An Investigation of the Radioecology of the Carbonatite Deposits in the Homa Mountain Region in South Nyanza, Kenya
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An Investigation of the Radioecology of the Carbonatite deposits in the Homa Mountain region in South Nyanza, Kenya by David Otwoma Registration Number: 180/80187/2010 A thesis submitted in fulfillment of the requirements for the degree of Doctor of Philosophy, University of Nairobi A Thesis submitted in December 2012 i DECLARATION This thesis is my original work and has not been presented for a degree in any other University David Otwoma 180/80187/2010 Date Department of Physics University of Nairobi P.O. Box 30197 Nairobi, Kenya This thesis has been submitted for examination with the approval of my supervisors: 1. Prof. J. P. Patel Signature Dale Department of Physics University of Nairobi P.O. Box 30197 Nairobi, Kenya M- ' I ~L~ ZI . 2. Dr. A. O. Mustapha Date Department of Physics Federal University of Agriculture P.M.B. 2240 Abeokuta, Nigeria ABSTRACT The level of radiation due to the naturally occurring radioactive materials was investigated and the radiological hazard was calculated for Homa Mountain located at the intersection of longitude 33° 26’ E and 34° 34’ E latitude 0° 30’ N and 0° 20’N in southwestern Kenya. The absorbed dose rate in air at 1 meter above the ground was measured using calibrated hand held survey meters that could be coupled to sodium iodide (Nal) radiation detector. The measured mean dose rate of 474.1 nSv h 1 translated to annual dose due to external radioactivity of 4.15 mSv y 1. The radioactivity concentrations of 40K, 226Ra and 232Th in rock and soil samples were measured using gamma ray spectroscopy from two different institutions for comparison purposes of accuracy and precision. The cumulative mean activity concentrations obtained were 915.6, 195.3 and 409.5 Bq k g 1 of 40K, 226Ra and 232Th, respectively. The calculated mean absorbed dose rate was 383.33 nGy h 1; the radium equivalent activity mean was 838.6 Bq k g 1; the representative level index mean value was 5.93; the gamma activity index mean value was 2.96; the external hazard index mean was 2.26 and the internal hazard index mean was 2.77. It was observed that the pattern of high values of the radiological indices follows that of the corresponding activity concentrations. These indices are important parameters for the radiological protection of the population since soils are used for making earthen huts and pottery while bricks and rocks for building human habitations. The resultant total effective dose (TED) equivalent was 5.03 mSv y- b Using RESRAD software program, the impact of these naturally occurring radioactive materials on the environment was evaluated for different age sets of the population and the results showed that infants and teenagers receive more effective doses than other segments of the populations. The RESRAD code enabled modeling of 40K, 232Th and 238U environmental pathways. This led to exposure pathways of external radiation, inhalation and ingestion resulting in total effective dose equivalent to exposed individuals estimated as 4.93 mSv y 1. The excellent correlation of similar TED derived from the different iii measurements and modeling methodologies increased the certainty of the conclusions arrived at for the work done. The Homa Mountain study has established data on radiation dose to the population and qualifies the region as a high background radiation area due to dominance of carbonatite rocks. Investigations of the health hazards of radiation and the impacts of high levels of background radiation on the local inhabitants can be done in future epidemiological studies. The exploration of the possible economic benefits that can be derived from the minerals associated with carbonatite deposits, Rare Earth Elements and Thorium is open for consideration. IV TABLE OF CONTENTS T itle....................................................................................................................... i Declaration.......................................................................................................... ii Abstract................................................................................................................iii Table of Contents................................................................................................ v Acknowledgements............................................................................................. vii List of Figures ....................................................................................................ix List of Tables....................................................................................................... xi List of Abbreviations..........................................................................................xii Chapter 1: INTRODUCTION................................................................................ 1 1.1 Radioecology of Natural Occurring Radiation Materials................. 1 1.2 Geography and Geology of Homa Mountain Area........................... 4 1.3 Gamma Dose Rates of NORM......................................................... 10 Chapter 2: LITERATURE REVIEW..................................................................... 14 2.1 Review of Activity Concentration of Natural Radioactivity..............14 2.2 Review of Significant Radiation H azard...........................................19 2.3 Studies done in Kenya on Radiological Exposure........................... 24 2.4 Statement of the Research Problem.................................................28 2.5 Objective of the Research................................................................. 29 2.6 Rationale of the Research............................................................... 30 Chapter 3: THEORETICAL BACKGROUND..................................................... 32 3.1 Theory of Radioecological Assessment of Naturally Occurring Radioactive Materials..........................................................................................32 3.2 Dosimetric Radiation Quantities and Units....................................33 3.3 Theory of Different Radiation Indices............................................. 38 3.4 Background Information of RESRAD...............................................42 ' f Chapter 4: MATERIALS AND METHODS..........................................................59 v 4.1 Summary........................................................................................59 4.2 Equipment and Calibration Procedures.......................................... 59 4.3 Method of Sampling after surveying................................................66 4.4 Sample Preparation......................................................................... 66 4.5 Measurement of Activity Concentration.......................................... 67 4.6 Computer Hardware and RESRAD Software...................................77 Chapter 5: RESULTS AND DISCUSSIONS...................................................... 80 5.1 Summary..........................................................................................80 5.2 Background Radiation Dose Measurements using Hand Held Survey Meters.................................................................................................................80 5.3 Concentration of Uranium, Thorium and Potassium in Samples...86 5.4 Exposure Pathways and Dose Estimation using RESRAD.............99 Chapter 6: CONCLUSIONS AND RECOMMENDATIONS.................................112 6.1 Dose Rates using hand held Survey Meters Conclusions................. 112 6.2 Radiation Hazard/Indices Conclusions..........................................112 6.3 RESidual RADiation (RESRAD) Adaptation Conclusions ................. 113 6.4 Recommendations..........................................................................114 REFERENCES................................................................................................118 Appendix A: Geology and Geography of Homa Mountain............................. 137 Appendix B: List of Surveyed A reas................................................................147 Appendix C: Reference Materials isotopic Data for 40K, 232Th and 238U ........165 f vi ACKNOWLEDGEMENTS I wish to express my profound gratitude to Prof. J. P. Patel who opened up in- situ measurements of radioactivity in Kenya. He occupies a special place and deserves accolades for always encouraging, guiding, and offering very useful suggestions throughout the course of the research. Appreciation to Dr. A. O. Mustapha for pioneering in-situ followed by laboratory background radiation measurements in Kenya before he left University of Nairobi (UoN) in 2007 and hence encouraging many of us into this field of research. I thank Dr. H. A. Kalambuka of UoN for his incisive insight and being available at the completion of this study; Mr. Joel Kioko for approving and Mr. Stanislaus Masinza for being present when performing calibration of equipment at the Secondary Standards Dosimetry Laboratory (SSDL) at Kenya Bureau of Standards (KeBS); Ms. Ruth Nguti of National Council of Science and Technology (NCST) for conscientiously typing most of the thesis. Ms. Irene Gichira, Stellah Wambui and Ms. Marcellah Ojiambo for assisting in sample preparations at the Materials Testing and Research Laboratories (MTRL) of the Ministry of Roads (MoR), Mr. David Kabiga of NCST for ever encouraging me to persist with the study; Mr. Simon Bartlol for facilitating gamma radiation measurements at the Institute of Nuclear Science and Technology (INST), UoN; Michael Mangala for his positive appraisal