Sep 7, 2008 - giving a partial half life for a specific decay mode. However .... During the decay the nuclei in the chain emit various types of radiation. (usually ...
Measurements of Naturally Occurring Radioactive Materials (NORM) in Environmental Samples
Fuad A. Ali
A dissertation submitted to the Department of Physics, University of Surrey, in partial fulfilment of the degree of Master of Science in Radiation and Environmental Protection
Department of Physics Faculty of Engineering & Physical Sciences University of Surrey September 2008 © Fuad A. Ali 2008
Abstract Naturally Occurring Radioactive Materials (NORMs) have always been present in a variety of concentrations in every part of the earth’s mantle and in the tissue of every living being. Natural radioactivity can be found almost everywhere; in soil, public water supplies, oil and atmosphere. NORMs arises a measurable exposure to human beings. In the present study soil samples were collected from four different site of state of Qatar. These soil samples have been analysed using high-resolution gamma-ray spectrometry. From the measured gamma-ray spectra the activity concentrations were determined for two series (238U and 232Th) and two non-series (40K and 137Cs) radio nuclides. In addition, the activity concentrations of two samples with high concentrations of 238
U and 232Th which are Uranium-Oxide (UO2) and Monazite (mineral sand rich 232Th) were also
measured. The determination of the specific activity of 214
238
U was found from the decay progeny
Bi (608.90keV and 1120.3keV) to be 7.5±0.7MBq/kg and 8.5±1.4MBq/kg respectively.
Whereas, the activity of 232Th from the gamma-ray emission lines 228Ac 910.81keV was measured to be 474±49kBq/kg. The activity concentrations obtained in this study are higher than presented for the same soils in other studies. The obtained results confirm that one of the samples (soil-228) is more radioactive than the other soil samples. One concern arising from this study was the observation of significant gamma-ray self attenuation in the samples which causes an apparent reduction in the measured activity concentration by up to a factor of two at low energies gammaray photons.
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Acknowledgments The most considerable academic challenge I have ever had to face is the writing of this dissertation therefore I would like to express my gratitude to my supervisor, Dr. Patrick Regan who helped by throughout my research and also thank him for his great insights, ideas, and excellence guidance as well as being a good friend who was always willing to help anytime.
I would like to thank all those who supported me during my research study; the laboratory technician Mr. John William Brown, Mr. James Wall bank and Miss Huda Al.Sulaiti for their useful suggestions, and guidance throughout this study.
Also, my sincere gratitude goes to the Kurdistan Regional Government who supported me financially throughout my study thus helping me to attain this academic success.
I would also use this opportunity to sincerely thank my parents, and my siblings for their love and encouragement. Thanks for being part of my success story. Once again, thank you.
Fuad A. Ali
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Contents Abstract…………...…………………………………………..……………………….………... (i) Acknowledgements..…………………………..………….……………………….……...……. (ii) Contents………………………………………………….…………………….………….. ….. (iii) Chapter -11-Introduction..……………………………………………………………….………..…….. (1) Chapter -22-Theory……………………………………………………………………….……..……….. (5) 2-1 Law of Radioactive Decay...……………………………….…………..………..…….... (5) 2-2 Decay Chains.…………………………………………….……………..……….……… (6) 2-3 Equilibrium in radioactive Decay Chains.……………….……………………..………. .(8) 2-4 Specific Activity.………………………………………...………….…………………… (9) Chapter -33- Experimental Setup……...……………………………..……..………………………..….. (12) 3-1 Interaction of Gamma-radiation with matter..…………….………………...………… (12) 3-1.1 The Photo-electric effect..…………………………….………………………….... (12) 3-1.2 Compton scattering..……………………………….………………………………. (13) 3-1.3 Pair-production..……………………………...……….…………………………… (14) 3-2 Gamma-Ray Detectors….………………………..………..………….………………... (16) 3-3 Semiconductor diode (Solid state) detector.…………………………………………… (17) 3-4 Gamma-ray Attenuation..………………………………………………………...……. (19) 3-5 Effect of lead shielding.………………………………………..………………………. (20) 3-6 Effect of bias voltage.………………………………………….………………………. (21) 3-7 Gamma-Ray Spectroscopy..…………………………………….………...…………… (22) 3-7.1 Energy Calibration…………………………………………….………..…………. (22) 3-7.2 Energy Resolution...……………..……………………………………...…………. (22) 3-7.3 Energy Efficiency..……………………………………….……………..………… (23) 3-8 Sample Collection and Preparation..…………………………….…………..……..….. (24) 3-9 Sample Analysis…………………………..………………………………..…..……… (25)
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Chapter -44- Experimental Results and Discussion..……..………………………………........………. (26) 4-1 Detector Characterisation..……………………………………………..….…….…….. (26) 4-1.1 Energy Calibration..………………………………………………..……………… (26) 4-1.2 Energy Resolution...………………………………………………..………………. (27) 4-1.3 Energy Efficiency and effect of bias voltage……...………………………………. (28) 4-2 Effectiveness of shielding………………..……………………………………………. (30) 4-3 Identify Radioactive Nuclides in Background..……………………………………….. (31) 4-4 Activity concentration of a Uranium-Oxide (UO2) crystal sample and Monazite (a mineral rich in 232Th)………………………………..………………………….……(35) 4-5 Activity concentration of environmental samples…..………………………………….. (41) Chapter-55-Conclusion……..………………………..………….….………………………….………(45) Suggestion……………..…………………………….…………………...………….……….. (47) References.………..……………………………………….………………………….……… (48) Appendix I………………..………………………….……………...……………….………. (50) Appendix II……………………..…………………………………………………….……… (53) Appendix III……………..………………………..…………………………….……………. (55)
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Chapter -1-
1- Introduction: NORM stands for Naturally Radioactive Materials. NORMs are radioactive materials that can be found in nature. Since the earth was first created, NORMs were formed in supernovae and the primary particles from our region of universe continually bombard the forming earth’s crust. NORM can be found almost everywhere, in soil, air, public water supplies, oil and even in radioactive potassium (40K) in our bodies. Therefore, NORM always has been a part of our world. [1]
Early investigations in soil and rock (i.e., components of earth’s crust.) showed that there are measurable amounts of radio nuclides such as Uranium and Thorium. It is possible to classify the sources of naturally occurring radioactive material on earth by their specific nuclei and their means of productions. Firstly, cosmogenic radioactive nuclides such as 3H, 14C, 14N, 81Kr, 22Na, 32P .etc. are produced by primary particles such as cosmic rays which are continually bombarding the earth’s atmosphere from space. These particles are slowed down in the earth’s upper atmosphere and interact with atoms in the air. As a result of these reactions those radioisotopes are generated. After formation they fall down under gravity, either settling in ground soils or following precipitation processes via rain and/or snow falls. In addition, the concentration and the rate of production of cosmogenic nuclides are strongly dependent on altitude. For example, the rate of production of 7Be is 70% in the stratosphere layer whereas only 30% of the total is produced in the lower altitude troposphere. The rate of production of 3H and 14C are quite small and therefore these nuclides are of little radiological impact. The half life of the cosmogenic radioactive nuclide 3H (12.3 years) has a larger relative activity than 14C with a 5730 year half-life. However, because 3H is an isotope of Hydrogen, it mainly appears in the form of tritiated water. Consequently, from the biological point view, the overall dose in a typical human life time is from ingested smaller than 3H [2].
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14
C which is
Secondly, terrestrial radionuclides are other sources of NORM, including One of the most important primordial radio nuclides is
40
147
Sm,
152
Gd and
40
K.
K which is widely distributed in the
earth’s crust and present in measurable quantities in many building materials. The amount of potassium is estimated to be 0.1% in limestone and 4% in some granite rocks. 40K has a very long half life (26x109) years abundance of 0.0118% which gives a specific activity (31.4Bq/g) of 40
natural potassium. [2], so that the probability of decay is low.
K undergoes beta emission
followed by decay with a characteristic photon with energy 1460keV which can be seen clearly using gamma ray spectroscopy. Furthermore, in any biological living thing there is an amount of potassium. A 70 kg man contains about 140g of potassium distributed in muscle and bones (the bones contain concentration of nearly 17mg of corresponds to activity 4.4kBq of 40K) [2].
Thirdly, there are four independent decay chains with mass number 4n, 4n+1, 4n+2 and 4n+3 (where n is an integer). The decay process is headed by the longest member of a series associated with a number of intermediate daughters. They are Uranium, Thorium, Actinium and Neptunium. The Neptunium series has decayed away and, thus can not be seen any more because the half life of Neptunium is less than the age of earth (4.5x109 years). Only the residual isotope from this decay chain, Bismuth- 209 can be observed today. In gamma-ray spectroscopic studies of these decay series, most frequently decays from the daughter can be observed. For example, in the Thorium (232Th) series gamma-ray decays from decays from
228
Ac (338.32, 911.20, 968.97keV
with relative intensities of 11.27%, 25.8%, and 15.8%, respectively), and1120.29keV with relative intensity of 44.6%, 14.7%) and
208
214
Bi (609.31,
Tl (2614.53keV with 35.64%
relative intensity) are observed. Whereas, in the Uranium (238U) series, gamma-ray following the radioactive decays of (186.21keV 3.59%),
214
Pb (351.93keV with 35.1% intensity) and
214
226
Ra
Bi can be readily identified.
Using high-quality, low background spectroscopy a photo peak from the decay of 7Be of energy about 477.60keV with branching ratio 10.44% which is produced in upper atmosphere can also be detected. [3]
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These isotopes can be investigated in rock and soil samples. In general, their presence represents little harm to health, with the exception of the mining of ores or fractures where some of radioactive gaseous isotopes are released to the atmosphere. Radium-226 produces Radon-222 via alpha decay which diffuses from the earth into the atmosphere producing a number of short lived radio nuclides [4].
In the decay chain series, there are three primary possible decay modes (alpha, beta and gamma) for each particular radioactive nuclides decay process. In NORM, the decay chains from Uranium and Thorium gives rise to nuclear energy level and decay schemes which can be complicated and involve decays from various competing decay modes. The term, branching ratio has been defined to specify the relative intensities of the competing modes. The branching ratio can be evaluated by giving a partial half life for a specific decay mode. However, the activity is observed to decay only with total half life regardless considering the calculation performed taken by any partial decay mode of alpha, beta or gamma [5].
In addition, human activity is another source of radioactivity in the environment. The most important artificially and environmentally radionuclides are 137Cs, 90Sr and
239
Pu. Those are Man-
made radio nuclides produced in atomic weapon explosions at Hiroshima and Nagasaki, the Chernobyl accident, and also underground and atmospheric tests of nuclear weapons, plus unauthorized dumping of radioactive into marine environment. These isotopes were dispersed in the form of small particles across the world. [6]
Radionuclides are also found in certain foods such as rice and bananas. For instance, it is found that the common radioactive nuclides
40
K and
226
Ra, the activities in banana are 3520pCi/kg and
1pCi/kg respectively [7]. After the nuclear atmospheric tests and the Chernobyl accident, the dominant residual radioactive nuclides in the fallout were
137
Cs and
90
Sr. They precipitated on
plants and grass following rainfall. These isotopes are slowly diffused into the soil and some of them are absorbed by the plants via roots. Consequently, a certain amount of those radioactive substances are transported through the plants into the chain food and finally to the humans. To sum up, earth, air, water, biota (including humans) are all radioactive to some degree.
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There are many techniques used to investigate the amount of NORM in the environment. One of the well-known techniques is gamma ray spectrometry. This uses equipment to provide the intensity, gamma-ray energy and the overall activity of the samples. An increased sensitivity for measurements of activity concentrations from NORM can be achieved by having a good shielding around the gamma-ray detector. This should be made from suitable substance (usually lead). Such shielding plays an important role in NORM gamma ray spectroscopy.
This project is focused on the measurement of naturally occurring radioactive material in some environmental soil samples which were collected from different sites in the State of Qatar. Highresolution gamma-ray spectroscopy with an HpGe detector was used to analyse the activity in the samples. The effect of changing bias voltage on the efficiency response of the gamma-ray detector was also measured. The effectiveness of lead shielding around the detector is also illustrated. The activity concentration of a Uranium-Oxide (UO2) crystal sample and Monazite (a mineral rich in 232
Th) were also determined. Finally, the specific activities of gamma-ray lines of naturally
radioactive nuclides in the soil samples were investigated.
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Chapter -2-
2- Theory 2-1 Law of Radioactive Decay: Radioactivity is the spontaneous (statistical) process at which the parent nucleus decays by emission of particles into a daughter nucleus. The nuclei subject to such decays are called radioactive. It is possible to derive how the number of atoms present in a radioactive ensemble will change over time. The relation between the number of atoms ( N ) that are present at time t and the number of atoms initially present ( N 0 ) can be written as follows:
N = N 0 e − λt
(Eq.1)
where λ is the radioactive decay constant. It can be defined as the probability that a certain fraction of the nuclei within a sample of a particular nuclide will decay per unit time. This is different for different nuclei and units for the decay constant are inverse time units which is expressed as sec-1, min-1, hour-1 or year-1. Furthermore, the activity (A) of a sample is defined as the rate of decay of that sample. This rate of decay is usually measured by the number of disintegrations that occur per second. The activity is the product of the decay constant and the number of atoms present in the sample. The relationship between the activity, number of atoms, and decay constant is shown in Equation (2).
A = λN
(Eq.2)
where: A
: Activity of the nuclide (disintegrations/second)
λ
: Decay constant of the nuclide (second-1)
N
: Number of atoms of the nuclide in the sample.
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Since λ is a constant, the activity and the number of atoms are always proportional. The unit of activity is either the old, traditional unit of the curie (Ci) or the SI unit of the Becquerel (Bq), where one Becquerel is one disintegration per second. From equation (2), since the activity and the number of atoms are always proportional, they may be used interchangeably to describe any given radionuclide population. Therefore, the following is true.
A = A0 e − λ t
(Eq.3)
where: A
: Activity present at time t
A0
: Activity initially present
λ : Decay constant (time-1) t : Time.
The radioactive half-life is the time interval in which the original number of radioactive nuclei or their activity is reduced to one-half. A relationship between the half-life and decay constant can be developed from Equation (3). The half-life can be calculated by solving Equation (3) for the time, t when the current activity A, equals one-half the initial activity A0. [8]
⇒ t1 / 2 = ⇒ t1 / 2 =
− Ln (1 / 2 )
λ Ln (2 )
λ
=
0 .693
λ
(Eq.4)
2-2 Decay chains: Three naturally occurring radioactive decay chain families are found in significant amounts on earth. Others also exist which resulted from atomic weapons explosions to make 244
239
U, 241Am and
Cm… etc. The three primordial series of nuclides are called the Uranium, Thorium and
Actinium series. Radioactive decay series can be defined as groups of isotopes representing various stages of radioactive decay in which heavier members of the group are transformed into successively lighter ones, the lightest being stable.[9]
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Most naturally occurring radioactive material obeys a chained series of transformation rather than decaying in a single step. During the decay the nuclei in the chain emit various types of radiation (usually either beta or alpha particles), until ending in a radioactively stable nuclide. Usually the concentrations of parent isotopes decrease as the decay progresses. In the mean time, the concentration of their daughter products increases [10]. In the earth’s crust
238
U constitutes 99.27% of natural Uranium by mass, in comparison,
makes up only about 0.72%. As illustrated in figure (1-2) the Uranium decay chain, headed by ends via 18 intermediate stages at stable 206Pb. The Actinium series starts with
235
235
U
238
U
U producing 15
daughter radio nuclides and stops at 207Pb. Finally, the beginning of Thorium decay chain is 232Th and ends at 208Pb, including 10 daughter products. [10]
Figure (1-2) Diagram of natural decay chains. Figure taken from reference [10]
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2-3 Equilibrium in radioactive decay chains: In a radioactive decay chain the parent nucleus (A) decays with a certain decay constant (λA), and while the parent decays, the daughter nucleus (B) concentration starts to growth to some point and then decay away with another decay constant (λB). The process continues until it reaches a final stable nuclide. Radioactive equilibrium takes place when each radioactive nuclide decays at the same rate of the parent nuclide. Thus, λA λB A → B → C
From equation (Eq.2), the activity of each nuclide is given by:
AA = λA N A
and
AB = λB N B
(Eq.5)
For the sake of simplicity, assuming that the decay chain is only two steps and the daughter (B) is also radioactive, and the half live of parent and daughter in equilibrium can be classified as:
1- When the half life of the parent (A) is much larger than that the half life of the daughter product. The daughter nuclide produces more radiation. After about 7 half lives of the daughter (B), the mother and daughter activities become equal. This kind of equilibrium is called secular equilibrium. Therefore, assuming that there are several succeeding generations of radioactive nuclear decay, we can generalize equation 5 such that:
λA N A = λB N B = ........ = λn N n
(Eq.6)
The equation is usually quoted as the “Bateman equation in secular equilibrium”.
2- When the half life of the daughter product is longer or the same as the parent (A). As a result of the combined decay of both parent the (A) and daughter (B), the total activity increases and finally equilibrium is obtained. The total activity then decays at about the same rate of the parent nuclide. This is called transient equilibrium. [11]
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2-4 Specific Activity: The activity concentration of gamma emitting radionuclide in the environmental samples can be found using a gamma ray spectrometric analysis. It is calculated using the expression:
A =
N I (γ ) ε MT
(Eq.7)
where: A is the activity concentration of a certain radioactive nuclide in the decay series. N is the net peak area count subtract background of the sample.
ε is the absolute efficiency of the detector. I (γ ) is the emission probability of a specific energy photo peak. T is time for collecting the spectrum of the sample. M is the weight of the sample.
In order to determine whether there is any radioactivity concentration in a certain sample, Curie first introduced a binary (Yes or No) decision of the presence of activity in an unknown sample. There is a certain minimum detectable activity to be measured in any radiation measurement especially in low level radioactivity in environmental sample. To perform this comparison, the net counts of a sample to the critical level ( Lc ) are required for a given sample. The critical level ( Lc ) could be assumed at zero when there is no statistical fluctuation and other instrumental variation. Since radioactivity is a statistical process, fluctuations exist to some degree in any counting measurement. Furthermore, there is a critical issue to make the decision of choosing Lc . The probability of a false positive is decreased for a high value of Lc , while by contrast, the likelihood of false negative is decreased for a low value of Lc . In the case of no activity present, if the distribution of counting measurements obey a Gaussian distribution and only a statistical fluctuation from counting statistics are considered, the critical level is given by:
Lc = 2.326σ N B
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(Eq.8)
where, σ N B is the standard deviation of Gaussian distribution for the number of counts in the background. Therefore, any conclusions will be a false positive and the probability will be no larger than 5%. [12] Conversely, in the case of activity present it is important to introduce the Minimum Detectable Activity (MDA) which can be found in the background spectrum and can be determined from the expression:
MDA =
σ NB I(γ ) εMT
(Eq.9)
where σ is the statistical coverage factor equal to 1.645 with the 95% confidence level and N B is the background count under a specific photo peak energy [13].
In fact five corrections factors are necessary to add to the equation (7), which are characterised by the parameters K1, K2, K3, K4 and K5.
K1 corresponds to the nuclide decay from the time that the sample was collected to the start of the measurement and can be calculated using: ln 2.∆t K 1 = exp − T1/ 2
(Eq.10)
where ∆t is the lapsed time between the sample collection and the beginning of the measurement and T1/ 2 is the radioactive nuclide half life.
K2 is the correction factor which accounts for the decay of the nuclide during the measurement. This is giving by:
K2 =
ln 2.t r T1 / 2 exp1 − ln 2.t r T1 / 2
where t r is the elapsed real clock time during the measurement.
- 10 -
(Eq.11)
The correction for self attenuation in the sample is taken in to account by the factor K3. This is equal to the ratio of the full energy peak efficiency ε(µ,E) for a sample with the linear attenuation coefficient ( µ ) and the full energy peak ε(µ ref ,E) for a sample. The linear attenuation coefficient ( µ ref ) is equal to unity if the matrix of both the calibration sample and measured sample are the same. However it can be investigated through the expression:
K3 =
ε(µ,E) ε(µ ref ,E)
(Eq.12)
In the pulse sampling cycle, when two or more photons enter the detector and are converted to signals at the same time, the sum of these two energies appear in the spectrum rather than two separate signals. Consequently, there is a net reduction in count rate or efficiency of the detector. To account for this, the correction factor K4 is introduced which corresponds for pulse loss due to random summing phenomena:
K 4 = exp(− 2 Rτ )
(Eq.13)
where R,τ is the resolution time of the system and the mean count rate respectively. In for low count rates this factor could be taken as unity.
The energy level decay scheme for those nuclides decaying through a cascade of successive photon emissions, the sample geometry and composition and the detector parameters affect the final correction factor, K5. If there is no cascade emission, this factor is equal to one. [14] Therefore the equation (Eq.7) becomes:
A=
N I (γ ) ε MTK1 K 2 K 3 K 4 K 5
- 11 -
(Eq.14)
Chapter -3-
3- Experimental set up 3-1 Interaction of gamma radiation with matter: Gamma rays interact with matter by three main ways for energies below 3MeV, these are the Photoelectric effect, Compton scattering and Pair production. For energies less than 1022keV photoelectric and Compton are possible, for energies above 1022keV Pair production is also possible.
3-1.1 The Photo-electric effect: In the photo-electric effect when a low-energy gamma strikes an atom, the total energy of the gamma ray ( hν ) is expended in ejecting an electron from its orbit. The result is ionisation of the atom and expulsion of a high energy electron (fig.2).
Figure (3-1): Photoelectric Effect. [15].
Any gamma photon energy in excess of the binding energy of the electron is carried off by the electron in the form of kinetic energy, kinetic energy of the electron ( Te )can be written as follow:
Te = hν − Eb
(Eq.15)
where: Eb : is the binding energy of the atomic electron.
- 12 -
The photoelectric effect is the most favorable process for gamma spectroscopy because generally, all of the photon energy transfers to the electron in the detector. After the electron is ejected, a hole is left in the place of the electron. This hole is filled by the transition of an electron from one of the higher-lying energy levels. This transition gives the emission of a characteristic X-ray. In the case of gamma ray spectroscopy this X-ray is also absorbed by the detector. The magnitude of the photoelectric effect increases rapidly with the power of the atomic number (Z) (Z4-5) of the target nucleus [16].
3-1.2 Compton scattering: In Compton scattering the gamma ray interacts with an orbital or free electron; however, in this case, the photon loses only a fraction of its energy. Inelastic scattering of photon by electron is illustrated in Figure 3-2.
Figure (3-2): Compton Scattering. [15]. The actual energy loss depends on the scattering angle θ of the gamma radiation on the electron. The scattered gamma ray continues on at a lower energy and the energy difference is transmitted to the electron. Conservation of total energy and linear momentum leads us to the following expression for the energy of the scattered photon:
hν − =
hν 1 + h ν / m e c 2 (1 − cos θ )
(
)
- 13 -
(Eq.16)
where: hν
: is the incident photon energy.
hν −
: is the energy of the scattered photon.
θ
: is the scattering angle.
me c 2 : is the electron rest mass energy (511keV).
The energy difference between incident and scattered photon appear as the kinetic energy of the electron and given by:
(h ν ) (1 − cos θ ) −
k .E e = h ν − h ν
−
=
2
m e c 2 + h ν (1 − cos θ
)
(Eq.17)
It is clear from the above equation the energy of electron ranges from zero for θ =0 to
(
2(h ν ) / m e c 2 + 2 hν 2
) for θ =180. This means the total energy of the incoming photon is never
lost in any one collision. The scattered photon can scatter again into another scattering process, and so on, or escape from the detector material. This process of escaping photons is very important in the gamma spectroscopy; because the photons can not lose their entire energy in the detector leading to a continuous background in the energy spectrum known as the Compton continuum. This continuous background extends up to the maximum transfer energy (Compton edge) and the − 2 energy of the scattered photon for θ =180 is h ν = h ν /(1 + 2 ( h ν / m e c )) . This energy gives
rise to the peak in the range of Compton spectrum known as the backscattered peak. The probability of Compton scattering increases linearly with Z [16].
3-1.3 Pair production: In pair-production when a high energy gamma passes close enough to a heavy nucleus (pair production not happen in vacuum, because of violation of conservation of momentum), the gamma ray completely disappears and an electron-positron pair is formed, as shown schematically in figure (3-3).
- 14 -
Figure (3-3): Pair Production. [15].
From the conservation of total mass-energy, the energies that are carried away by particles should be satisfied the following equation:
h ν = 2 M e c 2 + K .E − e + K .E + e
(Eq.18)
For this reaction to take place, the original gamma energy ( hν ) should be at least equal to the total rest-mass energy of both the electron and positron ( 2M e c 2 =1022keV). Any energy greater than 1022keV becomes kinetic energies K .E−e and K .E+ e of the electron and positron respectively. The positron can then annihilate with one of the atomic electrons in the detector to produce two photons of energy 511keV. These two photons can either be absorbed or escape from the detector. This gives rise to the escape peaks in the gamma spectrum. If one photon escapes from detector then a peak is observed at h ν - M e c 2 , but if both of them escape, a peak observed at hν - 2 M e c 2 in spectrum. The process clearly has an energy threshold of 1022keV. [16].
The processes have different relative importances in different gamma-ray energy ranges. The graph between the energy and the atomic number (Z) of the material (see figure 3-4 below) shows the border where the probability of photoelectric and Compton scattering occurring are equal. Similarly the relative intensity of the Compton and pair production are similar at that range of energy on the other line.
- 15 -
Figure (3-4). Variation of photon energies at which different effects become importance figure taken from reference. [12]
3-2 Gamma-Ray Detectors: Detecting gamma rays with a detector, the interaction of photon with matter should be considered. The gamma ray photons have an electromagnetic nature; they can interact strongly with the charged electrons in the atoms of all matter. Ionisation is the main process by which gamma rays can be detected, where the gamma ray deposits part or all of its energy to an electron. The ionised electrons pass through the matter and collide with other atoms and knock out further electrons. The charges can be collected either directly (as with a semiconductor detector) or indirectly (as with a scintillation detector), in order to measure the energy of incoming gamma-ray. The outputs from the detectors are the electrical signal whose voltage is proportional to the energy that deposited by the gamma ray in the detecting medium.
- 16 -
3-3 Semiconductor diode (solid state) detector: A radioactive source may emit gamma rays. The energy and the intensity of this radiation can be determined by using a gamma ray spectrometer. Gamma ray spectroscopy can be performed with either scintillation or semiconductor crystal detectors. Scintillation detector crystals have a limited energy resolution. Semiconductor detectors have a higher resolution compared to scintillation detector. This advantage can be used to obtain an accurate result in the measurements of NORM with a low level of radioactivity in the environmental samples. A semiconductor is a solid crystalline material that has an electrical conductivity between an insulator and a good conductor. It has a narrow gap between the valance and conduction bands. A typical value for this band gap in germanium is about 0.7eV. This means that a small amount of energy is required to excite an electron into the conduction band whereas in the valance band a hole is created. The electrical conductivity of semiconductor is therefore changed when a semiconductor is exposed to radiation. The principle of operation of a semiconductor detector is as shown in the figure (3-5). A depletion layer spreads out when a reverse voltage is applied to the p-n junction diode, in a reversed biased diode, no leakage current passes through the circuit. When a radiation particle is incident in the depletion layer, the generated electric charges make an electron-hole pair. This electric charge becomes the output for radiation detection. The resulting charges are integrated and converted to a voltage pulse by a preamplifier and amplifier. The amplitude of the pulses is proportional to the energy of the measured incident radiation. The sensitive volume of the radiation detector element depends on the area of the p-n junction and the active thickness of the detector, called a depletion layer. That is the larger the sensitive volume, the larger the detection efficiency of our detector. The depletion depth is inversely proportional to the net electrical impurity concentration and the count efficiency depends on the purity of the material. Large volumes of very pure material are needed for high counting efficiency, particularly for high photon energy. The energy resolution is strongly related to charge carrier generation (e-h pair). [12]
- 17 -
Figure (3-5): Schematic of a P-N Semiconductor Detector (figure taken from reference 17).
Among semiconductor detectors HpGe detector is the most useful for gamma ray spectroscopy. Silicon may also be used for the same purpose, but germanium is preferred because of the higher atomic number of Germanium which makes it more efficient at stopping and detecting high energy gamma radiation. The configuration of HpGe detector is shown in figure (3-6). The semiconductor detector is cooled to 77 0 K with nitrogen liquid, in order to reduce dark current (which may be present as the result of the thermion emission or other effects when there is no radiation incident) and detector noise. The HpGe detector is unstable at room temperature [5] [12].
Figure (3-6): Schematic of the germanium detector configuration (reproduced from reference [15]).
- 18 -
3-4 Gamma-Ray Attenuation: When a highly collimated mono-energetic photon beam strikes an absorber material of thickness (t), the photons may interact through that material via the photoelectric effect, Compton scattering or pair production. The total linear attenuation coefficient is defined as the total probability per unit length ( µ ) for removal of a photon and is determined by the sum of probabilities for photoelectric absorption ( τ ), Compton scattering ( σ ) and pair production ( κ ) respectively. Thus:
µ =τ +σ +κ
(Eq.19)
The fractional of loss in intensity in passing any thickness ( dt ) of the absorber material is given by:
dI = − µ dt I
(Eq.20)
where, I represents the number of transmission photons and can be derived in terms of the intensity with out any absorber ( I 0 ) crossing through a thickness ( t ):
I = I 0 exp(− µt )
(Beers Law)
(Eq.21)
Since, the linear attenuation coefficient changes with the density of absorber material, it is convenient to use the mass attenuation coefficient rather than linear attenuation coefficient. For specific gamma ray energy, the mass attenuation coefficient does not change with the physical state of that absorber. It is given: Mass attenuation coefficient =
µ ρ
(Eq.22)
The units for mass attenuation coefficient are mg / cm 2 or g / cm 2 . Therefore, Beers law can be rewritten in a convenient way as:
I = I 0 exp(−
µ ρt ) ρ
(Eq.23)
The thickness of the absorber is represented by a mass thickness rather than physical thickness [5].
- 19 -
3-5 Effect of lead shielding: The most significant requirement of low level measurement is shielding of the detector from external environmental radiations. This shielding will reduce the background resulting from cosmic radiation, and natural radioactive traces in building materials or in the surface of the ground. A common material for such shielding is lead with thicknesses of approximately 10cm. Note that the shielding material itself is a source of background radiation which can also produce X-ray florescence peaks ( K α and K β ) at energy 75keV and 85keV respectively. In the measurement of NORM, these radiations may be superposed on top of the spectrum from the photons under study. To reduce this effect, a sheet of either Cadmium or Copper is placed around inner side of the Pb shield castle.
Figure (3-7) shows the mass attenuation coefficient versus the energy of the photon for Pb. In the photoelectric absorption curve, which is simply is the production of X-rays that there are discontinuities at energies corresponding to the binding energy of a particular electronic shells. The energy of the incident photon is greater than the energy of the electron shell so that it can knock out the electron and give rise to an X-ray which is detected by the detector [18]. In the low background system the sources of background radiation will originate from: 1- Radioactive nuclides in the surrounding material. 2- Radioactive nuclides in air. 3- Cosmic ray interaction with both the surrounding material and the detector itself. 4- Radioactive nuclides in the detector assembly. [19] One of the drawbacks of lead is that it will probably contain some
210
Pb which belong to the
uranium series decay. However, its cheapness and availability makes it more desirable to remove higher energies which contribute to the detector low energy spectrum through the Compton scattering. [19] In this project a castle of lead shield with thickness 5cm was constructed to reduce the background contribution and attenuate the photons coming from outside the shielding. This effectiveness of this shielding was measured by collecting the background spectrum with and with out the shielding in place. The X-rays from lead still appear in the final spectrum due to lack of a Copper or Cadmium inner wall in this set-up.
- 20 -
Lead
Figure (3-7): the attenuations of three primary processes in lead material. Taken from reference [20]
3-6 Effect of bias voltage: In order to measure more penetrating radiation using a HpGe detector, the depletion depth of p-n junction diode is most important and the thickness of the depletion region is proportional to the square root of the bias voltage. In a pure germanium diode the maximum depth (fully depleted) of active volume can be achieved by applying a bias voltage across the detector to collect most of the charge carries which were formed when an incident radiation strikes the detector crystal. This effect was investigated by measuring the efficiency curves of the detector for bias voltages of 500V, 1000V, 2500V and 3500V.
- 21 -
3-7 Gamma –ray Spectroscopy: A gamma-ray spectroscopy system is used to determine the energy and intensity of the nuclear radiation. This system consists of a series of equipment, including a high resolution HpGe coaxial detector, high voltage power supply, preamplifier, amplifier, multichannel analyzer (MCA) and a computer with specific software (PCA3). The process of detecting the nuclear radiation is that the electronic signal from the detector directly goes to preamplifier which changes particle charge pulses to voltage pulses. Next the voltage signals pass through an amplifier which provides the voltage gain to bring the milliVolt preamplifier output to the range of a few volts. The amplifier has a linear response, so that the amount of energy deposited is proportional to the pulse height. The pulse height can then be displayed on a MCA. After that the spectra of either a point source or environmental sample will ready to analyze by means of software associated with it (PCA3).
3-7.1 Energy calibration: In gamma-ray spectrometry with a HpGe detector which is operated using PCA3 software, the pulse heights are represented by channel numbers. Therefore, an energy calibration should be performed. The aim of energy calibration is to obtain a relationship between pulse heights (i.e., peak position in the spectrum) and the corresponding gamma-ray energy. Energy calibrations were performed by measuring the spectrum of source emitted gamma-rays of precisely known energy and covering a wide range of energies. In other words, it was required to use a standard source such as
152
Eu which covers low and high energies. It is important to perform this calibration
regularly during the measurement of the low activity environmental sample and for long enough time to avoid any shift in the spectrum and achieve a good statistical precision for the peaks used for the calibration.
3-7.2 Energy resolution: Another important term in gamma ray spectrometry is the energy resolution which is related to the detector response. The resolution can be defined as the ability of the detector to distinguish between two radiations whose energies lie near to each other. It is usually represented by the Full Width at Half Maximum (FWHM) of the pulse height distribution: R=
FWHM 2.35σ = H0 N
- 22 -
(Eq.24)
where, R , H 0 , σ and N corresponds to energy resolution, peak energy, standard deviation, and the number of charge carrier produced in the detector. The energy resolution is improved as the number of the charge carriers is increased. In the current project, an HpGe was used which is a semiconductor detector. It has a significant property that has large number of charge carriers are generated per unit energy loss by the incident radiation. Consequently, at high energy, the FWHM resolution is poorer than for low energies. However, there are a several sources of broadening the peak. Here, the most significant cause is due to statistical fluctuation in the number of charge carrier [12]. Therefore,
FWHM α E γ
1/ 2
⇒ Rα
Eγ
1/ 2
Eγ
= Eγ
−1 / 2
(Eq.25)
3-7.3 Energy efficiency: One of the important characteristics of a detector is its efficiency. In gamma-ray spectrometry the intention is to find the relation between peak area and the amount of activity present in the sample or the source. To obtain this, the absolute photo-peak efficiency should be considered. In general, considering how many quanta of radiation which come from radioactive sources compared to how many quanta reach the detector, the efficiency can be defined as the ability of the detector to measure how many pulses occur for a certain number of gamma rays emitted.
There are two main types of counting efficiency. These are:
1- Absolute efficiency: The ratio of the number of counts measured by the detector to the number of gamma-rays emitted by the source in all direction. It can be expressed as:
ε abs =
Cp Nγ
× 100%
(Eq.26)
where:
ε abs : is absolute photo peak efficiency, C p : is the total count recorded per unit time under the photo peak and N γ : is the number of gamma quanta emitted by the sources per unit time. It can be investigated by using this relation: N γ = D s I γ ( Eγ )
- 23 -
(Eq.27)
where Ds : is the activity of the source I γ ( Eγ ) is the intensity or the probability of the certain radiation emission, for example for
137
Cs is equal to 0.85.
2- Intrinsic Photo peak efficiency: The ratio of the number of pulses produced by the detector to the number of gamma-rays striking the detector. It can be written as:
ε in =
N γ′ =
where:
Cp Nγ′
×100%
(Eq.27)
Ω × Nγ 4π
(Eq.28)
Ω is the solid angle which is the angle subtended by the detector at the source place and can be
found using:
[
Ω = 2π 1 − d / d 2 + a 2
]
(Eq.29)
where ( d ) and ( a ) are the distance between source-detector and radius of the detector respectively. [21] The absolute and intrinsic photo peak efficiency were measured over various energy ranges using source of
241
Am,
133
Ba with energies 59.5keV and 81keV respectively and the standard sealed
source 152Eu (major peaks at 121.78, 244.70, 344.27, 778.90, 964.00, 1112.05 and 1407.92keV). The calibration spectra were collected for 600sec each.
3-8 Sample Collection and Preparation: The samples were collected from different sites in the state of Qatar, as shown in the map in figure (3-8). The samples were prepared in the preparation laboratory room. To remove any moisture from the samples, they were dried in an Oven with temperature 60C0 for one day (24 hours). After that a 500µm mesh was used to sieve the samples. Then they weighed and put in polyethylene bottles with volume 100ml. The samples were sealed for one month in order for radioactive secular equilibrium to be achieved. (The sealing time should be more than 7 half life of radioactive isotopes 222Rn and 220Rn [22].)
- 24 -
Sample 254 Sample 248 Sample 228
Figure 2 Map of Qatar showing places where samples were taken which were analysed in the current work.
3-9 Sample Analysis: For low level samples it is necessary to consider the background distribution around the detector. For that reason, after characterising the detector, an empty polyethylene bottle of volume 100ml which has the same geometry of the samples bottles, was placed 10cm above the top of the detector and the spectrum acquired for two days. Next, to determine the minimum detectable radiation, every specific peak which was identified by the software in MCA in the background around the detector was measured. In addition, the activity concentration of a Uranium-Oxide (UO2) crystal sample and Monazite (a mineral rich in
232
Th) were determined using equation
(Eq.7). Finally, the activities of other samples were found by the same way. - 25 -
Chapter -4-
4-Experimental Results and Discussion 4-1 Detector characterisation: 4-1.1 Energy Calibration: The standard point sealed source
152
Eu with a range of energies (121.78keV, 344.27keV,
778.90keV, 1112.05keV and 1407.92keV) spectrum was collected for 600s. The channel number and associated peaks were recorded. The graph between channel number and the energy was plotted. 1600 1400
y = 0.4174x - 6.5888
Energy keV
1200 1000 800 600 400 200 0 0
500
1000
1500
2000
2500
3000
3500
4000
Channel no.
Figure (4-1): Energy Calibration relation between channel numbers corresponds to energy.
From the energy calibration figure the relation between channel and energy is linear and mathematically represented by the equation(Y=0.417X-6.588), which was used to find the energy for any other peak which does not belong to 152Eu. This calibration should be performed regularly under the same condition of amplifier cross gain 7 and the shaping time 3 µsec to prevent any shift in the spectra. This drift may occur due to the effect of temperature change on the amplifier and other electronic systems. The important point in calibration is that it allows identification of the radio nuclides in the background and environmental samples. - 26 -
4-1.2 Energy Resolution: The energy resolution of the detector was found at different energy ranges. In general, the resolution of the detector was less than 1%. At energy of 121.77keV the FWHM of the pulse height distribution in the spectrum is 1.02keV. Whereas, in higher energies, the FWHM increased approximately more than double of the value at low energies and is equal to 2.08keV. It is obvious that the smaller figure of FWHM indicates that sharp peaks appear on the spectrum and thus the spectral lines become better resolved. Appendix I table (1). -2 2
2.2
2.4
2.6
2.8
3
3.2
-2.1 -2.2
Log R
-2.3 -2.4 -2.5 -2.6
y = -0.7083x - 0.639 2 R = 0.985
-2.7 -2.8 -2.9 -3
Ene rgy ke V
Figure (4-2) Energy resolution of the HpGe detector.
The relation between resolution and energy in the log-log graph was shown in figure (4-2). It is assumed that the most significant cause of fluctuation is due to statistical counting of number of charge carrier collection are characterised by Poisson statistics. The gradient of the straight line was 0.7083. It is quite different from the theoretical value of 0.5. This may be related to the other sources of fluctuation contributions which are electronic noise, inaccuracy in the characterisation of detector, and temperature change of the cooling liquid which cause to incomplete of charge collection. However, the gain setting and the channel comprising spectrum will limit the resolution of the detector. For instance, a rough calculation can be done, in 8000 channels covering 0 to 4000keV.
- 27 -
4-1.3 Energy Efficiency and effect of bias voltage: In general the efficiency is the ratio between the responses of the instrument (gamma spectrometry) and the value of the physical quantity which is in this case the photon emission rate. The efficiency can be classified into two parts which are absolute photo peak efficiency and intrinsic photo peak efficiency. In this project the total absolute photo peak efficiency held a major concern. 0.00700
0.00600
Efficiency
0.00500
Y = 0.3401X-0.902 R2 = 0.9921
0.00400
0.00300
0.00200
0.00100
0.00000 0
200
400
600
800
1000
1200
1400
1600
Energy keV
Figure (4-3) energy efficiency of HpGe detector at 3500keV bias voltage.
It depends on the energy in a complex manner. As shown in figure (4-3), the efficiency varies with the range of energy. At low energies, the absolute efficiency increases with energy to some point (about 81keV) because in the detector material the dominant interaction is the photoelectric effect. Whereas, in the energy range from 100keV up to 1000keV the Compton scattering in the electrons of the detector crystal takes place and some of the gamma rays are scattered and escape from the detector without detection, therefore, not all the photon energies contribute to the full energy peak, so the absolute efficiency decreases gradually. At very high gamma ray energy, pair production is dominant, although the single and double escape peaks should be taken into account at high energy. In addition, the attenuation from the detector window, detector geometry and source detector distance also is an issue and affect the efficiency.
- 28 -
However, a significant portion of the gamma photons can cross the detector without interaction, and consequently the photo peak efficiency decreases progressively. The efficiency curve here covers the energy to a maximum photon energy of 1407.92keV with the fitting power function (Y=0.3401X -0.902). This equation was interpolated to calculate the absolute efficiency of unknown radio nuclides in the samples. Furthermore, a germanium detector is simply a diode with the depletion region. The theoretical relationship between thickness of the depletion layer and the bias voltage is ( dα V ) [12]. The proportionality check of increasing the bias voltage to attain a larger active volume was carried out experimentally, as illustrated in figure (4-4). When the bias voltage increase the charge collection improves. Hence, the large number of pulses recorded; therefore, the efficiency curve is enhanced. The fully depleted region (the entire thickness between two electrodes of the diode are an active volume) was achieved at 3500V. The significant point here is that the bias voltage should be stable in order to sustain the same voltage gradients in the detector and thus the same rate of charge collection occurs during the measurements. 0.00700
0.00600
Absolute efficency
0.00500
0.00400
0.00300
0.00200
0.00100
0.00000 0
200
400
600
800
1000
1200
1400
1600
Energy keV
3500Volt
500Volt
2000Volt
1000Volt
Figure (4-4) represents effect of variation of the bias voltage on efficiency curve of HpGe detector.
- 29 -
4-2 Effectiveness of Shielding: In terms of lack of sufficient shielding, the uncertainty in the analysis of the peaks of interest becomes considerable. The low energy peaks may interfere on a relatively high background as a result of Compton continua of the high energy gamma rays which gives rise to a considerable peak area of uncertainty. Moreover, the overlap of small peaks that comes from high abundant photons and less abundant photons which are emitted from those samples may also occur. Finally, the presence of cascade transitions, in the radioactive nuclides with emission multi gamma rays, the correction of coincidence-summing shall be taken into account. Therefore, the overall are effects on the efficiency of the detector. The spectrums of background for one day were acquired first without any shielding, and secondly after making a 5cm lead castle around the detector. As show in figure (4-5), in the area under background, the Compton continuum is decreased by 1.5 to 2 orders of magnitude as compared to the area without shielding. It is concluded that the strong reduction in background can be obtained with the presence of an engineering shielding design with a thickness of more than 10cm.
Figure (4-5): Background Compton spectrum with out shield and with 5cm shield castle.
- 30 -
4-3 Identify of Radioactive Nuclides in Background: From the gamma-ray spectroscopy, the spectrum of the background where there is an empty polyethylene bottle on the top of the detector was collected for 172800s. The photo peaks and the area of their peaks were located and recorded. In order to detect any activity present in the samples the term of critical level was found from equation (Eq.8) for each photo energy peaks. This can be done by comparing the net count recorded in soil samples with the value of critical level. As shown in table (4-1). The identification of the observed peaks is attained using the gamma library look up of those peaks which were found. Gamma-ray lines with a high abundance for 848 nuclides are available in the gamma library program. From the emission probability and the absolute efficiency the Minimum Detectable Activity was investigated that was associated with the photo energy peaks were located in the background spectrum and samples using equation (Eq.9).
Table (4-1) lists most of the radioactive nuclides which were observed in the background spectrum and their decay series. It concludes that this level of background is often too high to measure low level of environmental radioactivity. Therefore low detection limit is not achieved accurately. In determining the detection limit, the critical levels of every nuclide gamma-ray energies were found for the counting system. The minimum critical level is found to be 27±8 counts for 214Bi at energy 1408.99keV, conversely, the maximum critical level was found at energy 1461.88keV for
40
K to
be 185±2 counts. The critical level values were compared to the net corrected count recorded in the photo peaks for every nuclide. If the number of counts is exceeding the value of the critical level, this line is emitted by the sample and the activity of that line was determined. If not it means that there is no measurable activity in the sample.
It appears that most of the nuclides come from the naturally occurring radioactive series of Uranium-238 and Thorium-232 daughters such as 226Ra with energy 185.94keV and 228Ac with the energy line 910.11keV. However,
137
Cs (661.2keV) and 40K (1461.69keV) were also identified in
the spectra. They mainly come from nuclear fallout and from building materials, predominantly concrete walls, which could be explained by the laboratory walls being concrete.
- 31 -
In the spectra of background each of the photo peaks that correspond to a certain radionuclide were highlighted and the spectra were divided into three ranges of energy 0-1000keV, 1000-2000keV and 2000-3400keV. As shown in figure (4-6a, b&c).
Table (4-1) the critical level of radionuclide that observed in background spectrum.
Radio nuclide
Ra-226 Pb-212 Pb-214 Ac-228 Pb-214 Ac-228 Tl-208 Tl-208 Bi-214 Cs-137 Bi-212 Ac-228 Tl-208 Ac-228 Bi-214 Ac-228 Bi-214 Bi-214 Bi-214 Bi-214 K-40 Bi-214 Bi-214 Bi-214 Tl-208
Decay series
U-238 Th-232 U-238 Th-232 U-238 Th-232 Th-232+Annihilation Th-232 U-238 Non-series Th-232 Th-232 Th-232 Th-232 U-238 Th-232 U-238 U-238 U-238 U-238 Non-series U-238 U-238 U-238 Th-232
Photo peak Energy (keV)
185.94 238.57 295.20 338.31 352.01 463.95 511.26 583.72 609.88 662.31 727.99 795.74 861.42 912.11 935.07 965.61 1121.35 1239.10 1378.73 1408.99 1461.81 1730.38 1765.22 2204.02 2613.30
- 32 -
ROI net (Counts)
1337 3240 1989 1012 3791 239 4800 2685 4255 468 805 430 388 2149 250 333 1273 572 344 131 6351 244 1377 396 2477
Critical Level Lc ± Lc(Counts)
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
385 280 261 240 268 131 258 178 190 154 136 126 130 136 103 86 115 117 81 67 141 52 82 58 80
85 132 104 74 143 36 161 121 152 50 66 48 46 108 37 42 83 56 43 27 185 36 86 46 116
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
13 6 7 9 5 21 4 4 3 9 6 8 8 3 9 6 4 6 5 8 2 4 3 4 2
Counts
0 1000
600
400
1100
200
300
1200
1300
400
1000
1400
500 Energy keV
1500 Energy kev
energy ranges (1000-2000keV).
- 33 600
1600
Bi-214, 1766.44keV
200 Pb-214, 295.34keV
700
1700
800
1800
900
1900
Ac-228, 967.74keV
Ac-228, 910.11keV
Bi-214, 767.43keV
Bi-212, 726.47keV
Cs-137, 661.2keV
Bi-214, 608.73keV
Tl-208, 582.72keV
Tl-208+Annihilation, 510.41keV
Ac-228, 463keV
Ac-228, 238.31keV Pb-214, 351.91keV
1000
Tl-208, 1590.21keV
1200
K-40, 1461.69keV
100 Pb-212, 238.57keV
4000
Bi-214, 1379.82keV
0
Bi-214, 1239.53keV
2000 Ra-226, 185.55keV
X-ray from Lead
5000
Bi-214, 1121.88keV
3000 Th-234, 93.71keV
C ounts
6000
0 1000
Figure (4-6a) the spectrum of background collected for 172800sec with 5cm lead shielding from
energy ranges (0-1000keV).
1400
800
2000
Figure (4-6b) the spectrum of background collected for 172800sec with 5cm lead shielding from
Tl-208, 2614.51keV
350
300
250
B i-214, 2248.38keV
100
B i-214, 2204.01keV
150
Tl-208, 2100.17keV
C ounts
200
50
0 2000
2100
2200
2300
2400
2500
2600
2700 2800 Energy keV
2900
3000
3100
3200
3300
3400
Figure (4-6c) the spectrum of background collected for 172800sec with 5cm lead shielding from energy ranges (2000-3400keV).
The Minimum Detectable Activity for the counting system from the background radiation was calculated as listed in the table (4-2). The MDA is the minimum amount of radioactive nuclide which can be found in a given particular spectrum measurement. It depends on the detector environment, efficiency and the duration of counting. In order to decrease MDA, or increasing sensitivity, it is necessary to reduce the background counts and FWHM (which is essentially resolution). Meanwhile, it is required to raise the efficiency and counting time. A thick lead shielding with 10 to 15 cm around the detector may increase the sensitivity of the counting system. MDA is investigated as the system is functioning with a critical level and ensuring that the rate of false negative and false positive does not exceed 5%.
- 34 -
Table (4-2) represents the Minimum Detectable Activity for the nuclides founded in the background. Radio nuclide
Ra-226 Pb-212 Pb-214 Ac-228 Pb-214 Ac-228 Tl-208 Tl-208 Bi-214 Cs-137 Bi-212 Ac-228 Tl-208 Ac-228 Bi-214 Ac-228 Bi-214 Bi-214 Bi-214 Bi-214 K-40 Bi-214 Bi-214 Bi-214 Tl-208
Photo peak Energy (keV)
Emission probability
± Error in Emission probability
Absolute efficiency
MDA (Bq)
± Error in MDA (Bq)
185.94 238.57 295.20 338.31 352.01 463.95 511.26 583.72 609.88 662.31 727.99 795.74 861.42 912.11 935.07 965.61 1121.35 1239.10 1378.73 1408.99 1461.81 1730.38 1765.22 2204.02 2613.30
0.0359 0.4330 0.1812 0.1127 0.3510 0.0440 0.0813 0.3040 0.4660 0.8510 0.0658 0.0425 0.1242 0.2580 0.0300 0.0499 0.1470 0.0578 0.0410 0.0280 0.1067 0.0288 0.1510 0.0498 0.9990
0.003 0.003 0.009 0.04 0.017 0.011 0.029 0.009 0.04 0.0001 0.0011 0.000 0.000 0.004 0.01000 0.04 0.003 0.006 0.002 0.004 0.001 0.001 0.005 0.0025 0.004
0.00306 0.00244 0.00201 0.00178 0.00172 0.00134 0.00123 0.00109 0.00105 0.00097 0.00089 0.00082 0.00077 0.00073 0.00071 0.00069 0.00060 0.00055 0.00050 0.00049 0.00048 0.00041 0.00040 0.00033 0.00028
8.97 1.45 3.29 4.27 2.75 7.06 18.70 4.22 3.60 0.70 13.02 15.96 5.57 6.65 19.93 14.25 10.82 20.18 24.30 22.40 42.30 35.76 16.50 32.77 4.76
2.59 0.13 0.43 1.01 0.19 3.87 1.01 0.28 0.16 0.23 2.20 4.68 1.86 0.42 8.20 3.69 0.97 4.11 5.70 11.43 0.94 7.57 0.98 4.80 0.15
4-4 Activity concentration of a Uranium-Oxide (UO2) crystal sample and Monazite (a mineral rich in 232Th): There are a number of ores and minerals that contain a relatively high concentration of natural radio nuclides. Mineral sand is one of them, such example and has a significant portion of monazite with a typical activity of 300kBq/gm of Thorium-232 and 40Bq/gm of Uranium-238 (secular equilibrium). To determine the activity concentration of the 238U and 232Th from the decay products (because they are very long lived nuclides), the spectrum of the Uranium-oxide crystal sample and monazite (a mineral rich in
232
Th) were collected for 86400s as shown in Figure (4-
7&4-8). The spectrum contains several peaks.
- 35 -
Each peak represents a gamma ray emission line which is emitted by the daughters of those parent radioactive nuclides. Using the equation (Eq.7) and assuming that they are in secular equilibrium with its decay products, the specific activity of 238U was found from the decay progenies 214Pb (the gamma ray lines of 294.95keV and 351.60keV) and
214
Bi (608.90keV and 1120.3keV) to be
4.4±0.2MBq/kg, 5.5±0.3MBq/kg, 7.5±0.7MBq/kg and 8.5±1.4MBq/kg respectively. Whereas, the activity of 582.88keV and
228
232
Th from the gamma-ray emission lines
212
Pb 238.43keV,
208
Tl
Ac 910.81keV were equal 275±28kBq/kg, 171±17kBq/kg and 474±49kBq/kg
respectively. The activities for other lines were also calculated for both UO2 and Monazite samples shown in the table (4&5) in Appendix II.
Bi-214, 933.57keV
Ac-228, 968.47keV Ac-228, 9604.04keV
Ac-228, 910.71keV
Bi-214, 805.71keV
Bi-214, 608.90keV
50000
Bi-214, 767.90keV Pb-214, 785.47keV
100000
Annihilation, 510.71keV
150000
Pb-214, 242keV
Counts
200000
Ra-226, 186.04keV
Kα &Kβ X-ray from lead
250000
Tl-208, 583keV
Pb-214, 294.95keV
300000
Pb-214, 351.60keV
350000
0 0
100
200
300
400
500 Energy keV
600
700
800
900
Figure (4-7a) the spectrum of the Uranium-oxide crystal sample showing at energy between 0 and 1000keV.
- 36 -
1000
0 1000
1100
1200
1400
1500 Energy keV
B i-214, 1729.01keV
B i-214, 1763.92keV 1300
1600
1700
B i-214, 1846.83keV
5000
B i-214, 1154.65keV
10000
A c-228, 1051.47keV
15000
Pa-234, 1000.55keV
20000
B i-214, 1660.72keV
C ounts
25000
B i-214, 1582.59keV
30000
B i-214, 1508.67keV
35000
B i-214, 1377.13keV B i-214, 1384.74keV B i-214, 1400.98keV B i-214, 1407.40keV
40000
B i-214, 1280.42keV
45000
B i-214, 1237.58keV
B i-214, 1119.75keV
50000
1800
1900
2000
Figure (4-7b) the spectrum of Uranium-oxide crystal sample showing at energy between 1000 and 2000keV.
2000
Bi-214, 2292.60keV
3000
Bi-214, 2117..91keV
Counts
4000
1000
0 2000
2100
Bi-214, 2447.12keV
5000
2200
2300
Tl-208, 2614.01keV
Bi-214, 2203.50keV
6000
2400
2500
2600
2700 2800 Energy keV
2900
3000
3100
3200
3300
3400
Figure (4-7c) the spectrum of Uranium-oxide crystal sample showing at energy between 2000 and 3500keV.
- 37 -
3500
Specific Activity distribution for the nuclides in Uranium crystal
14
specific Activity MBq/kg
12
Bi-214 Ra-226
10
Pb-214
8
Ac-228 6
Tl-208
4
Pa-234
2 0 0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
2800
EnergykeV
Figure (4-8) the distribution of the specific activity of the radioactive nuclide in the Uranium-Oxide crystal.
As shown in the figure (4-8), the graph between energy against specific activities was drawn assuming 10% uncertainty in the efficiency (the efficiency curve is not fitted with the trend line and the extrapolated of efficiency to determine efficiency at high energy). The data for the nuclide 226
Ra is notably lower than the other lines (from 214-Bi etc.). This could be explained possibly due
to (a) the sample was not in secular equilibrium and (b) the specific gamma-ray line has a relatively low energy (186.04keV) indicating increase self-attenuation in the sample. This attenuation effect can also be also observed in the results for the radionuclide
214
Pb at energy
294.95 and 351keV. The attenuation correction should be calculated experimentally and taken into account. The activity of the 238U can be estimated from the activity of 214Bi (assuming equilibrium decays) to be ≈8MBq/kg.
- 38 -
0 1000
800
700
1100
1200
400
300
200
1300
400
600
500
1400
500 Energy keV
900
1500 Energy keV
2000keV.
- 39 600
1600
700
1700
B i-214, 1845.84keV
300
Bi-214, 1762.91keV
200
B i-214, 1728.05keV
100
Ac-228, 1586.92keV Tl-208, Escape peak, 1591.05keV B i-212, 1619.41keV A c-228, 1629.27keV
0
1800
800 900
1900
Figure (4-9b) the spectrum of Monazite mineral sand sample showing at energy between 1000 and A c-228, 968.61keV
A c-228, 910.81keV
Tl-208, 860.19keV
A c-228, 835.35keV
A c-228, 755keV A c-228, 772.07keV A c-228, 794.61keV
B i-212, 726.94keV
B i-214, 609.02keV
Tl-208, 582.88keV
A nnihilation, 510.39keV
A c-228, 327.56keV A c-228, 337.95keV Pb-214, 351.5keV
Pb-214, 294.82keV
A c-228, 269.84keV
R a-226, 185.79keV A c-228, 208.86keV Pb-212, 238.43keV
100000
Bi-214, 1508.03keV
20000 A c-228, 129.17keV
120000
K -40, 1459.6keV
40000
K α & K β X-ray from lead Th-234, 93keV
60000
Bi-214, 1119.8keV
C ounts 80000
B i-214, 1407.07keV
C ounts
140000
0 1000
Figure (4-9a) the spectrum of Monazite mineral sand sample showing at energy between 0 and
1000keV.
1000
100
2000
Tl-208, 2609.81keV
3500
3000
C ounts
2000
1500
1000
500
0 2000
2100
Bi-214, 2201.43keV
Tl-208, Escape peak, 2100.79keV
2500
2200
2300
2400
2500
2600
2700 2800 Energy keV
2900
3000
3100
3200
3300
3400
3500
Figure (4-9c) the spectrum of Monazite mineral sand sample showing at energy between 2000 and 3500keV. Activity distribution of nuclides in Monazaite sample
700
Specific Activity kBq/kg
600 Ac-228 Ra-226
500
Pb-212
400
Pb-214 Bi-212
300
Tl-208 200
Bi-214
100 0 0
500
1000
1500
2000
2500
3000
Ene rgy ke V
Figure (4-10) represent the distribution of the specific activity of the radioactive nuclide in the Monazite mineral sand sample.
Figure (4-10), showing the distribution of specific activity versus the energy of the nuclides. For the same reason as in Uranium-oxide crystal, the activity of 232Th was calculated from the
≈400kBq/kg.
- 40 -
228
Ac
4-5 Activity concentration of environmental samples: After the radio nuclides were identified using gamma-ray spectroscopy, it is important to know the minimum detectable activity of individual radio nuclides of the samples to ensure that the false negative and false positives are no larger than 5%. This was performed from the background of individual nuclide in each sample as illustrated in the table (4-3). The range of MDA in the samples for
226
Conversely, for 137
Ra is 86±29 to 114±29Bq/kg and for 40
137
Cs range from 5±2 to 6±4Bq/kg.
K this ranged from 392±6Bq/kg to 428±7Bq/kg. The small value of MDA for
Cs means that the minimum amount of that nuclide that can be detected by the counting system.
However, in some of the gamma-ray lines the number of counts recorded was less than the critical level (such as
212
Pb at 238.57keV) in the majority of samples. This concludes that
212
Pb exists in
measureable amounts in the background environment of the detector only and does not come from the samples themselves. Table (4-3) indicates the Minimum detectable activity in the environmental samples: Radio nuclide
Photo Emission peak probability Energy (keV) Ra-226 185.94 0.0359 Pb-212 238.57 0.4330 Pb-214 295.20 0.1812 Ac-228 338.31 0.1127 Pb-214 352.01 0.3510 Ac-228 463.95 0.0440 Tl-208 511.26 0.0813 Tl-208 583.72 0.3040 Bi-214 609.88 0.4660 Cs-137 662.31 0.8510 Bi-212 727.99 0.0658 Ac-228 795.74 0.0425 Tl-208 861.42 0.1242 Ac-228 912.11 0.2580 Bi-214 935.07 0.0300 Ac-228 965.61 0.0499 Bi-214 1121.35 0.1470 Bi-214 1239.10 0.0578 Bi-214 1378.73 0.0410 Bi-214 1408.99 0.0280 K-40 1461.81 0.1067 Bi-214 1730.38 0.0288 Bi-214 1765.22 0.1510 Bi-214 2204.02 0.0498 Tl-208 2613.30 0.9990 BLC: Below Critical Level.
Absolute efficienc y
MDA (Bq/kg) Soil-375
MDA (Bq/kg) soil-254
MDA (Bq/kg) soil-228
MDA (Bq/kg) soil-248
MDA (Bq/kg) soil-178
0.00306 0.00244 0.00201 0.00178 0.00172 0.00134 0.00123 0.00109 0.00105 0.00097 0.00089 0.00082 0.00077 0.00073 0.00071 0.00069 0.00060 0.00055 0.00050 0.00049 0.00048 0.00041 0.00040 0.00033 0.00028
90 ± 19 BLC 32 ± 3 40 ± 8 26 ± 1 84 ± 32 178± 7 40 ± 2 34 ± 1 5 ± 2 11 ± 17 145± 41 BLC 65 ± 3 190± 64 124± 34 103± 7 BLC 231± 44 254± 77 406± 6 BLC 160± 7 308± 37 46± 1
86 ± 18 BLC 31 ± 3 BLC 25 ± 1 BLC BLC BLC 31 ± 1 BLC BLC 137 ± 35 47 ± 14 BLC BLC BLC BLC 177 ± 23 BLC 247 ± 74 373 ± 6 BLC BLC BLC BLC
114± 29 BLC 42 ± 7 49 ± 9 34 ± 2 104± 48 195± 8 BLC 42 ± 1 BLC BLC BLC 58 ± 14 67 ± 3 224± 96 131± 74 120± 10 220± 40 242± 76 297± 90 428± 7 381± 77 181± 8 364± 83 47± 2
88 ± 19 BLC BLC 44 ± 8 26 ± 1 81 ± 19 BLC 40 ± 2 BLC 6 ± 2 BLC BLC 54 ± 13 64 ± 3 BLC 111± 59 100± 7 200± 30 230± 45 216± 91 392± 6 338± 76 BLC BLC BLC
BLC 15 ± 1 31 ± 3 42 ± 7 25 ± 1 95 ± 33 167± 7 39 ± 2 BLC 6 ± 4 BLC 133± 33 49 ± 0 60 ± 6 BLC 113± 0 96 ± 15 177± 36 226± 0 234± 0 381± 7 306± 99 143± 8 277± 30 42 ± 2
- 41 -
The activities of the individual radio nuclides for all samples were determined from equation 7. The summary of activity of activity of the samples is shown in the table 4-4 below:
Table (4-4): The calculated Activities of the samples of each radionuclide: Radio nuclide
Photo peak Energy (keV)
Emission probability
Absolute efficiency
Ra-226 185.94 0.0359 0.00306 Pb-212 238.57 0.4330 0.00244 Pb-214 295.20 0.1812 0.00201 Ac-228 338.31 0.1127 0.00178 Pb-214 352.01 0.3510 0.00172 Ac-228 463.95 0.0440 0.00134 Tl-208 511.26 0.0813 0.00123 Tl-208 583.72 0.3040 0.00109 Bi-214 609.88 0.4660 0.00105 Cs-137 662.31 0.8510 0.00097 Bi-212 727.99 0.0658 0.00089 Ac-228 795.74 0.0425 0.00082 Tl-208 861.42 0.1242 0.00077 Ac-228 912.11 0.2580 0.00073 Bi-214 935.07 0.0300 0.00071 Ac-228 965.61 0.0499 0.00069 Bi-214 1121.35 0.1470 0.00060 Bi-214 1239.10 0.0578 0.00055 Bi-214 1378.73 0.0410 0.00050 Bi-214 1408.99 0.0280 0.00049 K-40 1461.81 0.1067 0.00048 Bi-214 1730.38 0.0288 0.00041 Bi-214 1765.22 0.1510 0.00040 Bi-214 2204.02 0.0498 0.00033 Tl-208 2613.30 0.9990 0.00028 BMDA: Below Minimum Detectable Activity.
Activity (Bq/kg) Soil-375 IAEA
Activity (Bq/kg) Soil-254
Activity (Bq/kg) Soil-228
Activity (Bq/kg) Soil-248
Activity (Bq/kg) Soil-178
259±232 BMDA 110±41 179±84 81±28 219±221 181±159 63±32 61±22 3115±19 125±137 BMDA BMDA 99±41 BMDA 273±170 135±78 BMDA 287±245 BMDA 860±162 BMDA BMDA BMDA BMDA
BMDA BMDA 49±39 BMDA 37±22 BMDA BMDA BMDA BMDA BMDA BMDA 218±176 BMDA BMDA BMDA BMDA BMDA 236±155 BMDA 413±237 BMDA BMDA BMDA BMDA BMDA
443±251 BMDA 298±47 65±87 228±27 180±214 339±152 BMDA 206±23 BMDA BMDA BMDA BMDA BMDA 371±295 238±181 289±82 489±191 498±209 391±256 572±164 BMDA 300±84 1001±221 140±20
BMDA BMDA BMDA BMDA BMDA BMDA BMDA BMDA BMDA 9±8 BMDA BMDA 128±63 64±42 BMDA 161±157 BMDA BMDA BMDA BMDA BMDA BMDA BMDA BMDA BMDA
BMDA 32±17 36±37 55±64 53±18 172±154 BMDA BMDA BMDA 17±9 BMDA BMDA 206±51 273±42 BMDA 401±104 408±72 573±159 947±144 683±215 473±149 404±253 BMDA BMDA 106±18
As presented in table (4-4) the samples activities were calculated. To determine the activity of 238U and
232
Th it is suitable to look at the daughters
(609.88keV and 1121.35keV) and decay products
214
212
Pb (295.20keV and 352.01keV) and
Pb (238.57keV),
208
Tl (583.72keV) and
214 228
Bi
Ac
(912.11keV) gamma-ray lines of activities because their photon intensity is quite large and they are assumed to be near secular equilibrium with their parents. Also for activity calculation for and
40
137
Cs
K is based on the detection of their emission lines at 662.31keV and 1461.81keV
respectively.
- 42 -
Sample-228 contains relatively large amount of series and non-series radioactive nuclides compared to other samples.
226
Ra and
235
U peaks, which both have lines near 186keV are not
unambiguously resolved and are thus taken as a single (226Ra) peak. The activity concentration of 238
U is in the range from 206±23 to 406±251Bq/kg for the gamma- ray energy lines above. On the
other hand, the concentration of 232Th was 140±20Bq/kg from radionuclide208Tl (2613.30keV) and the other lines were below the minimum detectable activity. The obtained results are not comparable to the worldwide average value of activity in soil samples which is 40Bq/kg [UNCSEAR], this may be because the fitting and extrapolating the efficiency of the detector. In addition the non series radionuclide
40
K activity concentration was found to be 572±164Bq/kg.
This value is in the limit of the typical value that is recommended in the certificate of the reference soil-375 sample from IAEA. [15]. The spectrum of the sample-228 is shown in figure (4-11). The spectra of the other samples are given in appendix III. For soil-248, it can be seen from the table (4-4) that the activities for most of the series radioactive nuclides are below the minimum detectable activity and the concentration of
137
Cs was measured
to be 9±8Bq/kg.
Ac-228, 965.74keV
Ac-228, 912.32keV
Bi-214, 935.18keV
Ac-228, 795.89keV
Bi-214, 769.29keV
Bi-212, 728.07keV
Bi-214, 610.01keV Cs-137, 662.46keV
Ac-228, 463.49keV
Tl-208, 583.81keV
Pb-214, 352.07keV Ac-228, 338.39keV
500
Pb-214, 295.25keV
1000
Pb-212, 238.59keV
Counts
1500
Ra-226, 185.89keV
Kα &Kβ X-ray from lead
2000
Tl-208+Annihilation, 511.36keV
Spectrum of Sample- 228 -
2500
0 0
100
200
300
400
500 EnergykeV
600
700
800
900
Figure (4-11a) the spectrum of soil-228 sample showing at energy between 0 and 1000keV.
- 43 -
1000
K-40, 1462.14keV
1400
1200
1000
0 1000
1100
1200
1300
1400
1500 EnergykeV
1600
1700
Bi-214, 1848.63keV
Bi-214, 1765.58keV
Bi-214, 1730.81keV
Bi-214, 1589.36keV
200
Bi-214, 1409.61keV
400
Bi-214, 1378.14keV
600
Bi-214, 1239.43keV
Bi-214, 1121.61keV
Counts
800
1800
1900
2000
Figure (4-9) the spectrum of soil-228 sample showing at energy between 1000 and 2000keV.
Tl-208, 2613.91keV
350
300
250
Bi-214, 2204.45keV
Counts
200
150
100
50
0 2000
2100
2200
2300
2400
2500
2600
2700
2800
2900
3000
3100
3200
EnergykeV
Figure (4-9) the spectrum of soil-228 sample showing at energy between 2000 and 3400keV.
- 44 -
3300
3400
Chapter -55-Conclusion: A high resolution HpGe detector based gamma-ray spectroscopy was used to identify the radioactive nuclides in the environmental soil samples. These environmental soil samples were collected from four different sites in the State of Qatar and the activity concentrations of those nuclides were determined. The energy calibration is important in the determination of the specific activity of the environmental samples. The confidence of the radioactive nuclides identification and precise peak location are obtained by the energy calibration. In addition, the efficiency of the detector plays an important role in the low level of radioactivity in the soil samples. The efficiency of the detector may be increased by using larger volume detector. The efficiency of the detector may be affected from the source-detector distance and geometry of the sample.
An HpGe detector is simply a solid state p-n junction diode. The depletion layer thickness increases as the bias voltage supplied on the diode is increased. In a fully depleted detector the depletion region fills the region between the electrodes in the semiconductor detector. Therefore the efficiency of the detector is maximum at fully depleted. The effect of shielding around the HpGe detector was demonstrated. The area under the background spectrum without shielding is reduced by a factor two when there is a 5cm lead shield placed around the detector. In the low level counting spectroscopy of environmental samples 1015cm lead shield are recommended.
The daughter products decay series of the
238
226
U,
Ra,
235
212
Pb,
U and
non-series radioactive nuclides
214
232
137
Pb,
214
Bi,
212
Bi,
228
Ac and
208
Tl of the natural radioactive
Th which were formed since the origin of the earth and the
Cs which arise from the nuclear weapon test and reactor
accidents were detected in Uranium-Oxide crystal, monazite mineral and sand samples.
The Minimum Detectable Activities were calculated for the soil samples. The lowest value of the activity concentrations that can be detected by the counting system ranged from 86±18 to 114±29Bq/kg for 226Ra, 25±1 to 42±7Bq/kg for 214Pb, 40±8 to 145±41Bq/kg for 228Ac, 5±2 to 6±4Bq/kg for 137Cs and 373±6 to 428±7 for 40K.
- 45 -
The specific activity of the individual radioactive nuclides was evaluated. The activity concentration of 238U was computed from radionuclide 214Pb (at 352.01keV). The highest value of the activity was observed in the sample-228 to be 298±47Bq/kg, while in the sample-248 was below the detection level. Values for samples-254 & -178 were found to be 49±39 and 36±37Bq/kg respectively. The activity of 232Th was found from measurement of the gamma-ray line at 912.11keV from
228
Ac. For the samples 254 and 228 these are below the detection level,
whereas, in sample-248 and sample-178 the activities of
232
Th were equal to 64±42 and
273±42Bq/kg respectively. These values are dependent on the sample composition, energy of the radiation, source- detector distance, the detector efficiency, the background and the time duration of measurements.
40
K was observed in sample-228 and the sample-178 with specific activities of
that nuclide of 572±164 and 473±149Bq/kg respectively. These values are in good agreement with in the recommended value in the reference soil sample from IAEA [24]. The radionuclide
137
Cs
were observed in soil sample-248 and sample-178 with the concentration 9±8 and 17±9Bq/kg. This indicates that
137
Cs nuclide from the reactor accident and the atomic bomb test have been
spread out through a large area across the world including the locations were the samples were collected. The small amount of 137Cs that observed may be affected by the weather conditions and the reduction of its level in the environment because of its finite half life. The variation of the activity concentrations may be related to the particle size. Since the distribution of the radio nuclides was found to be very dependent on particle size. Moreover the geological nature of the samples is also strongly affecting the discrepancy in the activity concentrations. Additionally, the activity concentration of
238
U and
232
Th were found in the high concentration
Uranium-Oxide (UO2) and mineral sand monazite (rich in
232
Th). For
238
U is found to be
7.5±0.7MBq/kg for the gamma ray line 608.90keV for nuclide 214Bi. The activity of the 232Th was evaluated from 228Ac 910.81keV to be 474±49kBq/kg.
The aim of lead shield is to attenuate the background radiation. While attenuating this radiation, this shield also acts as an addition source of radiation and cosmic-rays strike it to produce secondary radiation. It should be noted that the size, thickness and the interior lining of the shield should be chosen to minimise the contribution of these secondary radiation.
- 46 -
Another major concern was observed in this study is the self attenuation in the samples which cause to reduce the activity concentration approximately by a factor two at low energies gammaray photons. Therefore correction must be carried out for this attenuation in future studies of this kind.
In conclusion, it is appears that naturally occurring radioactive materials can be found almost every where. This leads to humans being regularly exposed to the ionisation radiation from these nuclides. The activity concentrations obtained in this study are higher than presented in some other studies. The results obtained confirm that the soil sample-228 is relatively more radioactive than the other soil samples, consistent with other parallel measurements of the same samples.
Suggestion: The improvement of the measurements of NORMs in the environmental soil samples would be performed by reducing the background radiation. A very careful selection of detector assembly and geometry and shielding materials thickness is important to consider. Since the attenuation of the gamma rays emitted by the sample itself was observed at low energy, further work is required to investigate the self-attenuation in the samples. In this study, it appears that for the accurate measurement of NORM, it is necessary to consider the detector assembly which is made from Aluminum. Other previous works have shown that Aluminum can be a major source of natural radio activities. To overcome this effect, it is possible to make detector housing from material of low natural radio activities such as Magnesium/Copper and the detector window from plastic. Finally, the assessment of the potential risk to human health from the exposure to the ionising radiation from NORMs can be achieved by computing the dose from the activity concentration.
- 47 -
References: 1- Stuart Hunt and Associates Ltd, A Brief Discussion about Naturally Occurring Materials (NORM)[online], USA:SHA, 2002. Available from: http://www.stuarthunt.com/Downloads/Docs/NormText.pdf [Accessed 31 Aug 2008] 2- Ronald L Kathren, NORM sources and their origins, Elsevier, Applied Radiation and Isotopes, 49, 149-168, 1998. 3- 24thSeismic Research Review: Nuclear Explosion Monitoring: Innovation and Integration, Be-7 Cross Talk in RASA Continuous air samplers. [online] , Available from: http://www.ldeo.columbia.edu/res/pi/Monitoring/Doc/Srr_2002/screen/05-01.pdf [Accessed 7 September 2008] 4- Dawdall, Mark, Radon Exhalation from Soil in a High Arctic Region, [on line], Norway, 2002. Available on line: http://www.nilu.no/pomi/itemframeset.cfm?lang=3&id=5236&type=7&senter=0&sub=0&subsub= 0 [Accessed date 11Aug08]. 5- Krane S. Kenneth, Introductory Nuclear Physics. Oregon State University: John Wiley &Sons Ltd, 1988. 6- Js Hughe and Mpharvey, A Study on the Transport of Naturally Occurring Radioactive Material, Health Protection Agency, NORM Report, 2006. 7- Radiation Information Networks. Radioactivity in Nature. USA.[online], Available on line: http://physics.isu.edu/radinf/natural.htm [Accessed date 7 September 2008] 8- Meyerhof E.Walter. Elements of Nuclear Physics. McGraw-Hill Inc. 1967. 9- Dictionary, Definition of Radioactive Series, Available on line: http://www.answers.com/topic/radioactive-series [Accessed date 2 August 2008] 10-
European
Nuclear
Society,
Decay
Chains,
Natural,
[online],
Available
from
http://www.euronuclear.org/info/encyclopedia/d/decaybasinnatural.htm [Accessed date 7 September 2008] 11- Radiation Protection, U.S. Environmental protection Agency, Radioactive Equilibrium, [on line], Available from: http://www.epa.gov/rpdweb00/understand/equilibrium.html#transient [Accessed date 7 September 2008] 12- Knoll, G.F, Radiation Detection and Measurement.3rd ed. New York: John Wiley &Sons Ltd, 2000. - 48 -
13- El Afifi E. M., Evaluation of U, Th, K and emanated in some NORM and TENORM samples, Elsevier, Radiation measurement, 41, 627-633, 2006. 14- Dovlet,C. and Povierec, P.P, Quantification of Uncertainty in Gamma Spectrometric Analysis of Environmental Samples, Quantify Uncertainty in Nuclear Analytical Measurements [online]IAEA-TECDOC-1401, 103-126, 2004. Available from: http://www-pub.iaea.org/MTCD/PDF/te_1401_web.pdf [accessed date 13 August 2008] 15- Earth Atmospheric & Planetary Sciences and Associates Ltd, Trace Element Analysis of Geological, Biological & Environmental Materials by neutron Activation Analysis [online], Cambridge, 2005. Available from: http://ocw.mit.edu/NR/rdonlyres/Earth--Atmospheric--and-Planetary-Sciences/12091January--IAP--2005/83CC311F-C4F0-42CA-ACBD-581427686B64/0/session2a.pdf [Accessed date 6September 2008] 16- Lilley, J, Nuclear Physics: Principles and Applications. Chichester: John Wiley &Sons, Ltd, 2001. 17-Semiconductor Detectors (Solid State Detectors), [online], available from: http://www.kvi.nl/~wortche/detectors2003/detectors2003_files/solid_state.pdf [Accessed date 8September 2008] 18- Debertin K. and Helmer R.G, Gamma and X-ray Spectrometry with Semiconductor Detectors, Elsevier publishers, North-Holland, 1998. 19- Gordon G., Practical Gamma-Ray Spectrometry, John Wiley &Sons, 1995. 20- National Institute of Standards and Technology, Xcom: Photon Cross Sections Database Available online http://physics.nist.gov/PhysRefData/Xcom/Text/XCOM.html [Accessed date 6September 2008]. 21- Department of Physics, Introduction to Gamma-Ray Spectroscopy. Guildford: University of Surrey, 2006. 22- Al.Sulaiti H. A., Determination of Natural Radioactivity Levels of the State of Qatar Using gamma-ray Spectrometry, University of Surrey, 2007. 23- Firestone, R.B, Table of Isotope. 8th ed. California: John Wiley & Sons Ltd, 1998. 24- International Atomic Energy Agency (IAEA), Reference Material IAEA-375. [online], Austria, 2002. Available from: http://www.iaea.org/programmes/aqcs/pdf/rs_iaea-375.pdf [Accessed date 9September 2008].
- 49 -
Appendix I Activity calculation: The activity concentration of individual nuclides in the Uranium-Oxide crystal, Monazite and soil samples were calculated using the following equation:
A =
N I (γ ) ε MT
(Eq.1)
Where: A is the activity concentration of a certain radioactive nuclide in the decay series. N is the net peak area count subtract background of the sample.
ε is the absolute efficiency of the detector. I(γ ) is the emission probability of a specific energy photo peak. T is time for collecting the spectrum of the sample. M is the weight of the sample.
For example: The activity concentration of 137Cs (662.94keV) in soil sample-248 was determined: The net count under that peak = 409± 90 counts/2days. The net count in the background under that peak =271±92 counts/2days.
ε = 0.097% at energy 662.94keV. I(γ ) = 0.8510 the emission probability of the gamma ray line 662.94keV. T =172800s. M = 0.109579±0.000001kg.
Substituting these values in equation (1) the activity will be: A =
( 409 − 271 ) ( 0 . 8510 × 0 . 00097 × 0 . 1001484
× 172800 )
= 8 . 82 ≈ 9 ±8Bq/kg
This calculation was done by assuming that the all correction factors are equal to unity.
Error propagation determination: The error propagations were evaluated using the following formulas: In the case of addition and subtraction, let f is a function of x and y, and a and b are constant.
f = ax m by Therefore, the error will be given by: - 50 -
∆ f = a 2 ∆x 2 + b 2 ∆y 2
In the case multiplication and division:
f = axy
f =a
Or
x y
The error in function f was evaluated from this formula: 2
∆f ∆x ∆y = + f x y
2
Also, the error in the logarithm function ( f = Logax ) was found: ∆f =
∆x x
Determination of critical level and minimum detectable activity of the counting system: The critical levels for the counting system were found using the following equation:
Lc = 2.326σ N B
(Eq.2)
where, σ N B is the standard deviation of Gaussian distribution for the number of counts in the background. This is a decision limit to ensure that the false positive and the probability will be no larger than 5%. In addition, MDA was evaluated for the background spectrum and soils using:
MDA =
4.653 N B I(γ ) εMT
N B is the background count under specific photo peak energy in background and soils. Such as, calculation of Lc and MDA
226
Ra (185.94keV) were performed.
The number of counts recorded under that peak was 1337±385 counts. Therefore, Lc and MDA are given by:
Lc = 2.326 1337 = 85±13 counts
MDA =
4.653 × 1337 = 8.97±2.59Bq/kg 0.0359 × 0.00306 × 172800
- 51 -
(Eq.3)
Energy Resolution and Efficiency Data: Table (1) shows energy resolution determination: Eγ (keV) 122.77 244.62 343.73 443.03 776.83 864.95 961.33 1082.68 1108.84 1403.79
FWHM (keV)
1.02 1.1 1.22 1.29 1.6 1.44 1.78 1.78 1.83 2.08
Resolution %
Log Eγ
0.831 0.450 0.355 0.291 0.206 0.166 0.185 0.164 0.165 0.148
2.089 2.388 2.536 2.646 2.890 2.937 2.983 3.035 3.045 3.147
Log R
-2.080 -2.347 -2.450 -2.536 -2.686 -2.779 -2.732 -2.784 -2.782 -2.829
Table (2) represents the calculated absolute efficiency of the detector at 3500volts: Photon Energy (Eγ )
60.16 81.98 121.93 244.43 344.01 443.8 778.99 867.49 964.13 1085.92 1112.02 1407.94
Activity (DS) at 15/07/08
406.926 102.99 168.223 168.223 168.223 168.223 168.223 168.223 168.223 168.223 168.223 168.223
Branching ratio (Iγ)
Net counts Per second (Ct)
Absolute Efficiency ( ε Abs)
0.3600±0.0040 0.3411±0.0028 0.2837±0.0013 0.0753±0.0004 0.2657±0.0011 0.0313±0.0001 0.1297±0.0006 0.0421±0.0003 0.1463±0.0060 0.1013±0.0050 0.1354±0.0006 0.2085±0.0009
463.7183±1.1333 220.9950±0.7717 195.9267±0.7033 33.7267±0.3783 82.8483±0.4350 7.5600±0.2117 17.4333±0.2383 4.8767±0.1850 16.0200±0.2100 11.3800±0.1383 15.3317±0.1600 16.2117±0.1717
0.00317±0.00004 0.00629±0.00006 0.00411±0.00002 0.00266±0.00003 0.00185±0.00001 0.00144±0.00004 0.00080±0.00001 0.00069±0.00003 0.00065±0.00003 0.00067±0.00003 0.00067±0.00001 0.00046±0.00001
Table (3) represents the weight of the samples in kg: Sample code
Weight of the sample in kg
375 254 228 248 178 Uranium-crystal Monazite sand
0.104107±0.000001 0.115996±0.000001 0.100148±0.000001 0.107590±0.000001 0.113220±0.000001 0.003550±0.000001 0.007861±0.000001
- 52 -
Appendix-II Determination of the activities of Uranium-Oxide and Monazite sample: Table (4) shows the calculated values of activity of the individual radio nuclides in the UO2 sample: Radio nuclide
Ra-226 Pb-214 Pb-214 Bi-214 Bi-214 Pb-214 Bi-214 Ac-228 Ac-228 Ac-228 Pa-234 Ac-228 Bi-214 Bi-214 Bi-214 Bi-214 Bi-214 Bi-214 Bi-214 Bi-214 Bi-214 Bi-214 Bi-214 Bi-214 Bi-214 Bi-214 Bi-214 Bi-214 Bi-214 Bi-214 Tl-208
Photo peak Energy (keV) 186.04 294.95 351.60 608.90 767.90 785.47 805.71 910.71 964.02 968.47 1000.55 1051.47 1119.75 1154.65 1237.58 1280.42 1377.13 1384.74 1400.98 1407.40 1508.67 1582.59 1660.72 1729.01 1763.92 1846.83 2117.91 2203.50 2292.60 2447.12 2614.01
Net count U02 109384 500999 1031995 1076853 101647 23663 25113 45492 7931 22755 14430 4853 234804 25258 85817 20686 53964 10715 14611 30906 26620 7675 11488 31952 167123 20821 10806 43330 2383 12425 24421
± Net count
1446 1165 1413 1189 534 474 525 535 384 458 471 232 654 413 496 374 376 307 312 353 405 265 250 280 462 248 193 271 135 162 174
Emission Probability
± Emission Probability
Absolute efficienc y
Specific Activity (MBq/kg)±
0.73000 0.18500 0.35800 0.44800 0.04800 0.00850 0.01120 0.26600 0.49900 0.16173 0.00837 Not found 0.14800 0.01640 0.05860 0.01440 0.03920 0.00890 0.01550 0.02800 0.02120 0.00703 0.01140 0.02880 0.15360 0.02040 0.01140 0.04860 0.00301 0.01500 0.99160
0.04000 0.00300 0.00500 0.00500 0.00700 0.00100 0.00030 0.00069 0.00010 0.00319 0.00001 Not found 0.00200 0.00040 0.00080 0.00030 0.00080 0.00090 0.00170 0.00400 0.00040 0.00014 0.00030 0.00060 0.00200 0.00040 0.00030 0.00090 0.00009 0.00040 0.00000
0.00305 0.00201 0.00172 0.00105 0.00085 0.00083 0.00081 0.00073 0.00069 0.00069 0.00067 0.00064 0.00060 0.00059 0.00055 0.00054 0.00050 0.00050 0.00049 0.00049 0.00046 0.00044 0.00042 0.00041 0.00040 0.00038 0.00034 0.00033 0.00032 0.00030 0.00028
3.26 ±0.18 4.39 ±0.07 5.47 ±0.08 7.49 ±0.08 8.13 ±1.19 10.91±1.30 8.99 ±0.31 0.77 ±0.01 0.07 ±0.01 0.67 ±0.02 8.40 ±0.27 Not found 8.56 ±0.12 8.54 ±0.25 8.65 ±0.13 8.75 ±0.24 8.95 ±0.19 7.87 ±0.83 6.23 ±0.70 7.32 ±1.05 8.86 ±0.21 8.05 ±0.32 7.76 ±0.27 8.86 ±0.20 8.84 ±0.12 8.65 ±0.20 9.09 ±0.29 8.86 ±0.17 8.15 ±0.52 9.05 ±0.27 0.29 ±0.00
- 53 -
Table (5) shows the activity of the individual radio nuclides in the Monazite sample: Radio nuclide
Ac-228 Ra-226 Ac-228 Pb-212 Ac-228 Pb-214 Ac-228 Ac-228 Pb-214 Anni. Tl-208 Bi-214 Bi-212 Ac-228 Ac-228 Ac-228 Ac-228 Tl-208 Ac-228 Ac-228 Bi-214 Bi-214 K-40 Bi-214 Ac-228 Tl-208* Bi-212 Ac-228 Bi-214 Bi-214 Bi-214 Tl-208* Bi-214 Tl-208
Photo peak Energy (keV) 129.17 185.79 208.86 238.43 269.84 294.82 327.56 337.95 351.5 510.39 582.88 609.02 726.94 755 772.07 794.61 835.35 860.19 910.81 968.61 1119.8 1407.07 1459.6 1508.03 1586.92 1591.05 1619.41 1629.27 1728.05 1762.91 1845.85 2100.79 2201.43 2609.81
Net count U02 16359 8265 29943 197388 24655 16523 19448 62810 26956 45870 107028 19887 20882 2776 3739 12141 3962 11343 62309 30403 3963 553 1188 247 2664 814 1777 1966 551 2584 393 4702 633 31035
± Net count
Emission Probabilit y
593 591 427 886 405 355 336 532 440 502 440 314 306 207 174 235 129 203 300 272 165 118 159 70 125 151 117 101 105 129 84 149 114 193
0.024472 0.035900 0.038836 0.433000 0.034314 0.185000 0.029526 0.112518 0.358000 not found 0.844843 0.447906 0.065790 0.010055 0.015029 0.043358 0.016758 0.124248 0.266000 0.161728 0.148000 0.028000 0.106700 0.021200 not found not found 0.014862 0.015960 0.028794 0.153600 0.020400 not found not found 0.991600
- 54 -
± Emission Probabilit y 0.001862 0.000000 0.000106 0.003464 0.000798 0.003000 0.001596 0.002660 0.005000 not found 0.006941 0.005000 0.000513 0.000532 0.000612 0.001064 0.001064 0.000992 0.006916 0.003192 0.002000 0.004000 0.001000 0.000400 not found not found 0.000320 0.000798 0.000000 0.002000 0.000400 not found not found 0.000000
Absolute efficiency
Specific Activity (kBq/kg)±
0.00424 0.00305 0.00275 0.00244 0.00218 0.00201 0.00183 0.00178 0.00172 0.00123 0.00109 0.00105 0.00089 0.00086 0.00085 0.00082 0.00079 0.00077 0.00073 0.00069 0.00060 0.00049 0.00048 0.00046 0.00044 0.00044 0.00043 0.00043 0.00041 0.00040 0.00039 0.00034 0.00033 0.00028
232.15±19.57 110.97±7.94 413.03±6.00 275.19±2.52 484.96±13.81 65.29±1.76 529.51±30.05 461.58±11.59 64.51±1.39 not found 171.27±1.57 62.45±1.21 523.72±8.68 471.38±43.10 433.42±26.80 500.65±15.64 442.21±31.55 175.33±3.44 473.68±12.53 401.84±8.71 65.24±2.86 59.12±15.21 34.45±4.63 37.13±10.60 not found not found 406.32±28.08 420.90±30.16 68.95±13.14 61.72±3.18 73.67±15.75 not found not found 163.57±1.02
Appendix III Gamma-ray spectroscopy analysis for the soil samples: Soil sample-375:
Table (6) Activity calculation: Radio nuclide
Ra-226 Pb-212 Pb-214 Ac-228 Pb-214 Ac-228 Tl-208 Tl-208 Bi-214 Cs-137 Bi-212 Ac-228 Tl-208 Ac-228 Bi-214 Ac-228 Bi-214 Bi-214 Bi-214 Bi-214 K-40 Bi-214 Bi-214 Bi-214 Tl-208
Photo peak Energy (keV) 185.94 238.57 295.20 338.31 352.01 463.95 511.26 583.72 609.88 662.31 727.99 795.74 861.42 912.11 935.07 965.61 1121.35 1239.10 1378.73 1408.99 1461.81 1730.38 1765.22 2204.02 2613.30
Emission probability
± Error in Emission probability
Absolute efficiency
Net counts ±
0.0359 0.4330 0.1812 0.1127 0.3510 0.0440 0.0813 0.3040 0.4660 0.8510 0.0658 0.0425 0.1242 0.2580 0.0300 0.0499 0.1470 0.0578 0.0410 0.0280 0.1067 0.0288 0.1510 0.0498 0.9990
0.003 0.003 0.009 0.04 0.017 0.011 0.029 0.009 0.04 0.0001 0.0011 0.000 0.000 0.004 0.01000 0.04 0.003 0.006 0.002 0.004 0.001 0.001 0.005 0.0025 0.004
0.00306 0.00244 0.00201 0.00178 0.00172 0.00134 0.00123 0.00109 0.00105 0.00097 0.00089 0.00082 0.00077 0.00073 0.00071 0.00069 0.00060 0.00055 0.00050 0.00049 0.00048 0.00041 0.00040 0.00033 0.00028
512±458 BLC 719±267 647±303 877±302 232±235 324±284 375±191 538±190 46320±280 132±144 58±152 BLC 333±139 42±119 169±105 216±125 BLC 106±91 52±78 785±148 BLC 171±86 51±65 161±82
BLC: Below Critical level BMDA: Below Minimum detectable Activity.
- 55 -
MDA ± MDA (Bq/kg) Soil-375 90±19 32±3 40±8 26±1 84±32 178±7 40±2 34±1 5±2 112±17 145±41 65±3 190±64 124±34 103±7 231±44 254±77 406±6 160±7 308±37 46±1
Activity± (Bq/kg) Soil-375 259±232 BMDA 110±41 179±84 81±28 219±221 181±159 63±32 61±22 3115±19 125±137 BMDA BMDA 99±41 BMDA 273±170 135±78 BMDA 287±245 BMDA 860±162 BMDA BMDA BMDA BMDA
250
200
0 2000 Tl-208, 2610.17keV
0 1000
150
100
Bi-214, 2202.55keV
Counts
Counts 600
400
2100
200
1100 1200
2200
2300
2400 1300
2500
300 400
1400
(1000-2000keV) and (2000-3400keV).
- 56 -
1500 EnergykeV
2600 2700 EnergykeV
500 EnergykeV
2800
1200
800
600 700
1600 1700
2900
3000
Bi-214, 768.38keV
3100
3200
Ac-228, 965.53keV
Bi-214, 934.44keV
Ac-228, 911.47keV
Ac-228, 795.18keV
Cs-137, 661.7keV
10000
Bi-214, 1764.21keV
Bi-212, 727.5keV
Bi-214, 609.39keV
Tl-208, 583.24keV
Tl-208+Annihilation, 510.8keV
Ac-228, 462.83keV
Pb-214, 351.67keV
Ac-228, 338.06keV
Pb-214, 294.87keV
Pb-212, 238.32keV
Ra-226, 185.64keV
6000
Bi-214, 1729.1keV
200
Bi-214, 1509.39keV
1000 K-40, 1460.86keV
100
Bi-214, 1408.47keV
0
Bi-214, 1377.99keV
2000 Kα &Kβ X-ray from lead
4000
Bi-214, 1238.39keV
Bi-214, 1120.5keV
Counts 12000 Spectrum of sample-375
8000
0 800 900 1000
1800 1900 2000
350
300
50
3300
Figure (1): Spectra of soil sample-375 was collected for 172800s the energy range (0- 1000keV),
3400
Soil sample-254:
Table (7) Activity calculation: Radio nuclide
Ra-226 Pb-212 Pb-214 Ac-228 Pb-214 Ac-228 Tl-208 Tl-208 Bi-214 Cs-137 Bi-212 Ac-228 Tl-208 Ac-228 Bi-214 Ac-228 Bi-214 Bi-214 Bi-214 Bi-214 K-40 Bi-214 Bi-214 Bi-214 Tl-208
Photo Emission ± Error in peak probability Emission Energy probability (keV) 185.94 0.0359 0.003 238.57 0.4330 0.003 295.20 0.1812 0.009 338.31 0.1127 0.04 352.01 0.3510 0.017 463.95 0.0440 0.011 511.26 0.0813 0.029 583.72 0.3040 0.009 609.88 0.4660 0.04 662.31 0.8510 0.0001 727.99 0.0658 0.0011 795.74 0.0425 0.000 861.42 0.1242 0.000 912.11 0.2580 0.004 935.07 0.0300 0.01000 965.61 0.0499 0.04 1121.35 0.1470 0.003 1239.10 0.0578 0.006 1378.73 0.0410 0.002 1408.99 0.0280 0.004 1461.81 0.1067 0.001 1730.38 0.0288 0.001 1765.22 0.1510 0.005 2204.02 0.0498 0.0025 2613.30 0.9990 0.004
Absolute efficiency
Net counts ±
0.00306 0.00244 0.00201 0.00178 0.00172 0.00134 0.00123 0.00109 0.00105 0.00097 0.00089 0.00082 0.00077 0.00073 0.00071 0.00069 0.00060 0.00055 0.00050 0.00049 0.00048 0.00041 0.00040 0.00033 0.00028
179±429 BLC 359±286 BLC 453±264 BLC BLC BLC 204±159 BLC BLC 153±123 73±127 BLC BLC BLC BLC 151±99 BLC 114±65 315±142 BLC BLC BLC BLC
BLC: Below Critical level BMDA: Below Minimum detectable Activity.
- 57 -
MDA ± MDA (Bq/kg) Soil-254 86±18 31±3 25±1 31±1 137±35 47±14 177±23 247±74 373±6 -
Activity± (Bq/kg) Soil-254 BMDA BMDA 49±39 BMDA 37±22 BMDA BMDA BMDA BMDA BMDA BMDA 218±176 BMDA BMDA BMDA BMDA BMDA 236±155 BMDA 413±237 BMDA BMDA BMDA BMDA BMDA
600
400
200
0 1000
100
0 2000
2100 1100 1200
2200
2300
2400
400
1300
2500
500
800
1400
300
250
2600
2700 EnergykeV
(1000-2000keV) and (2000-3400keV).
- 58 600
1500 EnergykeV
2800
700
1600 1700
2900
3000 Bi-214, 1847.11keV
300
Bi-214, 1763.7keV
200 Pb-214, 351.3keV
3100
800
1800
3200
Ac-228, 911.01keV
Ac-228, 964.47keV
Bi-214, 933.93keV
Ac-228, 794.64keV
Bi-212, 727.05keV
Cs-137, 661.43keV
Bi-214, 609.02keV
Tl-208+Annihilation, 510.43keV
Ac-228, 462.73keV
Ac-228, 337.69keV
700
Bi-214, 1728.69keV
100
Bi-214, 1508.84keV
1000
K-40, 1460.37keV
100 Pb-214, 294.52keV
Pb-212, 237.69keV
600
Bi-214, 1407.84keV
Counts 500
Bi-214, 1377.34keV
Bi-214, 1237.91keV
0
Bi-214, 1120.17keV
200 Kα &Kβ X-ray from lead
300
Tl-208, 2611.08keV
Counts
400
Bi-214, 2202keV
Counts
800 Spectrum of sample-254
0 EnergykeV
1200
900 1000
1900 2000
350
200
150
50
3300
Figure (2): Spectra of soil sample-254 was collected for 172800s the energy range (0- 1000keV),
3400
Soil sample-228:
Table (8) Activity calculation: Radio nuclide
Ra-226 Pb-212 Pb-214 Ac-228 Pb-214 Ac-228 Tl-208 Tl-208 Bi-214 Cs-137 Bi-212 Ac-228 Tl-208 Ac-228 Bi-214 Ac-228 Bi-214 Bi-214 Bi-214 Bi-214 K-40 Bi-214 Bi-214 Bi-214 Tl-208
Photo Emission ± Error in peak probability Emission Energy probability (keV) 185.94 0.0359 0.003 238.57 0.4330 0.003 295.20 0.1812 0.009 338.31 0.1127 0.04 352.01 0.3510 0.017 463.95 0.0440 0.011 511.26 0.0813 0.029 583.72 0.3040 0.009 609.88 0.4660 0.04 662.31 0.8510 0.0001 727.99 0.0658 0.0011 795.74 0.0425 0.000 861.42 0.1242 0.000 912.11 0.2580 0.004 935.07 0.0300 0.01000 965.61 0.0499 0.04 1121.35 0.1470 0.003 1239.10 0.0578 0.006 1378.73 0.0410 0.002 1408.99 0.0280 0.004 1461.81 0.1067 0.001 1730.38 0.0288 0.001 1765.22 0.1510 0.005 2204.02 0.0498 0.0025 2613.30 0.9990 0.004
Absolute efficiency
0.00306 0.00244 0.00201 0.00178 0.00172 0.00134 0.00123 0.00109 0.00105 0.00097 0.00089 0.00082 0.00077 0.00073 0.00071 0.00069 0.00060 0.00055 0.00050 0.00049 0.00048 0.00041 0.00040 0.00033 0.00028
BLC: Below Critical level BMDA: Below Minimum detectable Activity.
- 59 -
Net counts ±
MDA ± Activity± MDA (Bq/kg) (Bq/kg) Soil-228 Soil-228 841±476 114±29 443±251 BLC BMDA 1877±295 42±7 298±47 226±303 49±9 65±87 2378±281 34±2 228±27 184±218 104±48 180±214 585±262 195±8 339±152 BLC BMDA 1737±192 42±1 206±23 BLC BMDA BLC BMDA BLC BMDA 60±127 58±14 BMDA 114±148 67±3 BMDA 137±109 224±96 371±295 142±108 131±74 238±181 444±126 120±10 289±82 270±105 220±40 489±191 177±74 242±76 498±209 93±61 297±90 391±256 502±144 428±7 572±164 64±61 381±77 BMDA 314±88 181±8 300±84 283±62 364±83 1001±221 683±97 47±2 140±20
150
100
0 2000
Bi-214, 2204.45keV
Counts 300
250 Tl-208, 2613.91keV
Counts 1400
1200
1000
800
600
400
200
100
0 1000 1100
2100
1200
2200
2300
2400
1300
2500
1400
2600
2700
(1000-2000keV) and (2000-3400keV).
- 60 600
1500 EnergykeV
2800
700
1600 1700
2900
3000
Bi-214, 1848.63keV
500 EnergykeV
Bi-214, 1765.58keV
400
Bi-214, 1730.81keV
300
Bi-214, 1589.36keV
200
Bi-214, 1409.61keV
500 Ac-228, 338.39keV Pb-214, 352.07keV
3100
800
1800
3200
3300
Ac-228, 965.74keV
Bi-214, 935.18keV
Ac-228, 912.32keV
Ac-228, 795.89keV
Bi-214, 769.29keV
Bi-212, 728.07keV
Cs-137, 662.46keV
Bi-214, 610.01keV
Tl-208, 583.81keV
Tl-208+Annihilation, 511.36keV
Ac-228, 463.49keV
Pb-212, 238.59keV
Pb-214, 295.25keV
Ra-226, 185.89keV
2000
Bi-214, 1378.14keV
1000
Bi-214, 1239.43keV
K-40, 1462.14keV
0
Bi-214, 1121.61keV
1500
Kα &Kβ X-ray from lead
Counts 2500 Spectrum of Sample- 228 -
0 900 1000
1900 2000
350
200
50
EnergykeV
3400
Figure (3): Spectrum of soil sample-228 was collected for 172800s the energy range (0- 1000keV),
Soil sample-248:
Table (9) Activity calculation: Radio nuclide
Ra-226 Pb-212 Pb-214 Ac-228 Pb-214 Ac-228 Tl-208 Tl-208 Bi-214 Cs-137 Bi-212 Ac-228 Tl-208 Ac-228 Bi-214 Ac-228 Bi-214 Bi-214 Bi-214 Bi-214 K-40 Bi-214 Bi-214 Bi-214 Tl-208
Photo Emission ± Error in peak probability Emission Energy probability (keV) 185.94 0.0359 0.003 238.57 0.4330 0.003 295.20 0.1812 0.009 338.31 0.1127 0.04 352.01 0.3510 0.017 463.95 0.0440 0.011 511.26 0.0813 0.029 583.72 0.3040 0.009 609.88 0.4660 0.04 662.31 0.8510 0.0001 727.99 0.0658 0.0011 795.74 0.0425 0.000 861.42 0.1242 0.000 912.11 0.2580 0.004 935.07 0.0300 0.01000 965.61 0.0499 0.04 1121.35 0.1470 0.003 1239.10 0.0578 0.006 1378.73 0.0410 0.002 1408.99 0.0280 0.004 1461.81 0.1067 0.001 1730.38 0.0288 0.001 1765.22 0.1510 0.005 2204.02 0.0498 0.0025 2613.30 0.9990 0.004
Absolute efficiency
Net counts ±
0.00306 0.00244 0.00201 0.00178 0.00172 0.00134 0.00123 0.00109 0.00105 0.00097 0.00089 0.00082 0.00077 0.00073 0.00071 0.00069 0.00060 0.00055 0.00050 0.00049 0.00048 0.00041 0.00040 0.00033 0.00028
103±429 BLC BLC 120±313 173±274 69±135 BLC 191±199 BLC 138±129 BLC BLC 231±113 227±150 BLC 105±103 135±115 120±117 53±77 28±74 236±146 70±59
BLC: Below Critical level BMDA: Below Minimum detectable Activity.
- 61 -
BLC BLC BLC
MDA ± MDA (Bq/kg) Soil-248 88±19 44±8 26±1 81±19 40±2 6±2 54±13 64±3 111±59 100±7 200±30 230±41 216±95 392±6 338±76 -
Activity± (Bq/kg) Soil-248 BMDA BMDA BMDA BMDA BMDA BMDA BMDA BMDA BMDA 9±8 BMDA BMDA 128±63 64±42 BMDA 161±157 BMDA BMDA BMDA BMDA BMDA BMDA BMDA BMDA BMDA
0 1000
0 2000
600
400
150
100
2100
200
1100 1200
2200
2300
2400 1300
2500
500
800
1400 1500 EnergykeV
300
250
2600
2700
1000keV), (1000-2000keV) and (2000-3400keV).
- 62 -
600
2800
700
1600 1700
2900
3000 Bi-214, 1849.3keV
400
Bi-214, 1766.46keV
300
Bi-214, 1731.66keV
200
Bi-214, 1509.39keV
1000
K-40, 1462.86keV
100
Bi-214, 1409.81keV
500
Bi-214, 1379.83keV
0
Bi-214, 1240.22keV
1000
3100
800
1800
EnergykeV
3200
Figure (4): Spectrum of soil sample-248 was collected for 172800s the energy range (0-
3300
Ac-228, 966.52keV
Bi-214, 935.9keV
Ac-228, 912.99keV
Ac-228, 796.56keV
Bi-214, 770.01keV
Bi-212, 728.78keV
Cs-137, 662.94keV
Bi-214, 610.65keV
Tl-208, 584.43keV
Tl-208+Annihilation, 511.98keV
Ac-228, 464keV
Pb-212, 239.24keV
Pb-214, 352.71keV
Ac-228, 339.1keV
Pb-214, 295.91keV
Ra-226, 186.54keV
Kα &Kβ X-ray from lead
1500
Bi-214, 1122.21keV
Counts 2000
Tl-208, 2614.82keV
Counts
2500
Bi-214, 2205.05keV
Counts
3500 Spectrum of sample-248
3000
0 EnergykeV
1200
900 1000
1900 2000
350
200
50
3400
Soil sample-178:
Table (10) Activity calculation: Radio nuclide
Ra-226 Pb-212 Pb-214 Ac-228 Pb-214 Ac-228 Tl-208 Tl-208 Bi-214 Cs-137 Bi-212 Ac-228 Tl-208 Ac-228 Bi-214 Ac-228 Bi-214 Bi-214 Bi-214 Bi-214 K-40 Bi-214 Bi-214 Bi-214 Tl-208
Photo Emission ± Error in peak probability Emission Energy probability (keV) 185.94 0.0359 0.003 238.57 0.4330 0.003 295.20 0.1812 0.009 338.31 0.1127 0.04 352.01 0.3510 0.017 463.95 0.0440 0.011 511.26 0.0813 0.029 583.72 0.3040 0.009 609.88 0.4660 0.04 662.31 0.8510 0.0001 727.99 0.0658 0.0011 795.74 0.0425 0.000 861.42 0.1242 0.000 912.11 0.2580 0.004 935.07 0.0300 0.01000 965.61 0.0499 0.04 1121.35 0.1470 0.003 1239.10 0.0578 0.006 1378.73 0.0410 0.002 1408.99 0.0280 0.004 1461.81 0.1067 0.001 1730.38 0.0288 0.001 1765.22 0.1510 0.005 2204.02 0.0498 0.0025 2613.30 0.9990 0.004
Absolute efficiency
Net counts ±
0.00306 0.00244 0.00201 0.00178 0.00172 0.00134 0.00123 0.00109 0.00105 0.00097 0.00089 0.00082 0.00077 0.00073 0.00071 0.00069 0.00060 0.00055 0.00050 0.00049 0.00048 0.00041 0.00040 0.00033 0.00028
BLC 660±348 257±261 218±252 630±212 199±177 236±265 178±206 BLC 277±151 BLC 55±115 383±95 1004±153 BLC 271±70 708±125 358±99 381±58 184±58 470±148 93±58 138±86 58±52 583±96
BLC: Below Critical level BMDA: Below Minimum detectable Activity.
- 63 -
MDA ± MDA (Bq/kg) Soil-178 15±1 31±3 42±7 25±1 95±33 167±7 39±2 6±4 133± 33 49±0 60±6 113±0 96±15 177± 36 226±0 234±0 381±7 306±99 143±8 277±30 42 ±2
Activity± (Bq/kg) Soil-178 BMDA 32±17 36±37 55±64 53±18 172±154 BMDA BMDA BMDA 17±9 BMDA BMDA 206±51 273±42 BMDA 401±104 408±72 573±159 947±144 683±215 473±149 404±253 BMDA BMDA 106±18
0 2000 0 1000
150
100
2100
600
400
200
1100 1200
2200
2300
2400 300 400
800
1300 1400
300
250
2500
2600
2700 EnergykeV
1000keV), (1000-2000keV) and (2000-3400keV).
- 64 -
500 EnergykeV
1500 EnergykeV
2800
1200 600 700
1600 1700
2900
3000
Bi-214, 1763.77keV
3100
3200
Figure (5): Spectrum of soil sample-178 was collected for 172800s the energy range (0-
3300
Ac-228, 911.12keV
Ac-228, 964.49keV
Bi-214, 934.03keV
Ac-228, 794.92keV
Bi-214, 768.2keV
Bi-214, 609.05keV
Tl-208, 582.91keV
Tl-208+Annihilation, 510.49keV
700
Bi-212, 727.18keV
Cs-137, 661.42keV
Ac-228, 462.46keV
100
Bi-214, 1729.02keV
1000
K-40, 1460.51keV
200 Ac-228, 337.67keV Pb-214, 351.31keV
Pb-212, 237.97keV
600
Bi-214, 1407.62keV
100 Pb-214, 294.56keV
200
Bi-214, 1377.5keV
0
Bi-214, 1237.92keV
Kα &Kβ X-ray from lead
300
Bi-214, 1120.34keV
Counts 400
Tl-208, 2611.4keV
Counts 500
Bi-214, 2202.29keV
Counts 800 Spectrum of sample-178
0 800 900 1000
1800 1900 2000
350
200
50
3400
