The Determination of Radioactivity Levels in the Environment using High Resolution Gamma-ray Spectrometry

Transcription

1 The Determination of Radioactivity Levels in the Environment using High Resolution Gamma-ray Spectrometry Mohammed Al-Harbi A dissertation submitted to the Department of Physics, University of Surrey, in particular fulfilment of the degree of Master of Science in Radiation And Environmental Protection Department of Physics Faculty of Engineering and Physical Sciences University of Surrey September 2011 Mohammed Al-Harbi 0

2 Content Acknowledgment. 4 Abstract 5 Chapter 1: Introduction 6 Chapter 2: Theoretical Background The Radioactive Decay The Type of Radioactive Decay Alpha Decay Beta Decay Gamma Decay Interactions of Gamma Radiation with Matter Photoelectric absorption Compton scattering Pair production The Characteristics of Semiconductor Detectors Band structure in Solids Charge carries Effect of Impurities Intrinsic Semiconductors n-type Semiconductors p-type Semiconductors Radioactive Equilibrium Secular Equilibrium Transient Equilibrium No Equilibrium The Measurement of Activity Concentration (A C ) Gamma Dose Rate (D) Radium Equivalent Dose (Ra eq ) External Hazard Index (H ex )

3 2.10 Minimum Detectable Activity (MDA) Weighted Mean (W m ) 24 Chapter 3: Experiment Details Samples preparations Experimental Arrangement Detector Characteristics Energy calibration Efficiency calibration Samples analysis 28 Chapter 4: Experimental Results and Discussion Detector Characteristics Energy calibration Efficiency calibration Spectral Analysis and Nuclide Identification Background spectrum analysis Sample spectrum analysis Minimum Detectable Activity (MDA) Activity Concentration Measurement (A C ) and Weighted Mean (W m ) An Estimation of Dose Rate and Relative Values Chapter 5: Conclusion 49 Chapter 6: References Chapter 7: Appendixes Appendix A: The data of detector efficiency with standard sources counted using both gains A.1: The data of detector efficiency with 152 Eu standard source counted using normal gain.52 A.2: The data of detector efficiency with 226 Ra standard source counted using normal gain.53 A.3: The data of detector efficiency with 232 Th standard source counted using normal gain.54 A.4: The data of detector efficiency with 152 Eu standard source counted using high gain. 55 A.5: The data of detector efficiency with 226 Ra standard source counted using high gain. 56 A.6: The data of detector efficiency with 232 Th standard source counted using high gain 57 2

4 A.7: The data of detector efficiency with NG3 standard source counted using normal gain 58 A.8: The data of detector efficiency with NG3 standard source counted using high gain.. 58 Appendix B: The data of determining the activity concentrations for all soil samples B.1: The data of nuclides identified in soil sample (1-X-228) to counted using normal gain.59 B.2: The data of nuclides identified in soil sample (2-X-228) to counted using normal gain.60 B.3: The data of nuclides identified in soil sample (3-X-228) to counted using normal gain.61 B.4: The data of nuclides identified in soil sample (22-X-228) to counted using normal gain 62 B.5: The data of nuclides identified in soil sample (26-X-228) to counted using normal gain 63 B.6: The data of nuclides identified in soil sample (23-X-228) to counted using high gain.. 64 B.7: The data of nuclides identified in soil sample (24-X-228) to counted using high gain.. 65 B.8: The data of nuclides identified in soil sample (27-X-228) to counted using high gain.. 66 B.9: The data of nuclides identified in soil sample (28-X-228) to counted using high gain., 67 B.10: The data of nuclides identified in soil sample (29-X-228) to counted using high gain.68 3

5 Acknowledgment I thank God Almighty for His grace and kindness on what gave me strength, patience and intelligence to achieve this work. I would provide my deep gratitude to Professor. Paddy Regan for his faithful and gentle supervision and useful suggestions during this work. Also, I would like to express my acknowledgment to Dr Huda Al-Sulaiti for her supporting and patience and further thanks to Mr Nasser Al-Azemi for his help me during this work. Also, I am grateful to the Cultural Bureau of Saudi Arabia for its support of the material of the MSc programme. Finally, I particularly provide thanks to my wife Mrs Wafa Al-Harbi and my son Meshari for their patients and supports. 4

6 Abstract The aim of this study is to determine the radioactive levels in the environment and measure the activity concentration for soil samples, which were collected from an area around the Dukhan oil field in the State of Qatar. The samples have been analyzed by using a high-purity germanium detector (HPGe). The soil samples were collected to be analyzed in order to measure the activity concentrations related to the 235 U, 238 U and 232 Th decay chains and also 137 Cs and 40 K. For example, the 238 U is analyzed through its daughters, such as 226 Ra, 214 Pb and 214 Bi and the 232 Th by its daughters, including as 212 Pb, 228 Ac and 208 Tl. The sample (29-X-228) has the highest count values observed for 226 Ra compared to all the other soil samples. The activity concentration for 238 U and 232 Th decay chains, 137 Cs and 40 K in this sample were calculated by Bqkg -1 and the weighted mean activity concentrations for 238 U and 232 Th decay chains were found to be ± 0.04 and 8.42 ± 0.34 respectively. The activity concentration of 238 U in the sample (29-X-228) is considerably higher than the values determined for the other soil samples, which were measured in this current study and also significant higher than the worldwide mean range as reported by the UNSCEAR (2000). 5

7 Chapter 1 Introduction NORM is a contraction for naturally occurring radioactive material and NORM is any material including radioactive substances that have been present since the earth s formation. These materials occur in air, water, soil and human bodies. Although, the general name of materials is NORM, there are other specific names, which have been assigned, such as Technologically Enhanced Naturally Occurring Radioactive Material (TENORM) and Low Specific Activity (LSA) material. NORM is described by the International Atomic Energy Agency (IAEA) as Radioactive materials containing no significant amount of radionuclide than naturally occurring radionuclides [9]. The nuclides which can be determined by gamma-ray spectrometry include; 235,238 U, 40 K and 232 Th. The 235 U, 238 U isotopes and 232 Th are accompanied by their daughter nuclides. Natural materials including these nuclides are often indicated as the NORM. There are other naturally occurring nuclides, such as 14 C, which are created incessantly by nuclear reactions between high-energy particles with oxygen and nitrogen in the earth s atmosphere. Gamma spectrometry of NORM is complicated for a variety of reasons. First, the activity levels are usually rather low and can require long counting periods. The second cause is the matter of background spectrometry, therefore a large number of peaks can be present in the background spectra owing to the NORM nuclides in the surroundings of the detector. Depending on the local environment, there may be evidence of contamination from neutroncapture and fission-product nuclides, such as 60 Co and 137 Cs [3]. Uranium and thorium are not stable and principally decayed by alpha-particle emission to nuclides which are also themselves radioactive. Natural uranium consists of three long lived isotopes, namely 238 U, a smaller ratio of 235 U and an even smaller fraction of 234 U which is decay-series daughter of 238 U. Natural thorium has one single isotope, which is 232 Th. Each of these nuclides decays to an unstable daughter leading to a series of nuclides which terminates in one of the stable isotopes of lead. In natural material, the 235 U and 238 U proportion will be fixed and all nuclides in each of the series will be in equilibrium. The 238 U isotope makes up 99.25% of natural uranium and decays by alpha emission to 234 Th. The 238 U decay series generates daughters, such as 226 Ra, 214 Pb and 214 Bi.The half-lives of the different nuclides are much shorter than the half-life of 238 U, which means each daughter nuclide can be in secular equilibrium with the 238 U present [3]. This means that the activity of each daughter nuclide in the chain will be the same as the activity of 238 U and the total activity of each source will be 14 times that of the parent if there are 14 radionuclides in the decay chain [3]. 6

8 235 U contains 0.72% of natural uranium. The 235 U decay series comprises 12 nuclides in eleven decay phases and the emission of seven alpha particles and therefore, the total activity of the nuclides through this series is 11 times of the activity of 235 U. In addition, the main gamma-ray is emitted by 235 U at kev is approximately the same energy that is emitted by 226 Ra ( kev). The 232 Th decay chain produces daughters, such as 228 Ac, 212 Pb, 212 Bi and 208 Tl. The decay of 212 Bi is branched and 35.94% of decays create 208 Tl by alpha decay. 212 Po is produced by the beta decay [3]. Some radionuclides in these chains emit characteristics gamma rays, which can be measured by using a gamma spectrometry system. Figure 1 illustrates the natural uranium and thorium decay series, where radon gas is present in each chain and the three chains end up with stable lead. 40 K is very evident in the typical background spectrum and is present as 0.17% of natural potassium. It is present in wood and building materials. The gamma spectrometry of 40 K is straightforward but the correction of peak-background is necessary. There is a spectral intervention from the kev peak of 228 Ac, but if the activity of the 232 Th daughters is low and the peak formed is not obviously affected, [3]. Figure 1: The natural uranium and thorium decay chains [15] The exposure of external radiation is 33% higher through the residences than outside [2]. Consequently, the radiation is absorbed by the building materials, which creates outside the building. 7

9 The exposure through the buildings is more than recompensed by the presence of radionuclides in the materials of structure. Coal also includes radionuclides of the uranium and thorium series and 40 K. The average value for all coals sampled is 1.7 μg g -1 for uranium and 4.5 μg g -1 for natural thorium, which is the same to the mean concentrations found in soils and rocks. The consumption of amount of radionuclides liberated to the atmosphere per ton of coal relies on the concentration of radionuclides in the coal [2]. The objective of this study is to determine the radioactivity level in the environment and calculating the activity concentration, weighted mean and minimum detectable activity for naturally occurring and technically enhanced levels of radiation in the samples, which were collected from the state of Qatar. The samples have been analyzed by a high-purity germanium detector (HPGe) and the data of this work will be used to infer levels of hazards of associated with exposure to such materials. Chapter 2 discusses the fundamental basic of radioactive decay and the main basics and characteristics of semiconductor detector including the processes by gamma rays interacting with matter. Chapter 3 describes the methodological experiment of collecting, preparing and analyzing the samples and detector characteristics. The experimental results are studies and discussions in chapter 4. Finally, chapter 5 provides the conclusion and general comments of this project in the future. 8

10 Chapter 2 Theoretical Background 2.1 The Radioactive Decay The process of an unstable nuclide or parent transferring into a more stable nuclide, which is called the daughter is radioactive decay. Radioactivity is a random process, which is described by a half-life (t 1/2 ), which this time is taken for half the nuclei in a sample to decay. Radioactive nuclei can decay by alpha or beta decay or both and can emit a combination of alpha, beta and gamma radiation. These can continue to be in the environment together with cosmic rays from outer space, which generate the background radiation [8]. The radioactive decay is a spontaneous and alters the nucleus of an atom, which can lead to the emission of particles or electromagnetic radiation. The activity of a radioisotope source can be expressed as its rate of decay, which is given by the fundamental law of radioactive decay [6]. 2.1 where, A(t) is activity of a radioisotope source. N is the number of radioactive nuclei. λ is a decay constant which is expressed as (ln2/half-life t 1/2 ). The half-life (t 1/2 ) is described the time necessary for half of the nuclei to decay and half-life (t 1/2 ) is represented following as 2.2 By integrating equation (2.1) with respect to N 0 that represents the amount of atoms at time equal zero and N(t) represents the amount of time at any future time (t): 2.3 Equation (2.3) can be written as: Referring to equation (2.1) is written as:

11 2.2 The Type of Radioactive Decay There are three main fundamental radioactive decay processes, which are called Alpha (α), Beta (β) and Gamma (γ) decay Alpha Decay The alpha particle is emitted from a heavy nucleus and in particular from helium-4 4 He which includes two neutrons and two protons. This particle is commonly the preferred mode of decay at high atomic number, Z> 83. If the nucleus loses α-particles, it will lose four units of mass and two units of charge particles. The α-decay process can be represented as [6]. 2.6 Figure 2.1: The alpha decay process [11] An example for alpha decay process is and by α-decay emission and change them to and respectively, the decay is expressed as [6]: 2.7 In which the radium 226 Ra has the half-life about 1600 years and the α-particle occurs with a kinetic energy which is estimated about 4.8 MeV. A fixed energy is common between the α-particle and the recoil nucleus. As a result each α- particle from a given decay occurs with the same energy, which is given by Q (A-4)/A. Most energies of alpha-particle are limited between roughly 4 to 6 MeV. There is a strong association between α-particle energies and the half-life of the parent isotopes with the highest energies associated with the shortest half-life. [5,6] Beta Decay A positive electron (e + ) is called positron, a negative electron (e - ) is called negatron and an electron. 10

12 They are three nuclear processes which are closely correlated, which are called Beta-decay. β decay process is transferred directly from a proton into a neutron or a neutron into a proton. β decay is altered the atomic number, Z and neutron number, N by one unit in a nucleus [6]. The first process is defined as a negative β decay or negatron decay and relates the emission and creation of a standard electron. This process can be expressed as follows [6]: n p + e - β - decay 2.8 For example Figure 2.2: Beta minus decay process [12] The second process is known as positive β decay or positron decay which means a positive charge electron is emitted and this process is represented as [6]: p n+ e + β + decay 2.9 An example of this process is Figure 2.3: Beta plus decay process [12] In the third process, an atomic electron close to the nucleus is swallowed, allowing the transfer of a proton to a neutron. This procedure can be written as [6]: p + e - n electron capture (ε) 2.10 For example 11

13 In all three process another particle, which is called a neutrino is also emitted, but the neutrino does not have electric charge, and its inclusion in the decay process has no effect in the identity of the other final particles. In positive and negative beta decay a particle has been created out of the decay energy and the electron or positron was no accessible inside the nucleus before the decay. In these processes, the atomic number, Z and neutron number, N are changed by one unit, but the total mass number (Z+N) remains constant [6] Gamma Decay Gamma emission is similar to the emission of atomic radiation, such as optical or X-ray conversions. The emission of gamma-ray is observed in all nuclei that have excitated bound states (A> 5). Figure 2.4: The gamma decay scheme of 60 Co [14] Gamma emission has half-lives which are usually quite short and generally less than 10-9 s, but sometimes the half-lives for γ emission find significantly longer, even hours or days. These longer lived decays are defined as isomeric transitions and the long-lived excitation states are called isometric states or isomeric. The γ emission does not contribute to change in the number or the type of nucleons in the nucleus and also, there is no alteration in atomic number (Z) neutron number (N) or mass number (A). Typical energies of gamma rays are in the range of 0.1 to 10 MeV. These corresponding to wavelengths which are far shorter than other types of electromagnetic radiations such as visible light which has wavelengths 10 6 times longer than gamma rays [3,6]. 12

14 2.3 Interactions of Gamma Radiation with Matter There are three main fundamental processes which describe gamma ray interactions with matter. These processes are Photoelectric absorption, Compton scattering and Pair production. Figure 2.5: The major processes for interacting γ-rays with matter [5] Photoelectric absorption Photoelectric absorption is established by interaction of the gamma-ray photon with one of the bound electrons in an atom. An energetic photoelectron is ejected by the atom from one of its bound shells. The photoelectron appears with an energy given as [5]: 2.11 where E p is the energy of photon which is equal to hν, E b is the binding energy of the photoelectron in its original shell. The atom is left in an excitation state with an excess energy of E b and recovers its equilibrium in one of two ways. The atom may de-excite by redistribution of the excitation energy between the remaining electrons in the atom. The vacancy left by the ejection of the photoelectron may be filled by a higher-energy electron falling into it with the emission of a characteristic X-ray which is called X-ray fluorescence. In this condition, the level of the energy, which is ejected from the electron, depends on the energy of the gamma ray. However, the photoelectric absorption edges appear at the binding energies resulting from the electron shells. The probability of a photon suffering photoelectric absorption, can be expressed as a cross section (τ). The measurement of the degree of absorption and attenuation differs with the atomic number (Z) of the absorber and the gamma ray energy (E γ ) in a complicated manner [3]. 13

15 2.12 where n and m are through the range 3 to 5, depending on energy. Functions of the form and have been quoted. The photoelectric attenuation coefficient (μ PE ) can be derived from the related cross-section [3] where ρ is the density of the absorbing material, A is the average of atomic mass and N A is the Avogadro constant Compton scattering Compton scattering is defined as a direct interaction of the γ ray with an electron, which is conversion part of the γ-ray energy. In Compton scattering, the incoming gamma-ray photon is deflected within an angle (θ) with respect to its original direction. The photon converts a portion of its energy to the electron which is called a recoil electron. The energy transferred to the electron can range between zero to a large fraction of the gamma-ray energy that because of the different angle of scattering which are possible. The energy imparted to the recoil electron is expressed as follow [3,5]. { [ ] } 2.14 where, m 0 c 2 is the rest mass energy of the electron which is equal MeV. Figure 2.6: The Compton scattering process [13] The differences of the energy absorbed depend upon the scattering angle (θ). For example, when θ=0, scattering directly forward from the interaction point, E e is found to be 0 and no energy is converted to the detector. In the other extreme, the gamma ray is backscattered and θ=

16 The amount of energy, which is transferred to the electron, must be between those two extremes in intermediate scattering angles. The absorption cross-section of Compton scattering (σ) is associated to the atomic number of the material and the energy of the gamma-ray [3] The Compton scattering coefficient (μ CS ) can be calculated and writing as follows [3]: 2.16 The possibility of Compton scattering per atom of the absorber depends on the number of the electrons that are available as scattering objectives and so the linear increases with atomic number (Z) [5] Pair production Pair production corresponds to the interaction of a gamma ray with the atom as whole. This process takes place through the coulomb field of the nucleus, due to the transition of a γ ray into an electron-positron pair. However, pair production can appear only through the gammaray spectrum when the energy is more than MeV. In addition, pair production can occur in the influence of the field of an electron but the probability [3]. Figure 2.7: The mechanism of pair production [16] As figure 2.7 above, the electron (e - ) and positron (e + ) are created by the surplus gamma-ray energy similarly. When the energy of positron is reduced to near thermal energies, it must meet inevitably an electron and the two will annihilate, releasing two MeV annihilation photons [3]. These annihilation gamma rays are emitted in opposite directions in order to protect momentum and they may in turn interact in the absorbing medium by either photoelectric absorption or Compton scattering [8]. 15

17 On the other hand, the net energy is absorbed through the detector by the immediate consequences of pair production event is given by [3]: E e = E γ Furthermore, the cross-section of pair production (k) depends on E γ and Z in a complicated manner which is expressed as follows [3]: The Characteristics of Semiconductor Detectors The measurement of gamma rays or high-energy electrons can be made in a much smaller volume than the equivalent gas-filled detector for the reason that has a solid density, which is some 1000 times greater than that for a gas. However, the best way for reducing the statistical limit on energy resolution is to raise the number of charge carries per pulse. The appeal of semiconductor materials as radiation detectors is that they can produce a much large number of carries for a given incident radiation event. Furthermore, the best energy resolution of radiation spectrometry can be achieved by using semiconductor detectors. The principle information carriers are electron-hole pairs, which are created along the length of the path of the detectors [5] Band structure in Solids The electrons are disposed in an accurate determination of energy levels at free atoms. A collection of atoms are combined together into a solid structure, which expands those energy levels into energy bands, and each energy band can include a fixed number of electrons. The band structure of electron energies is divided into the lower band, which is called the valence band, corresponding to the outer-shell electrons that are bound to specific lattice sites through the crystal. The upper band, which is called the conduction band, corresponds to electrons that are free to migrate within the crystal and the band gap separates between these bands [3,5]. Figure 2.8: The band structure for electrons energies in insulator and semiconductor [5] 16

18 Materials can be classified as semiconductor or insulator. The valence band is full and the next available energy cases are in conduction band, are separated by a range of forbidden energy in an insulator. An electron which migrate within the material, must increase by sufficient energy compared to bound electrons in the valence band and get across the band gap into the conduction band. The band gap is about 10 ev, which is much larger than can be disposed by thermal excitation. In a semiconductor, the valence band remains full but the band gap is smaller approximately 1 ev, and the energy can be achieved by thermal excitation [3] Charge Carries An electron is excited normally to be a part of a covalent bond, which can leave the specific bonding site and drift through the crystal. The excitation process does not only move an electron in the empty conduction band, but also it leaves a vacancy, which is called a hole, in the previously full valence band. The valence band and conduction band combined to create an electron-hole pair, which is approximately the solid-state similar to an electron-ion pair in a gas detector. In the conduction band, the electron can move under the effect of an applied electric field. The hole, which represents a net positive charge, will have a tendency to travel in an electric field, but in an opposite direction of the electron. The probability per unit time that an electron-hole pair is thermally created is shown as [5]: ( ) 2.19 where, T is an absolute temperature, E g is a bandgap energy, k is Boltzmann constant and C is proportionality constant characteristic of the material. The possibility of thermal excitation depends seriously on the proportion of the bandgap energy to the absolute temperature. When an applied electric filed becomes absent, the electron-hole pair recombines eventually and equilibrium is created wherein the concentration of electron-hole pairs observed at any given time is relative to the rate of formation [5] Effect of Impurities Intrinsic Semiconductors Holes in the valence band and electrons in the band conduction can be created by thermal excitation. For the reason the number of holes in the valence band must be similar to the number of electrons in the conduction band. However, the electrical characteristics of real materials have a tendency to be dominated by the very small level of residual impurities. 17

19 Equilibrium is created by the thermal excitation of electron from the valence band to conduction band and to lead the equal number of holes and electrons in the intrinsic material, which can be given as [5]: 2.20 where, n symbolize the concentration of electrons in the conduction band (number per unit volume), p represents the concentration of holes in the valence band n-type Semiconductors In intrinsic material, thermal excitation causes the breaking loose of covalent electrons, which leaving behind an unsaturated bond or hole. A small concentration of impurity remains in the semiconductor even after the best purification processes. Indeed, a small amount is intentionally added to the material, which is called a dopant to tailor its characteristics. In small concentration, the impurity atom occupies a replacement site in the lattice, which takes the place of a normal silicon atom. Since five valence electrons surround the impurity atom, one is left over after all covalent bonds have been formed. In the n-type semiconductors, the impurities are indicated as donor impurities because they supply electrons to the conduction band [5]. Figure 2.9: A scheme of (a) n-type semiconductor and (b) the corresponding donor levels created in the silicon bandgap [5] The extra electrons, which are associated with donor impurities, can reside at positions through the normally forbidden gap. The energy spacing between the bottom of the conduction band and these donor levels is adequately small in order that the probability of the thermal excitation is enough high to make sure that a large fraction of all the donor impurities are ionized. The net effect in n-type material is to generate a situation in which the number of holes is less and the number of conduction electrons is greater than in the pure material. The electrical conductivity is determined exclusively by the flow of holes and electrons. 18

20 In this situation, the holes are called the minority carriers and the electrons are called majority carriers [5] p-type Semiconductors In p-type semiconductors, the impurity atom has one valence electron than the surrounding silicon atoms if it takes a substitutional location and so one covalent bond is unfilled. A valence electron is excited to the conduction band, but its energy characteristics are slightly dissimilar to the pure material. When an electron is captured to fill this vacancy, an electron contributes in a covalent bond, which is not identical to the bulk of the crystal because one of the two participating atoms is a trivalent impurity. However, the acceptor impurities can create electron positions through the normally forbidden energy gap. In this situation, the acceptor levels lie near the bottom of the gap as their characteristics are closely related to locations resided by normal valence electrons. In the crystal, a normal thermal excitation ensures that some electrons are available to fill the vacancies which are created by the acceptor impurities. The difference in energy between the top of the valence band and typical acceptor positions is small and a large fraction of all the acceptor locations are filled by thermal excitation electrons. [5]. Figure 2.10: A scheme of (a) p-type semiconductor and (b) corresponding acceptor levels created in the silicon bandgap [5] These electrons move towards other normal covalent links in the crystal and consequently leave holes behind in the valence band. However, the raised availability of holes increases the recombination probability between holes and conduction electrons and the equilibrium number of conduction electrons falls. In this type material, holes are called the majority carrier and control the electrical conductivity. In addition, the filling acceptor sites symbolize fixed negative charges that are in equilibrium with the positive charge of the majority holes [5]. 19

21 2.5 Radioactive Equilibrium When one radionuclide, which is called the parent, decays into another radionuclide, that is called the daughter, the rate of the change of daughter atoms depends on both the rate of growth from the parent and the rate of decay of the daughter [3] Equation 2.19 can be rewritten as follow: 2.20 where the subscripts P and D indicate to parent and daughter respectively, and the subscript 0 refers to the number of atoms at the time t = 0. There are three particular situations, depending on whether the half-life of parent is greater or less than the half-life of daughter Secular Equilibrium When the half-life of the parent (Pt 1/2 ) is very long compared to the half-life of the daughter (Dt 1/2 ), (i.e. λ 1 << λ 2 ) the equilibrium case is secular equilibrium. Secular equilibrium can be expressed as [3,4]: λ 1 N 1 = λ 2 N 2 (2.21) The activity of daughter will be equal the parent activity. An example is the first three phases of the 238 U decay series, which is shown in figure 2. The activity of each daughter can be the same to that of its parent and therefore total activity will be three times that of 238 U [3]. Figure 2.11: A scheme of secular equilibrium [17] 20

22 There are two states that are essential to reach this type of equilibrium [4]: 1- The parent radionuclide must be to have a long half-life than its daughters, such as 238 U which has a half-life of years. 2- A long duration of time must have elapsed, for example ten half-lives of the decay product having the longest half-life, to allow for in-growth of the decay products Transient Equilibrium In a transient equilibrium, the activity of the daughter nuclide is in a fixed proportion to that of the parent nuclide and decays with the apparent half-life of the parent. The relative numbers of parent and daughter atoms at equilibrium can be written as [3]: 2.22 The equilibrium activity of the daughter is related to the parent activity can be expressed as [3]: 2.23 Equation 2.23 can be rewritten as the term of half-lives [3]: ( ) 2.24 where, t 1/2P and t 1/2D are the half-lives of parent and daughter respectively. Figure 2.12: A scheme of transient equilibrium [17] 21

23 2.5.3 No Equilibrium When the half-life of parent is less than the half-life of daughter (i.e. t 1/2P < t 1/2D ), the parent will decay and leaving behind the daughter alone. In this case, non-equilibrium occurs and the decay curve will be that of the daughter nucleus [3]. Figure 2.13: A scheme of no equilibrium [17] 2.6 The Measurement of Activity Concentration (Ac) The activity concentration of the radionuclide in a soil sample, expressed in Bq/Kg, can be calculated by using the equation that [1,4]: 2.24 where, C net is the net peak count, I γ is the density decay of gamma-ray, ε is the absolute photopeak efficiency of germanium detector and m is the mass of soil sample in Kg. 2.7 Gamma Dose Rate (D) The gamma dose rate (D) in the outdoor air at 1 m above the ground level can be determined by using the equation [4]: D = A Ra A Th A K 2.25 where, A Ra, A Th and A K symbolize to the activity concentration in Bq/Kg for 226 Ra, 232 Th and 40 K respectively , and are the dose conversion factors used in calculation for 226 Ra, 232 Th and 40 K respectively. 2.8 Radium Equivalent Dose (Raeq) Radium equivalent dose (Ra eq ) is used to estimate the hazards related with materials that include 226 Ra, 232 Th and 40 K in Bq/Kg. This equivalence is determined using the supposition that 260 Bq/Kg of 232 Th or 370 Bq/Kg of 226 Ra or 4810 Bq/Kg of 40 K create the same gamma dose rate. 22

24 The Radium equivalent dose can be calculated by using the following equation [4]: where, A Ra, A Th and A K represent to the activity concentration in Bq/Kg for 226 Ra, 232 Th and 40 K respectively External Hazard Index (Hex) The external hazard index is an assessment of the hazard of the natural gamma radiation. The major purpose of this index is to limit the radiation dose to the admissible dose equivalent limit of 1 msv/y. This index can be calculated using the following equation [4]: 2.27 This model takes into consideration that the external hazard that is caused by γ rays and corresponds to a maximum radium equivalent activity of 370 Bq/Kg for materials. The external hazard index should be less than unity for a safe radiation hazard Minimum Detectable Activity (MDA) The Minimum Detectable Activity (MDA) is defined as the smallest amount of radioactivity that can be distinguished from the blank sample. If the count rate of a sample is roughly the same as the count rate of the background, the MDA is significant in low-level counting. The MDA relies on the lower limit of detection and on the counting efficiency of a counting system [1]. The MDA can be determined by using the following equation [3]: 2.28 where, L D is the limit of detection in counts, ε is the efficiency of the detector at the energy of the measured gamma ray, P γ is the gamma-ray emission probability and t C is the live time of the count. The limit of detection in counts can be calculated as follows: 2.29 where, B is the number of background counts. 23

25 2.11 Weighted Mean (Wm) The weighted mean can be determined by using the following equation [1]: 2.30 where, w i is known as weighting factor, which can be shown to be : 2.31 where, σ is the standard deviation of the distribution. 24

26 Chapter 3 Experiment Details 3.1 Samples Preparation The samples were collected and prepared by Dr. Huda Al-Sulaiti, from the state of Qatar, which has an area of km 2 with 430 km in the length and 230 km in the width. The location of samples as collected is shown in Figure (3.1). The samples were collected from 5-15 cm in the depth and then filled into labeled polyethylene bags, sealed, packed in a box and shipped to the UK. The collection samples were placed in a drying oven and the temperature of drying oven was set at 60 º C for 24 hours to ensure any significant moisture would be removed from the samples. After that, the samples were sifted using a 500 μm mesh to obtain a uniform particle size. Then each sample was weighed after sieving. The sealed samples were then kept for about one month to radioactive equilibrium of 222 Rn with its parent 226 Ra in the 238 U chain. Finally, the samples were measured for seconds (i.e. 24 hrs) to achieve good count statistics. Figure 3.1: The location of soil samples investigated by Dr. H. Al-Sulaiti and in the current work that are indicated the map of Qatar [4]. 25

27 3.2 Experimental Arrangement The soil samples were examined by using high-resolution and low background gamma ray spectrometry system. The type of detector was used to measure gamma ray spectroscopy in the samples is called high-purity germanium detector (HPGe) and is shown in figure (3.1). The HPGe housing detector contains 10 cm of the lead, which is the most common material used for shielding because it has a high atom number and density. It is usually used for the surface of the shield to reduce the surrounding radiation, such as natural radionuclides in building materials including 40 K, cosmic rays and uranium decay series nuclides. The HPGe detector is operated by preamplifier and the high voltage supply set to approximately 3 kv (i.e V). The shaping time was selected at 8 μs and the fine gain and applied course gain were 0.64 and 10 respectively. Finally, the spectra data was converted directly to the PC to be introduced by using GENIE 2000 software [4]. b) Top view of HPGe detector and shielding. a) The HPGe detector. c) Electronic instruments including a shaping amplifier and high voltage supply. Figure 3.2: The high-purity germanium detector (HPGe) set-up used in the experiment. 26

28 3.3 Detector Characteristics Energy calibration The aim of energy calibration is to obtain a relationship between the peak position in the spectrum and the resulting gamma ray energy [3]. The energy calibration of the detector was carried out by using photo-peaks from a standard source 152 Eu, which had activity about 3020 Bq. The preset time of the energy calibration was set to approximately 1200 seconds (i.e. 20 mins). A range of energies for this source including , , , , , and kev were selected to perform the initial energy calibration for the HPGe detector Efficiency calibration This study is focused on the absolute photo-peak efficiency which depends on the fraction of the detector s geometry in view of the source. The absolute full-energy peak efficiency is defined as the number of γ rays emitted by the source to the number of counts that are detected in the full energy peak in the spectrum. The absolute full-energy peak efficiency can be determined by using following equation [7]: 3.1 where, C t is the total number of count rates recorded per unit time. N γ is the number of gamma quanta emitted by the source per unit time and can be calculated by [7]: ( ) 3.2 where, D s is the disintegration rate of the source. I γ (E γ ) is the fractional number of gamma rays emitted per disintegration. However, the radioactive samples that have prepared to be measured have non-negligible volume and mass, and gamma rays can be attenuated by self-absorption through the material itself [3]. The efficiency calibration values result from using the full-energy peak data from 226 Ra, 232 Th and 152 Eu sources (i.e. Radium-226, Thorium-232 and Europium-152) and NG3 which is a mixed nuclide source that contains 241 Am, 169 Cd, 57 Co, 60 Co, 203 Hg, 139 Ce, 113 Sn, 137 Cs, 85 Sr and 88 Y. The 226 Ra, 232 Th and 152 Eu and NG3 sources have identical geometries and similar densities to the samples. Thus, the variation in the self-attenuation for gamma rays in the sources and the samples is assumed to be negligible, which will minimize the uncertainties of activity concentration results. The calibration spectrum was measured for 86,400 second (i.e. 24 hrs) for each sample. 27

29 3.4 Samples analysis The background was measured using a Marinelli beaker filled with deionized water with the same geometry as the samples and was placed around the detector for 86,400 seconds (i.e. 24hrs) to be counted. Consequently, the background spectrum was used to correct the net peak area for gamma ray emission from the radionuclide with the samples. The samples were directly moved to the front face of the detector for 86,400 seconds. Gamma rays lines were observed a wide range of energies that are from 70 kev up to 2.6 MeV from the nuclear decay chains for 235 U, 238 U, 232 Th, their daughters and 40 K. The first activity concentration of 238 U provided in the samples was determined by using the γ ray at 186 kev which is related to the decay of 226 Ra. In the 238 U decay chain, all the visible gamma ray lines from decays of shorter-lived radionuclide, such as 214 Pb and 214 Bi were also used to assess the activity concentration of 238 U. In addition, the activity concentration of 232 Th was estimated using all the measured γ ray transitions related to the decay of radionuclide, such as 212 Ph, 228 Ac and 208 Tl. The gamma ray peaks related to decays from 137 Cs and 40 K were observed at 661 kev and kev respectively. The peak area of gamma rays was calculated and background subtracted. The absolute fullenergy peak efficiency of each γ ray line in the samples was determined by using equation 3.1. The absolute full-energy peak efficiency curve of gamma ray transition was plotted versus energy by using Excel software. The activity concentrations of the radionuclide found in each soil sample were estimated by using equation The minimum detectable activity of the radionuclide was determined using equation 2.28 and is expressed in Bq. 28

30 Energy (kev) Chapter 4 Experimental Results and Discussion 4.1 Detector Characteristics Energy calibration Energy calibration is performed by measuring the spectrum of a source that emits gamma rays of accurately recognized energy and comparing the measured peaks with these energies [3]. Energy calibration is the corresponding relationship between the channel number and the photon energies. Once the energy calibration has been established over the range of interest, a calibration graph was plotted by the photon energies with the channel number, as shown in Figure 4.1. The standard sources are 152 Eu, 232 Th, 226 Ra and NG3 which includes 241 Am, 137 Cs and 60 Co had activities in each source, as follows; 152 Eu is Bq, 226 Ra is Bq, 232 Th is 1080 Bq and NG3 (i.e. 241 Am, 137 Cs and 60 Co) is , and Bq respectively. These activities were corrected for the time between the validation and the each calibration spectrum was measured for 86,400 seconds y = x R² = Eu-152 Th-232 Ra-226 NG Channel Number (Count/sec) Figure 4.1: The energy calibration for standard sources Figure 4.1; The best-fit straight line that plotted between the channel number and the energy of gamma-rays. Even the best spectrometer systems usually have nonlinearities of one channel or two over a full range of several thousand channels. 29

31 Absolute Efficiency In a high-quality germanium system, the uncertainty in the peak location can be one part in 10 5 which is the similar to the order of the uncertainty in the calibration energy standards. [3,5] Efficiency calibration Efficiency calibration is the proportional to the number of counts in the full energy peak and the number of radiation incident on the detector. However, difference efficiencies occur as a result of the thicker outer contacts on the p-type material compared to a thinner contact typical on n-type detectors. The region, where photoelectric absorption controls in germanium and few millimeters of thickness, is adequate to intercept almost all the incident photons. However for energies between 100 kev and 1 MeV, the majority of γ rays photons still interacted in the detector, but suffer Compton scattering and can escape before contributing to the full-energy peak [5] Eu-152 Ra-226 Th-232 NG Energy (kev) Figure 4.2: The efficiency calibration for standard sources A significant portion of all the incident gamma rays can pass through the detector without interaction of any kind and the efficiency peak falls rapidly with energy [5].The efficiency calibration values resulting from using photopeaks of 152 Eu, 226 Ra, 232 Th and NG3. The preset time of efficiency calibration was set by second. Figure 4.2 illustrates the absolute efficiency curve of the HPGe. Though, the efficiency of germanium detector can be assessed by measuring or calculations for detectors of the same size. Radioactive samples, which are measured, have non-negligible volume and mass and gamma rays can be attenuated by self-absorption through the sample material. 30

32 Count (count/sec) In other cases, when a precision determination of the γ ray emission rate from the entire sample is required, a correction must be performed for this attenuation. In the calibration of large volume samples, a standard sample container, called a Marinelli beaker is used to fit over the endcap of the detector cryostat. The calibration standard sources used the same Marinelli beaker shape to simplify the efficiency determination for such bulk sample [5]. 4.2 Spectral Analysis and Nuclides Identification Background spectrum analysis Environmental background radiation occurs from various sources of radionuclides that can emit from the constituent materials of the detector assembly and its shielding from the earth s surrounding including the air. Also, it may be created by the interaction of cosmic radiation with the detector and its surrounding. The relative significance of these sources relies on the construction of the detector, of its shielding and the location of the detector. A typical detector, which has no extraordinary protection taken, may have 10% of background created through itself, 40% from its immediate environment, 10% from radon in the air and the remaining 40% from the interactions of cosmic ray [3] Energy (kev) Figure 4.3: The background spectrum measured with de-ionized water counted in 24 hours In this study, the background radiation was observed for a de-ionized water sample, which was filled in a Marinelli beaker with the same geometry applied for the sample. The deionized water sample was measured for 86,400 seconds (i.e. 24 hours). 31

33 Most of gamma ray energy peaks that were observed from the 238 U, 232 Th decay chain and 40 K, which are primordial radionuclides, are shown in figure 4.3. The spectrum also demonstrates the annihilation peak at 511 kev, which is created by pair production processes within the detector. These peaks reside on a continuum due to Compton scattering of the γ rays and backscattering and bremmstrahlung owing to direct interaction of cosmic particles with the detector and shielding material [3] Sample spectrum analysis The spectrum of each sample was analyzed and the most of the radionuclides of each sample were recognized by determination of the centroid energies of the peaks and compared with the known gamma energies from the published references. Spectra from these soil samples are shown from figure 4.4 to figure The spectrum of each soil sample show lines from the 238 U, 232 Th decay series and 137 Cs and 40 K. The significant radionuclides identified in the samples are 226 Ra, 214 Pb and 214 Bi from the 238 U decay chains, 228 Ac, 212 Pb and 208 Tl from 232 Th decay chains. Due to the low concentration of 235 U (0.72 % of natural uranium) the 235 U and its daughters did not clearly appear in all of the soil sample spectra. 137 Cs was no observed in all soil samples 40 K can be observed in each soil sample and the background spectra. At high amplifier gain, the peak of 208 Tl which has energy about kev was no seen because the range of the energy was shorter than in a normal gain. The soil sample (29-X- 228) has a comparatively high number of counts for the most of the detected radionuclides contrast with other samples, particularly 226 Ra ( kev), indicating a relatively high activity concentration for 226 Ra radionuclide for this sample. 32

34 Count (count/sec) Count (count/sec) Energy (kev) Figure 4.4: Gamma-ray spectrum of soil sample (1-X-228) counted at a normal gain Energy (kev) Figure 4.5: Gamma-ray spectrum of soil sample (2-X-228) counted at a normal gain 33

35 Count (count/sec) Count (count/sec) Energy (kev) Figure 4.6: Gamma-ray spectrum of soil sample (3-X-228) counted at a normal gain Energy (kev) Figure 4.7: Gamma-ray spectrum of soil sample (22-X-228) counted at a normal gain 34

36 Count (count/sec) Count (count/sec) Energy (kev) Figure 4.8: Gamma-ray spectrum of soil sample (26-X-228) counted at a normal gain Energy (kev) Figure 4.9: Gamma-ray spectrum of soil sample (23-X-228) counted at a high gain 35