples with extremely low neutron-capture cross sections or sample masses in the range of mg to µg. These special applications require also the development of new sample-holder materials to reduce the introduced background. In the frame of an experiment evaluating the background of low-mass papers prepared from carbon nanowires, also different sample holder materials were evaluated. A circular patch of such a paper with an approximate diameter of 1 cm has a weight around 10 mg. For comparison, the introduced background of the Teflon strings used for standard PGAA, introduces a signal equivalent to 10 mg of carbon as well. Since the introduced background is in the same range of the sample material, the contribution of different sample holder materials like aluminum strings, lead strings, a pocket of aluminum foil, and carbon nanowires were tested and the corresponding mass evaluated. The tested sample-holder materials were irradiated in an evacuated sample chamber with the standard PGAA setup. The materials were irradiated in a cold neutron beam with different irradiation times. All irradiations were longer than 18,000 s to obtain sufficient statistics. Specific details about the irradiation times and neutron fluxes are given in Tab. 2. Tab. 2: Overview of the sample-holder materials with live times and neutron fluxes. Sample Live Time Neutron Flux (th. equivalent) Frame without Strings s 2.0x10 10 n cm -2 s -1 Frame with FEP Strings s 4.0x10 10 n cm -2 s -1 Frame with aluminum strings s 4.0x10 10 n cm -2 s -1 Frame with lead wires s 4.0x10 10 n cm -2 s -1 Frame with aluminum pocket s 7.2x10 9 n cm -2 s -1 Frame with carbon fibers s 4.0x10 10 n cm -2 s -1 In the following experiments, one Teflon frame was irradiated without strings. A second frame was equipped with 280-µm thick FEP strings as used for standard PGAA experiments (shown in Fig. 26 on the right). The other frames were equipped with aluminum strings (Goodfellow, 20 µm diameter), lead strings (Goodfellow, 125 µm diameter), a pocket of folded aluminum foil (Toppits, average thickness of 13 µm, shown with two samples in Fig. 26 on the left), and carbon fibers with thicknesses in the range of µm and a mass of 2-4 µg/cm[52]. The irradiation of the sample holder equipped with the carbon wires was performed in combination with an experiment to determine the neutron capture cross-section of an isotope enriched 90 Zr sample with a mass of 1 mg. The spectra were evaluated with Hypermet-PC[50] and ProSpeRo[31] according to our standard evaluation procedure for PGAA measurements.

46 46 Experimental Fig. 26: Images of the modified sample holders for the comparison of sample holder materials. The left image shows a frame equipped with aluminum pocket and a sample paper in it. The right image shows a frame with Teflon wires as wired during standard PGAA experiments.

47 Experimental Determination of nitrogen in ZnO powders with PGAA and other modern characterization techniques Introduction Zinc oxide is a II b -VI semiconductor compound that crystallizes preferentially in the hexagonal wurtzite-type structure and is experiencing a veritable research boom in recent years due to its unique properties. It is characterized by a direct wide band gap of approximately 3.4 mev and a high excitation binding energy of ca. 60 mev at ambient temperature[53], thus rendering it a transparent, clear and colorless material. Due to these properties, ZnO-based materials find their way in a wide variety of applications exploiting, e.g., the potential to act as catalyst[32-34][54-56]or as anti-bacterial material[57]. In the field of optoelectronics ZnO holds great promises towards the development of LEDs in the blue/uv range of the light spectrum[53,58-60] as an alternative to GaN (e.g. for LEDs or laser diodes), as radiation hard material for electronic devices, as material transparent in the visible part of the light spectrum for the use in electronic circuits, and as cheap, transparent, conducting oxide to replace ITO. In particular for its use towards high-end applications, a stable form of p- doped ZnO is essential. As-grown however, zinc oxide typically exhibits n- type conductivity[61] and, depending on the production process, hydrogen can be observed as an impurity acting as a shallow donor when located on interstitial sites[61-64]. The production of p-type ZnO requires these n-type donors to be overcompensated and shallow acceptor levels to be stabilized instead[65]. Neighboring group V elements such as nitrogen, phosphorous, and arsenic offer promising prospects of acting as stable acceptors within ZnO and have therefore been the target of considerable recent research[66]. Among these, nitrogen has probably been the most extensively discussed candidate to act as an acceptor on oxygen sites[67] due to its low ionization energy and the similarity of the two species in terms of ionic radii and electronic structure. On the other hand, recent experiments[68,69] and refined theoretical models[70] report nitrogen to act as a deep acceptor. The suitability of nitrogen to form stable p-zno therefore remains an unresolved issue, while establishing a reliable and controllable process for its production lies at the forefront of considerable ongoing research worldwide[57,71-78]. Another open question related to the synthesis of ZnO is the color of the product, which has been observed to range from pale yellow, over orange, to red[61,79-81]. Halliburton et al. annealed ZnO crystals in zinc vapor at high temperatures and reported red coloration, which was thereby attributed to an excess of zinc. Red color was additionally observed for ZnO samples annealed in phosphorus vapor[80] or in the presence of titanium and zirconium[81]. By invoking the ability of the latter to absorb oxygen, the crystal s red color was associated in these studies with the formation of oxygen vacancies in the ZnO lattice. In independent work, Selim et al. reported a reversible change of ZnO samples from red to colorless when annealing ZnO crystals in oxygen atmosphere at 1100 C for half an hour and back to red when annealing them in zinc atmosphere[81]. In contrast, Weber et al. used positrons to investigate the effect of hydrogen incorporation in ZnO and at-

48 48 Experimental tributed the observed red coloration to the presence of hydrogen in oxygen deficient ZnO[61]. Towards the production of nitrogen-doped ZnO, a number of different synthesis routes have been suggested and tested including molecular beam epitaxy[71,72,76,78], reactive sputtering[74] and chemical vapor deposition[73,74,77]. A rather straightforward yet nevertheless promising approach towards nitrogen incorporation into an oxide is to use an appropriate nitrogen source and promote nitrogen intake to the oxide via the solution combustion method (SCM). Along these lines, Mapa and Gopinath heated aqueous solutions of zinc nitrate and urea and reported the formation of orange-colored ZnO nano-crystals with bulk nitrogen concentrations of up to 15%[57]. In the present work, the method of Mapa and Gopinath[57] was adapted and a range of colored ZnO samples prepare by altering the initial reactant ratios. The products are subsequently characterized by a series of techniques including powder and single crystal X-ray diffraction, nuclear magnetic resonance (NMR) and Raman spectroscopy. In addition, Prompt Gamma Activation Analysis (PGAA) was applied for the first time for ZnO:N samples to detect nitrogen. Applying PGAA for the determination of bulk nitrogen in possibly nitrogen-doped ZnO powders helped in disentangling the observed effect of coloration and possible nitrogen doping Synthesis All samples were synthesized via the solution combustion method (SCM), as described by Mapa and Gopinath[57]. Zinc nitrate hexahydrate (Zn(NO 3 ) 2 6 H 2 O, Chempur 98+%) and urea (CO(NH 2 ) 2 Chempur 98+%) of analytical grade were used without further purification. The reactants were mixed in given molar ratios (their total mass was about 15 g) and filled into glass beakers. After dissolving the reactants in 10 ml of deionized water, the beakers were placed in a Nabertherm muffle furnace, and were heat treated at 500 C. After the combustion reaction, the beakers were removed from the furnace and left in air at ambient temperature for cooling. Upon reaching room temperature, the products were collected, ground and stored in closed glass tubes. A ZnO reference sample was produced by annealing 15 g of pure zinc nitrate hexahydrate in a glass beaker at 400 C in air for 20 minutes. The container was subsequently left to cool in air at ambient temperature, to yield a white/pale yellow powder. In the following, the samples will be denoted according to the employed urea : zinc nitrate hexahydrate molar ratios (i.e. a ratio of 2 : 1 corresponds to two molar parts of urea and one part of zinc nitrate hexahydrate). Based on the described synthesis route, various samples were prepared with reactant ratios ranging from 0.5 : 1 to 10 : 1. For extracting the additional organic phase (described later), detected in certain samples prepared with molar ratios around 2.3 : 1, the powders were completely dissolved in hydrochloric acid maintained at 70 C. Upon cooling and at temperatures of ca. 50 C, small white crystals were found on the surface of the liquid. These crystals were subsequently removed by filtration and re-dissolved in 20% hydrochloric acid at 100 C. After complete dissolution,

49 Experimental 49 the acid was cooled to room temperature over a period of 8 hours. White crystals were collected from the surface of the acid Characterization techniques Powder X-ray diffraction X-ray powder diffraction data were collected on a STOE StadiP diffractometer in the chemistry department of the Technische Universität München, using Cu K α1 radiation (λ = Å) and a flat bed sample holder in transmission setup. The patterns were recorded with a 130 image plate detector at room temperature and ambient pressure. Samples prepared with molar ratios of 1 : 1 and 2.3 : 1 were additionally measured at the high-resolution powder diffractometer P02 at the German electron synchrotron DESY. These samples were placed on a quartz capillary with a diameter of 0.3 mm and exposed to radiation of λ = Å at ambient pressure and room temperature Single crystal X-ray diffraction Single crystal diffraction measurements were performed on a IPDS 2T STOE diffractometer at the chemistry department of the Technische Universität München, using Mo K α radiation (λ = Å) at ambient temperature. The program suite Jana 2006 was used for the refinement and structure solution. The acquired data and resulting structural parameters can be found in the appendix of this thesis Nuclear magnetic resonance spectroscopy 1 H NMR (400 MHz), 13 C NMR (100 MHz), and 15 N NMR (41 MHz) measurements were performed on a Bruker Avance III 400 spectrometer at the chemistry department of the Technische Universität München. The chemical shifts of 1 H and 13 C NMR were calculated relative to the solvent signal for dmso-d6 (2.50 and 39.5 ppm)[82,83], while 90% CH 3 NO 2 in CDCl 3 was used as an external standard for the measurement of 15 N NMR Raman Spectroscopy Raman spectra were recorded on a Jobin Yvon Horiba HR800 UV Raman microscope with an attached Jobin Yvon Symphony CCD detector of 1,024 x 359 pixel 2 at the chemistry department of the Ludwig-Maximilian Universität München. A 633-nm laser beam was used for excitation, which was dispersed by a grating of 600 lines/mm. The fine powder samples were pressed by hand on a glass flat bed carrier. Spectra in the range of 150 to 1,300 cm -1 were collected in three measurement cycles and with an accumulation time of 15 s per scan.

50 50 Experimental Prompt Gamma Activation Analysis Prompt Gamma Activation Analysis (PGAA) was performed at Forschungsneutronenquelle Heinz-Maier Leibnitz. Powder samples with masses of ca. 200 mg were sealed in Teflon bags and irradiated in vacuum (0.3 mbar) in a cold neutron beam with a thermal equivalent flux of 4 x n cm -2 s -1. The standard PGAA setup[15] was modified by inserting the 10 mm lead attenuator in front of the high-purity germanium (HPGe) detector as described earlier. The use of the attenuator was found to be necessary in order to allow the application of a higher neutron flux. This eliminates the low energy peaks of zinc similarly to these of copper in the earlier described experiment and allows the measurement with higher count rates in the high-energy part of the spectrum, where the main gamma lines can be seen. Thus, the statistical uncertainties and detection limit of nitrogen for the actual case are improved. The modified PGAA setup with the attenuator was used as described previously [32]. The bulk concentrations of zinc, hydrogen and nitrogen were determined based on selected, element-specific gamma lines[30]: For zinc all characteristic gamma lines above 5,000 kev were taken into account, while hydrogen is known to provide only a single gamma line at 2,223 kev. The presence of nitrogen was examined by checking for its characteristic line at 10,829 kev in the normally background-free region of the PGAA spectrum. If detected, and depending on the sample's nitrogen concentration, other weaker gamma lines were additionally taken into account and fitted.

51 Experimental Neutron imaging at the PGAA instrument Introduction At the end of the PGAA neutron guide, two setups for the neutron beam are possible: the collimated and the focused neutron beam. Prior to the development of a neutron imaging setup at the PGAA instrument, the divergence and homogeneity of the neutron beam was examined. During irradiations, performed with the elliptical neutron guide (so-called nose) illustrated in Fig. 27, the sodium in the glass of the neutron guide activates. Fig. 27: Schematic drawing and photograph of the changed parts at the end of the neutron guide. Both systems can be changed after the reconstruction without opening the lead shielding. In the setup, used during the ANCIENT CHARM collaboration[84,85], a manual change between the two possible irradiation modes was possible and it was necessary to open the covering lead shielding. The change was only possible after a long cooling time to let the activated sodium decay away to prevent radiation hazards. In the present setup an automatic change of the irradiation modes is possible and thus, a neutron collimator of 20 x 20 mm 2 can be introduced in the neutron beam. For the installation of a new imaging setup at the PGAA instrument, the different possible irradiation modes were analyzed and the neutron beams with the new changing mechanism characterized. The focus of the study was the determination of the divergence angles and the horizontal intensity profiles of the neutron beam due to the curvature of the neutron guide[46]. The advantage of a curved neutron guide is that the transmission of fast neutrons and gamma radiation from the reactor to the instrument is prevented. However, it also leads to a strong inhomogeneous pattern in the intensity profile. A linear scan was performed along the beam axis and the intensity distribution on the scintillator screen recorded as a function of the distance from the end of the neutron guide as illustrated in the experimental part in Fig. 29. Based on the characteristics of the neutron beam, a neutron radiography setup was installed at the PGAA instrument. Different scintillation screens were tested to determine their effect on the resolution with a test target. It was found that the best performance is achieved with a 100 µm LiF/ZnS scintillator leading to good intensities at short exposure times. The developed setup was

52 52 Experimental then tested with tomography experiments of an amplifier tube and an USB stick to evaluate the performance Characterization of the detector system The detector box of the DEL-Cam[84], developed in the frame of a diploma thesis of M. Mühlbauer, was upgraded with a more powerful camera system for the following experiments. The original CCD camera was changed to an Andor DWZ436 CCD camera (Red part in Fig. 28) with a chip size of 2048x2048 pixels 2 and 16-bit pixel depth. During the experiments, the CCD array is thermoelectrically cooled to a temperature of -60 C for the reduction of thermal noise. The camera is attached with a 12 mm spacer ring to a Nikkor 35 mm lens system with an F number of F=1.4. Fig. 28: Schematic view of the modified housing of the DEL Cam Instead of the scintillation screen, an optical resolution target (NBS 1963A Resolution Target[85], Edmund Industrial Optics) was mounted at the position of the scintillator during the first experiments. The outside of the target was covered with 5 sheets of office paper to create a homogeneous, bright background and achieve a diffuse illumination of the target. During the calibration process, an image was recorded with the CCD camera, triggered and read out with the software Andor Solis[86]. After each image, the contrast of the line pairs on the reference target was evaluated and the lens system adjusted till the contrast reached a maximum Characterization of the neutron beam Detector setup In preparation for experiments with neutrons, the optical test pattern, used for the determination of the optical resolution of the detector system, was exchanged to a 300 µm thick 6 LiF/ZnS scintillator, converting incident neutrons to visible light. When the 6 Li isotope in the scintillation screen absorbs an incident neutron, the formed 7 Li isotope decays in a 3 H and 4 He particle, following (2.1)

53 Experimental 53 The energy of 4.79 MeV is released as the kinetic energy to the two particles. These are absorbed in the material of the scintillation screen and excite the ZnS. During the absorption of one neutron, the ZnS screen emits approximately 1.77x10 5 photons. The emitted light is in the green range of the light spectrum[34,37], which corresponds to the maximum sensitivity of the used CCD camera. The whole detector box was then mounted on a linear axis (IGUS, DryLin ZLW-series) according to the schematic drawing in Fig. 29. It shows the setup, used for the scan along the beam axis to characterize the divergence of the neutron beam and the effect of the neutron attenuators. To increase the stability of the axis, it was mounted on a X-95 profile. This allowed also a testing of the developed software and all components could be tested before the setup was mounted at the PGAA instrument. The linear axis was equipped with a tooth belt, driven by a stepper motor for the automated positioning of the detector along the neutron beam. The axis allowed a scan along a total length of 2,000 mm. The coordination system started at the beginning of the axis (black box of redirection roll) with reference position 0 mm, defined by the axis touching the lead shielding of the neutron guide. Fig. 29: Setup for the beam characterization along the beam axis. Once everything was tested successfully, the X-95 profile was mounted on the PGAA instrument parallel to the beam axis. An adapter plate was designed to move the camera face to face to the lead shielding at the exit of the neutron guide and cover the gap where the redirection roll of the tooth belt is located (black box in front of the axis). Optionally, a possibility was kept to mount an adjustable aperture holder and an adjustable sample holder on the linear axis to perform first neutron radiography experiments. For the scan, a simple program was developed in the script language Python[49] for the automation of the experiments. It was based on the TACO protocol[87] to establish the communication between the motion controllers and Andor Solis[86] for the image acquisition.

54 54 Experimental During the scan, no permanent neutron shielding could be assembled due to the moving detector system. Therefore all scans had to be performed with an attenuated neutron beam to reduce the emission of neutron and gamma radiation from the experiment site. Special care was taken for radiation safety issues also at the surrounding areas of the experiment. All surfaces along the neutron beam were lined with sheets of rubber, containing 50% of boron carbide, to efficiently absorb neutrons, scattered in the air along the flight path. The main beam was absorbed with a boron carbide beam stop at the end of the linear axis Scan with neutron collimation The scan of the neutron beam with the neutron collimator setup started 50 mm behind the shielding material of the neutron guide. It covered a total length of 1800 mm and was performed with a step size of 20 mm. At the time of the scan, the adapter plate was not ready for use and thus an investigation of the first 50 mm was not possible. The experiment was carried out without a neutron attenuator in the neutron beam. Exposure time for all acquisitions was 0.2 s Scan with a graphite diffusor A strong inhomogeneity in the intensity of the horizontal neutron beam profile, due to the strong curvature of the neutron guide was observed during the initial scan with the neutron collimator. To compensate for the inhomogeneity a neutron diffusor assembled by Nikolay Kardjilov at HMI Berlin was tested. The diffusor contained graphite powder, packed in a box of 1 mm thick aluminum sheets and edge lengths of 40x40x30 mm 3. Due to random Bragg scattering in the graphite powder, the inhomogeneous profile was homogenized at the cost of beam intensity. For the experiment with the diffusor the collimated neutron beam was used without attenuators. The diffusor was placed directly at the exit of the neutron guide and a scan was performed from 50 mm from the lead shielding backwards to a total distance of 1810 mm from the end of the neutron guide. The exposure time for all acquisitions was set to 1.7 s Scan through the focal area of the elliptical extension of the neutron guide The focal area of the elliptical nose was investigated separately from the rest of the beam profile. The reason for a separation of the total scan and the investigation of the focal area was the high intensity of the neutron beam in the focal area. To prevent an overexposure of the image, all attenuators were aligned in the beam (A1+A2+A3). The scan was performed directly from the lead shielding of the neutron guide in steps of 10 mm backwards to a total distance of 120 mm. The exposure time for all acquisitions was set to 0.5 s.

55 Experimental Scan of the beam profile of the elliptical extension behind the focus As continuation of the scan through the focal area, a second scan was carried out, starting from a distance of 50 mm behind lead shielding of the neutron guide. It started close behind the focus of the elliptical nose and was continued in steps of 20 mm backwards to a total distance of 450 mm from the lead shielding. Attenuator 1 was in the beam and the exposure time set to 1 s Effect of the neutron attenuators For a reduction of the neutron flux, at the PGAA instrument, three different attenuators, made from Boral [47], are available at the PGAA instrument. Construction drawings of these can be found in the appendix. Attenuator 1 (A1) contains vertical slits and attenuates the neutron flux down to 18% of the initial value. Attenuator 2 (A2) contains horizontal slits and attenuates the neutron beam to 50% of its initial intensity. Attenuator 3 (A3) contains holes and attenuates the beam to 6% of the initial intensity. To allow a full illumination of the detector screen, similar as it is recorded during neutron imaging experiments, the detector was positioned 1850 mm from the end of the neutron guide. The collimated neutron beam with the flight tube was used for this experiment. Each attenuator was introduced into the beam and images with an exposure time of 1 s were recorded. In addition, one image without opening the camera shutter was recorded as dark image and one of the unattenuated beam was taken as open-beam image for the normalization. During the evaluation, the dark image was subtracted from all other images. To determine the transmission of the attenuator, the normalized image of the attenuated beam is then divided by the normalized open-beam image Data analysis The data evaluation was performed with the software ImageJ[88]. The images were evaluated subsequently with developed macro programs for ImageJ and Python[49] software. In the first step, the images were loaded and a rectangular region with a height of 120 pixel and a width of 2048 pixel was selected surrounding the brightest part of the image as shown in Fig. 30. The brightness values of each column were then averaged and the averaged values saved in a text file. In the second step, a Python[49] program was developed for the visualization and fitting of the results. It plots the intensity profile as a function of the position on the scintillation screen for each image and calculates the width of the profile. In a final step, the widths of the profiles are plotted as a function of the distance from the end of the neutron guide. The slope is determined with different fitting methods and the opening angle of the neutron beam calculated.

56 56 Experimental a) Slice of the neutron beam with flight tube without neutron attenuator at position 50 mm. (exp. Time: 0.2 s) b) Slice of the neutron beam at position 0 mm with the elliptical nose. (exp. Time: 0.5 s) Fig. 30: The images show two examples of a measured slice during the experiment and the region used for the evaluation of the intensity profiles indicated with the white rectangles

57 Experimental Neutron radiography and tomography All experiments were performed without an attenuator and the elliptical nose is used due to the high beam divergence and the limited space available. The neutron imaging setup was built according to the concept shown in Fig. 32. A boron carbide aperture with a thickness of 3 mm and a circular pinhole with a diameter of 4 mm was introduced in the focus of the elliptical guide to collimate the neutron beam and to improve the resolution. Fig. 31: Schematic sketch of the finally chosen imaging setup with a 4 mm pinhole aperture and usage of the elliptical nose The flight path from the aperture to the detector had a total length of 800 mm and thus; the L/D was 200. Two different scintillation screens were tested with the final setup. To determine the maximum achievable resolution, a 10-µm GADOX (gadolinium oxysulfide) scintillator was installed. All other radiography experiments were performed with a 100-µm LiF/ZnS scintillator. The PGAA instrument was modified for neutron imaging according to Fig. 32. The sample chamber in the center of the instrument was removed and a XYZω positioning device manufactured by XHuber (Fig. 33) was mounted in the lower part of the carriage (Fig. 32). The positioning device was adjusted along a rail system, to allow a precise and reproducible alignment. Each axis was driven by a stepper motor and controlled with a Phytron-MCC2 motion controller. The linear axis could be moved along a total length of 200 mm and the rotation part turned around 360. The sample holder is mounted on top of a 500 mm long aluminum pin (Fig. 33) on top of the rotation table to maximize the distance from the neutron beam. The inner part of the sample chamber is lined with boron-containing rubber to prevent the activation of the surrounding lead shielding by scattered neutrons. Along the flight path of the neutrons, sheets of boron-containing rubber were rolled to a tunnel and aligned inside a 50 mm thick layer of lead bricks to absorb the emitted gamma radiation of the neutron absorption.

58 58 Experimental Fig. 32: In transparent the PGAA instrument. The solid pieces are the parts assembled and mounted for the neutron imaging setup within this work. Fig. 33 (left) Image of the XYZω-table used for the alignment of the samples below the PGAA carriage and the aluminum pin (right) without sample holder in front of the scintillation screen of the detector device. The pink and white sheets on the right indicate the front of the collimator towards HPGe-detector of the PGAA instrument.

59 Experimental 59 The data acquisition is handled using python programs, developed for this experiment. The user interface for the preparation of tomography experiments and the acquisition of image series is shown in Fig. 34. In the tomography section on the right side of the image, all parameters can be adjusted to select file path, positioning coordinates, important camera settings and the parameters of the tomography. In the field Start angle a start position can be entered, for the case, that a measurement was canceled before finishing. Maximum angle defines the angle, to which the tomographic data are acquired. Number of slices defines the number of images, which will be taken from the start angle to the maximum angle. Number of images allows acquiring more than one image of each slice to allow later image processing procedures. At 180 the software automatically acquires an image for the determination of the rotation axis during the reconstruction process in the software Octopus[89]. Fig. 34: Graphical user interface for the preparation of neutron radiography and tomography experiments. The software was prepared to visualize the last acquired image when pushing the Load image button, but this option was not further developed as all images were visualized directly in the acquisition software of the camera, Andor Solis[86]. For triggering the camera, an Andor Basic script developed by M. Schulz at the instrument Antares (FRM II) was used. The determination of the maximum possible resolution of the imaging setup took place with a high contrast reference wafer[90], mounted directly on the front surface of the 10-µm GADOX scintillator. This procedure minimizes the distance x from the sample to the scintillation screen and prevents edge blurring effects introduced by divergence as calculated from the L/D ratio according to Eq. (1.20). The exposure time with the GADOX scintillator was

60 60 Experimental set to 20 s for the acquisition of reasonable image intensity. Based on the line pattern on the wafer, the maximum resolution was determined. Two experiments were performed, one with the 10-µm GADOX scintillator and one with a 100-µm LiF/ZnS scintillator. In both cases, the resolution test target[90] was positioned in a distance of 40 mm in front of the scintillation screen to simulate a possible sample distance of a real tomography experiment. The acquisition time was adjusted according to the open beam image in both cases to reach sufficient intensity. After the acquisition, the results were evaluated in accordance to the Siemens star structures. For all other experiments the 100-µm LiF/ZnS scintillator was chosen due to its better performance regarding exposure time and resolution. During the tests, two tomography experiments were performed. In one experiment an electric valve was irradiated, in the other an USB memory stick. In both experiments, 375 images were acquired and the sample rotated for a total angle of 360. The steps were selected in a way, that after a rotation of 180 not only the mirrored absorption of the sample is recorded, but also angle steps in between the former steps were covered. In addition, an image was recorded at 180 for the correction of the tilt angle. For the image normalization, 15 beam images of the beam without sample, and 15 dark images of the thermal noise of the CCD were recorded. An exposure time of 2.2 s was selected for all radiographs, performed with the 100-µm LiF/ZnS scintillator. The normalization and reconstruction was performed with the Octopus 8.5[89] software. All normalization steps were performed in Octopus prior to the reconstruction process. Because of the high divergence of the neutron beam, the reconstruction was performed with cone beam geometry. The results were visualized subsequently with the VG Studio Max 2.2[91] software.

61 Experimental First developments towards PGAI In preparation to combine PGAA with neutron imaging, a brief study was performed with the installed neutron tomography sample positioning system and the PGAA gamma collimator. To characterize the field of view of the gamma collimator, a radioactive 152 Eu source was positioned along the axis of the collimator in front of the HPGe detector. It was moved along a linear path perpendicular to the axis of the gamma collimator and the corresponding gamma spectra were recorded to evaluate the position of the source as a function of gamma energy according to Fig. 35. Fig. 35: Schematic illustration of the performed experiment. A 152 Eu source was mounted in the imaging sample holder and moved parallel to the lead shielding on a total distance of 40 mm. Following the experiment described in [92] a radioactive point source is moved in front of a gamma collimator. Fig. 36 illustrates a schematic sketch of a detector behind a gamma collimator with radius r and a length b. The distance to the center of a radioactive point source towards the front face of the gamma collimator is represented by distance a. If this source is positioned in the center of the axis along the gamma collimator, it is fully seen by the detector. The detected intensity remains constant, as long as the source is moved between the center of the setup (Pos 0) and the radius of the collimator r. All movements take place on a planar surface, perpendicular to the axis of the gamma collimator. If the source is passing the position indicated with r, it becomes partially shaded by the collimator until it is completely shaded when passing position e. Therefore the length of the partial illumination zone can be described by the ratio of a and b with: (2.2)

62 62 Experimental The ratio of a to b describes mainly the influence of the geometry on the partial illumination region e. If the distance becomes larger, also the length of a partial illumination region will increase linearly. Fig. 36: Schematic sketch of positioning a radioactive source (white circle on the right) in front of a collimated gamma detector and the active areas, which are in the field of view of the detector[93]. Thus also the energy dependence of gamma-ray absorption in the collimator material influences the spatial resolution (see Fig. 9) of such a gamma spectrometer. While low-energy gamma radiation can be shielded well with relatively thin layers of shielding material, the shielding effect of lead decreases with increasing gamma-ray energy. To evaluate the spatial resolution of the present gamma spectrometer, a first scan with the present PGAA collimator was performed. The recorded spectra at each position of the 152 Eu source were recorded and three representing peak intensities evaluated to determine the intensity of characteristic gamma lines. An image of the used experimental setup is shown in Fig. 37.

63 Experimental 63 Fig. 37: Experimental setup to determine the sensitivity of the gamma collimator for different energies. The distance from the source to the front face of the shielding material was 30 mm and it was moved along a total length of 40 mm. With incremental steps of 1 mm, the gamma spectra were acquired for 600 s. Automation of the experiment was achieved with a Python[49] script, utilizing the digital inputs and outputs of the motion controller in combination with the TACO protocol[87]. All spectra were evaluated with the Hypermet-PC[50] software and the extracted peak intensities plotted in Origin 8.5[94]. Tab. 3: Relative intensities with their corresponding energy for the gamma emission of 152 Eu and detection efficiency Energy Rel. intensity of the gamma Efficiency of detector line[95] 245 kev % 1.175E kev % 7.333E kev % 5.677E-5 For a better comparison of the data, three gamma energies were selected according to Tab. 3, covering most of the energy range of the radioactive 152 Eu source. The activity of the radioactive source was determined, following ( ) (2.3) where A 0 is the activity of the radioactive source, A is the net peak area, t is the live time of the measurement, ε(e γ ) is the efficiency of the detector at the energy E γ and I γ is the relative intensity of the emitted gamma line. All three intensities are then plotted as a function of their position and the full width at half maximum values are determined.

64 64 Experimental

65 Results and Discussion Results and Discussion As the topics of this work cover a wide field of experiments, in the following chapter these results will be discussed subsequently according to their order in the experimental methods. In the first part, only the results of the PGAA experiments will be discussed, followed by the characterization of zinc oxide powders for possible traces of nitrogen with PGAA and other modern characterization methods. The second part illustrates the characterization of the neutron beam at the PGAA instrument, followed by the first experimental results with the assembled neutron radiography setup. The last part presents the results of first experiments towards position sensitive prompt gamma activation analysis, called prompt gamma activation imaging (PGAI). 3.1 PGAA In most cases, the elemental composition of samples can be determined with standard PGAA, described in In such cases, PGAA is a useful method, which requires only simple sample preparation to detect a large number of elements in a sample simultaneously. However, if elements are present in a sample, emitting a large fraction of their gamma rays in the low-energy part of the gamma spectrum and still provide sufficient high-energetic gamma rays for a precise analysis, it may be beneficial to reduce the sensitivity of the spectrometer in the low-energy region to increase the dynamic range of PGAA. This can be achieved by the introduction of a gamma-ray attenuator made of heavy metals like lead. The effect of such a device was tested during the calibration of the gamma spectrometer and its advantage then compared for the characterization of different samples Increasing the dynamic range of PGAA In the first part of the experiment, the transmission function of the gamma-ray attenuator was determined by measuring a PVC foil. The transmission value at a given energy was calculated as the ratio of the peak count rates measured with and without the gamma-ray attenuator. All the peaks with uncertainties less than 3 % were taken into account. Fig. 38 shows the experimental values in comparison to the theoretical transmission curve (red function in Fig. 38). The theoretical attenuation curve calculated for a 10-mm lead gamma-ray attenuator is in very good agreement with the experimental values. In the high-energy range of the gamma spectrum the transmission is about 60 % while at 517 kev a transmission of 24 % was achieved. Its value at 478 kev is slightly lower. Both of these values meet the requirements of reducing the boron peak by a factor of 5 and the higher energetic part of the spectrum not more than a factor of 2. The efficiency curves determined for the quantitative analysis are shown in Fig. 39. If no gamma-ray attenuator is used, the efficiency reaches its maximum at approximately 100 kev with a value of about 10-4 and is somewhat lower in the range of the boron peak. When using the attenuator, the efficiency at an energy of few hundred kev is significantly lowered.

66 66 Results and Discussion Fig. 38: Theoretical and experimentally determined transmission with a 10-mm lead gamma-ray attenuator (sample PVC foil)[32] The efficiency at 500 kev was reduced from 8.8 x 10-5 to 1.7 x 10-5 a reduction of slightly more than a factor of 5. At energies higher than 2 MeV it is reduced to 65 % of the values determined without attenuator. To illustrate the effect of the gamma-ray attenuator for samples with high boron content, the two spectra of Alborite, measured with and without attenuator are shown in Fig. 40. When no attenuator is used, the count rate of the boron peak was 36,500 cps, while the total count rate in the spectrum was as high as 77,100 cps. Fig. 39: Efficiencies with and without lead gamma-ray attenuator. The symbols represent measured values with their uncertainty within symbol size. The lines show 6 th -order polynomials used as efficiency functions for quantitative analysis. (Certified radioactive sources: 60 Co, 152 Eu PGAA samples: PVC, Urea)[32]

67 Results and Discussion 67 Fig. 40: Spectra of Alborite with (lower) and without (upper) the gamma-ray attenuator[32] If two or even three boron gamma rays hit the detector within the resolution time, they are detected as one peak, having twice or three times the energy of the original peak, in the case of boron at energies of 956 and 1434 kev, respectively. Especially the triple coincidence peak is very unlikely to appear when the count rate is low. The count rates of the double and triple coincidence peaks without the gamma-ray attenuator were 565 cps (1.5 % of the boron peak) and 7.6 cps (0.02 % of the boron peak), respectively. Thus, the intensity of the boron peak will be reduced significantly more than the other peaks. If the attenuator is introduced in front of the detector, the count rate of the boron peak could be reduced from 36,500 to 6,080 cps (i.e. by a factor of 6), resulting in a count rate of only 15.5 cps in the double coincidence peak (i.e. reduction by a factor 36, and its area was just 0.25 % of the single boron peak). In this case, the count rate of the boron peak was already too low for producing a triple coincidence peak. The count rate of the main aluminum peak at 1778 kev in Fig. 40 was reduced from 31 to 18 cps (i.e. 58 % remaining intensity after attenuation). The attenuator efficiently reduced the overall count rate of the gamma spectrum as well: from 77,100 to 11,600 cps. While the boron peak was reduced by a factor of 6, the baseline around it was lowered by an order of magnitude. Thanks to this, more characteristic peaks could be detected, improving the determination of other components. To verify the method, the compositions of boron-containing samples were determined. 600 mg of Alborite (nominal composition of 9Al 2 O 3 2B 2 O 3 ) was measured with and without the attenuator under the same conditions as mentioned above. Though PGAA is a multi-elemental technique, oxygen is an element which can be barely detected due to its low neutron-capture cross section. Therefore, the spectra of these two oxides were evaluated based on their Al-to-B molar ratio. The nominal and measured values for Alborite with and without attenuator are given in Tab. 4.

68 68 Results and Discussion If the attenuator is used, the determined molar ratio agrees well with the nominal value. However, if no attenuator is used, the ratio is 4.88 ± 0.07, which is significantly higher than expected. A possible reason for the missing counts from the boron peak could be the high count rate. Theoretical calculations taking into account the double and triple coincidence peaks cannot entirely explain the lost counts. Adding twice the peak area of the double coincidence peak to the peak area at 478 kev yielded an Al/B ratio of 4.72 ± The addition of the triple coincidence peak does not contribute significantly to this value, since its area is only 0.02 % of that of the boron peak. The rest of the lost counts can be found in the Compton plateau between the double and the triple coincidence peaks, where a significant increase can be observed when no attenuator is used (Fig. 40). A precise quantification of this region is not possible, that is why the determination of boron as a major component without an attenuator cannot be used for reliable analysis. For further validation of the method, Alborex, NiB, and TiB 2 samples were analyzed using similar masses and conditions as described above. The results are presented in the lower part of Tab. 4. For boron concentrations up to 50 %, the use of the attenuator yielded excellent results. The nominal molar ratios could be confirmed with high precision for Alborex, Alborite, and NiB. In the case of TiB 2, a slight discrepancy was observed, however, this cannot be called significant, as it agrees within twice the uncertainty limit. All other results were found to agree with the nominal compositions within the uncertainty limits, and have proven that the method can be applied to samples containing high concentrations of boron. Tab. 4: Results of a quantitative measurement of Alborite with both methods and Alborex, NiB and TiB 2 with 10 mm lead gamma-ray attenuator. Sample: Alborite Al/B ratio Sample: Alborex Al/B ratio Sample: NiB Ni/B Sample: TiB 2 Ti/B Nominal value Measured, no gamma-ray attenuator Measured, 10 mm Pb gamma-ray attenuator ± ± ± ± ± 0.01 In conclusion, a method was developed for the measurement of high boron concentrations in PGAA, using a high neutron flux. The lead gamma-ray attenuator with a thickness of 10 mm in front of the HPGe-detector reduced count rates in the boron peaks by a factor of 6 and the count rate of the double coincidence peak by a factor of 36. The baseline was reduced by an order of magnitude. Thanks to this, further peaks could be detected in the lowenergy part of the spectrum (up to energies of 1.5 MeV), which were not observable when no attenuator was used.

69 Results and Discussion 69 Furthermore, the attenuator reduced the overall count rate leading to lower dead times and thus shorter measurements. The systematic error due to the random coincidence summing at high count rates could also be significantly reduced with this method. The use of an attenuator enables the analyses of samples containing boron as major components, and, therefore, it increased the dynamic range of the PGAA technique by about an order of magnitude.

70 70 Results and Discussion Application of PGAA with gamma-ray attenuator for copper containing samples After the characterization of the 10-mm thick gamma-ray attenuator with boron containing samples, it was applied also in the case of copper-containing alloys. Even though copper does not emit such a strong, single, gamma line like boron in the low energy part of the spectrum, samples with masses in the range of grams can still easily saturate the detector due to the high count rate. Especially in the case of copper, a large number of gamma rays with low energies can be observed on top of the spectral background shown in Fig. 41 for the example of bronze. Fig. 41: Comparison of the spectra of bronze with and without gamma-ray attenuator. The orange section shows the region from kev where 50% of the count rate of the spectrum with no attenuator is detected. In the spectrum with no attenuator, 50% of the overall count rate (1,780 cps out of 3,660 cps overall) is emitted in the energy range from kev, marked in orange in Fig. 41. Most of these gamma rays originate from copper or the increasing baseline due to partial absorption of gamma rays with higher energies. When the same measurement is performed with the lead gammaray attenuator in front of the HPGe detector, the count rate in the energy range from kev is suppressed to 20% of the total count rate of the spectrum (240 cps from 1,200 cps). Since not only the efficiency in the lowenergy range of the spectrum is reduced (see Fig. 39), but also the total efficiency, the total count rate of the spectra was reduced from 3,660 cps to 1,200 cps, approximately by a factor of three. However, due to the stronger reduction of the low-energy part of the spectrum, this leads to a higher sensitivity for the detection of peaks in the high-energy range.

71 Results and Discussion 71 Fig. 42: Comparison of two bronze spectra normalized to the same number of total counts, measured with and without using a lead gamma-ray attenuator. For a better illustration of this effect, both spectra were normalized to the same total of counts and plotted in Fig. 42. It is clearly shown up to an energy of 300 kev, that a significant fraction of the counts is suppressed in this energy range. On the other hand, much better statistics is achieved in the highenergy part above 300 kev. To compensate for the reduced total efficiency, measurements can be performed with a higher neutron flux to record a spectrum with a reasonable count rate. In addition, the high-energy statistics can be improved by a factor of 2 in the case of the investigated spectra. To benchmark the tested method, the acquired spectra were evaluated quantitatively with the ProSpeRo[31] program and the results shown in Tab. 5 and Tab. 6. Tab. 5: Expected concentration and measured concentration values, applying a 10-mm thick lead gamma-ray attenuator. Expected concentration Measured concentration Brass sample (CuZn36) Cu 63.5 to 65.5 wt % 64.7 wt% ± 1.8 % Zn 34.5 to 36.5 wt % 35.2 wt% ± 2.3 % Bronze sample (CuSn6) Cu 93.0 to 94.5 wt % 93.3 wt% ± 0.8 % Sn 5.5 to 7.0 wt % 6.7 wt% ± 11.0 % The compositions for both industrial alloys agree well with the measured values within the uncertainty limits and could confirm the applicability of the

72 72 Results and Discussion method for such samples. Nevertheless, due to the low amount of tin in the sample in combination with its low neutron-capture cross section (almost an order of magnitude less than copper), the analysis of the three main peaks resulted in a high uncertainty. The results of the simulated bronze sample with additional elements are shown in Tab. 6. In contrast to the homogeneous samples, the determined values for the simulated sample are in poorer agreement. Tab. 6: Expected values, weight of the used material and determined values from the measurement with the lead gamma-ray attenuator of the simulated bronze sample as shown in Fig. 42c Prepared Bronze Weight of Material Theory Determined with gamma-ray attenuator S 98.1 mg ± 4.0% Fe 99.4 mg ± 3.3% Cu mg ± 1.0% Sn mg ± 3.4 % Pb mg ± 8.2 % The main reason for the discrepancy with the theoretical composition is the inhomogeneity of the sample in combination with the inhomogeneity of the neutron beam. If self-absorption of gamma radiation was the main reason for an offset in the determined concentration, the value for sulfur (in the inner part of the sample) needs to be below the expected value. However, its concentration is above. An explanation is the inhomogeneity of the neutron beam at the sample position, which averaged the values of the two industrial alloys according to the spatial differences of the neutron flux. In contrast, this was not possible for the prepared sample due to its inhomogeneity, leading to the discrepancy shown in Tab. 6. In conclusion, it has been shown that the developed method can be applied for the measurement of homogeneous copper alloys like brass or bronze if the constituents emit at least some of their main gamma rays above 1,000 kev. However, special attention is needed if inhomogeneous samples are irradiated or if the sample contains elements, which do not emit their main gamma rays above 1,000 kev, for example in the case of arsenic. For precise analysis of such samples, longer measurements with a lower flux are still the recommended method.

73 Results and Discussion Investigation of different sample holder materials In the case of special samples, with e.g. extremely small masses or low neutron-capture cross sections, possible sample holder materials were compared. The results of the measurements are given in Tab. 7. The values correspond to the determined masses of the PGAA measurement with each sample holder configuration. Tab. 7: Composition of different sample holders to evaluate the contribution of each material to the spectral background (masses given in grams). Element H B C Frame only 5.98E-7 ± 3.3% 7.26E-9 ± 3.3% 1.30E-4 ±12.0% Frame with FEP strings 6.87E-6 ± 2.2% 7.09E-8 ± 1.5% 1.34E-2 ± 3.3% N - - F 2.74E-4 ±5.0% Na - Al Si 2.37E-5 ±3.2% 1.39E-5 ±5.0% Cl E-2 ± 5.0% 2.83E-5 ±16.0% 1.39E-4 ± 3.5% 1.51E-4 ± 4.0% 1.62E-6 ±15.0% Frame with Al strings 5.6E-7 ± 1.4% 1.3E-8 ± 0.9% 3.78E-4 ± 3.5% 4.96E-6 ± 5.1% 1.16E-3 ± 4.7% E-5 ± 1.5% - Frame with Pb strings 1.89E-5 ±1.6% E-3 ±9% Frame with Al pocket 9.33E-7 ±4% 5.26E-8 ±1.0% 6.43E-4 ±14% E-3 ±5% 1.67E-5 ± E-4 ±2.8% 1.21E-4 ±6% 6.64E-4 ±6% E-3 ±2.3% 1.95E-5 ±9% Mn Fe E-5 ±3.4% 4.55E-5 ±3.5% Zr Pb 1.25E-4 ± E-3 ±3.3% 9.87E-4 ± 8.3% 0.67E-1 ±2.7% - Frame with C wires 6.87E-7 ± 17% E-4 ±3.3% 4.64E-6 ± 4% 1.22E-3 ±6% 3.27E-6 ± 2.5% 2.80E-5 ± 1.6% 1.23E-5 ± 2.3% 1.31E-7 ±21% E-6 ± 2.5% 1.79E-4 ± 5% 3.49E-4 ± 2.6% The measurement Frame only was performed for the determination of background elements, if the sample frame is put in the beam, but no sample holder is used. Sodium and silicon originate from the activation of the glass in the neutron guide and can be detected due to their high energetic gamma rays, transmitted through the lead shielding. The contribution of aluminum originates from the construction material of the sample chamber. Lead is the main shielding material surrounding the instrument. However, even with the best

74 74 Results and Discussion lining of the lead with neutron absorbing material, a minor activation cannot be avoided completely. Carbon and fluorine are present in the ladder of the sample changer and the frame of the sample holder. In addition, hydrogen and boron are detected because of scattered neutrons that are absorbed in the 6 LiF-containing plastic in the inner part of the sample chamber and the sheets of boron containing rubber, used as neutron shielding materials around the sample chamber. In standard PGAA measurements the frames are equipped with FEP strings, contributing significantly to the background of carbon and fluorine. When comparing the results for the empty frame and the FEP strings, an increase of these elements by two orders of magnitude can be observed in Tab. 7. In addition, the detected level of hydrogen is increased by one order of magnitude due to scattered neutrons interacting with the 6 LiF-contatining plastic. The appearance of chlorine may originate from the glue in the sticky tape, used to fix the strings at the sample-holder frames. In the case of aluminum strings, the values of boron, carbon and fluorine are slightly elevated. The appearance of nitrogen originates from the background pressure of 0.3 mbar and can be detected because of the long irradiation time. The total increase of the aluminum mass, relative to the background ( Empty frame ) is 4.1. Nevertheless, the detected mass of the introduced aluminum in the form of wires as sample holder material is still below the aluminum background of standard PGAA experiments. Unfortunately the 20- µm thick aluminum wires were too weak to act as proper sample holder material and they break easily, even if a sample with a total mass of 10 mg is fixed with them. The determined mass of the lead wires, compared to the empty sample holder frame resulted in an increase of detected mass by a factor of 536. In addition, the total neutron-scattering cross section of lead with 11.1 barn is twice as high as the total neutron-scattering cross sections of the other sample holder materials (C, F, Al), all in the range of 4 5 barn. The high amount of hydrogen, determined from the experiment with the lead wires as sample holder material, may originate from the scotch tape, used to fix the lead wires at the sample holder frame. However, the 125-µm thick lead wires, tested during this experiment were too weak for a future application in routine measurements. Similarly to the case of the aluminum wire, the lead wire is too weak to hold a 10 mg sample properly. The tested aluminum pocket, prepared from aluminum foil as shown in Fig. 26 has proven to be the best candidate for later routine measurements of 10 mg carbon samples. The material was strong enough to hold a sample, is cheap, and the preparation can be performed easily. Its introduced hydrogen background is about twice as high as the background of an empty frame. The determined boron mass is in a similar range as in the case of FEP strings. Carbon and fluorine masses are almost two orders of magnitude below the FEP strings and only slightly higher than in the case of an empty sample holder. Though, the aluminum foil introduced the two additional alloying elements manganese and iron into the PGAA spectrum. The total mass of activated aluminum in the case of the aluminum pocket was about 3.4 mg This is approximately two orders of magnitude above the value with the empty frame and 40 times higher than with the tested aluminum strings. In contrast

75 Results and Discussion 75 to the aluminum strings, the pocket served as a reliable solution for mounting samples and reducing the carbon and fluorine background significantly. The last tested material was a carbon fiber, fabricated from carbon nanotubes[52] at RICE University. Preparing the sample holder was easy due to the strength of the wires, holding up to 15 g. Thus, the samples could be fixed easily. The hydrogen and aluminum concentrations were almost as low as the background values with an empty frame. A slight increase of the carbon and fluorine masses was observed, similarly to the case of the aluminum strings. The values of carbon and fluorine agree acceptably with each other. Only a slight excess of fluorine was observed. Thus no additional carbon was identified due to the use of carbon fibers. It is expected that the carbon concentration is in the range of that of the background measurement as the total mass introduced (40 µg total mass in the form of carbon strings) would lead to a total mass of 1.7x10-4 g. The increased background could thus be attributed to the neutron scattering of the zirconium sample since both, the carbon and fluorine mass increased. A separate measurement without a sample would help to verify if the expected carbon mass can be met. In any case, the carbon fibers have shown to be the best material for future measurements of solid, low mass, samples due to their low contribution to the spectral background.

76 76 Results and Discussion Chemical analysis of nitrogen in ZnO powders A number of different samples were prepared via the SCM by varying the ratio of urea to zinc nitrate. The ratio of reactants was found to have a pronounced effect on the combustion reaction that takes place upon heating: For reactant ratios of 4 : 1 and less, the first steps of the process involve water starting to evaporate, boil and subsequently a change in color to dark orange-brown. The dark color can be attributed to the presence of nitrogen oxides that evolve from zinc nitrate. Once the solution is saturated, the emission of brownish nitrogen oxides starts and is followed by an intense flame striking out of the beaker. In particular for low relative amounts of urea (between 0.5 : 1 and 3 : 1), the flame is extremely bright and of a white-yellowish color as depicted in Fig. 43. With increasing ratios of urea, however, the combustion becomes less intense and the emission of nitrous gases decreases. For reactant ratios of 5 : 1 and higher, black carbon is observed on the upper part of the beaker indicating the deficiency of oxygen for a complete combustion of urea. For ratios higher than 6 : 1, white smoky gases are emitted from the solution due to the decomposition of urea, which takes place after the reactants in the solution have been saturated. This starts with the formation of bubbles and foam, which solidifies over time to form an amorphous material. In addition to the reaction process, also the color of the product changes with the employed reactant ratios: The initial red-orange color observed for low amounts of urea fades towards pale orange with ratios larger than 2 : 1, and then goes from yellowish to pale brown for samples with ratios of 10 : 1 (see Fig. 44). Fig. 43: Combustion reaction of a sample with a molar ratio of 1 : 1 of urea and Zn(NO 3) 2 6 H 2O at 500 C (1 represents the boiling and subsequent coloring before the reaction, 2 and 3 show the combustion reaction and 4 the result after the reaction).

77 Results and Discussion 77 Fig. 44: Overview of sample colors starting with a Urea : Zinc nitrate ratio of 1 : 1 on the left to 10 : 1 on the right. The powder XRD pattern of all prepared samples are depicted in Fig. 45, along with that of the pure ZnO reference (black line, bottom). A clear decrease in the intensity of the diffraction features is observed as a function of the relative amount of urea used in synthesizing the samples. This is associated with the corresponding increase in the amount of urea s decomposition products and, therefore, decreases in the absolute mass of the crystalline products of interest. The main diffraction features of the synthesized powders can be indexed to a hexagonal wurtzite-type ZnO. The unit cell dimensions of the products are essentially unaffected by the varying synthesis conditions within error of determination (see below). Fig. 45: Powder X-ray diffraction patterns of ZnO-samples prepared with molar ratios of urea to zinc nitrate from 1 : 1 to 10 : 1. A number of additional reflections are observed in the low angle region (< 40 in 2Θ) for samples prepared with a reactant molar ratio of ca. 2 : 1. These appear consistently within a narrow range of employed reactant ratios (ca. 2 : : 1) and are most intense for the 2.3 : 1 sample, as indicated by the powder XRD patterns of Fig. 46. The appearance of additional reflections in consecutive products along with the reproducibility of the results exclude the

78 78 Results and Discussion possibility of sample contamination, but clearly suggest the formation of an additional crystalline phase within the aforementioned samples. Fig. 46: Samples prepared with molar ratios of 1.6 : 1 to 3 : 1 of urea and Zn(NO 3) 2 6 H 2O showing the appearance of an additional phase. Main reflections of the additional phase are indicated with arrows. After dissolution of a 2.3 : 1 powder sample in hydrochloric acid, crystals of isocyanuric acid were isolated and identified via single crystal XRD. The obtained crystal structure data agrees well with values reported by Verschoor et al.[96] and Coppens et al.[97]. Further details on crystallographic data and atomic coordinates can be found in the appendix (Tab. S1 and Tab. S2). The presence of isocyanuric acid was additionally confirmed by liquid state NMR. The measured chemical shifts for 1 H, 13 C, and 15 N are listed in Tab. 8 and show perfect agreement with values reported for isocyanuric acid by Yamada et al.[98] using liquid state NMR and by Damodaran et al.[99] with solid state NMR. Tab. 8: Chemical shifts in ppm from the measurement of isocyanuric acid and literature values from liquid state (LS) NMR and solid state (SS) NMR measurements. Isotope Measured LS SS NMR[99] NMR[98] 1 H (NH) , C (CO) , N (NH) , The NMR spectra obtained within this work can be found in the appendix (Fig. S5-S7). The identification of isocyanuric acid strongly suggests that the additional crystalline phase existing within the 2.3 : 1 powder sample can be related to cyanuric acid or a related compound. This compound forms as a main decomposition product during the pyrolysis of urea[100,101] Based on the above, focus is directed towards samples with a high crystalline character prepared with low relative amounts of urea and in particular those

79 Results and Discussion 79 with 1 : 1 and 2.3 : 1. While interest in the 2.3 : 1 sample lies in the presence of a cyanuric acid related compound, motivation for investigating the 1 : 1 sample derives from the report by Mapa and Gopinath stating a high nitrogen bulk concentration for a similarly prepared sample[57]. Tab. 9 lists the lattice parameters (α and c) as determined by Rietveld analysis: for the 1 : 1 and 2.3 : 1 samples in the present work, for the aforementioned 1 : 1 sample reported by Mapa and Gopinath and named ZU1[57], for ZU1 after annealing at 950 C (ZU-950)[57], and for two literature values of pure ZnO[66,102]. Mapa and Gopinath attributed the overall lattice contraction observed for ZU1 to a nitrogen incorporation of 15 at.% and linked it to the sample's orange color. Upon high-temperature annealing, the authors suggest nitrogen release as the lattice parameters reflect those of pure ZnO and the sample color changes from orange to white. However, in the present work despite the similar orange coloration of the samples no deviation of the unit cell parameters from those expected for pure ZnO is observed for the 1 : 1 or the 2.3 : 1 product. Tab. 9. Lattice parameters of samples 1 : 1 and 2.3 : 1 at 298 K and literature values for comparison. a c Sample 1: (1) Å (5) Å Sample 1:1 DESY (3) Å (5) Å ZnO ref, DESY (3) Å (5) Å Sample 2.3: (1) Å (2) Å ZU1[57] Å Å ZU-950[57] Å Å ZnO[102] (1) Å (1) Å ZnO[66] Å Å The Raman spectra of samples prepared with reactant ratios between 1 : 1 and 5 : 1 along with that of pure ZnO (black line, bottom) are depicted in Fig. 47. It is noted that strong fluorescence was observed in the spectra of samples prepared with higher initial urea concentrations, due to the contribution of the organic decomposition products. Therefore, these spectra are not shown. It is clear from Fig. 47 that while retaining the spectral signature of ZnO (through bands of the 1st order Raman signal at 384, 414, 441 and 586 cm -1 )[103] the synthesized powder samples give rise to four additional Raman bands at 275, 510, 582, and 643 cm -1 (marked by stars in the figure). These bands have in fact been detected in numerous past studies. However, no unambiguous interpretation regarding their origin was presented and, thus, they are a matter of a long-lasting controversy. These bands have repeatedly been assigned to local vibrational modes of nitrogen[57,77, ] and used as evidence to support nitrogen incorporation into the ZnO lattice[57]. Raman bands at these wave numbers, however, have also been reported when intentionally excluding nitrogen species from the growth process and doping ZnO thin films with different elements (such as Fe, Sb, Ga or Al)[107,108]. Accordingly, in their relevant overview Manjón et al.[109] attributed the presence of these bands to disorder-activated Raman scattering, which occurs due to induced defects and the consequent breaking of symmetry.

80 80 Results and Discussion Fig. 47: Raman spectra of samples 1:1 to 5:1 and pure ZnO. Bands, in literature assigned to nitrogen containing zinc oxide are marked with stars. Crosses mark the bands appearing with the additional phase. The spectrum of ZnO is multiplied by a factor of 4 for better comparison. A feature, which was not reported so far, is the appearance of yet two additional Raman bands (both marked by crosses in Fig. 47) in the spectra of the 2 : 1, 2.3 : 1 and 2.6 : 1 samples with wavenumber values of 748 cm -1 (accompanied by a weaker band at ca. 735 cm -1 ) and 1043 cm -1 (accompanied by a broad shoulder in a slightly higher frequency region). These bands could not be directly related to existing data[110,111] for ZnO or for isocyanuric acid. Finally, the bulk concentrations of zinc, hydrogen, and nitrogen were determined with PGAA measurements for samples with reactant ratios of 1 : 1 and 2.3 : 1. In achieving optimal conditions for the low detection limit of nitrogen, two 1 : 1 samples were irradiated for different durations (21,800 and 11,808 seconds for 1 : 1 (1) and 1 : 1 (2), respectively) and the results are summarized in Tab. 10. Tab. 10: Results of the PGAA measurements of various samples in atomic percent. Sample 1 : 1 (1) was irradiated for 21,800 s, sample 1 : 1 (2) for 11,808 s, and sample 2.3 : 1 for 7,200 s. Samples Hydrogen Nitrogen Zinc 1 : 1 (1) 0.63% ± % ± % ± : 1 (2) 0.51% ± 0.06 < 0.09% 99.5% ± : % ± % ± % ± 0.9 Based on the peak at 2,223 kev (Fig. 48), both 1 : 1 samples were found to contain more than 0.5 at.% of hydrogen. Whether this remains adsorbed on the samples' surface in the form of moisture or is capable of penetrating the surface and stabilizing itself within the host lattice, remains unknown. The possible incorporation of such hydrogen species in the ZnO bulk may in fact be the source of the sample colors, as proposed by Weber et al.[61].

81 Results and Discussion 81 Fig. 48. Enlarged region from the prompt gamma spectra around the hydrogen peak from 2,200 to 2,300 kev normalized to count rates. BKG indicates the data of a background spectrum for comparison. The difference in the height of the zinc peaks centered around 2,110 kev as well as of the background of each spectrum can be attributed to the different sample masses. In contrast the spectral baseline is much weaker in the energy region from 9,000 kev to 11,000 kev, where the characteristic nitrogen peak is located (depicted in Fig. 49). In particular for samples prepared with molar ratios of 1 : 1, the N-related peak areas could not be fitted using the peak fitting algorithm of HYPERMET-PC[50] due to the low number of detected nitrogen counts. The detected counts were therefore manually integrated and their statistical uncertainties estimated based on the sum of the peak counts. This explains the high uncertainty of the determined nitrogen concentration in sample 1 : 1 (1), while the detection limit for both measurements with respect to the spectral background equals 0.09 at.%. In any case and in contrast to the report of Mapa and Gopinath[57], the PGAA data indicate very low amounts of nitrogen existing in samples prepared by reactant ratios of 1 : 1. In case of the 2.3 : 1 sample, the PGAA measurements indicate hydrogen and nitrogen concentrations of 10.3 at.% and 11.0 at.%, respectively in the ZnO matrix. Within the given error limits, these values agree with the presence of the cyanuric-acid-related additional phase. Motivated by reports regarding the formation of stable ZnO with a high concentration of nitrogen doping (up to 15 at.% [57]), the simple combustion synthesis process was adopted to produce a variety of ZnO-based powder samples. The synthesis was found to be highly dependent on the initial molar ratio of the reactants, namely urea : zinc nitrate hexahydrate, in terms of generated heat, gas formation and the color of the collected products, which ranged from dark red to white when increasing the fraction of urea. Powder X- ray diffraction showed no change whatsoever in the dominant ZnO-wurtzite structure for all prepared samples, a fact, which was confirmed by subsequent Rietveld refinements on selected data.

82 82 Results and Discussion Fig. 49. Comparison of PGAA spectra of different samples in an enlarged region around the nitrogen peak at 10,829 kev. Although of a similar orange color, this result disagrees with the earlier observation of a minor lattice contraction for similarly prepared 1 : 1 products, which were denoted ZnO 0.85 N Instead, the 1 : 1 sample shows identical Raman activity with the additional appearance of the four controversial Raman bands superimposed onto the spectrum of pure ZnO. The quantification of nitrogen by prompt gamma activation analysis (PGAA) showed it to be below or close to the detection limit, estimated as 0.09 at.%. It is therefore refuted, that a significant introduction of nitrogen in the ZnO lattice via the solution combustion method as previously suggested is possible and excludes this mechanism as a reason for the additional Raman bands discussed above. Furthermore the sample s orange color as a consequence of nitrogen uptake is ruled out and it is suggested, that instead either the high concentration of hydrogen impurities (indicated by PGAA to amount to ca. 0.5%), or the presence of oxygen vacancies induced by the extreme combustion conditions may be a reason for the orange coloration of the powders. Finally, within a narrow range of employed reactant ratios an additional coexisting organic phase was detected, which was found to be reproducible and most dominant in the 2.3 : 1 sample. Dissolving the powder product in hot hydrochloric acid and performing single crystal XRD on the resulting crystals achieved the identification. The presence of isocyanuric acid was additionally confirmed by liquid state NMR and is in agreement with the PGAA data, which shows significant amounts of both nitrogen and hydrogen.

83 Results and Discussion Development of an imaging system at the PGAA instrument Another aim of this work was the test and installation of a neutron radiography setup at the PGAA instrument. Therefore, an existing imaging system was upgraded with a more powerful camera and different scintillator screens as well as the neutron beam were characterized for the development of a neutron radiography setup. Finally the developed setup was tested with two neutron tomography experiments Characterization of the detector system For the calibration of the lens system, the scintillation screen was changed to an optical test target[85]. This target was photographed and the lens system adjusted till the highest image sharpness was achieved. After the successful focus adjustment, the image shown in Fig. 50a was acquired. Due to the construction of the camera system and a constant distance to the scintillation screen, the distance to the object plane was constant during all further experiments. Thus the camera box was sealed after the calibration of the camera system and the focus kept constant during all following experiments. a) b) Fig. 50: On the left are the detector data of the optical target, on the right the magnified part, marked with the red rectangle. The right image shows the number of line pairs, which could be resolved in the end. Fig. 50b shows magnified the red rectangle from Fig. 50a, illustrating the section of the contrast measurement. It depicts the section of 8 and 9 line pairs/mm. The single black lines crossing to the line pairs indicate where the camera resolution was measured and the contrast along them is plotted in Fig. 51. For the determination of the maximum resolution, a minimum contrast between the bright and dark lines of at least 10% was assumed. While this criterion is fulfilled for 8 line pairs/mm, illustrated in Fig. 51a, it is not fulfilled for 9 line pairs/mm (Fig. 51b). Due to the performed experiments, the maximum camera resolution is given with 62.5 µm and a field of view of 84.0 x 84.0 mm 2. The pixel size is given with mm.

84 84 Results and Discussion a) b) Fig. 51: Grey scale values in absolute intensity of 8 and 9 line pairs per mm on the optical grid. The difference in absolute intensity originates from an inhomogeneous illumination. The point markers show the measured values, the lines are for illustration to show the trend between the markers.

85 Results and Discussion Characterization of the neutron beam Scan of the beam profile with a neutron collimator In Fig. 52, the measured beam intensity is plotted as a function of the horizontal distance on the scintillation screen on one axis and as a function of distance from the neutron shielding on the other. The plotted intensity profiles represent the average value of each column with a height of 120 pixels over the horizontal image as illustrated in Fig. 30a. Fig. 52: Intensity values of the collimated neutron beam starting 50 mm from the shielding of the neutron guide backwards to a distance of 1750 mm. The scan started 50 mm behind the lead shielding with a maximum intensity of 50,944 counts. Incrementally, the data are plotted in steps of 100 mm till the final position of the scan reached 1750 mm with a remaining intensity maximum of For a quantitative analysis the FWHM values of the recorded profiles were evaluated and three representative profiles of the positions 50 mm, 790 mm, and 1,490 mm are plotted in Fig. 53.

86 86 Results and Discussion a) FWHM determined from the intensity profile at position 50 mm. b) FWHM determined from the intensity profile at position 790 mm c) FWHM determined from the intensity profile at position 1510 mm Fig. 53: Full with at half maximum values of selected sections of the beam profile shown in Fig. 52. The asymmetry of the profile in general can be explained with the curvature of the neutron guide. Although the last 6.9 m of the beam guide are kept straight

87 Results and Discussion 87 to homogenize the beam, the effect of the curvature is clearly visible due to two propagation mechanisms of the neutrons in a curved neutron guide. One effect is the total reflection on both sides of the guide, leading to a symmetric propagation of the neutrons. For such cases, the divergence of a straight neutron guide without tapering can be approximated according to B. Schillinger[34] with 0.1 per Ångström wavelength. For the PGAA instrument[46] this would result in a divergence of 0.67 with a mean wavelength of 6.7 Å and 1.3, if 5% of the maximum intensity of the neutron spectrum[46] are taken into account with approximately 13 Å. However, due to the curvature and the elliptical tapering of the neutron guide this approximation is not valid for the neutron guide at the PGAA instrument due to its curvature and elliptical tapering. In addition to the reflections in a straight neutron guide, also garland reflections are propagated at the outer radius of the curvature. When these neutrons enter the last, straight, and in both directions (vertically and horizontally) elliptically tapered part of the neutron guide, they are homogenized but an effect of the curvature remains visible. For a better understanding of the observed effects shown in the following, appropriate simulations are recommended but these were not part of this work. Three methods were evaluated to determine the divergence of the neutron collimator: It was determined based on intensity at the full width at half maximum value (Fig. 54) of the intensity profile, the intensity at a value, three times larger than the background (Fig. 56), and a dynamic approach with the profile width at 5% of the maximum intensity value (Fig. 57). For the following determinations of the divergence, all profile widths are plotted as a function of the distance from the shielding material. Fig. 54: Determined values for all FWHM data acquired during the scan with the collimator plotted versus the absolute distance from the shielding material at the end of the neutron guide. From the gradient of the fitting function the divergence of the beam was calculated. In the case, of the FWHM approach, a linear regression function was used to determine the slope of the experimental values. Based on the fit, the opening angle was determined with 1.077(2). However, geometric considerations (Fig. 55) with the collimation give a maximum opening angle of 2.08, which is almost twice the determined one. It becomes clear from the results presented in

88 88 Results and Discussion Fig. 53a-c, that the profiles widen significantly more at the bottom than at their half width. Fig. 55: Illustration of the calculated opening angle based on geometric considerations of the collimator (lower part of the figure). The upper part of the figure shows the determined opening angle based on the FWHM method for the collimator. Based on the geometry of the collimator, a geometric, maximum divergence of 2.08 is expected for the determined measurement. Thus a second evaluation was performed where the intensity of the triple background (>1,500 counts) is set as threshold to determine the profile width. These values are plotted in Fig. 55 with two possible fits. Fig. 56: Width of the intensity profile at a brightness of the triple background (>1500) and approaches to fit the experimental values It has to be noted that a linear fit (based on the approach of linear propagation of neutrons in air) does not represent the experimental values. Additionally, structures in the intensity profile as observed (e.g. steps at 68 mm in Fig. 53c) cause an inhomogeneity in the experimental values for larger distances from the neutron guide. However, a second approximation was the fit

89 Results and Discussion 89 of a 2 nd order polynomial to better represent the determined data, and calculating the gradient angle from its first derivation at different positions. The derived angles are given in Tab. 11. Tab. 11: Divergence angles based on different calculation methods. Method Divergence Linear fit 1.45(2) Polynomial at 50 mm 2.06(2) Polynomial at 1750 mm 0.90(3) In contrast to the angle determined with the FWHM method of 1.07(2), the linear fit of the profile with a lower intensity threshold approaches the expected value. None of the fitted functions meets the expected values based on the geometric consideration of the collimator. The approximation with a polynomial fit meets the expected divergence angle of 2.08 well for distances close to the shielding material at the start of the scan, but with increasing distance from the neutron guide the angle decreases. In conclusion, an approximation with an absolute brightness threshold is not recommended for the determination of the divergence of the examined neutron beam. The third tested method was developed using a dynamic threshold. It determined the profile width at 5% of the maximum intensity. The determined width is plotted as a function of the distance in Fig. 57. Fig. 57: Linear fit of the profile width at 5% of its maximum intensity in the linear region. The discrepancy observed at higher distances is due to a step at one side of the profile. At 1270 mm a flattening is observed, which is caused in some cases due to low intensity and in some cases due to the illumination of the whole scintillation screen. The linear fit was adjusted along the linear region of the determined values and corresponds perfectly to the expectation values. The angle of 2.096(6) meets the expectation of 2.08 within 3 times the uncertainty interval. Thus, this is the best method among all investigated methods and will be used for the further studies of the neutron beam.

90 90 Results and Discussion Characterization of the collimated beam with a graphite diffusor In this experiment a graphite diffusor was used to homogenize the beam. The profiles are plotted in Fig. 58 for a set of distances, from 50 mm to 910 mm from the lead shielding. Fig. 58: Intensity profile and with on the scintillation screen as a function of distance with graphite diffusor. Some asymmetry from the curvature is visible; however, the minimum in the middle part of the profile was flattened by the diffusor. For all measured values, the width at 5% of the maximum intensity was determined and is plotted in Fig. 59. Due to the scattering of neutrons in the graphite powder, not only a homogenization of the neutron beam was achieved but also an increase of the divergence. The homogenization with the diffusor was achieved with a reduction in the neutron flux by a factor of 6.4. This resulted in the prolongation of the used exposure time from 0.2 s to 1.7 s to acquire the beam profiles with a similar maximum intensity than in the case of the collimator experiment. In conclusion, the diffusor is a suitable tool for the homogenization of the neutron beam. The optimization of the thickness of the diffusor can help to gain a higher neutron flux with a more homogeneous profile for imaging applications.

91 Results and Discussion 91 Fig. 59: Profile width as a function of the distance from the neutron guide with the diffusor. The values are determined at 5% of the maximum intensity and fitted in their linear region. At a total distance of 1,350 mm from the scintillator the neutron beam illuminated the full scintillator area, causing the flattening of the measured data.

92 92 Results and Discussion Characterization of the beam profile with the elliptical guide extension Due to the high intensity of the neutron beam at the PGAA instrument, the scan with the elliptical guide was performed in two steps. First, a profile along the beam axis was recorded with an attenuated beam and an exposure time of 1 s. This saturated the camera at the beginning of the scan but yielded sufficient intensity at larger distances to the neutron guide as plotted in Fig. 60. Fig. 60: Intensity of the neutron beam with the elliptical nose. The saturation close to the neutron guide originates from the exposure time, which was kept constant the sake of comparison. The scan was performed with an attenuator in the beam. The exposure time was selected with 1 s and kept constant. In addition, the first part of the intensity profile was recorded with all neutron attenuators in the beam in a second measurement and plotted in Fig. 61. In the profiles, the vertical slits of attenuator 1 (vertical slits) are clearly visible in form of the inhomogeneous beam profile.

93 Results and Discussion 93 Fig. 61: Intensity of the neutron beam around the focus of the elliptical guide. The intensity maximum was reached 40 mm away from the shielding of the neutron guide. The scan was performed using all neutron attenuators in the beam with an exposure time of 0.5 s. The intensity maximum of the neutron beam is clearly visible at a distance of 40 mm behind the lead shielding, which corresponds to a total distance of 90 mm from the end of the elliptical nose. Selected intensity profiles with corresponding width values at 5% of the maximum intensity are plotted Fig. 62. At the beginning of the flight path past the elliptical guide extension, the neutron beam is focused and thus only little divergence can be observed at the first centimeters. At the exit of the guide, the beam is relatively homogeneous as depicted in Fig. 62a. Once the camera system is moved into the focus of the elliptical nose, the effect of the curvature becomes clearly visible with the spike on the left side of the profile width and a maximum of 47,369 counts. At this position the pinhole aperture was aligned later for neutron imaging experiments. With the increasing distance, the divergence of the beam becomes more homogeneous according to Fig. 62d-f. However, since at least one neutron attenuator had to be in the neutron beam, it was not possible to record un-attenuated beam images for radiation safety reasons. The periodic maxima in the beam profile originate due to the slits of the used neutron attenuator in addition to the structure introduced by the mapping of the neutron guide. The results of the full scan in addition to the determined divergence of the beam are plotted in Fig. 63. After the neutron beam becomes more homogeneous, the divergence follows a linear trend up to a total distance of 310 mm from the shielding material. Then a significant increase in the profile width is observed till the intensity decreases at a distance of 430 mm. Beyond this distance, an intensity of 5% from the maximum intensity corresponds to the background noise of the CCD chip of the camera.

94 94 Results and Discussion a) Intensity of the beam profile directly at the end of the shielding of the neutron guide. (A1+A2+A3) b) Intensity of the beam profile 40 mm from the shielding at the end of the neutron guide. (A1+A2+A3) c) Intensity of the beam profile 100 mm from the end of the neutron guide shielding. (A1+A2+A3) d) Intensity of the beam profile 210 mm from the shielding at the end of the neutron guide. (A1) e) Intensity of the beam profile 310 mm from the shielding at the end of the neutron guide (A1) f) Intensity of the beam profile 410 mm from the shielding at the end of the neutron guide. (A1) Fig. 62: Intensities and FW5% (full width at 5% of maximum) values of the neutron beam profile close to the focus (a, b, c) taken with attenuators A1, A2, and A3 in the neutron beam with an exposure time of 0.5 s. Figures (d, e, f) show the neutron beam profiles and the corresponding FW5% values for the scan performed from 100 mm to 500 mm with attenuator A1 and exposure times of 1.0 s at three sections.

95 Results and Discussion 95 Fig. 63: The FW5% values of the scan are plotted as a function of the distance from the shielding. Directly after the focus the opening angle can be determined with a linear fit. With increasing distance from the neutron guide, a significant change in the width can be observed due to steps in the intensity profile.

96 96 Results and Discussion Effect of the neutron attenuators The effect of the different neutron attenuators, available at the PGAA instrument is illustrated in Fig. 64. Each attenuator was moved into the neutron beam and a transmission image was recorded. For comparison, also the unattenuated image is shown in Fig. 64a. a) Image of the un-attenuated neutron beam b) Image of the neutron beam with neutron attenuator A1 c) Image of the neutron beam with neutron attenuator A2 d) Image of the neutron beam with neutron attenuator A3 Fig. 64: Images of the neutron beam with different attenuators and the same exposure time of 1 s. Images recorded with 300 µm scintillator and the flight tube setup. For the case of PGAA measurements, which are performed close to the neutron guide (corresponding to a position at 110 mm), the inhomogeneities introduced by the neutron attenuators are homogenized in the last section of the neutron guide. However, by increasing the distance and thus the effect of the divergence, the stripe pattern from the neutron attenuators become clearly visible. The images shown in Fig. 64b-d are normalized with the image of the un-attenuated beam and presented in Fig. 65.

97 Results and Discussion 97 a) Transmission image of the neutron beam with neutron attenuator A1 b) Transmission image of the neutron beam with neutron attenuator A2 c) Transmission image of the neutron beam with neutron attenuator A3 Fig. 65: False color transmission images of the neutron beam with different attenuators.

98 98 Results and Discussion The images show the normalized mapping from the pattern of the introduced neutron attenuator through the neutron guide on the scintillation screen of the camera box. Also, the expected averaged intensities of 18%, 50%, and 6% were in good agreement with the expected attenuation values, given for A1 (vertical lines), A2 (horizontal lines) and A3 (holes), respectively[15]. In conclusion, the attenuators offer a good possibility to reduce the neutron flux in the case of certain PGAA experiments; however, for a possible application in neutron radiography the usage of the attenuators has to be avoided. Based on the performed characterization of the neutron beam, it is strongly recommended to use the elliptical extension of the neutron guide for a neutron-imaging instrument at PGAA. With a permanent shielding, the full neutron flux can be used, leading to short exposure times and rapid tomography measurements. Also, the large divergence of the neutron beam allows a relatively small instrument with a total length of 800 mm to achieve a L/D ratio of 200. The application of a diffusor device is a good possibility for the homogenization of the beam. An optimum thickness of the diffusor may be found with further experiments to allow neutron imaging with a more homogeneous neutron beam and shorter exposure times. Due to the activation of the elliptical guide during its use, such experiments need to be done at the beginning of neutron imaging measurements but could not be performed in the time frame of this work anymore. In addition, the application of the setup with the 20 x 20 mm 2 neutron collimator for a possible setup proved to be insufficient due to the low divergence of the neutron beam. A possible imaging setup would have a minimum length of 2,000 mm to illuminate an area of 80 x 80 mm on the scintillation screen, but this space is not available at the PGAA instrument. Also the neutron flux at the sample position is in this case significantly lower than with the elliptical extension and thus all further experiments are performed with the elliptical extension of the neutron guide without neutron attenuators.

99 Results and Discussion Neutron Radiography and Tomography Neutron radiography The first acquired image with the developed neutron radiography setup at the PGAA instrument was an open beam image. The open-beam images acquired with two different scintillation screens were recorded for the comparison of the different screens and are shown in Fig. 66. In addition, the acquisition of the open beam images allowed an adjustment of the exposure time for appropriate image intensities for later radiography experiments. Fig. 66: Open beam image of the developed setup with a 10 µm GADOX scintillator (30 s exposure time) and a 100 µm LiF/ZnS scintillator (2.2 s exposure time). In the case of the 10-µm GADOX scintillator, an exposure time of 30 s was selected to acquire the beam image with a maximum intensity of about 56,000 counts. The main advantage of such a thin screen is its high spatial resolution, much below the resolution of the camera and thus, allows a good characterization of the detector setup. However, the gadolinium in the scintillation layer emits significant amounts of high-energy gamma rays during the irradiation because of the neutron capture reaction. These can be observed in the images as white spots (see Fig. 67 on the upper right) but can be filtered in later image processing steps. In contrast, the LiF scintillator allowed an exposure time of 2.2 s for the acquisition of a maximum pixel intensity in the range of 56,000 counts. Thanks to the (n,α) reaction on lithium nuclei without accompanying gamma-ray emission, the problem of gamma spots could be significantly reduced with the use of a LiF/ZnS scintillator. The GADOX scintillator was used only in this experiment to determine the best resolution of the camera system. In later experiments, the screen was changed to the LiF/ZnS scintillator for the sake of short exposure times. In both cases, the beam images show strong vertical structures due to the curvature of the neutron guide.

100 100 Results and Discussion Fig. 67: Cut-out of the resolution test device with a magnified section of the actual line pair measurement. Fig. 67 shows the image of the resolution test target from PSI[90], acquired with the GADOX scintillator. The right part shows a magnified section of the wafer with enhanced contrast, at which the resolution was determined in the area inside the white rectangle. For illustration purposes, in this image the - spots are not filtered. The lower part of Fig. 67, depicts the contrast profile of the line pairs as a function of the distance on the scintillation screen. With decreasing distance of the line pairs, the contrast fluctuation reduces. A fluctuation with a maximum of 10% was found for the range, indicated with the red line. The length of this line corresponds to a total distance of 0.5 mm and 3 line pairs, which results in 6 line pairs/mm or a resolution of 83 µm.

101 Results and Discussion 101 Furthermore, the structure of another test object, the so-called Siemens star was used to investigate the resolution of the built imaging system in all directions. With the GADOX scintillator, a homogeneous circular structure was investigated and the resolution limit estimated with about 100 µm, based on the structures of the star as illustrated in Fig. 68. Fig. 68: Resolution measurement with the Siemens star structure and a 10 µm GADOX scintillator. In addition, the resolution in a distance of 40 mm from the scintillation screen was investigated in preparation for later computer tomography experiments. Due to the estimated L/D of 200, a maximum resolution of 200 µm is expected with both scintillation screens, the 10 µm GADOX scintillator and the 100 µm LiF/ZnS scintillator. The recorded and normalized sections from the images of the Siemens star are given in Fig. 69. Fig. 69: Comparison of the radiographs acquired with two different scintillators in a distance of 40 mm from the detector to estimate the resolution for tomography measurements. If the image of the test target acquired with the GADOX scintillator at a distance of 40 mm in Fig. 69 is compared with the image of the same scintillator directly in front as shown in Fig. 68, the geometric effect of the L/D ratio be-

102 102 Results and Discussion comes clearly visible. The separation of the outer structures of the Siemens star, starting at a ring indicating a resolution of 500 µm towards the structures of 300 µm is possible in both cases. When comparing the inner rings from 300 µm to 100 µm a separation of the shapes can be recognized if the test target is mounted directly on the scintillation screen. However, this is not possible, if the target is at a distance of 40 mm from the scintillation screen due to blurring introduced by geometry of the instrument. To optimize the exposure time of neutron radiography measurements, the same experiment was performed with the 100 µm LiF/ZnS scintillator, leading to a maximum resolution of 200 µm. The main difference was the reduction of the exposure time from 30 s to 2.2 s without a loss of resolution. In addition, the shorter exposure time leads to a reduction of the gamma background on the CCD chip of the camera. Due to the better performance of the LiF/ZnS screen and no significant effect on the resolution of the instrument, the 100 µm LiF/ZnS screen is found to have the best performance for the requirements of neutron radiography and tomography experiments at the PGAA instrument Neutron tomography The developed radiography setup was tested for neutron tomography with an amplifier tube and an USB stick. Fig. 70a shows the amplifier tube with a height of 70 mm and a diameter of 23 mm. The reconstructed images of the tube are shown in Fig. 70b-d. The computer tomography allowed the clipping of the outer grid and metallic layer at the glass surface, revealing the inner structures of the sample. Further clipping and zooming into the sample uncovers the structures at the bottom of the amplifier tube with its connectors from the outer pins towards the electrodes (Fig. 70c). Even if the inner structures appear homogeneous in the clipped image, a proper segmentation by assigning colors to different materials, simply based on the gray scale values proved to be difficult. First trials to segment the different parts of the reconstructed sample, based on their grey-scale values due to the materials attenuation coefficients, are shown in Fig. 70d. The discoloration of the surrounding glass is clearly visible. The different color on top shows the effect of beam hardening, a shift of the neutron spectrum with increasing depth in a material and thus, a change in the transmission of neutron radiation through the sample. Based on the 1/v law, the energy dependent attenuation coefficients of the irradiated material change accordingly with the changing spectrum. The second observed effect is a combination of the geometrical unsharpness and scattered neutrons at the surface of the cylindrical electrodes[112,113]. A better approach for the separation of different parts may be achieved with an algorithm, which connects all voxels within a certain range of grayscale connected to a previously defined object. A further improvement could be the knowledge of defining geometric shapes of inner structures, which may be used in combination with the segmentation process according to different greyscale values but was not a topic of this work. The computer tomography of the amplifier tube showed that the setup is capable of imaging small samples with a resolution of 200 µm.

103 Results and Discussion 103 a) Photograph of the amplifier tube (70 mm x 23 mm) b) Cut along the center axis of the tomographic reconstruction of the amplifier tube c) Magnified section of the bottom of the tomographic reconstruction of the amplifier tube d) Trial to segment the different materials based on their grey scale values Fig. 70: Image and tomographic reconstruction of the amplifier tube with different sections. The grid distance is equal to a distance of 10 mm.

104 104 Results and Discussion The second investigated object was an USB memory stick, shown in Fig. 71a. Also in the case of the USB stick, a high resolution was achieved with the developed imaging system and the inner structures could be visualized well, as depicted in the clipped slices of Fig. 71b-d. a) Photograph of the USB memory stick b) Back layer with some chips clearly visible on the board c) Slice through the board of the USB stick and the connector channels of the plug. d) Slice through one of the main chips on the board of the USB stick and the connectors soldered on the board Fig. 71: Photograph and different sliced sections of the tomographic reconstruction of the investigated USB memory stick. The sections of the lines illustrate a distance of 10 mm. Slicing the sample along planes parallel to the plug reveals the different layers of chips on both sides of the stick. In addition, the connectors within the board and contacts of the connector plug can be visualized well. However, in Fig. 71b reconstruction artifacts can be observed around the long, grey semiconductor device on the back of the board. A separate, more detailed view of the USB stick is shown in Fig. 72. A hazy shadow around the inner structures below the housing can be observed well, caused by parts that strongly attenuate the transmission of the neutron beam. The thicker plastic housing at the joints of the metallic cover, which acts as protection of the USB connector is a possible reason for the strong absorption. Due to the low neu-

105 Results and Discussion 105 tron energy, most neutrons of the beam are scattered by the hydrogen of the surrounding plastic housing and thus, a sufficient transmission is not possible under all angles and information, needed for the reconstruction are partly lost. Fig. 72: Overview of different sections of the USB stick. On the left, the memory chip is probably visible and the connectors can be just seen. However, on the back, artifacts can be seen due to the scattering and absorption of neutrons. The second phenomenon observed after the computer tomographic reconstruction were ring artifacts. They are caused by defects and inhomogeneities in the acquisition system of the camera and become visible, especially if low intensities are recorded. In addition to the inhomogeneity, which was observed in the open-beam image, the strong absorption in the plastic parts of the sample reduced the detected intensity further, increasing the effects of camera defects. Fig. 73a and Fig. 73b illustrate the effect of the plastic surrounding at the example of the USB connector. In the range of the connector without a hydrogen containing case, a good resolution with sharp edges was achieved in the reconstructed image and the cavities of the connectors are clearly visible.

106 106 Results and Discussion a) Image of the USB connector with cut above the beginning of the plastic b) Image cut at the USB connector slightly lower where the connector is surrounded by the housing. Fig. 73: The worsening of resolution is clearly visible in the image, when hydrogen-containing materials are present in the sample. Depending on where the plug is clipped, the edges of the cavities within the USB connector are sharp (Fig. 73a) or blurred (Fig. 73b). This effect appears due to the neutrons, which are scattered in the plastic surrounding of the USB stick. In addition, in some positions of the acquired radiographs, the transmitted neutron intensity is reduced significantly, especially if the neutron beam had to penetrate the sample through thick layers of plastic. Ring artifacts of the reconstruction are then commonly observed. In conclusion, both experiments have shown that the developed system works well with the limited space available and a good resolution was achieved for such a compact instrument. One main benefit of the developed instrument at the curved neutron guide is the low gamma-ray background. Due to the wellshielded neutron guide, gamma spots were not observed when the 100-µm LiF/ZnS scintillator was used. The positioning of the camera above the main shielding of the PGAA instrument with lead layers of at least 100 mm around the camera prevented a direct view from the camera chip towards the sample. This shielding contributed to a significant suppression of gamma spots in the recorded images and enhanced the quality of the recorded images. The application of a 100 µm LiF/ZnS scintillator with a 4 mm pinhole resulted in the best resolution, allowing short exposure times in the range of 2.2 s. Thinner scintillator screens will result in longer exposure times and due to the limitations of the camera system a much better resolution than 80 µm cannot be achieved for flat samples with neutrons. The resolution for bulky samples will decrease with increasing distance from the scintillator screen and can be calculated according to the L/D ratio. During the performed experiments, the acquisition of one image took 2.2 s for the exposure of a single image and approximately 8 s for the read-out of the image from the camera, which resulted in much longer acquisition times during the tomography experiments than necessary. With a faster camera system as e.g. the Andor Neo camera system, the read-out of the image can be reduced to a negligible time interval and tomography experiments with 400 images may be performed in about 15 minutes. A possible application for such a fast tomography instrument with cold neutrons may be in-situ tomography of slow, dynamic processes at dif-