GAMMA RAY SPECTROMETRIC ANALYSIS OF THE NATURALLY OCCURRING RADIONUCLIDES IN SOILS COLLECTED ALONG THE SHORES OF LAKE VICTORIA, MIGORI COUNTY, KENYA

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1 GAMMA RAY SPECTROMETRIC ANALYSIS OF THE NATURALLY OCCURRING RADIONUCLIDES IN SOILS COLLECTED ALONG THE SHORES OF LAKE VICTORIA, MIGORI COUNTY, KENYA DANIEL OKELO ELIJAH [B.Ed. (Sc)] I56/CE/11179/2007 A thesis submitted in partial fulfillment of the requirements for the award of the Degree of Master of Science in the School of Pure and Applied Sciences of Kenyatta University September, 2015

2 ii DECLARATION This thesis is my original work and has not been presented for a degree in any other University or any other award. Daniel Okelo Elijah (I56/CE/11179/2007) Signature Date Department of Physics, Kenyatta University, P.O BOX 43844, Nairobi, Kenya. This thesis has been submitted with our approval as University Supervisors Dr. N. O. Hashim Department of Physics, Signature Date Kenyatta University, P.O BOX 43844, Nairobi, Kenya. Dr. A. S. Merenga Department of Physics, Signature Date Kenyatta University, P.O BOX Nairobi, Kenya.

3 iii DEDICATION This thesis is dedicated to my beloved wife Judith, our son Albert and my mother Hana.

4 iv ACKNOWLEDGEMENTS I am grateful to my research supervisors, Dr. N.O. Hashim and Dr. A. S. Merenga for their guidance, corrections and meaningful suggestions throughout this work. Their incredible attention, physics explanations and advice are highly appreciated. I wish also to extend my sincere gratitude to the chairman and staff of the physics department for their continued cooperation during this time. My appreciation also goes to my employer the Teacher s Service Commission for granting me a study leave that enabled me to have ample time for sample collection, preparation and data collection. I would also like to thank the National Council of Science and Technology for granting me a permit to conduct the research. I would like to thank my beloved wife, Judith for her encouragement to undertake this work, without her love, support and patience I would have done nothing. I will never forget the help that Paul Omollo provided in sample collection. I am also grateful to my uncle Eliud Onyango, Cousin Brian Odhiambo and friends: Caroline Owili, Doreen Naa, for all the support they accorded me during this time. Above all I owe my creator the greatest appreciation for if it were not for him where could I have been. He is the source of my strength and inspiration. MAY HIS NAME BE GLORIFIED

5 v TABLE OF CONTENTS TITLE PAGE i DECLARATION.ii DEDICATION...iii ACKNOWLEDGEMENTS iv LIST OF TABLES viii LIST OF ABBREVIATIONS AND ACRONYMS xi ABSTRACT...xii CHAPTER ONE... 1 INTRODUCTION Background to the study Sources of ionizing radiation Statement of research problem Hypothesis Objectives General objective Specific objectives Rationale... 5 CHAPTER TWO... 6 LITERATURE REVIEW Lake Victoria and its catchment Related studies on natural radioactivity... 7 CHAPTER THREE THEORETICAL BACKGROUND OF GAMMA RAY SPECTROMETRY Background information on gamma radiation Photoelectric effect Compton effect Pair production Relative predominance of the interaction processes The radioactive decay law... 16

6 vi 3.4 Secular equilibrium Gamma-Ray emission Relationship between various dosimetric quantities Energy fluence and kerma Fluence and dose Kerma and dose Collision kerma and exposure Ionizing and non-ionizing radiation Ionizing radiation and human health Gamma-Ray spectrometry Mechanism of NaI(Tl) detector CHAPTER FOUR MATERIALS AND METHODS Sampling and sample preparation NaI(Tl) Gamma- Ray spectrometer Standard samples Experimental procedures Energy calibration of the NaI(Tl) detector Energy resolution of NaI(Tl) detector Background determination Detector efficiency Minimum detection limit Analysis of samples for activity concentration Area under the peaks Spectral data decomposition Calculation of radioactivity Assessment of radiation hazards Radium equivalent activity External radiation hazard index(hex) Internal hazard index (Hin)... 41

7 vii Absorbed gamma radiation dose rate Annual effective dose Dose to risk conversion CHAPTER FIVE RESULTS AND DISCUSSION Activity concentration of natural radionuclides Human exposure to gamma radiation Radiological impact CHAPTER SIX CONCLUSION AND RECOMMENDATIONS Conclusions Recommendations REFERENCES APPENDICES 61 Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix Appendix

8 viii LIST OF TABLES Table 4.1: Calibration equation fit parameters...29 Table 4.2: Energy resolution fit parameters...31 Table 4.3: Emission probabilities, detector efficiency and minimum detection limit values using RGMIX measured in this work...32 Table 4.4: Gaussian fit parameters of 40 K-1460keV photopeak for a soil sample...36 Table 4.5: Gaussian fit parameters of a gamma-ray photopeak of 1460keV of 40 K in a soil sample...39 Table 5.1: Average activity concentrations of radionuclides sand along the lake shore compared to values obtained from other parts in Kenya.44 Table 5.2: Activity concentration of the sand samples measured in this study compared to world average.45 Table A1: Activity concentration of the natural radionuclides 40 K, 238 U and 232 Th measured in this work.61 Table A2: The annual effective dose rate (EFDR) and absorbed dose rate measured in this work...62 Table A3: World average external exposure rates calculated from the average radionuclides in soil (UNSCEAR, 2000). 63 Table A4: Gamma lines emission probabilities of the IAEA reference materials (IAEA, 1992) 64 Table A5: Dose limits as recommended by the ICRP (Leo, 1994) 65

9 ix LIST OF FIGURES Figure 1.1: Sources and distribution of average radiation exposure to the world population (WHO, 2013)...1 Figure 3.1: Figure 3.2: Figure 3.3: Figure 3.4: Figure3.5: Figure 3.6: Figure 3.7: Figure 4.1: Figure 4.2: Figure 4.3: Figure 4.4: Figure 4.5: Figure 4.6 Figure 4.7: Figure 4.8: Figure 4.9: Figure 4.10: Figure 4.11 Schematic representation of photoelectric effect...13 Schematic representation of Compton scattering...13 Schematic representation of pair production.15 Schematic representation of the three major types of gamma-ray interactions..16 A Secular equilibrium between 230 Th and daughter 226 Ra.18 Schematic representation 137 Cs decay scheme (Gilmore, 2008) 19 Schematic representation of photomultiplier tube.23 Map of Kenya showing Lake Victoria and the sampling points 26 Schematic diagram of the NaI (TI) gamma-ray spectrometer used in this work 27 Energy calibration of the NaI (Tl) detector 29 Shows the Gaussian fit from which energy resolution for the NaI (TI) detector was calculated in this study.30 A Typical Gamma Ray spectrum of the background radiation measured in this study 33 A typical gamma ray spectrum of a sample measured in this study...33 A typical gamma ray spectrum of the thorium standard (RGTH-1) after background subtraction 34 A typical gamma-ray spectrum of the Uranium standard (RGU-1) after background subtraction...34 A typical gamma-ray spectrum of potassium standard (RGK-1) after background subtraction 35 A typical gamma-ray spectrum of RGMIX measured in this study after background subtraction...35 A Gaussian curve fitting of 40 K-146keV photopeak for soil sample 36

10 x Figure 4.12 Figure 5.1: Figure 5.2: Figure 5.3: Figure 5.4: Figure 5.5: Figure 5.6: Figure 5.7: Figure 5.8: Figure 5.9: Figure 5.10: Figure A1: Figure A2: A Gamma photopeak of 1460keV of 40 K in a soil sample (a) before spectrum decomposition and (b) after spectrum decomposition 38 Activity concentrations in all the sampling points (concentration values of K have been divided by 5)...46 Frequency distribution curve of activity concentration of 238 U in sand samples..47 Frequency distribution curve of activity concentration of 40 K in sand samples..48 Frequency distribution curve of activity concentration of 232 Th in sand samples..48 Regression plot showing correlation between activity concentration of 238 U and 232 Th Regression plot showing correlation between activity concentration of uranium 238 U and 40 K 49 Regression plot showing correlation between activity concentration of 232 Th and 40 K...50 Thorium versus Uranium Ratio in different sand sampling points..50 Absorbed dose rate versus sand sampling points.52 Effective dose rates versus sand sampling points...52 Uranium-235 decay series. 66 Thorium-232 decay series..67 Figure A3 (a): Measurement of masses of the samples.68 Figure A3 (b): Placing of the sample in the detector system.68 Figure A4 (a): Spectral data acquisition Figure A4 (b): Reading and analyzing Spectral data

11 xi LIST OF ABBREVIATIONS AND ACRONYMS AEA EPA FWHM Hex Hin IARC ICRP Kerma LET MCA NaI (TI) NEMA NORM PMT Raeq TAP UNSCEAR International Atomic Energy Agency Environmental Protection Agency Full Width at Half Maximum External Hazard index Internal Hazard index International Agency for Research on Cancer International Commission on Radiation Protection Kinetic energy per unit mass Linear Energy Transfer Multichannel Analyzer Thallium Activated Sodium Iodide Detector National Environmental Management Naturally Occurring Radioactive Materials Photo-multiplier tube Radium equivalent Total Absorption Peak United Nations Scientific Committee on Effects of Atomic Radiation

12 xii ABSTRACT Natural radiation is present in all human environments: rocks, soil, water, air, food etc at varying concentrations. The major radionuclides responsible for the natural terrestrial background radiation are 40 K and the radioisotopes in the decay series of 238 U and 232 Th.These radionuclides pose exposure risks due to their radiation which could lead to health related problems like cancer to the people exposed. There is therefore a growing concern on the health risks associated with such exposure to natural sources of radiation in the place of work. In this research sand samples from the sand mines at the estuaries of River Kuja, River Migori and other small streams in Migori County were analyzed using NaI (TI) detector together with a computer based gamma ray spectrometer. The average concentrations of 238 U, 232Th and 40 K in the samples analyzed in this study are 64.5±3.3 BqKg -1,146.0±4.4 BqKg -1 and ±43.3 BqKg -1 respectively. It is observed that the activity concentrations are above the world average values. As a measure of radiation hazard to the general public, the absorbed dose rate in air at a height of 1m above the ground surface was estimated. The calculated radiation absorbed dose rate from the different sampling sites ranges from 123±2.4 ngyh -1 to 236±4.8 ngyh -1 with an average of 171.2±3.4 ngyh -1. The mean value is higher than the world average of 60 ngyh -1.The annual effective dose rates were calculated for human exposure to the gamma radiations from the radionuclides; 238 U, 232 Th and 40 K in the sand samples and were found to range from 0.302±0.006 msvy -1 to 0.579±0.012 msvy -1, which is below the ICRP limit of 1mSvy -1 for members of the general public. The internal and external hazard indices were calculated and their average found to be 0.99±0.02 msvy -1 and 1.17±0.02 msvy -1 respectively, with external hazard index being more than unity, hence slightly exceeding the permissible limits set by the International Commission on Radiation Protection (ICRP).

13 1 CHAPTER ONE INTRODUCTION 1.1 Background to the study Human beings are exposed to natural background radiation everyday from the ground, building materials, air, food, the universe and even elements in their own body. In addition to natural background radiation people are also exposed to, low and high-linear energy transfer radiation from the man made sources such as X- rays equipment and radioactive materials used in medicine, research and industry. The figure below shows a pie chart showing the distribution of average radiation exposure to the world population. Earth gamma radiation 15% Cosmic rays 13% Radon 43% Medical exposure 20% Food/water 8% Others 1% Figure 1.1 Sources and distribution of average radiation exposure to the world population (WHO, 2013). Ionizing radiation occur naturally either as terrestrial or extra terrestrial origin in form of high energy cosmic ray particles whose sources are galactic and extra galactic. Terrestrial origin are due to the presence of naturally occurring

14 2 radionuclides, mainly Potassium ( 40 K) Uranium ( 238 U ) and Thorium ( 232 Th) and they basically occur in geological materials making the rocks and soil (UNSCEAR, 2000). Exposure to high level of radiation can lead to health problems like abnormal cell growth. It is therefore, important to measure the concentration of the radionuclides in the soil and asses the possible radiological hazard to human health and to develop standards and guidelines for the use and management of these materials (Turhan et al., 2008). Ionizing radiation is a flux of subatomic particles like photons,neutrons,nuclei etc that cause ionization of atoms of the medium through which the particles pass. In order for ionization to occur a certain amount of energy must be transferred to the atom. By the law of conservation of energy, this amount of energy is equal to the decrease of kinetic energy of the particle that causes ionization. Ionization is only possible if the energy of the incident particles exceeds the ionization of the atom which is usually of the order of 10 electron volts. In passing through matter, charged particles lose some kinetic energy by excitation of bound electrons and by ionization. The energy loss of charged particles passing through matter depends on the particle velocity, its charge and the properties of the transverse material. To ionize atoms in human tissue an energy of approximately 30 electron volts and above is required.radiation with frequencies below 10 6 Hertz corresponding to 30 electron volts are termed as nonionizing. Biological processes in the human body are influenced by non-ionizing radiation. Low frequency radiation affects humans via its electric and magnetic fields.

15 3 High doses of radiation can be harmful or even fatal. The damage caused by exposure to a radiation is determined by the type of radiation, the duration of exposure and the part of the body that is exposed. The interaction of ionizing radiation with the human body arises from either external sources or internal contamination which can lead to biological effects (UNSCEAR, 2000). Radiation effects can lead to death of a cell, impairment in the natural functioning of the cell leading to somatic effects such as cancer and a permanent alteration of the cell which is transmitted to later generation i.e. genetic effect. Biological effects can also be considered in terms of stochastic effect and non stochastic effects. Stochastic effects increases with increase in dose rate (ICRP, 2005) while non stochastic effect has a threshold below which there is no effect. Human exposure to radiation also reduces the immunity of the person exposed (UNSCEAR, 2006). 1.2 Sources of ionizing radiation Exposure to ionizing radiation arises from sources such as medical diagnostic and therapeutic procedures; nuclear weapon testing; radon and other natural background radiation; nuclear electricity generation; accidents such as the one at Chernobyl in 1986; and occupations that increase exposure to artificial or natural sources of radiation (UNSCEAR, 2010). Ionizing radiation in our environment can occur either naturally or can be produced artificially, through human activity. The effects of artificial or naturally occurring radiation are the same (UNSCEAR, 2010). Naturally occurring radionuclide materials (NORM) existed since the creation of the earth. Radionuclides of uranium, thorium and potassium are

16 4 relatively abundant in rocks and soils. The gamma radiation emitted from these radionuclides gives to human beings a radiation dose. 1.3 Statement of research problem Activities involving naturally occurring radioactive materials are potential sources of radiation exposure to workers and members of the public in general (Mustapha et al., 2007). Exposure to ionizing radiation is generally undesirable at all levels to the public. No harmful effects are presently proven for very low exposure (UNSCEAR, 1993). The sandy soil has been used for construction of different structures used by human beings with little attention being paid to the dangers that the miners and the other users of the sandy soil could be exposed to in terms of radiation exposure. The medical data has also been reviewed and crystalline silica has been classified as carcinogenic to humans (IARC, 2012). There is therefore a growing need to investigate the health risks associated with such exposure due to natural sources of radiation in these areas. 1.4 Hypothesis The hypothesis of this research is that the activity concentration of 226 Ra, 232 Th and 40 K and the average for both external and internal hazard indices are above the limits recommended by I C R P (2000).

17 5 1.5 Objectives General objective To determine the activity contractions of naturally occurring radionuclides ( 238 U, 232 Th and 40 K) in soils collected from the shore and use it to evaluate the radiological hazard associated with them Specific objectives To determine the radioactivity concentrations of naturally occurring radionuclides found in the soil samples from the shore. To calculate the radiological parameters; radium equivalent dose, hazard indices and absorbed dose rate due to the soil samples. To estimate the radiological hazard of human exposure to gamma rays from the soil sample. 1.6 Rationale Studies on the background natural radiation are of great importance because it is the main source of exposure for humankind; Gamma ray spectrometry provides a reliable method for measuring natural radiation from naturally occurring radionuclides. This study is important in that, the data obtained from such studies may be used locally to establish if and where controls are needed and they also enrich the global data bank enabling accurate estimation of global average values of radiometric and dosimetric quantities.

18 6 CHAPTER TWO LITERATURE REVIEW 2.1 Lake Victoria and its catchment Lake Victoria is the second largest fresh water lake in the world. The lake extends between latitudes ' N, 'S and between longitudes 'E, 'E. It has a surface altitude of 1,135 meters with an average depth of 40m. The lake is shared by the three east African countries namely Uganda, Tanzania and Kenya. The lake is fed by several rivers and small streams. The only outlet for the lake is the Victoria Nile west of Jinja making it a possible sink for pollution. In this research the shoreline between Muhuru bay and Karungu bay in Migori county which lies 1 0 4'0'' South and '0'' North is considered due to the economic activities on the main land like agriculture, mining and sugarcane industries whose wastes could be carried down to the lake by river Kuja, Migori and other small streams to course pollution to the lake and its environs. The sandy soil along the shore in this area of study is also mined and used for construction of buildings inhabited by man. Apart from the sources of pollution mentioned above the soil could also be as a result of weathering of igneous granite rocks from the hills surrounding the shore and this could be a source of naturally occurring radionuclides leading to exposure to the miners and the general public. In this research gamma ray spectrometer was used to measure activity concentration of sandy soil samples collected along the shores of Lake Victoria. The estuaries of river Kuja and river Migori and other small streams found in Migori County are associated with large deposits of sandy soil which may contain the weathered

19 7 particles of the igneous-granite rocks from the main land which is associated with radioactive nuclides. Exposure to high level of radiation can lead to health problems like cancer. The sandy soil mined from these areas are mainly used as building material both in the rural and in the urban centers. It is therefore important to measure the concentration of the radionuclides in the sandy soil and asses the possible radiological hazard to human health and to develop standards and guidelines for the use and management of these materials (Turhan et al., 2008). 2.2 Related studies on natural radioactivity Studies on the background to natural radiation are of great importance because it is the main source of exposure for humankind. The study of the distribution of primordial radionuclides allows the understanding of the radiological implication of these elements due to the gamma ray exposure to body and irradiation of lung tissue from inhalation of radon and its daughters (Singh et al., 2005). In particular it is important to asses the radiation hazards arising due to the use of soil or sand in the construction of dwellings. Therefore the assessment of gamma ray radiation dose from natural sources is of importance as natural radiation is the largest contributor to the external dose of the world population (UNSCEAR, 2000). These dose rates vary depending upon the concentration of the natural radionuclides 226 Ra, 232 Th and 40 K present in soil, sand, and rock, which in turn depend upon the local geology of each region in the world. There are several studies that have been carried out to access the dangers of human exposure to radiations from naturally occurring radionuclides in the

20 8 environment. In Cyprus, a survey was carried out to determine activity concentration levels and associated dose rates from the naturally occurring radionuclides 232 Th, 238 U and 40 K in the various geological formations by means of high-resolution gamma ray spectrometry. From the measured spectra, activity concentration were found to be 232 Th (range from 1.0x10-2 to 39.8 Bqkg -1 ), 238 U (from 1.0x10-2 to 39.3 Bqkg -1 ) and 40 K (from 4.0x10-2 to Bqkg -1 ). Gamma absorbed dose rates in air were calculated to be in the range of 1.1x10-2 to 51.3 ngyh -1 with an overall mean of 8.7 ngyh -1 which was below the world average of 60 ngyh -1. Effective dose rates equivalent to population were calculated to be between 1.3x10-2 and 62.9 µsvy -1 with a mean of 10.7 µsvy -1 (Tzortzis and Tsertos, 2004). In Nigeria exposure to workers and villagers in and around some quarry sites in Ogun state was done using radiation detection methods. The results obtained from the study show that annual exposure rate was found to be 49.1 µsvy -1 which is below the world average of 70 µsvy -1, but recommended that workers at quarry sites should always put on masks to reduce the amount of radioactive inhalation (Odunaike et al., 2008). A study on the activity concentration and the gamma absorbed dose of the primordial naturally occurring radionuclides was done for sand samples collected from the Baoji Weihe sands park, China using γ-ray spectrometry. The natural radioactivity concentration of sand ranged from 10.2 to 38.3 Bqkg -1 for 226 Ra, 27.0 to 48.8 Bqkg -1 for 232 Th and to 1,126.7 Bqkg -1 for 40 K with mean value of 22.1, 39.0 and Bqkg -1 respectively. The radium equivalent activity values of all sand samples were lower than the limit of 370 Bqkg -1. The mean outdoor air absorbed dose rate was 69.6 ngyh -1 and the

21 9 corresponding outdoor effective dose rate was msvy -1 ( Xinwei and Zang, 2006). A study on the distribution of natural radionuclides concentrations in sediment samples in Didim and Izmin Bayin Turkey has been done. The results showed that the concentrations of activity in the sediment samples were 9±0.6Bqkg -1 to 12±0.7Bqkg -1,7±0.4Bqkg -1 to 16±1.0Bqkg -1,6±0.3Bqkg -1 to 16±1.0Bqkg -1 and 250±13Bqkg -1 to 665±33Bqkg -1 (Akozcan, 2012) for 226 Ra, 238 U, 232 Th and 40 K, respectively which were in the same order as international levels. In Kenya, various studies have been done to asses the level of human exposure to ionizing radiation. Study on natural radioactivity in some building materials in Kenya and the contribution to the indoor external doses has been done. Typical activity concentration encountered was in the range of; 50 to 1500 Bqkg -1 for 40 K, 5 to 200 Bqkg -1 for 226 Ra; and 5 to 300 Bqkg -1 for 232 Th (Mustapha et al, 1997). The concentration levels of 238 U, 232Th and 40 K in Mombasa, Malindi and Gazi along the coast was measured and found that Mombasa had the highest of 22.8±1.8 Bqkg -1 for 238 U, 26.2±1.7 Bqkg -1 for 232 Th and 479.8±24.2 Bqkg -1 for 40 K. The effective dose rate in Mombasa was found to have a mean of 0.12±0.01 msvy -1 (Hashim, 2001). Radioactivity and elemental concentration in Mombasa island, the north and south coast Kenya has been done (Hashim, 2001). The dose rates obtained were below the ICRP (1991) acceptable dose of 1 msvy -1. Natural radioactivity in some building materials in Kenya has been done (Mustapha et al., 1997). The indoor external doses were found to be average in comparison with other reports.

22 10 The contribution of radiation absorbed dose in dwelling places has been studied (Chege, 2007). The contribution of radon in some dwelling places was found to be above EPA recommended level of 148 Bqm -3. Soil samples from Mrima hills Kenya has been studied (Kebwaro, 2009). The average absorbed dose rate was found to be ngyh -1 which is higher than the world average of 60 ngyh -1. Activity concentration and radiation exposure level in soapstone quarries in Kisii has been studied (Kinyua et al., 2011).The internal and external hazard indices (1.03mSvy -1 and 1.27mSvy -1 ) were found to be more than unity thus exceeding the permissible limits set by ICRP for members of the public and annual effective dose rate was found to be 0.44 msvy -1 and less than the 1 msvy -1 the limit set by ICRP for the members of the public.

23 11 CHAPTER THREE THEORETICAL BACKGROUND OF GAMMA RAY SPECTROMETRY 3.1 Background information on gamma radiation Gamma rays are energetic photons with short wavelengths of the order m. After a decay reaction the nucleus is often in an excited state. Rather than emitting another beta or alpha particle, this energy is lost by emitting a pulse of electromagnetic radiation called gamma ray. Gamma rays interact with a material by colliding with the electrons in the shells of the atoms. They lose their energy slowly in a material, being able to travel significant distant before stopping. Depending on their initial energy, gamma rays can travel one to hundred of meters in air and can go right through people. Gammas rays interact with matter by the following three major processes: photoelectric effect, Compton Effect and pair production Photoelectric effect In the photoelectric effect the photon interacts with a tightly bound orbital electron of an attenuator and disappears, while the orbital electron is ejected from the atom as a photoelectron with a kinetic energy, Ee, given by the equation 3.1 (Gilmore, 2008); Ee=Eγ-Eb. (3.1)

24 12 where Eγ is the gamma-ray energy and Eb is the energy binding the electron in its shell. The atom is left in an excited state with an excess energy of Eb recovers its equilibrium in one of two ways. The atom may de-excite by redistribution of the excitation energy between the remaining electrons in the atom which results in the release of further electrons from the atom. Alternatively, the vacancy left by the ejection of the photoelectron may be filled by a higher energy electron falling into it with the emission of characteristic X-ray called X-ray fluorescence (Gilmore, 2008). The energy level from which the electron is ejected depends upon the energy of the gamma-ray. The most likely to be ejected is a K-electron. If sufficient energy is not available to eject a K-electron, then L or M electrons will be ejected instead. This gives rise to discontinuities in the photoelectric absorption curves. These absorption edges occur at the binding energies corresponding to the electron shells. The atomic attenuation coefficient for the photoelectric effect aτ depends on the atomic number Z of the absorber and energy hυ as shown by equation 3.2 (Podgorsak, 2005); Z 4 aτ α (hυ) (3.2) 3 while the mass attenuation coefficient for the photoelectric effect τm depends on the atomic number Z of the attenuator and energy hυ as shown below (Podgorsak, 2005). τmα ( Z hυ ) (3.3)

25 13 where Z is the atomic number of the attenuator and hυ is the photon energy Photoelectron nucleus Incident Photon hυ Figure 3.1 Schematic representation of photoelectric effect Compton Effect The Compton Effect (incoherent scattering) represents a photon interaction with an essentially free and stationary orbital electron. In this interaction the incident photon energy is much larger than the binding energy of the orbital electron. The photon loses part of its energy to the recoil electron and is scattered as photon Eγ through a scattering angle Ө while the angle between the incident photon and the direction of the recoil electron is represented by angle Ф (Figure 3.2). Recoil electron E e Incident gamma ray E γ =hυ Ф Ө Figure 3.2: Schematic representation of Compton scattering Scattered photon, E=hʋʹ

26 14 Considering energy and momentum conservation, the ratio of scattered energy (E'γ) to incident photon energy (Eγ) is given by equation 3.4( Grupen, 1998); E γ E γ = I... (3.4) 1+ε(1 CosΘ γ ) where Θ γ is the scattering angle of the photon with respect to the original direction and ε = E γ m o c 2. The cross section of Compton scattering is approximated at high energies by equation 3.5 (Grupen, 1998); In ε σ c = Z.... (3.5) ε Pair production This interaction involves the conversion of a photon into an electron positron pair in the Coulomb field of an atomic nucleus. In order for pair production to take place the gamma-ray must carry energy at least equivalent to the combined rest mass of the two particles-511kev each. The net energy absorbed within the detector due to this process is given by equation 3.6 (Gilmore, 2008) E e = E γ (3.6) where the energies are expressed in kev The cross section for the interaction, depends on Z is given by equation 3.7 (Gilmore, 2008);

27 15 к α Z 2. (3.7) The figure below shows how a photon interacts with the nucleus of an atom to produce an electron positron pair e - Electron Incident gamma photon nucleus hυ Positron e + Figure 3.3: Schematic representation of pair production 3.2 Relative predominance of the interaction processes The probability for a photon to undergo any one of the various interactions phenomena with an attenuator depends on the energy of the incident photon and the atomic number Z of the attenuating material. Generally, the photoelectric effect predominates at low photon energies, the Compton Effect at intermediate energies and pair production at high energies. The interactions can be characterized as shown in figure 3.4.

28 16 Figure 3.4: Schematic representation of the three major types of gamma-ray interactions (Knoll, 1989) 3.3 The radioactive decay law The radioactive decay is a spontaneous change within the nucleus of an atom which results in the emission of particles or electromagnetic radiation (Gilmore, 2008). The rate of decay or transformation of a radionuclide is described by its activity. The unit of activity is the Becquerel (Bq), defined as one disintegration per second. The activity of a pure radionuclide decreases exponentially with time. If N represents the number of atoms of a radionuclide in a sample at any given time, then the change dn is the number during a short time dt, is proportional to N and to dt. If λ is taken as the constant of proportionality then; dn = -λn dt..... (3.8) where the negative sign denotes decrease in N as time increases and λ is the decay constant

29 aaaaaaa 17 If the number of atoms present at time t=0 is No on integration equation (3.8) becomes; N = Noe -λt (3.9) 3.4 Secular equilibrium The concentration of the various daughter radionuclides can be approximated if a condition of secular equilibrium is assumed for naturally occurring radionuclides If the half-life of the parent is much larger than the half-life of the daughter, λ1 << λ2, the decay products emit radiation very fast and the parent decays at a constant rate all the times (t), then e λt 1 which when substituted into equation 3.9 gives equation 4.0; N(t) N0 λ 1 λ 2 (1 e λ 2t ) (4.0) equation 4.0 gives a condition of secular equilibrium where the daughter and the parent decay at the same rate, λ1n2 = λ2n1 and therefore A 2 A 1 1, this is shown in figure 3.5.

30 Activity 18 Time in years Figure 3.5: A secular equilibrium between 230 Th and daughter 226 Ra 3.5 Gamma-ray emission A radioactive decay can lead to emission of one or more photons from the excited states of daughter nuclei in form of α, β- or β+. Transitions that result in gamma emission leave Z and A unchanged and are called isomeric and nuclides in the initial and final states are called isomers (Turner, 2007). Simple case of single beta-emission radionuclide is that of 137 Cs, where some beta decays go directly to ground state of 137 Ba and most go to an excited nuclear state of 137 Ba. The gamma radiation is released as that excited state de-excites and drops to the ground state by releasing energy of 661.7keV.The figure below shows typical gamma ray decay

31 19 Figure 3.6: Schematic representation 137 Cs decay scheme (Gilmore, 2008) 3.6 Relationship between various dosimetric quantities Energy fluence and kerma The energy transferred to electrons by photons can be used in two distinct ways: through collision interactions which can either be soft collision or hard collisions and through radioactive interactions which can either be bremsstrahlung or electron-positron annihilation. The total kerma is therefore divided into two components the collision kerma Kcol and the radiative kerma Krad Fluence and dose When radiative photons escape the volume of interest and secondary electrons are absorbed instantly, the absorbed dose to medium Dmed is related to the electron fluence Фmed as follows; Dmed = Фmed ( S col )... (3.10) ρ med

32 20 Where (Scol/ρ)med is the unrestricted mass collision stopping power of the medium at the energy of the electron Kerma and dose The transfer of energy from the photon beam to charged particles at a particular location does not lead to the absorption of energy by the medium at the same location. This is because of the finite range of the secondary electrons released through photon interactions. Since radiative photons mostly escape from the volume of interest, absorbed dose can be related to collision kerma. The ratio of dose and collision kerma is given by the following equation; D Β =.... (3.11) K col Collision Kerma and exposure Exposure X is the quotient of dq by dm, where dq is the absolute value of the total charge of the ions of one sign produced in air when all the electrons and positrons liberated or created by photons in mass dm of air are completely stopped in air. Exposure is given by the following equation; X= dq (3.12) dm 3.7 Ionizing and non-ionizing radiation Radiation is classified into two main categories, non-ionizing and ionizing, depending on its ability to ionize matter. The ionization potential of atoms ranges from a few electron volts for alkali elements to 24.5eV for helium (noble gas).

33 21 Non ionizing radiation cannot ionize matter while ionizing radiation can ionize matter either directly or indirectly. Directly ionizing radiation which is mainly charged particle, deposits energy in the medium through direct coulomb interactions between the directly ionizing charged particle and orbital electrons of atoms in the medium. Indirectly ionizing radiation which are mainly neutral particles deposits energy in the medium through a two step process: In the first step a charged particle is released in the medium (photons release electrons or positrons, neutrons release protons or heavier ions).in the second step the released charged particles deposit energy to the medium through direct coulomb interactions with orbital electrons of the atoms in the medium. Both directly and indirectly ionizing radiation are used in radiotherapy Ionizing radiation and human health High doses of radiation can be harmful or even fatal. The damage caused by exposure to a radiation is determined by the type of radiation, the duration of exposure and the part of the body that is exposed. The interaction of ionizing radiation with the human body arises from either external sources or internal contamination which can lead to biological effects (UNSCEAR, 2000). Radiation effects can lead to death of a cell, impairment in the natural functioning of the cell leading to somatic effects such as cancer and a permanent alteration of the cell which is transmitted to later generation i.e. genetic effect. Biological effects can also be considered in terms of stochastic effect and non stochastic effects. Stochastic effects increases with increase in dose rate (ICRP, 2005) while non stochastic effect has a threshold below which there is no effect. Dose limits

34 22 are set so that occupational exposures will not cause deterministic effects (Radiation safety manual, 2010). Human exposure to radiation also reduces the immunity of the person exposed (UNSCEAR, 2006). 3.8 Gamma-ray spectrometry Gamma-ray spectrometry uses the direct proportionality between the energy of the incoming Gamma-ray and the pulse amplitude at output of the detector. After amplification and digitization, pulse amplitudes are analyzed and the output of the spectrometer is an energy spectrum of the detected radiation. Since individual radionuclides emit specific gamma-ray energies, gamma ray spectra can be used to identify and quantify radionuclides in a sample Mechanism of NaI(Tl) detector The gamma ray detector consists of a single crystal of Thallium activated Sodium Iodide optically coupled to the photocathode of a photomultiplier tube. When a gamma ray enters the detector it interacts by causing ionization of the sodium iodide. This creates excited states in the crystal that decay by emitting visible light protons a process called scintillation. The Thallium doping of the crystal is done to shift the wavelength of the light photons into the sensitive range of the photocathode. The intensity of the scintillation then decays approximately exponentially in time with a delay constant of 250 ns. Surrounding the scintillation crystal is a thin aluminium enclosure with a glass window at the interface with the photocathode, to provide a hermetic seal that protects the hygroscopic NaI against moisture absorption. The inside of the aluminium is lined

35 23 with a coating that reflects light to improve the fraction of the light that reaches the photocathode. At the photocathode, the scintillation photons release electrons by photoelectric effects. The photoelectrons emitted are proportional to the energy deposited in the crystal by the gamma- ray. The other part of the photomultiplier tube consists of a series of dynodes enclosed in the evacuated glass tube as shown below. Figure 3.7: Schematic representation of photomultiplier tube (wikibooks online) Each dynode is biased by a high voltage supply. Since the first dynode is biased to a positive voltage than the photocathode, the photoelectrons are accelerated to the first dynode and as each electron strikes the first dynode it has acquired sufficient kinetic energy to knock out more secondary electrons thus the dynode multiplies the number of electrons in the pulse of charge. This multiplication is repeated at each dynode until the output of the last dynode is collected at the anode. For the selected bias voltage the charge arriving at the anode is proportional to the energy deposited by the gamma ray in the scintillator. The preamplifier collects the charge from the anode on a capacitor, turning the charge into a voltage pulse which is transmitted to the linear amplifier where the pulse height is proportional

36 24 to the energy deposited in the scintillator by the detected gamma ray. The multichannel analyzer (MCA) measures the pulse heights delivered by the amplifier and sorts them into a histogram to record the energy spectrum which represents the gamma ray energies intercepted by the NaI(Tl) detector. The MCA uses software in supporting personal computer to operate the instruments and display the spectrum. The MCA connects to the computer via a USB cable.

37 25 CHAPTER FOUR MATERIALS AND METHODS 4.1 Sampling and sample preparation Thirty surface soil samples were collected in the accessible area along the shore between Karungu Bay and Muhuru Bay. The samples were collected systematically at a distance of two kilometers from each along the shore from Muhuru Bay to Karungu Bay. For each soil sample collected an area of about 0.5 m x 0.5 m was marked and carefully cleared of debris to a few centimeters depth. Surface soil was taken from different places randomly within the marked and cleared area and mixed together thoroughly, in order to obtain representative sample of the area. The sampling site map showing the sample points along the shore from Karungu Bay to Muhuru Bay is shown in figure 4.1. The soil samples were then crushed to smaller pieces and sieved through fine mesh (~0.5 mm). The sieved pieces were then dried at C in order to remove the moisture. The samples were then sealed in plastic beakers and kept for about four weeks for equilibrium to be reached between 226 Ra and its progeny (Mustapha, 1997). To prevent radon leakage the bottle caps were then lined with aluminium foil and the caps were further secured to the bottles with vinyl tape. A gamma ray spectrometer consisting of NaI(Tl) detector and its associated electronics was then used to measure the radionuclides in the samples.

38 Figure 4.1: Map of Kenya showing Lake Victoria and the sampling points 26

39 NaI(TI) Gamma- Ray Spectrometer The gamma ray spectrometer consists of a shielded NaI(TI) detector. The system also includes a PCA-P multichannel analyzer (MCA) and its software for spectral data acquisition and analysis. The PCA-P contains a high voltage supply, a charge sensitive pre-amplifier, a shaping amplifier, analogue to digital converter (ADC) with multichannel analyzer (MCA). Data acquisition and analysis are performed with an MCA comprising of a 4 k channels, Multichannel Buffer (MCB) card and ACE emulation software package (AMCA). The MCB card collects data independently of the other operations of the computer. Figure 4.2 shows a schematic diagram of the NaI(TI) gamma-ray spectrometer. NaI PMT Amplifier MCA Source E.H.T Power PC Figure 4.2: Schematic diagram of the NaI (TI) gamma-ray spectrometer used in this work 4.3 Standard samples System calibration and decomposition of measured spectrum into components was done using three standard materials, obtained from International Atomic Energy Agency (IAEA). The standards are RGK-1, RGU-1 and RGTH-1 for potassium, uranium and thorium respectively (IAEA, 1987). The potassium calibration

40 28 standard is potassium sulphate with Bq/kg potassium activity and uranium and thorium content lower than and 0.01 ppm respectively. The uranium standard (RGU-1) is uranium ore diluted with silica, with 238 U activity of 4900Bq/kg with a negligible amount of potassium and some traces of thorium. Thorium standard (RGTH-1) is thorium ore diluted with silica, with 232 Th activity of 3280 Bq/kg but containing some 238 U and 40 K (IAEA, 1987). In addition to these standard materials, another standard referred as RGMIX, which is a combination of the three was also used in this study. 4.4 Experimental Procedures Energy calibration of the NaI (TI) Detector Energy calibration of the NaI(TI) was done in the energy range of 662 kev to 1330 kev using the IAEA certified standards. The following gamma ray lines were used: 137 Cs (662 kev), 60 Co (1170keV) and 60 Co (1330keV). The photon energy was represented as a function of channel number using a second order polynomial of the form shown in equation 4.1; E=EO+B(channel..no.)+A (channel..no.) 2. (4.1) where A, B and EO are constants. The polynomial was generated by least square fit to the calibration points using micro-cal origin Software.

41 29 ENERGY (KeV) E = C E-4 C 2 (1330) Co-60 (1170) Co (662) Cs CHANNEL NUMBER Figure 4.3: Energy calibration of the NaI(Tl) detector Table 4.1 Calibration equation fit parameters Parameter A Fitted value E-4 B EO Energy resolution of NaI (TI) detector This is a measure of photo peak sharpness and is defined as the ratio of full width at half maximum (FWHM) of full energy peak (662keV) of NaI(TI) spectrum for Cs-137. In this research the resolution of the gamma ray detector was obtained by fitting a Gaussian curve to Cs-137 full energy peak at 662keV using the equation 4.2;

42 30 y = y o + A w π 2 e 2 (x xc)2 w 2.. (4.2) The Gaussian curve obtained in figure 4.4 below represents a probability distribution that is used to calculate energy resolution and it is a continuous, symmetric distribution whose density is given by equation measured data Gaussian fit Intensity(c/s) Energy(keV) Figure 4.4 shows the Gaussian fit from which energy resolution of 7.5% ±0.064 was obtained for the NaI (TI) detector used in this study Table 4.2 Energy resolution fit parameters Parameter Fitted value yo(y-offset) 0.79±0.20 xc (peak position) ±0.52 w (peak width) 49.88±1.13 A (peak area) ±45.94

43 31 In table 4.2,the yo is the baseline offset,xc is the center of the peak and it gives the centroid energy,w is the peak width and it gives the energy at half maximum of the curve and A is the area under the curve which gives the intensity of the radiation used Background Determination The background activity was determined by measuring an inert sample basically a plastic container filled with distilled water for duration of 30,000 seconds. The background intensity was subtracted for each of the recorded spectrum (Righi et al., 2009); Yg-Yb=Yn. (4.3) where Yg is the gross spectra count,yb is the background radiation and Yn is the net spectra count of the sample Detector efficiency This is the ability of the detector to change energy of the radiation into a useful signal and it is expressed as the ratio of net count rate to the absolute activity of a radionuclide. In this study standard IAEA certified samples of 238 U, 232 Th and 40 K were used, with full energy peaks of 1460 kevof 40 K,1760 kev of 214 Bi and 2615 kev of 208 TI,where both 214 Bi and 208 Tl are at secular equilibrium with 238 U and 232 Th respectively. Certified activities and number of counts in the peaks were used to calculate the detector efficiency using the formula in equation 4 (Mustapha et al, 1999);

44 32 ε = Y n AP γ M s. (4.4) Where ε is the efficiency of the detector, Yn is the net peak count, Pγ is the photon emission probability of the given radionuclide in the reference sample, A is the activity concentration of a given radionuclide in RGMIX and Ms is the mass of the standard reference sample (RGMIX) of the sample Minimum detection limit This is the ability of the detector to record minimum values of the useful signal.the minimum detection limit, LD of the NaI(TI) was computed using the equation 4.6 (Mustapha et al., 1999) L D = 1 [ M s εp γ T 4.65 Y b ]... (4.5) T where Ms is the mass of the reference sample (RGMIX), ε is the detector efficiency, Pγ photon emission probability, T is the counting time and Yb is the background count. Table 4.6 shows the values of the photon emission probabilities, detector efficiency and minimum detection limit for the various radionuclides analyzed in this work using the standard reference sample. Table 4.3: Emission probabilities, detector efficiency and minimum detection limit values using RGMIX measured in this work. Radionuclide E(keV) Activity Pγ ε LD(Bq/kg) K U-238(Bi-214) Th-232(TI-208)

45 Analysis of samples for activity concentration The samples were analyzed by subtracting the background count from the measured count to obtain net spectrum. The spectral data acquisition time was 30,000 seconds for each sample that was analyzed Intensity (c/s) K 1460(keV) 40 Bi 1765(keV) Tl 2615(keV) Energy(keV) Figure 4.5: A typical gamma ray spectrum of the background radiation measured in this study. 0.3 Intensity(C/S) K kev Bi kev Tl keV Energy(keV) Fig 4.6: A typical gamma- ray spectrum of a sample measured in this study before background subtraction.

46 Intensity(c/s) Tl keV Energy(keV) Fig 4.7 A typical gamma ray spectrum of the thorium standard (RGTH-1) after background subtraction 0.3 Intensity(c/s) Bi 1765keV Energy(keV) Fig 4.8 A typical gamma-ray spectrum of the Uranium standard (RGU-1) after background subtraction

47 B Intensity(c/s) K 1460 kev Energy(keV) Fig 4.9: A typical gamma-ray spectrum of potassium standard (RGK-1) after background subtraction Intensity(c/s) Bi keV Bi keV K keV Bi kev Tl kev Energy(keV) Fig 4.10: A typical gamma-ray spectrum of RGMIX measured in this study after background subtraction

48 Area under the Peaks This was computed by fitting the photopeaks to Gaussian curves to the data points using microcal origin software. A model Gaussian curve equation stated in equation 4.2 was used to generate the fit parameters: base line offset, yo, total area under the curve from the baseline,a, the center of the peak Xc and the width of the curve at half maximum,w. Figure 4.11 shows a schematic diagram of a Gaussian curve fitting of 40 K-1460keV photopeak obtained from a soil sample 0.09 Data pts Gauss fit 0.08 Intensity(c/s) Energy(keV) Figure 4.11: A Gaussian curve fitting of 40 K-1460keV photopeak for a soil sample Table 4.4 Gaussian fit parameters of 40 K-1460keV photopeak for a soil sample Fit parameter Value yo 0.04±0.01 xc ±0.65 w 71.43±5.37 A 4.66±0.68

49 Spectral Data Decomposition Since the NaI(Tl) detector has low energy resolution the measured spectrum was decomposed into parts that appear in the decay series of 238 U and 232 Th and spectrum of 40 K. This was done as follows; the spectrum of a sample can be presented as a sum of composing spectra of the Y of three separate natural radionuclides and the background spectrum (Muminov et al., 2005) Y = Yb +YTh +YU +YK... (4.6) where Yb is the background spectra and YTh, YU and YK are the spectra of 238 U and 232 Th decay series and 40 K respectively. By subtracting the background spectrum Yb from equation (3.5) it becomes Y net = YTh +YU +YK.. (4.7) For decomposition of the spectra into parts and determination of activities, standard sources of 238 U, 232 Th and 40 K were used. In this study the standard sources used were RGU-1, RGTH-1 and RGK-1 respectively. The extraction of the interfering components was done using the stripping off method. In this method, the TAP, of the radiations of a radionuclide i1 which weakly interferes with radionuclide picked for 232 Th series, 2615keV of 208 TI was selected. A ratio c1 between its intensity and the corresponding intensity of the standard source E (i1) was determined. The spectrum of the radionuclides i1 in the sample will be given by (Muminov et al., 2005); Y (i1) = c1 E (i1). (4.8)

50 38 where Y (i1) is the spectrum of the radionuclides (i1), c1 is the normalizing factor and E (i1) is the spectrum of the corresponding standard radionuclide. Y(i1) was then subtracted from the spectrum that had been corrected for background leading to simplification of the resultant spectrum equation (4.10); Ynet Y (i1) =Ynet2... (4.9) where Ynet is the spectrum that has been corrected for background counts and Ynet2 is the final spectrum of 232 Th after decomposition. A Gaussian function (described in section 4.8) was then fitted on the photopeak, to compute net intensity represented by the area under the Gaussian curve. The intensities were used for activity calculation. This procedure was repeated to obtain the uranium and potassium components in the sample. Figure 4.12 shows a gamma ray photopeak of a soil sample before and after spectrum decomposition Data points Gauss fit Data points Gauss fit Intensity(c/s) Intensity(c/s) Energy(keV) Energy(keV) (a) Before spectrum decomposition (b) After spectrum decomposition Figure 4.12: A gamma photopeak of 1460keV of 40 K in a soil sample (a) before spectrum decomposition and (b)after spectrum decomposition.

51 39 Table 4.5: Gaussian fit parameters of a gamma-ray photopeak of 1460keV of 40 K in a soil sample ( Figure 4.11) Parameter Fitted value Fitted value Before decomposition After decomposition yo ± ± xc ± ± w ± ± A ± ± Calculation of radioactivity Activity concentration of the radionuclides was calculated using the method of comparison for gamma ray analysis, given by the equation (4.10) (Mustapha, 1999); A R I M R R A S S.. (4.10) I M S where AR is the activity of the radionuclide in the reference sample, MR is the mass of the reference sample, IR is the intensity of the radionuclide in the reference sample, AS is the activity of the radionuclide in the sample, MS is the mass of the sample and IS is the intensity of the radionuclide in the sample. 4.8 Assessment of radiation hazards The radiation hazards associated with the radionuclides were estimated by calculating the radium equivalent activity (Raeq). It is a weighted sum of activities of 226 Ra, 232 Th and 40 K and it is based on the assumption that 370 Bq.Kg -1 of 226 Ra, 259 BqKg -1 of 232 Th and 4810 BqKg -1 of 40 K produce the same gamma radiation dose rate (Matilullah et al., 2004).

52 Radium equivalent activity Radium equivalent was calculated using the following formula (Beretka and Mathew, 1985); Ra eq ARa ATh AK.. (4.11) where ARa, ATh in and AK are the activity concentrations of 226 Ra, 232 Th and 40 K, respectively. 10 respectively. 7 and are conversion factors for Thorium and Potassium External radiation hazard index (H ex ) External radiation hazards due to natural radionuclides of 40 K, 226 Ra, and 232 Th were defined in terms of external or outdoor radiation hazard index. It was calculated from the expression of Raeq through the assumption that its maximum value allowed ( unity) corresponds to the upper limit of Raeq (370 Bq kg -1 ). The external hazard index was calculated using the following equation (Beretka and Mathew, 1985); H ex ARa ATh AK (4.12) where ARa,ATh and AK are the activity concentrations of 226 Ra, and 232 Th and 40 K, respectively.

53 Internal hazard index (H in ) Radon and its short lived products are also hazardous to the respiratory organs. The internal exposure to radon and its daughter products was quantified by the internal hazard index (Hin), it was calculated by the following equation (Beretka and Mathew, 1985); H in ARa ATh AK (4.13) where ARa, ATh and AK are the activity concentrations of 226 Ra, and 232 Th and 40 K respectively Absorbed gamma radiation dose rate Activity concentration in soil compound to the total absorbed dose rate (D) in air at 1m above the ground level was calculated using equation (4.14) (Abbady et al., 2005); D 0.427A 0.622A A U Th K (4.14) where ATh, AU and AK are the average activity concentration of 232 Th, 238 U, and 40 K respectively Annual effective dose The annual effective dose received was calculated using the following formula. (UNSCEAR, 2000) E T. Q. D (4.15)

54 42 where D is the absorbed dose rate in air, Q is the conversion factor of 0.7 Sv Gy -1, which converts the absorbed dose rate in air to human effective dose received (UNSCEAR, 2000) and T is the outdoor occupancy time which is 1 year Dose to risk conversion Dose to risk conversion will be used to estimate the number of people likely to die due to annual effective dose in a given population. This will be done using the relation; G fep..... (4.16) where G, f,e and P are the numbers of death, dose to risk conversation factor, annual effective dose and total population respectively. A dose to risk conversion factor of 5% per Sievert (ICRP, 1991) to the maximum effective dose observed in this study was used.

55 43 CHAPTER FIVE RESULTS AND DISCUSSION In this study radioactivity levels in soil collected from the shore of Lake Victoria between Muhuru bay and Karungu bay was measured using NaI(TI) detector. Further analysis done using decomposition method. The collected soil samples were analyzed at Kenyatta University Physics Laboratory. The quantities measured include; the radioactivity concentration in the soil samples, radium equivalent activity, external and internal hazard indices, absorbed radiation dose, annual effective dose and dose to risk conversion. These quantities have been calculated and the results presented in tables and graphs. The results are further discussed in details in this chapter. 5.1 Activity concentration of natural radionuclides The activity concentration of radionuclides 232 Th, 238 U and 40 K were measured after doing spectrum decomposition and an average of 146.0±24.2 Bqkg -1, 64.5±3.3 Bqkg ±43.3 Bqkg -1 respectively were obtained. The minimum activities for 232 Th, 238 U and 40 K were found to be 92.2±1.8 Bqkg -1, 34.4±0.7 Bqkg -1 and 652.9±13.1 Bqkg -1 while the maximum values were 190.6±3.8 Bqkg -1, 103.2±2.1 Bqkg -1 and ±40.4 Bqkg -1 respectively. A summary of the activity concentration calculated in this study compared to other parts in Kenya are presented in table 5.1.

56 44 Table 5.1 Average activity concentrations of radionuclides sand along the lakeshore compared to values obtained from other parts in Kenya Place of study Activity Concentration(Bqkg -1 ) 238 U 232 Th 40 K Migori county(lake shore).this work 64.5±3.3 ( ) 146.0± ± Kibwezi District 130.6± ± ±303.9 (Mutie, 2011) ( ) ( ) ( ) Mrima Hill ± ± ±20.7 (Kebwaro, 2009) ( ) ( ) ( ) Kwale T. Mines 20.9± ± ±16.5 (Osoro, 2007) ( ) ( ) ( ) Mombasa 22.8± ± ±24.2 (Hashim et al., 2004) ( ) ( ) ( ) Malindi 21.3± ±42.1 (Hashim et al., 2004) ( ) ( ) ( ) Different parts in Kenya 28.7± ± ±38.5 (Mustapha et al., 1997) ( ) ( ) ( ) It is observed that the activity concentrations are above the world population weighted average of 33Bqkg-1 for 238 U, 45Bqkg-1 for 232 Th and 420Bqkg-1 for 40 K as reported in UNSCEAR (2000). This shown in table 5.2. The activity concentration of potassium-40 was found to be the higher in the area

57 45 of study as compared to other parts in Kenya (Mutie, 2011); Kebwaro,2009; Mustapha et al.,1997 implying that the shore is heavily polluted by the potassic fertilizers used in the sugarcane farms and also industrial wastes from the sugarcane processing factories. The concentration of Uranium-238 and Thorium- 232 was also found to be high this was due to the weathered particles of igneous rocks on the main land which are associated with naturally occurring radionuclides having been deposited at the shore by the rivers and streams. However, the concentration of Uranium-238 and Thorium-232 is lower than what was reported in Mrima Hill (Kebwaro,2009). Due to the high activity concentration calculated in this area there was need to calculate the radiological parameters to assess the danger that this could pose to the miners and general public. Table 5.2: Activity concentration of the sand samples measured in this study compared to world average (UNSCEAR, 2000) Radionuclide This study(avg) (Bqkg -1 ) World (Avg)(Bqkg -1 ) Th ± U ± K ± Figure 5.1 below shows the activity concentration of the three radionuclides in all the sampling points

58 K-40 U-238 Th-232 Activity concentration (Bq/kg) point 5 point 10 point 15 point 20 point 25 point 30 Sampling points Figure 5.1: Activity concentrations in all the sampling points (concentration values of K have been divided by 5) The activity concentration for 40 K is generally high than those of 238 U and 232 Th in all the sampling points except for point six. There is generally higher activity concentration of 40 K at the estuary of river Kuja (point 18) refer sampling site map. This can be attributed to the deposition of the potassium residues from the sugarcane farms on the main land. There is also generally high concentration of 232 Th and 238 U in all the sampling points. This might be attributed by the geological outline of the area showing that the soil deposited along the shore might have been formed from carbonatite and monazite rocks which are rich in these radionuclides.

59 47 The thorium to uranium ratio was calculated and found to range from 1.3 to 4.9. This can be explained by high solubility of uranium ions compared to thorium ions. However, the regression plot between Thorium and Uranium shows that there is no strong correlation between them. The frequency distributions of the activity concentrations of the three radionuclides, 238 U, 40 K and 232 Th are shown in figures ( ). Their correlations are also shown in figure From the frequency distribution graphs it is observed that the activity concentrations of 232 Th and 40 K are less skewed as compared to 238 U. Figure 5.2 shows the frequency distribution curve of activity concentration of 238 U in soil samples Frequency Activity concentration of U-238 in sand Figure 5.2: Frequency distribution curve of activity concentration of 238 U in sand samples

60 Frequency Activity concentration of K-40 in sand Figure 5.3: Frequency distribution curve of activity concentration of 40 K in sand samples Frequency Activity concentration of Th-232 in sand Figure 5.4: Frequency distribution curve of activity concentration of 232 Th in sand samples

61 R= Regression line 90 Uranium Thorium-232 Figure 5.5: Regression plot showing correlation between activity concentration of R= Regression line 90 Uranium Potassium-40 Figure 5.6: Regression plot showing correlation between activity concentrations of 238 U and 40 K

62 Thorium R= Regression line Th/U Ratio Potassium-40 Figure5.7:Regression plot showing correlation between activity concentration of 232 Th and 40 K Th/U Ratio Sample Site Sample Site Figure 5.8: Thorium versus Uranium Ratio in different sand sampling points

63 Human exposure to gamma radiation The radium equivalent activity, external radiation hazard index and internal radiation hazard index were calculated using equation (4.11), (4.12) and (4.13) respectively for the sand samples at the shore of Lake Victoria between Karungu bay and Muhuru bay. The mean radium equivalent activity was found to be 367.1±7.3 Bqkg -1 which is less than the maximum allowed limit of 370 Bqkg -1. The mean external radiation hazard index was found to be 0.99±0.02mSvy -1 and the mean internal radiation hazard was found to be 1.17±0.02mSvy -1 which is above 1mSvy -1 which is the recommended value for the radiation hazard to be insignificant. The outdoor absorbed gamma dose rate in air at a height of 1m above the ground surface was calculated based on the guidelines provided by UNSCEAR (2000). The absorbed dose rate was calculated in this study using the formula obtained from equation (4.14), while effective annual dose rate was determined using equation (4.15). The estimated results for absorbed dose rate and corresponding annual effective dose in soil samples are tabulated in tables 6. The absorbed dose rate in air at a height of 1m above the ground level obtained from the different sampling points ranged from 123.3±2.4 ngyh -1 to 236.2±4.8 ngyh -1 with and an average of 171.2±3.4 ngyh -1. This value is higher than the world average of 60 ngyh -1 (UNSCEAR, 2000). The annual outdoor effective dose ranged from 0.302±0.006 msvy -1 to 0.579±0.012 msvy -1 with an average of 0.420±0.008 msvy -1. This

64 52 value is below the world average of 1mSvy -1 (ICRP, 1991). Absorbed dose rates and annual effective dose graphs are shown in figure 5.9 and 5.10 respectively. 250 Dose Rate 200 Dose Rate(nGyh -1 ) ngyh -1 60nGyh Sample Site Figure 5.9: Absorbed dose rate versus sand sampling points Effective Dose Rate Effective Dose Rate (msvy -1 ) mSvy Sample Sites Figure 5.10: Effective dose rates versus sand sampling points

65 Radiological Impact The annual effective dose rate was used to estimate the health effects on the persons exposed to radiation in terms of fatal cancers occurring per Sievert (Sv). A dose to risk conversion factor of 5% per Sievert (ICRP, 1991) was applied on a population of 144,625 residents of the area and the maximum annual effective dose rate of 0.579mSvy -1 was applied. Using equation 4.16, for the sample population in Nyatike Sub County the number of exposure induced deaths per year was estimated to be four people.

66 54 CHAPTER SIX CONCLUSION AND RECOMMENDATIONS 6. 1 Conclusions Gamma ray spectrometric analysis of soil samples along Lake Victoria between Muhuru Bay and Karungu bay has been done. The activity concentration of the radionuclides 232 Th, 238 U and 40 K in the sand samples were found to be in the range of (92.2± ±3.8) BqKg -1, (34.4± ±2.1) BqKg -1 and (652.9± ±40.4) BqKg -1 respectively. The average concentrations of 232 Th, 238 U and 40 K measured in the samples collected in this study are ±24.15BqKg -1, ±3.33BqKg -1 and ±43.30BqKg -1 respectively. These values are higher than those reported from other studies done in other parts of the country (Mustapha et al., 1997; Hashim et al., 2004; Osoro, (2007) and Langat (2012).However these values are within the range observed by Kebwaro ( 2009) and Mutie (2011). There was a marked difference in the activity concentration of 40 K in sample site 18 corresponding to the estuary of river Kuja the major inlet to the lake from the main land. The calculated radiation absorbed dose from the different sampling sites ranges from123.3±2.4 ngyh ±4.8nGyh -1 with an average of 171.2±3. This mean value is higher than the world average of 60nGyh -1. Sample site 18 still recorded the highest dose rate of 236.2±4.8nGyh -1. This shows that there is increased level of radiation pollution of the lake at the estuary of river Kuja which could be due to human economic activities on the main land along which the river flows like gold mining, sugarcane farming -where there mass use fertilizer rich in

67 55 40 K and also could be due to industrial wastes from the Sugar Industries on the main land. The effective dose rate due to gamma radiation from the decay of 232 Th, 238 U and 40 K in soil samples were found to be in the range of (0.302± ±0.012) msvy -1 with a mean of 0.420±0.008mSvy -1. All the sampling sites registered an effective dose rate values below the global average value of 0.420mSvy -1, this below the ICRP limit of 1mSvy -1 for members of the general public (ICRP, 1991). The mean external radiation hazard index was found to be 0.99±0.02mSvy -1 and the mean internal radiation hazard index was found to be 1.17±0.02mSvy -1 which is slightly above unity. This shows that the radiation hazard from the naturally occurring radionuclides at the region of study is slightly significant. Applying dose to risk conversion factor of 5% per Sievert (ICRP, 1991) to the maximum effective dose of msvy -1 observed in this study the following conclusion can be made, that for an estimated population of 144,625 people exposed the gamma radiation from the surface sand, the risk casualties per year may be four people, though it should be noted that dose to risk conversion factor depends on other factors also, like the sensitivity of the individuals to radiation induced cancer. 6.2 Recommendations This study was done using the soil samples from the lake shore. Further studies should be done using soil samples from the surrounding catchment on the level of activity concentration.

68 56 Further studies to determine the concentration levels of trace elements in rock and soil sample from the lake shore is recommended for monitoring of heavy metal content in the area. Future work should also be done on activity concentration of Uranium and Thorium using the other gamma lines of Thorium and Uranium. Further studies to determine the levels of naturally occurring radionuclides 232 Th, 238 U and 40 K in water from Lake Victoria in this region of study should be done because the water is used directly by the locals and it is the source of fish which is a source of food to both the locals and other Kenyans.

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73 61 APPENDIX 1 Table A1: Activity concentration of the natural radionuclides 40 K, 238 U and 232 Th measured in this work Activity concentrations(bqkg-1) SAMPLE SITE K-40 U -238 Th ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±3.1 MEAN ± ± ±24.15

74 62 APPENDIX 2 Table A2: The annual effective dose rate (AEDR) and absorbed dose rate measured in this work. Sample Site Absorbed dose(ngyh -1 ) Annual effective dose(msvy -1 ) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±0.009 Mean 171.2± ±0.008

75 63 APPENDIX 3 Table A3: World average external exposure rates calculated from the average radionuclides in soil (UNSCEAR, 2000) Radionuclide Concentration in soil (Bqkg -1 ) Dose coefficient(ngyh -1 per Bqkg-1) Absorbed dose rate in air (ngyh -1 ) 40K U series Th series Total absorbed dose rate outdoors from soil 60

76 64 APPENDIX 4 Table A 4: Gamma lines emission probabilities of the IAEA reference materials (IAEA, 1992) Reference material Gamma ray Emission Nuclide source energy(kev) probability RGU Ra Bi Pb Pb Bi Bi Bi RGTH Pb Ac Tl Tl RGMIX K Bi Tl

77 65 APPENDIX 5 Table A 5: Dose limits as recommended by the ICRP (Leo, 1994) Whole body Single organs Lens of eye Skin Other organs or tissues Occupational 100 msv in 5yrs, But not more than 50 msv in any year 150 msv/yr 500 msv/yr 500 msv/yr General public 1 msv/yr averaged over Any consecutive 5 years 15 msv/yr 50 msv/yr 50 msv/yr

78 66 APPENDIX 6 Uranium-235 α 700 million yrs Protactinium- 231 Thorium-231 β 26 hours α 33,000 years Actinium-227 α 22 years (1%) Francium-223 β 22 years (99%) days β 22 minutes days Thorium-227 α α 19 days Radium days Radon-219 α 4.0 seconds Polonium-215 α Lead milliseconds α β 36 minutes Bismuth minutes Thalium-207 β 4.8 minutes Lead-207 (stable) Figure A1: Uranium -235 decay series (Gilmore, 2008)

79 67 APPENDIX 7 α Thorium billion years Thorium β 6.1 hours α 1.9 years Actinium-228 β 5.8 years Radium-228 Radium α 3.7 days Radon α 56 seconds Polonium α 0.15 seconds Lead Bismudh-212 β 11 hours α 61 minutes (36%) Polonium-212 β 61 minutes (64%) α 310 nanoseconds Lead 208 (stable) Thallium-208 β 3.1 minutes Figure A2: Thorium-232 decay series (Gilmore, 2008)

80 68 APPENDIX 8 Figure A3: (a) Measurement of masses of the samples Figure A3: (b) Placing of the sample in the detector system

81 69 APPENDIX 9 Figure A4: (a) Spectral data acquisition Figure A4: (b) Reading and analyzing Spectral data