Scintillators SEMINAR 1. YEAR, I. CYCLE AUTHOR: BPhys. Manja Ščetinec ADVISOR/MENTOR: Prof. Boštjan Golob Faculty of Mathematics and Physics, University in Ljubljana Ljubljana, October 2017 Abstract In this seminar I present the basic concepts of scintillators. I start with luminescence, focus on fluorescence and phosphorescence. I overview the various types of scintillators and their characheristics. Then i examine scintillation mechanism in organic and inorganic crystals. The following chapter covers response of organic scintillations, more precisely light output and time response. At the end of my seminar I introduce the application of scintillators. PET are shown to improve the performance of crystal scintillators used in medical imaging devices.
Contents 1 Introduction... 2 2 Luminescence... 3 2. 1 Fluorescence and phosphorescence... 3 3 Scintillators... 3 3.1 Characteristics... 3 3.2 Scintillator types... 4 3.3 Scintillation mechanism in organics... 4 3.4 Scintillation mechanism in inorganic crystals with activators... 5 4 Response of organic scintillations... 6 4.1 Light output... 6 4.2 Time response... 6 5 Application of scintillators... 7 5.1 Detection of scintillation photons... 7 5.2 Positron emission tomography - PET... 8 6 Conclusion... 10 1 Introduction A scintillator is a material which causes scintillation or emits a flash of light when a material is irradiated. When excited by ionizing radiation, they exhibit a property called luminescence. If such materials are connected to each other by the amplifier, such as photomultiplier tubes, these scintillations are converted into an electrical signal. That can then be analyzed and counted, to obtain information on the given radiation [1]. Sir William Crookes built the first device known as a spinthariscope which used a scintillator in 1903. Their use increased after 1944 when Curran and Baker replaced the naked eye measurement with the invention of the photomultiplier. The modern scintillation detector were developed. They are one of the most widespread and most commonly used devices for particle detection. The detection of ionizing radiation by the scintillation light produced in certain materials is one of the oldest known techniques. The process of scintillation remains one of the most useful methods available for the detection and spectroscopy of a wide assortment of radiations [2]. 2
2 Luminescence Luminescence is defined as a cold light, that is, as a process corresponding to the emission of electromagnetic radiation outside the thermal equilibrium. Luminescent materials receive energy from different radiation like- (electromagnetic, alpha, beta, neutrons, etc.) and emit energy in the form of visible light. We know different types of luminescence based on the type of input energy. It may be caused by chemical reactions, electrical energy, subatomic motions, spontaneous emission or induced by heat [3, 4]. 2. 1 Fluorescence and phosphorescence When luminescence is a result of absorption of photons than we talk about photoluminescence. It is a result of irradiation of materials with shortwave light (UV, visible). There are two different types of it, fluorescence and phosphorescence [4]. The first one is the prompt emission of visible radiation by a substance that has absorbed electromagnetic radiation. It is a phenomenon in which the emitted light usually has a longer wavelength, and therefore lower energy, than the absorbed radiation [1, 5]. The process of phosphorescence corresponds to the emission of longer wavelength light than fluorescence. Because of very slow transitions, which are associated with "forbidden" energy state transitions, absorbed radiation is re-emitted at lower intensity. In such cases, the delay can last from a few microseconds to a few hours, depending on the material. The third type of luminescence, which can occur in simple organic molecules, is delayed fluorescence or the alpha process. A good scintillator is a material which converts a large fraction of incident radiation energy to the prompt fluorescence while also reducing the generally undesirable contributions for phosphorescence and delayed fluorescence [1]. 3 Scintillators 3.1 Characteristics There are many types of scintillators. When we talk about the ideal scintillator, it must possess the following properties: A high scintillation efficiency for converting the kinetic energy of charged particles into detectable light. The energy conversion should be linear so the light yield is proportional to the deposited energy over as wide range as possible. Transparency for the wavelength of its own emission. The decay constant of the induced luminescence should be minimal so that fast signal pulses can be generated. Scintillator material should be of good optical quality and possible to manufacture in sizes large enough. Its wavelength of the emitted light should be in the spectral range, which photomultiplier tubes or other light sensors can detect [1]. 3
3.2 Scintillator types Scintillators are roughly divided into the organic scintillators, inorganic crystals and gaseous scintillators. Organic scintillators are organic crystals, organic liquids, plastics, thin film scintillators, loaded organic scintillators. The organic scintillators are hydrocarbon compounds, such as naphthalene [ ], anthracene [ ], stilbene [ ], etc. Inorganic scintillators are crystals and also liquid, gaseous or glass scintillators. Most of inorganic materials, known as phosphors, are salt crystals, containing impurities activation. The usual types are alkali halides, activated by heavy metals such as thallium, and zinc sulphide, activated by copper, silver or manganese. Inorganic glasses are formed from the oxides of silicon, boron, phosphorus or lithium. In general, the time response of the inorganic scintillators is slower (around 500 ns) than the response of organic. This is due to phosphorescence. The advantage is their high specific energy loss (de/dx), which is due to the higher density and high atomic numbers [1, 6]. 3.3 Scintillation mechanism in organics The process of fluorescence in these compounds is the result of crossing of free electrons of a single molecule and can be observed from a given molecular species independent of its physical state. A lot of practical organic scintillators are based on organic molecules with certain symmetry properties known as the π electron structure. The electrons in a cloud of electrons are at the center of the molecule and do not belong to any particular atom. A typical diagram of π orbitals is shown in figure 1, where energy levels are shown in singlet (spin 0) and triplet states (spin 1). The ground electronic state is singlet or. Above this level there are excited singlet states (,,, etc.). A series of triplet electronic states are labeled as,. Markings with two indices show vibrational levels. Differences between electronic levels are a few ev (3 ev or 4 ev between and ), while the differences between the vibrational levels are of a few tenths of ev (0,16 ev). Almost all of the molecules at room temperature are in the state, as its average thermal energies are much smaller than vibrational [1]. Figure 1: Pi electronic energy levels of an organic molecule [7]. Absorption of energy is presented with the vertical upward arrows. It is a process in which electrons absorb the kinetic energy from a charged particle passing nearby. The higher singlet excited electronic states decay very quickly to the electron state through internal degradation or radiationless internal conversion. States with excess vibrational energy (such as or ) are not in thermal equilibrium with its neighbors and quickly lose that vibrational energy. 4
Therefore, the final effect of the excitation process in a simple organic crystal is a population of excited molecules in the state after a very short period of time. From the state, the principal scintillation light is emitted to one of the vibrational states above the ground electronic state. The transitions between and these states are indicated by the downward verticals. This is the prompt component of fluorescence. It is evident that the material is transparent to its own scintillation light, because the emitted photons do not have enough energy to excite electrons of other molecules at to levels [1]. Some excited singlets may be converted into triplets through a transition called intersystem crossing. This transition is forbidden but occurs in the presence of spin-orbit coupling. Decay from to is therefore a delayed light emission called phosphorescence. The wavelenght of phosphorescence spectrum is longer than for flourescence, because energies lie below. Some molecules from state may be thermally excited back to the state and subsequently decay through normal fluorescence (delayed fluorescence). Delayed fluorescence differs from phosphorescence in the wavelengths of the emitted optical photon [1]. 3.4 Scintillation mechanism in inorganic crystals with activators The energy states of molecule consist of a series of discrete levels which affect the scintillation mechanism in inorganic materials. Therefore the mechanism is dependent of the crystal lattice, which means that the response of the scintillator can be anisotropic. Figure 2 shows a schematic diagram of the discrete energy bands in the crystal lattice of the inorganic material. The pure crystal has the valence band, the forbidden band and the conduction band. Figure 2: Energy bands of an activated crystalline scintillator [6]. An addition of small amounts of impurity to inorganic scintillators may enhance the probability of visible photon emission during the de-excitation process. Intentionally added impurities are called activators. There are three main types of it. The first are luminescence or recombination centres, in which the transition to the ground state is accompanied by visible photon emission. This is the basis of the scintillation process which gives the fast component of time (ns μs). The second are quenching centres, in which radiationless thermal dissipation of excitation energy may occur. The last one are traps. They have metastable levels from which the electrons (or excitons) may subsequently return to the conduction band by acquiring thermal energy from the lattice vibrations, or fall to the valence band by a radiationless transition [1, 6, 8]. Recombination due to trapping adds the slow component of scintillation (ms - s) [6]. 5
4 Response of organic scintillations 4.1 Light output Charge particle loses a small fraction of the kinetic energy in a scintillator, which is converted into fluorescent energy. The fraction, known as scintillation efficiency S, depends of the particle type and its energy. If the scintillation efficiency is independent of energy, that leads to a linear dependence of light yield on initial energy. The fluorescent energy emitted per unit path length response of organic scintillators according to describes the. (4.1) The is the specific energy loss of charged particle [1, 8]. Birks suggested a semi-empirical relation based on the assumption that high ionization density along the track of the particle leads to quenching from damaged molecules and lowering of the scintillation efficiency. Quenching means nonradiative energy loss, typically through intermolecular interactions (without emission of a photon). In the case when density of excited molecules along the wake of the particle is directly proportional to the ionization density, B (de/dx) represents the density of them, where B is a proportionality constant. Figure 3: Variation of specific fluorescence with specific energy loss for anthracene [8]. If the quenching can be considered unimolecular, then a quenching parameter k is added and the equation (4.1) is modified to. (4.2) The response of organic scintillators is approximately linear at small de/dx. At higher values of de/dx, which also means higher densities of excited molecules (greather probability for quenching), the second term of denominator becomes important. This happens at low velocity of ionizing particles. The de/dx varies in dependence of particle's type [1, 8]. 4.2 Time response The time profile of the emitted light pulse can be described by a simple exponential decay, assuming that the luminescent states in an organic molecule are formed instantaneously and only prompt 6
fluorescence is observed. The intensity of the fluorescence emission decays exponentially with time t:. (4.3) Time represents the decay time of level and is of the order of nanoseconds or less in most organic scintillators [1]. More accurately, the rise time has to be taken into consideration. For simplicity let us assume a two-level system, electrons from level 1 are excited to level 2 from which they radiatively return to the ground state. The change in the population of the second (higher) state in a time interval dt is. (4.4) represents an increase of the higher level population due to excitations, and the second term on the right-hand side is the deficit due to exponential decay with decay time. Assuming an exponential excitation (i.e. ), we get a nonhomogenous first order differential equation. The solution gives the following dependence of the scintillation yield:. (4.5) The finite time required to populate the luminescent states is denoted by decay time by is the total number of photons., and the scintillation 5 Application of scintillators The organic scintillators are particularly suitable for a wide range of applications in detection and spectrometry of beta particles, gamma rays and fast neutrons. Other uses include coincidence methods and high energy and elementary particle studies [1]. Inorganic scintillators are widely used in X-ray or Gamma ray detection and also in the process industry, oil exploration, nuclear medicine, medical imaging, radioprotection, etc. The understanding of the mechanisms of scintillation process has dramatically changed with an increased demand for scintillators of better performance for large particle physics experiments as well as for medical imaging [8]. 5.1 Detection of scintillation photons The purpose of photon detection is conversion of the light output of a scintillation pulse into a detectable electrical signal. The photoelectric effect and Compton interactions are used to convert photons to photoelectrons. These scintillation photons are detected by the photomultiplier tube (PMT), which is the most widely used device. Some other devices are Micro Channel Plates (MCP), Photo Diodes (PD), Hybrid Photo Diodes (HPD), Visible Light Photon Counters (VLPC) and Silicon Photomultipliers (SiPM). The high photon detection efficiency (PDE) or Quantum efficiency (QE) is required. QE means the number of quanta emitted per quantum absorbed and it depends of material [6, 8]. 7
5.2 Positron emission tomography PET Positron emission tomography PET scanning is a medical diagnostic technique based on the detection of positrons emitted by radionuclides. PET is used in nuclear medicine (imaging of tumors, searching for metastases) and it is particularly valuable in investigating the brain diseases and animal studies. The radioisotopes named radiotracers are injected into the bloodstream. They connect to the appropriate targets in the brain or other parts of the body. Principle of PET is illustrated in figure 4. The radionuclides decay by positron emission ( decay). The positron quickly annihilates at rest with an electron, as the emitted positrons rapidly lose their energy in matter. The energy of annihilation is released in the form of two photons. They are emitted on the same axis in opposite directions. Because the energy and the momentum are conserved and the energy is equivalent to the mass of the annihilated electron, emitted photons are carrying 511 kev energy. At the last stage the PET scanner makes three dimensional images of metabolic processes in the body and reconstruct images of radiotracer distribution. Many millions of these gamma-ray pairs are needed for successful image [10, 12]. Figure 4: The concept of PET: positron emission, annihilation and simultaneously emitted gama rays 180 apart [11]. A PET detector consists of a set of detectors that surround the observed part of body displayed as detector rings in a figure 5. One of detector blocks is comprised of scintillator crystals and photomultiplier. Scintillator detectors are the basis of most PET scanners nowadays and they serve for gamma-rays detection (Figure 6) [12, 13]. Figure 5: Schematic image of PET detector [11]. The properties of scintillator for PET scanners are high density (for stopping the incident photons), fast decay time, large brightness (the number of light photons produced per 511 kev interaction) and small index of refraction (efficiency of transmision of optical photons from the scintillator to the photodetector). Scintillator crystals are followed by the PMT that converts scintillation light into electrical signal and amplifies it. At the end of the chain there is front-end electronics which adjusts the signal for further processing [12, 13]. 8
Figure 6: Prototypical PET detector and the essence of the operation of the basic components [12]. We also need to look at some important properties of PET detectors. The most important is spatial resolution. It gets smaller by the thickness of the detector because of the detector parallax (due to the fact that the annihilation photons can interact at any depth in the scintillator material). The result is that the point spread function is asymmetic as shown in figure 7. The spatial resolution is lower because the point spread function is wider [12, 13]. The sensitivity of the detector is an important property for the creation of high quality image. It depends on several factors, which include the efficiency of the detectors at 511 kev, the solid angle coverage of the detectors, the location of the radioactivity, the timing and energy windows. Figure 7: The projections of the annihilation point [12]. Time resolution is the ability to determine the time difference in the arrival of the annihilation photons. The PETs need a good time resolution so that the random coincidences are avoided. The finite width of timing window enables us to detect two unrelated photons which are referred to as random coincidence (figure 8). A typical timing resolution is 1 ns to 10 ns and depends on the material. Figure 9: Scatter coincidence and attenuation of gammas [12]. Figure 8: Random coincidence [12]. Determination of the energy of the photon in PET detector is named the energy resolution. Most of them have 20 % energy resolution. Low resolution is due to high energy scattered photons (figure 9) [12, 13]. 9
6 Conclusion The seminar summarizes the properties of scintillators, their properties, the mechanisms of their work and their use. Otherwise, this is a broad subject covered in many other researches. Scintillators have proven to be a very versatile and useful instrument for the detection and study of nuclear radiation, so they have a very wide application in technology. On the other hand, the researches of the scintillation processes in organic and inorganic scintillators have many unresolved issues. The variety of scintillation detectors is enormous, they differ from each other in the material, the form, the coupling with the photomultiplier and their intended use. They are used in many different experiments while I have covered only one case of use in nuclear medicine. I focused on the PET detector, while a lot of room for describing the upgrade and other features of the scanner itself remains. The technology of scintillators has become an area where research and development is very intense. References [1] W.R.Leo, Techniques for Nuclear and Particle Physics Experiments.Springer-Verlag, 1987 [2] https://en.wikipedia.org/wiki/scintillator dostopno dne 20.9.2017 [3] http://www-f9.ijs.si/~golob/sola/seminar/scintilatorji/predavanja4.pdf dostopno dne 20.9.2017 [4] http://www-f9.ijs.si/~golob/sola/seminar/scintilatorji/predavanja5.pdf dostopno dne 20.9.2017 [5] https://courses.lumenlearning.com/boundless-physics/chapter/applications-of-quantummechanics/ dostopno dne 20.9.2017 [6] http://www-f9.ijs.si/~golob/sola/seminar/scintilatorji/lecture4.pdf dostopno dne 20.9.2017 [7] http://www.texample.net/media/tikz/examples/pdf/fluor-energy-levels.pdf dostopno dne 20.9.2017 [8] J. B. Birks, The Theory and Practise of Scintillation Counting. Pergamon Press,Ltd., 1964 [9] http://www-f9.ijs.si/~golob/sola/seminar/scintilatorji/iii-scint.pdf dostopno dne 20.9.2017 [10] http://www.daviddarling.info/encyclopedia/p/positron_emission_tomography.html dostopno dne 20.9.2017 [11] http://www-f9.ijs.si/~golob/sola/seminar/scintilatorji/pet_spect_schibli.pdf dostopno dne 20.9.2017 [12] http://www-f9.ijs.si/~golob/sola/seminar/scintilatorji/med-app2.ppt dostopno dne 20.9.2017 [13] http://eknygos.lsmuni.lt/springer/360/1-117.pdf dostopno dne 20.9.2017 10