\ \
ASSESSMENT OF LEVELS OF NATURAL
RADIOACTIVITY IN SURFACE SOILS AROUND
TITANIUM MINES IN KENYA //
BY
lMAs.mu:, KEFA OSORO
REG. NO.: 15617246/2002
DEPARTMENT OF PHYSICS
A thesis submitted in partial fulfilment of the requirements for the award of the degree
of Master of Science in the School of Pure and Applied Sciences of Kenyatta
University.
15 September 2007
Masore, Kefa Osoro
Assessment 0/ levels
a/natural
1IIIIIIIIIIIIIInlm.III~1
2008/322630
KENYATTA UNIVERSITY If-8RARY
DECLARATION
This thesis is my original work and has not been presented for the award of a degree
or any other award in any University.
All sources of information have particularly been acknowledged by way of references.
-=-~ ~............11~- ..-..-..--
Sign
..........~~.~I.~.7..
Date
Masore, Kefa Osoro
(156/7246/2002)
Department of Physics,
Kenyatta University
P.O. Box 43844,
Nairobi, KENYA.
We confmn that the candidate, under our supervision, carried out the work reported
in this thesis.
Prof. 1.V. S. Rathore,
Department of Physics,
Kenyatta University,
P.O. Box 43844,
Nairobi, KENYA .
Dr. Amidu O. Mustapha,
Department of Physics,
University of Nairobi,
P.O. Box 30197,
Nairobi, KENYA.
Mr. Michael J. Mangala,
Institute ofNuc1ear Science,
University of Nairobi
P.O. Box 30197,
Nairobi, KENYA.
Sign
......~
Sign
ii
.....~JI09 / 07-
Date
....~~/~./~r-
~~te
'?t.M~l.( p~te
DEDICATION
To my mother, my wife and our dear children
III
ACKNOWLEDGEMENTS
I take great pleasure in acknowledging the support received from the following who
jointly and severally made invaluable contributions towards the success of this thesis.
First, I greatly thank the university supervisors: Prof. 1. v. S. Rathore (Department of
Physics, Kenyatta University), Dr. Amidu O. Mustapha (Department of Physics,
University of Nairobi) and Mr. Michael J. Mangala (Institute of Nuclear Science and
Technology, University of Nairobi) for their guidance and direction. The ideas and
knowledge presented in this thesis were shaped by countless discussions with these
radiation experts, without whose supervision very little would have been achieved
regarding the research project. They read thesis drafts in time and always availed
themselves to share their experiences, insights and opinions despite their obviously
tight schedules. Special thanks also to Mr. Willis Ambuso of Department of Physics,
Kenyatta University for advice that always came handy.
Gratefully, I acknowledge the assistance of the chairman/director and staff and
students of the Department of Physics, Kenyatta University and the Institute of
Nuclear Science and Technology, University of Nairobi for permission to use their
respective experimental equipment. The chairman of Department of Physics of
Kenyatta University always ensured that any necessary external help was obtained in
good time.
Am also grateful for the assistance I received from the following institutions: the
Institute of Nuclear Science and Tec1mology, University of Nairobi for availing their
laboratory equipment for analysis of samples; Ministry of Education, Nairobi for
iv
authorising the research in a short time; the Office of the President through the Kwale
District Commissioner and the Msambweni District Officer for permission to collect
soil samples from the study area and the United Nations Environmental Programme
(UNEP), Nairobi for access and use of library and internet facilities for literature on
the study subject.
Further gratitude go to Mr. William Angwenyi, Mr. Chris Kanoti, Mr. Richard
Maroko, Mr. Stanley Areba and the bodaboda (taxi) operators of Msambweni for their
kind assistance during sample collection. Many thanks go to Madam Ann Maina of
the Department of Physics and to Madam Flora and Mr Musakala both of Materials
Research and Laboratory, Ministry of Roads and Public Works, for assistance in the
preparation of soil samples for testing and analysis.
Last but not least, I am highly grateful to my mother, my wife and my children for
their unlimited material and moral support, love and encouragement.
v
TABLE OF CONTENTS
Content
Page
Title 0)
Declaration Oi)
Dedication (iii)
Acknowledgements (iv)
Table of Contents (vi)
(ix)Abstract
List of Figures (x)
List of Tables (xi)
List of Abbreviations (xii)
List of Symbols and Units (xiii)
Chapter 1: Introduction 1
1.1 Backgrounds to the Study 1
1.1.1 The Titanium Mines in Kenya 1
1.1.2 Geology and Geomorphology of the Area of Study 3
1.2 Statement of the Research Problem 5
1.3 Objectives of the Research Project 6
1.4 Significance of the Research Project 7
Chapter 2: Literature Review 8
2.1 Environmental Radioactivity 8
2.2 Human Exposures to Environmental Radioactivity 11
2.3 Biological Effects of Radioactivity 12
2.4 Environmental Radioactivity Studies in Kenya 15
Chapter 3: Theoretical Background of Gamma-ray Spectrometry 17
vi
KENYATTA UNIVERSITY ltBRARY
3.4
3.1 Emission of Gamma Rays by Radioactive Nuclei 17
3.2 Interaction of Gamma Radiation with Matter 17
3.2.1 Photoelectric Effect 18
3.2.2 Compton Effect 19
3.2.3 Pair Production 20
.., ..,.J •.J Principles of Gamma-ray Spectrometry 21
Radiation Dose From Terrestrial Exposure 23
Chapter 4: Materials and Methods
4.1
4.2
4.3
25
Sampling and Preparation of Samples 25
The Experimental Procedure 27
4.2.1 Experimental Gamma-Ray Spectrometry System 27
4.2.2 Energy Calibration of the Spectrometer System 28
4.2.3 Sample Counting 29
4.2.4 Calculation of Radioactivity 30
4.2.5 Detection Limits 30
Gamma Radiation Dose Rates 31
Chapter 5: Results and Discussion
5.1
5.2
5.3
32
Quality Control and Assurance 32
Radioactivity Concentrations of Radionuclides 34
Human Exposure to Gamma Radiation 44
Chapter 6: Conclusions and Recommendations 50
6.l
6.2
Conclusion 50
Recommendations 51
References 52
Appendices 56
vii
Appendix 1: Gamma-Ray Spectrum of Standard Reference
Material (SRNI-I) 56
Appendix 2: 238U Decay Series 57
Appendix 3: 232Th Decay Series 58
Appendix 4: Typical Gamma Spectrum of Soil Sample 59
viii
ABSTRACT
All human beings are exposed to radiation from naturally occurring radionuclides
in soil and other environmental materials. Some of these exposures are not
amenable to control and they are usually referred to as background radiation. Some
work activities such as conventional mining inadvertently produce large quantities
of naturally occurring radionuclides, which can result in additional and/or elevated
levels of radiation exposure of people in the areas around the mining sites. Such
exposures - induced or enhanced by human activities - are subject to control by
regulatory authorities. In some instances there may be contributions from the two
types of exposures and they must be separated before applying regulatory control.
In this study, natural radioactivity levels in surface soils around the proposed
titanium mines in Kwale district were determined from measurements of 78
samples of surface soils randomly sampled from two villages within the proposed
mining area by using a hyper pure germanium (HpGe) gamma-ray spectrometer.
The values of radioactivity concentrations in the soils and the likely radiation doses
from contact with these soils were determined and are reported in this thesis. The
radiological implication of these levels is discussed with regards to the impending
mining operations in the area. The ranges and mean of radioactivity concentrations
(Bqkg-I) obtained are: 804±Oo4-43.6±1.5 (27.6±1.7) for 232Th; 704±O.6-40.6±1.4
(20.9±1.5) for 226Ra and 31.9±1.3-114.1±1.4 (69.5±3.2) for 4oK, respectively. The
likely absorbed dose rates in air above these soils were calculated from these
radioactivity concentrations and found to be 8.5±0.5-36.9±1.1 nGyh-1 with a mean
of 25.2±lo4 nGyh-l• The corresponding effective dose rates are 21.0±1.2-90.8±2.6
flSvy·1with a mean of 62.0±3.5 flSvy"1, which are lower than the global average of
0046 msvy' and therefore of little radiological risk to the environment of the study
subject.
ix
LIST OF FIGURES
Fig. 3.1: Photoelectric emission of an electron 18
Fig. 3.2: Compton Scattering of a gamma photon 19
Fig. 3.3: The Process of Pair Production 20
Fig.4.la: Sampling Map and Profile 26
Fig.4.lb: Sampling and Sample Analysis Scheme 27
Fig. 4.2: Schematic diagram of HpGe detection system 28
Fig. 5.1: Correlation of measured values with certified values
of radioactivity of reference materials 34
Fig. 5.2: Typical gamma-ray spectrum of a soil sample 35
Fig. 5.3a: 232Th radioactivity levels in soil 40
Fig.5.3b: 226Ra radioactivity levels in soil 41
Fig.5.3c: 40K radioactivity levels in analyzed soil 41
Fig.5.4a: Variation of radioactivity concentration oe32Th
with that of226Ra 43
Fig. 5.4b: Variation of radioactivity concentration of 232Th
with that of4oK 43
Fig.5.4c: Correlation of radioactivity concentration of 226Ra
with that of4°K 44
Fig. 5.5: .. 226 232Th d 40KContributions of Ra, an to gamma
radiation dose rate 47
Fig.5.6a: Distribution of absorbed dose rate (nGyh-l) in the
analyzed soil samples 48
Fig.5.6b: Distribution of effective dose rate (~Svy-l) in analyzed
soil samples 48
x
Table 2.1:
Table 2.2:
Table 2.3:
Table 2.4:
Table 5.1:
Table 5.2:
Table 5.3a:
Table 5.3b
Table 5.3c:
Table 5.4:
Table 5.5:
Table 5.6:
LIST OF TABLES
Primordial radionuclides with their half-lives and relative
abundance/activity
Cosmogenic radionuclides with their half-lives and activity
Man-made radio nuclides and their half-lives and sources
Annual radiation dose limits recommended by the ICRP
Mean detection limits ofthe HpGe Detector.
Measured and certified values of radioactivity of the reference
materials RGTh-l, RGU-I, and RGK-l.
Radioactivity concentration of232Th in soil samples
Radioactivity concentration of226Ra e38U)in
soil samples
Radioactivity concentration of4oK in soil samples
Minimum, maximum and mean radioactivity concentrations of
232Th,226Raand 40Kin Soil
Absorbed and effective gamma radiation dose rates in soil.
Minimum, maximum and mean absorbed and effective
gamma radiation dose rates in the tested soil samples
xi
9
10
II
15
32
33
36
37
38
39
45
46
Term
ARPANSA
CES
FEP
FWHM
GANAAS
HpGe
IAEA
ICRP
JICA
KTM
MCA
NORM
RGK-l
RGTh-l
RGU-l
SRM
UNEP
UNSCEAR
LIST OF ABBREVIATIONS
Meaning
Australian Radiation Protection and Nuclear Safety Agency
Costal and Envirorunental Services
Full Energy Peak
Full Width at Half Maximum; Resolution of the HpGe detector
Gamma spectrum analysis, Activity calculations and Neutron
Activation Analysis Software
Hyper pure germanium detector
International Atomic Energy Agency
International Commission on Radiological Protection
Japanese International Cooperation Agency
Kwale Titanium Mines
Multi-Channel Analyzer
Naturally Occurring Radioactive Materials
Standard Reference Material for Radioactivity of Potassium-40
Standard Reference Material for Radioactivity of Thorium-232
Standard Reference Material for Radioactivity ofUranium-238
Standard Reference Material
United Nations Envirorunent Programme
United Nations Scientific Committee on Effects of Atomic
Radiation .
xii
Symbol
1291
131r
I37CS
14C
20srl
212Pb
214Bi
214Pb
222Rn
226RalRa-226
12SAc
232ThlTh-232
235UIU_235
23SUIU_238
239pU
241Am
3H
40KlK_40
60CO
7Be
90Sr
Ti02
f..tSVil
Bqkg-I
Gy
Gyh'
keY
MeV
Sv
mSvil
LIST OF SYMBOLS AND UNITS
Meaning
Iodine-I29
Iodine-I 3 I
Caesium-137
Carbon-I4
Thallium-208
Lead-212
Bismuth-214
Lead-2I4
Radon-222
Radium-226
Actinium-228
Thorium-232
Uranium-235
Uranium-238
Plutonium-239
Americium-241
Tritium
Potassium-40
Cobalt-60
Beryllium-7
Strontium-90
Titanium Oxide
Microsievert per year
Becquerel per kilogram
Gray (Unit of Absorbed Dose Deposited on Unit Mass of
Material; 1 Gy = 1 Jkg-I)
Gray per hour; Unit of absorbed dose
Kiloelectronvolt (Unit of' Energy = 1.6xlO-19 J)
Megaelectronvolt (= 106 Electron Volts or 1.6xlO-13 J)
Sievert (Unit of Effective Radiation Dose)
Millisievert per year
xiii
Chapter 1
Introduction
1.1 BACKGROUND TO THE STUDY
1.1.1 The Titanium Mines in Kenya
Rapid growth of the world population over the last several decades, currently
exceeding the six billion mark (UNEP, 1999), and technological advancements, have
increased the needs to exploit natural resources like metallic minerals, petroleum,
wood and natural gas among others. Human activities such as mining and milling of
materials containing naturally occurring radionuclides (NORM) may alter the natural
distribution of these radionuclides in the environment.
A heavy mineral (titanium) mining operation is proposed for Kidiani and Mwaweche
areas, east of Shimba Hills Settlement in Msambweni Division of Kwale District,
Kenya. The proposed mining area is located approximately 65 km south of Mombasa
Island and 10 km from the Indian Ocean. It stretches from approximately 39°26" E to
approximately 39°30" E latitude and from approximately 4°23" S to approximately
4°30" S longitude, covering an area of about 20 km2 (Figure 4.1a). The mineral ore is
in two areas, which are separated by the Mukurumudzi River and the Ramisi Sugar
Estate. The two mineral deposit areas include the two villages of Nguluku and
Maumba, which will be affected by the mining project. It is estimated that the mineral
deposits at Maumba, also kncwn as the Kwale Central Dune, cover an area of 2 km2
while those at Nguluku (also called the Kwale South Dune) cover an area of 3 km2
(CES, 2000). About 3,000 people live in the area covered by the proposed titanium-
mining project (Ong'olo, 2001). In both areas, the minerals occur as large sand dunes
that are locally concentrated and abundant in some places. The depth of the mineral
deposits varies from 30 metres below the surface to 40 metres below the surface.
The mining area IS m the tropics and, therefore, experiences reasonably constant
temperatures throughout the year. The warmer temperatures are between November
and April when they range from 26°C to 28°C (CES, 2000), while the cooler
temperatures are between May and October when they range from 24°C to 26°C. The
area experiences bimodal rainfall with long rains occurring from March to July and
short rains from October to December. The area receives an average of 1300 nun of
rainfall per year. During the rainy season the wind blows westwards bringing moisty
air from the Indian Ocean and consequently rains, while during the dry season the
northeast monsoon is dominant.
According to the project's mining proposal by the mining company, Tiomin Kenya,
each of the mineral deposits will be mined in about seven years, beginning with the
(Central dune) Maumba deposits. The ore body is estimated to contain 3-6% heavy
minerals and 15-30% fine particles. The rest of the ore body is red or orange coloured
sand. The total estimated resource is 1 million tonnes of rutile, 0.6 million tonnes of
zircon and 4 million tonnes of ilmenite (CES, 2000). Rutile, zircon and ilmenite
minerals appear in both Maumba and Nguluku as heavy sands (CES, 2000). These
minerals are normally deposited at similar sites through sedimentation in riverine and
marine waters. Ilmenite (with 49-51 % titanium oxide, Ti02) is the dominant mineral
present followed by rutile (with 95-96% titanium oxide, Ti02). These two minerals
are used to make either titanium metal or titanium oxide (a white non-toxic pigment)
in a highly specialized, toxic and energy-consuming refining process that begins with
a rutile concentrate.
2
Titanium is used to provide the white colour in paints and as filler in plastics, paper,
toothpaste, and many medicinal tablets and in sunscreens. It is also used in making
aeroplane bodies, artificial limbs and bones and heart pacemakers. Current global
production of titanium is in the range 120,000 to 140,000 tonnes per allium. The
proposed output of titanium from the Kwale project is about twice the world's
titanium metal needs annually (Ong'olo, 2001). Zircon is used in the manufacture of
ceramic tiles and in refractory and foundry industries. Thus rutile, zircon and ilmenite
are the targets for titanium mining in the area under study.
Tiomin Kenya has also indicated that the main stages of the mining operation will
include: construction and operation of the heavy mineral sands mine; construction and
operation of a ship loading facility; extraction and processing of the minerals; and
transportation by road of the final products from the mines to the ship loading facility
at Shimoni, 35 km from the mining site.
The mmmg process will involve clearing the vegetation of the mmmg area,
stockpiling the topsoil for future use in rehabilitation of the mined areas, dry mining
by an excavator (e.g., Bucket Wheel Excavator, BWE) or scrapper, transporting the
sand by conveyor belt to the wet plant for further processing and mixing the ore-
bearing sand with water to form slurry from which the heavy metals will be separated.
These mining processes have the potential for environmental pollution, mainly due to
dispersal of dust and other materials from the actual mining sites to the surroundings.
1.1.2 Geology and Geomorphology of the Area of Study
The proposed titanium mining area in Kwale district lies approximately 4°23" S to
approximately 4°30" S latitude, and approximately 39°26" E to approximately 39°30"
. 3
KENYATtA UNIVERSITY LtBRARY
E longitude, extending over a length of about 10 km with an average width of about 2
km. It slopes gently and rises from sea level to an altitude of about 150 metres above
sea level at the top of the hills. The Mukurumudzi River is the major surface water
source in the area. This is a perennial river that flows from the northwest to the
southeast and drains into the Indian Ocean near Gazi Bay. A large part: of the mineral
deposits area drains into the Mukurumudzi River while the eastern side of the Central
Dune drains into the smaller Kidogoweni and Mtawa rivers.
According to Austromineral report (1978) on Kenya's coastal mineralization,
Nguluku area is mainly comprised of Maji-ya-Chumvi formation and Igneous rocks
both of Duruma group of formations. The other members of the Duruma group are
Mariakani formation, Mazeras formation and Magarini formation, which forms a belt
of low hills running parallel to the coast. The Magarini formation consists of red or
orange sand dunes that were deposited during the Pliocene, 2 to 5 million years ago.
The Mazeras sandstones, derived from the disintegration of the Mozambican Belt
metamorphic rocks, provided the supply of heavy minerals to the Magarini sands.
The- mineral deposits at Kwale comprise two large sand dunes: the Kwale Central
Dune at Maumba and the Kwale South Dune at Nguluku (Figure 4.1a). The
geological arrangement is such that the mineral ore body is considered to be of a vent
by igneous activities of alkaline rock composed of the so-called "agglomerate".
Niobium and rare earth metals are the main composites of the alkaline igneous rock.
The same mineralization is reported in the Mrima-Jombo area under the same alkaline
igneous conditions (Austromineral, 1978). Post-reef sediments and coral reef
sediments in the area have been associated with zircon and limestone respectively.
The vent is surrounded by a sequence of Maji-ya-Chumvi profile covered on top by
4
sandstone and Taurus sediments that consist of alluvium, colluvium, dune sands, and
coral reef in that order from the top. The sandstone consists of dune sands and other
sands also in that order.
Maji-ya-Chumvi Formation is divided into three members: Upper, Middle and Lower
members. Shale and siltstone beds dominate lower and Middle members while the
Upper member consists of sandstone beds. A high value content of salt has been
reported in shale beds of Lower members. (The term "Maji-ya-Chumvi" means "salty
water" in the local language. ) Sandstone beds in Upper member are comprised of silty
sandstone beds with flaggy texture and well-developed joints (JICA, 1993).
Before the commencement of a mmmg operation, which has the potential of
impacting negatively on the natural environmental, there is always the need to obtain
some baseline data, which will serve as reference for future environmental
assessments. Therefore, in this study, the natural radioactivity levels in samples of
surface soil around the proposed mining area were measured prior to commencement
of the mining operations. This is important because once mining operations begin the
opportunity to obtain such baseline data will be lost, making it difficult to do an
accurate assessment of the radiologicaJ and the environmental impact of the mining
operation.
1.2 STATEMENT OF THE RESEARCH PROBLEM
Conventional mining operations inadvertently produce large quantities of naturally
occurring radionuclides because the ores -of interest, for example copper, bauxite,
titanium, phosphates, etc., are normally mineralised with primordial radionuclides,
particularly potassium-40 (4oK), uranium-238 CZ38U), and thorium-232 CZ32Th). These
5
radionuclides, although not originally designed to produce environmental radiation
hazard, result in the enhancement of natural radiation background around the mining
area. The enhancement is mainly due to contamination of liquid effluents from the
mines, leachants from tailings, atmospheric release of dust particles and radon
exhalation, which could lead to elevated exposures of both workers and the general
public. The principal exposure pathways are internal exposure from alpha particles
following inhalation of airborne dust particles and inadvertent ingestion of
radionuclides with food, water, etc., and external exposure to gamma radiation from
the bulk residue and by-products.
It is necessary to establish the background levels of the radioactivity and radiation in
the environment of the mining facility prior to commencement of the mining
operations. The present study focussed at determining the pre-operational
radioactivity concentrations of the natural radionuc1ides present in surface soils
around the proposed titanium mines.
1.3 OBJECTIVES OF THE RESEARCH PROJECT
The following were the specific objectives of this study:
(i) To determine radioactivity concentrations of the naturally occurring radionuclides
present in surface soils around the proposed titanium mines in Kwale district.
(ii) To calculate the radiation dose rates arising from the ambient levels of natural
radioactivity in the surface soils around the proposed titanium mines.
6
1.4 SIGNIFICANCE OF THE RESEARCH PROJECT
It is normally expected that, during the mining operations, precautionary measures
will be taken to avoid unnecessary occupational and/or public radiation exposures by
adopting standard working procedures and installing appropriate engineering devices.
However, it is also the obligation of the management of the mining operation and/or
the national regulatory authority to ensure regular monitoring (personal and
environmental) procedures required to assess the effectiveness of the existing
exposure-control measures and to review the radiological impact of the mining
operations. It is therefore necessary to establish the 'normal' background levels of the
radioactivity in the area surrounding the proposed mining facilities prior to the
commencement of the mining operations.
7
KENYATTA UNIVERSITY LIBRA r
Chapter 2
Literature Review
2.1 ENVIRONMENTAL RADIOACTIVITY
Environmental radioactivity arises as a result of the presence of three categories of
radionuc1ides: Primordial radionuc1ides (which have been in existence since the time
of formation of the earth), cosmogenic radionuc1ides (which are formed as a result of
cosmic ray interactions) and human-produced radionuc1ides (which are enhanced or
formed due to human activities) (Radiation Information Network, 2004). These
radionuc1ides are found naturally in air, water and soil. They are even found in the
human body, which is made up of chemicals, some of them being radionuc1ides
ingested daily with water and food.
Primordial radionuc1ides are leftovers from the time of formation of the earth. TIley
are typically long-lived, with half-lives in the order of hundreds of millions of years.
The progeny or decay products of these long-lived radionuc1ides are also in this
category. Table 2.1 shows some basic information on some of the common primordial
radionuc1ides (Radiation Information Network, 2004).
Generally the primordial radionuc1ides are evenly distributed in rocks and soils
everywhere around the world. However, there are some areas with particularly high
background radiation levels. The highest radiation levels are found primarily in
Brazil, India and China. In Orissa state, India, for example, average activities of 2500,
220 and 120 Bqkg-1 for 232Th, 238U and 4oK, respectively, have been reported
(Mohanty et al., 2004a, 2004b). At this beach, effective radiation dose rates range
between 0.46 and 6.12 rnxvy'. In Guarapari, Brazil, gamma radiation dose rates
8
above 120 mxvy' have been reported (Radiation Information Network, 2004). In
China, radiation dose rates of between 3 and 4 m.Svy" have been reported (Radiation
Information Network, 2004).
Table 2.1: Primordial radionuclides with their half-lives and relative
abundance/activity (Radiation Information Network, 2004)
Nuclide Half life Natural activity (Abundance)
mU 7.04 x 108 y Abundance: 0.72% of all natural uranium
238U 4.47 x 109 y Abundance: 99.27% of all natural uranium (0.5 - 4.7
ppm in common rock types)
232nl 1.41 x 10lOy Abundance: 1.6 to 20 ppm in rock.
226Ra 1.60 x 103 y Activity: 16 Bqkg-1 (in limestone); 48 Bqkg-I (in igneous
rock)
222Rn 3.82 days Noble (inert) Gas; (activity: 28 Bqm-3)
40K 1.28 x 109 y Activity in soil: 37-1100 Bqkg-1
Cosmic radiation permeates space, the source being primarily outside of the solar
system. The radiation is in many forms, including high-speed heavy particles and
high-energy photons and muons. The upper atmosphere interacts with many of the
cosmic radiations, resulting in the production of the cosmogenic radionuclides. These
radionuc1ides can have long half-lives, but most have shorter half-lives than the
primordial nuclides. Table 2.2 presents some of the common cosmogenic
radionuclides (Radiation Information Network, 2004).
9
Table 2.2: Cosmogenic radionuclides with their half-lives and radioactivity
(Radiation Information Network, 2004)
Nuclide Half life Source and Natural Activity
5730 y Source: Cosmic ray interactions;
Activity: 220 Bqkg" in organic matter.
12.3 Y Source: Cosmic ray interactions with N and 0;
Activity: 1.2 x 10-3 Bqkg-1.
53.3 days Source: Cosmic ray interactions with Nand 0;
Activity: 0.01 Bqkg'.
Human activities, especially those involving nuclear technologies, e.g., testing of
nuclear weapons, nuclear research and energy generation with nuclear reactors, etc.,
introduce some man-made radionuclides (e.g., 137Cs, and 90Sr) into the environment.
The Chemobyl accident of April 26t\ 1986, for example, added man-made
radionuclides to the environment that resulted in increase in the global average
effective radiation dose (UNSCEAR, 2000). However, the collective radiation
exposure from all man-made sources is small compared to the natural sources of
radiation exposure. A few examples of radionuclides generated from human activities
are presented in Table 2.3 (Radiation Information Network, 2004). There are other
human activities, such as mining, oil exploration, water works, etc., which are not
intended to produce radionuclides but which still result in the enhancement of natural
levels of the background radiation.
10
Table 2.3: Man-made radionuclides and their half-lives and sources (Radiation
Information Network, 2004)
Nuclide Half-life Source
3H 12.3 y Weapons testing, fission reactors, and nuclear
weapons manufacturing.
1311 8.04 days Weapons testing and fission reactors; Used in
medical treatment of thyroid problems.
1291 l.S7xl07y Weapons testing and fission reactors.
137Cs 30.17 y Weapons testing and fission reactors
90Sr 28.78 y Weapons testing and fission reactors.
239pu 2.41 x 104y 738Produced by neutron bombardment of- U.
2.2 HUMAN EXPOSURES TO ENVIRONMENTAL RADIOACTIVITY
Exposure of human beings to ionizing radiation from naturally occurrmg
radionuclides is a continuing and inescapable feature of life on earth. For most
individuals, this exposure exceeds that from all man-made sources combined. There
are two main contributors to natural radiation exposures: high-energy cosmic ray
particles incident on the earth's atmosphere and natural radionuclides (terrestrial
radiation) that originate in the earth's crust and are present everywhere in the
environment, including the human body itself. Cosmic radiation increases with
altitude (ARPANSA, 2004). Terrestrial radiation comes from naturally occurring
radionuclides, mainly from 40K and 87Rb and from 238Uand 232Thand their decay
products. On average, two thirds of the radiation dose people receive comes from
terrestrial sources (ARPANSA, 2004). Most of this dose is from radon gas, a decay
product of both uranium and thorium decay series (see appendices 2 and 3,
respectively). Radon ema~ from the soil and tends to accumulate in buildings,
11
contributing a high proportion of background radiation dose (ARPANSA, 2006). Both
external and internal exposures to humans arise from these sources of radiation.
Terrestrial radiation dose levels are related to the types of rock and place from which
the soils originate. Therefore, the natural environmental radiation mainly depends on
both geological and geographical conditions. Higher radiation levels are associated
with igneous rocks such as granite, and lower levels with sedimentary rocks. There
are exceptions, however, as some shale and phosphate rocks have been reported to
have relatively high content of natural radionuclides and therefore natural
radioactivity (Tzortzis et aI., 2003; UNSCEAR, 1993; Zaman et al., 1993). The
process of nucleosynthesis in stars forms primordial radionuclides. Only those
radionuclides like 4oK, 238U and 232Th, with half-lives comparable to the age of the
earth, and their radioactive decay products, can still be found today on earth.
2.3 BIOLOGICAL EFFECTS OF RADIOACTIVITY
When radiation is absorbed in the body it causes chemical reactions, which can alter
the normal functions of the body. At high doses (i.e., above 1 Sv), radiation can cause
massive cell death, organ damage and possibly death to the exposed individual (ICRP,
1991). At low doses (i.e., less than 50 mSv) the situation is more complex and
unpredictable (ARPANSA, 2004).
The effects of radiation can be divided into two groups: somatic and genetic effects.
Somatic effects affect only the person exposed to radiation while genetic effects affect
the offspring of the person exposed to radiation because of damage to the germ cells
in the reproductive organs.
12
The human body is made up of different cells, for example brain cells, muscle cells.
blood cells, etc. Genes within a cell determine how the cell functions. If damage
occurs to the genes, then it becomes possible for a cancer to occur. This is because the
cell loses the ability to control the rate at which it reproduces. Radiation can cause this
effect and at low doses it is the only known health effect. This type of event is very
unlikely to occur, and an estimate of its frequency of occurrence is only obtained by
measuring the effect at higher doses and then calculating the probability at low doses.
A dose of 1 mSv corresponds to a chance of 6 in 100,000 of contracting cancer (Jelfs,
1992). If the damage occurs in the testes or ovaries then hereditary effects in
descendants may become apparent. No first generation hereditary effects were
observed amongst Hiroshima survivors.
Radiation effects can also be divided into stochastic and deterministic (or non-
stochastic) effects. Stochastic effects are such that the probability of the effect
occurring depends on the dose received, i.e., the higher the dose received the higher
the risk of the effect occurring, and vice versa. Development of cancer is the most
obvious stochastic effect. Deterministic or non-stochastic effects occur only after a
certain radiation dose has been received and the severity of the effect depends on the
dose received. Thus deterministic effects have a dose threshold below which they
cannot occur. Deterministic effects include development of cataracts, sterility in men,
and death.
Because it is not possible to sense the presence of ionizing radiation, one can easily be
exposed to a lethal dose of radiation and still not be aware of it until after the event, as
happened with early radiation workers. International community concern about the
effects of ionizing radiation led to the formation of the International Commission on
13
Radiological Protection (ICRP) in 1928. The commission has since brought together
all the available information on effects of ionizing radiation and recommended dose
limits and good working practices in order to minimize the risks from working with
ionizing radiation. Based on many studies the ICRP recommends a risk factor of 2
congenital abnormalities per thousand per sievert (Sv) effective dose. This means that
a dose of 1 mSv to a large population will produce two (2) cases of severe hereditary
effects per million births (UNEP, 1985). This figure is low compared with the normal
incidence of severe congenital abnormalities, which incidence is 23,000 per million
births (Czeizel et al., 1988).
In areas of high natural background radiation, an increased frequency of chromosome
aberrations has been noted repeatedly (Radiation Information Network, 2004). The
increases are consistent with those seen in radiation workers and in persons exposed
at high radiation dose levels, although the magnitudes of the increases are somewhat
higher than predicted. However, no increase in the frequency of cancer cases is noted
for most places of high natural background radiation.
In an effort to minimize radiation exposure to members of the public and radiation
workers, the ICRP has set limits on exposure to ionizing radiation (Table 2.4). These
limits exclude exposure due to background and medical radiation. It is worthwhile to
note that exposure to the dose limit is not considered acceptable, and all doses should
be kept As Low As Reasonably Achievable (ALARA).
14
Table 2.4: Annual radiation dose limits recommended by the ICRP (ICRP, 1991)
Population group Annual effective radiation
dose limit (mxvy')
Members of the public
Non-classified workers
1
15
Classified workers
Whole body
Feet
Eyes
Hands
20
500
150
500
2.4 ENVIRONMENTAL RADIOACTIVITY STUDIES IN KENYA
There are no nuclear reactors in Kenya but traces of radionuclides from effiuents of
nuclear reactors in other parts of the world have been reported in Kenya (Hashim et
al., 2004). Studies carried out in Kenya have shown that natural radioactivity is due to
the primordial radionuclides: 238U, 232Th and 40K (Hashim et al., 2004; Mustapha et
al., 2000). Hashim et al., (2004) reported specific radioactivity of 238U, 232Th and 40K
in sediments along the Kenyan coastline to be 11.9-22.8 (l8.7±2.3), 10.8-26.2
(18.7±3.1) and 206.1-519.2 (401.7±30.9) Bqkg-1, respectively. These values are
comparable to those reported in other parts of the world (Narayana et al., 2000;
Tzortzis et al., 2003, 2004; Tzortzis and Tsertos, 2004) but they are lower than the
worldwide averages of 30 and 35 Bqkg", for 232Th and 238U, respectively
(UNSCEAR, 2000).
The population-bias average effective dose rate from natural sources of radiation in
Kenya has been estimated to be 3.9 msvy" (Mustapha et al., 2000). Mrima hill in the
south coast of Kenya has been identified in one study as an area of high background
radiation, the effective dose rate ranging from 13.7-106.7 m'Svy" (Patel, 1991), which
15
is rnrrch higher than the normal background radiation of 0.46 mSvil (UNSCEAR,
1993). The current study was a screening investigation intended to determine the
radioactivity concentrations of the naturally occurring radionuclides in the surface
soils around the proposed titanium mining sites in Kwale district of Kenya.
16
Chapter 3
Theoretical Background of Gamma-Ray
spectrometry
3.1 EMISSION OF GAMMA RAYS BY RADIOACTIVE NUCLEI
Most alpha (a) and beta (~) decays, and in fact most nuclear reactions as well, leave
the final nucleus in an excited state. These excited states decay rapidly to the ground
state through the emission of one or more gamma rays, which are photons of
electromagnetic radiation like x-rays or visible light. Gamma rays have energies in the
range of 0.1 to 10 MeV, and the corresponding wavelengths are shorter than those of
the other types of electromagnetic radiations.
Gamma radiation is due to electromagnetic effects arising out of changes in charge
and current distributions in the nucleus. Gamma emission may be accompanied or
even replaced by internal conversion due to electromagnetic interactions between the
nucleus and extranuclear electrons. Internal conversion results in emission of an
electron with kinetic energy equal to the difference between the energy of nuclear
transition and the binding energy of the electron. Internal conversion leaves an atom
with a vacancy in one of the shells, which is then filled by an electron from the next
higher shell. The subsequent atomic rearrangement process may result in emission of
characteristic x-rays and in internal photoelectric effects with emission of more
electrons from M and L shells.
3.2 INTERACTION OF GAMMA RADIATION WITH MATTER
Charged particles dissipate their energy continuously in a series of many ionizations
and excitations. However, the interaction of a gamma ray with matter is a single-event
17
process that leads to complete absorption or scattering. Gamma rays interact with
matter in any of these three ways: (1) Photoelectric Effect, (2) Compton Effect, or (3)
Pair Production. These modes of interaction of matter and radiation play important
roles in radiation detection and measurement.
3.2.1 Photoelectric Effect
At low photon energies (i.e., below 100 keV), photoelectric effect is the predominant
mode of interaction of gamma rays with matter. A photon gives up all its energy to a
bound electron, which may then become energetic enough to leave the atom. This is
illustrated in Figure 3.1 (Knoll, 1989).
• Photoelectron;
Energy = hv-Ei
Gamma-ray;
Energy = hv
Fig. 3.1: Photoelectric emission of an electron
The kinetic energy, Te, of the released electron is the difference between the photon
energy, hv, and the binding energy of the electron, Eb as expressed in equation 3.1
(Knoll, 1989).
(3.1)
Total absorption of a photon only occurs when an electron is initially at rest so that a
third body, the nucleus, conserves momentum (Adams and Dams, 1970).
18
KfNY4TT4 1I~l\/tD~ITV I .·nn. ""V
Photoelectric effect takes place mainly between the inner K- and L- shell electrons.
Photoelectric effect is enhanced in absorber materials of high atomic numbers, Z, as
shown by the approximation of interaction cross section, T, in equation 3.2 (Knoll,
1989).
Z'T = Constant x --E3.5 (3.2)
where E is the photon energy and the exponent n varies between 4 and 5 over the
gamma-ray energy of interest.
3.2.2 Compton Effect
A photon may also be scattered by an atomic or individual electron in a new direction,
B, with or without loss of energy. At photon energies that are very high compared to
the binding energies of the electrons, photons are scattered as if the electrons were
free and at rest. This process is called the Compton Effect and is dominant at photon
energies in the range of 0.6 to 4.0 MeV. The electron that scatters a photon is known
as a recoil electron. Figure 3.2 illustrates Compton scattering of a gamma photon.
o
Recoil Electron
Incident Photon; •••••~:--i'---------Energy = hv \J\./\../'\.Jf..... -,
Scattered Photon;
.... Energy = hv '.....,
Fig. 3.2: Compton scattering of a Gamma photon (Knoll, 1989)
19
Using the laws of conservation of energy and momentum, the energy of the scattered
photon, hv ', is given by (Knoll, 1989)
hv
hv'= hvJ + -------y (1 -cos B)
mrc
(3.3)
where mac2 = 0.511 MeV is the rest mass energy of the electron. The total Compton
interaction cross section, 0'; is (Knoll, 1989):
Z
E
(3.4)
which increases linearly with the atomic number, Z, of the absorber material.
3.2.3 Pair Production
When photon energy exceeds 1.022 MeV, pair production becomes possible. In this
process, an electron pair is created in the coulomb field of a charged particle (Adams
et aI., 1970). The total kinetic energy of the pair of particles equals the difference
between photon energy and the rest mass energy of the particles formed (rest mass
energy of the particles = 2moc2= 1.022 MeV). Figure 3.3 (Knoll, 1989) illustrates pair
production.
e -4-----\-->..:--.!
Gamma-ray;
hv ~ 2mac2
+e
Fig. 3.3: The process of Pair Production
20
The probability of pair production, k, varies approximately as the square of the atomic
number, Z, of the absorber material (Knoll, 1989),
k ex: Z2 (3.5).
3.3 PRINCIPLES OF GAMMA-RAY SPECTROMETRY
All nuclear radiation detection is based on the principles of interaction of radiation
with matter. Radiation causes excitation or ionization of atoms in the detector material
by the passage of charged particles. Electromagnetic radiation gives rise to energetic
electrons by Photoelectric Effect, Compton Effect or Pair Production. These electrons
are then detected like other charged particles. In gamma-ray spectrometry, gamma
radiation detectors (e.g., high-purity germanium (HpGe) or lithium-drifted sodium
iodide, NaI(Tl)) are used to obtain information about energies and intensities of the
gamma radiation.
Gamma-ray spectrometry is an analytical method in which radionuclides are
identified and quantified based on the gamma rays they emit (IAEA, 1989). The
method allows the radionuclides to be measured directly in the original sample
without the need for chemical pre-treatment. The form in which the sample is
presented to the gamma-ray detector depends on sample type, available equipment,
composition of radionuclides and their levels of activity. Some standard sample
containers for gamma-ray spectrometry include nylon planchets, aluminium cans and
moulded Marinelli beakers. Measuring time in gamma-ray spectrometry varies
according to sample type, required detection limits, detection efficiency and nature of
the radionuclides of interest.
21
Determination of identity of a radionuclide depends on inspection of a sample's
gamma-ray spectrum (Appendix 4). Identification is done by determination of an
unknown radionuclide's characteristic gamma radiation. The degree of confidence of
identification directly depends on the precision of measurement of gamma-ray energy.
Towards this end, accurate energy calibration of the detector is important. Energy
calibration is achieved by use of radionuclide standards (e.g., 241 Am, 137Cs and 60Co)
with known characteristic gamma-ray energies. The basic equation for radioactivity
(in Bqkg-1) calculation is (Mustapha, et aI., 2000):
A= (3.6)b.e(EJ.m
where A is radioactivity concentration, Np is net peak area or net intensity of the
gamma line in counts per second, b is emission transition probability or branching
ratio of the gamma rays during the nuclear de-excitation process, e(EJ is detection
efficiency of energy E; and m is the mass of the sample in kg.
Alternatively, the radioactivity concentration can be calculated using the method of
comparison expressed as equation 3.7 below.
AisMs (3.7)
where Ais = current radioactivity concentration of radionuclide i in the sample, M, =
mass of the sample, lis = intensity or net counts per second (count rate) of radio nuclide
i in the sample, AiR = current radioactivity concentration of radionuclide i in the
22
reference sample, MR = mass of the reference sample, fiR = intensity or net counts per
second (count rate) of radionuclide i in the reference material.
The advantage of this method is that it eliminates the need for efficiency calibration at
various gamma energies when the matrix of the reference material used is similar to
that of the sample analyzed.
3.4 RADIATION DOSE FROM TERRESTRIAL EXPOSURE
Assuming that naturally occurring radioactive nuclides are uniformly distributed in
the ground, absorbed radiation dose rates in air at 1 metre above the ground surface
are evaluated using the expression below (Kohshi et al., 2001):
(3.8)
where D is absorbed radiation dose rate in nfiyh', Au, ATh and AK are the radioactivity
concentrations of 238U,232Thand 4oK, respectively in Bqkg-I and Cn , CTh and CK are
h diati d . 1': 1': 238U 232Th d 40K . I· G h-It e ra ration ose conversion factors lor , an , respective y in n y
per Bqkg-I. The conversion factors in this equation take into account the attenuation
of gamma radiation as it penetrates through the soil.
The radiation dose rate conversion factors used in this study are 0.5281, 0.3892 and
0.0386 nGyh-1 per Bqkg" for 232Th series, 238Useries and 4oK, respectively. These
factors were multiplied by the measured radioactivity concentrations in order to
obtain the absorbed radiation dose rate in air due to the entire series. Using this
calculation, the absorbed radiation dose rate obtained for 232Thand 238Useries is the
average for the different dose rates for the different nuclides belonging to each series.
For 4oK, the radiation dose rate conversion factor is simply the value that is multiplied
23
by the measured radioactivity concentration at the energy of 1460 keY, in order to
deduce the dose rate due to 4oK.
In order to make a rough estimate of the annual effective dose outdoors, one has to
take into account the conversion coefficient from absorbed dose in air to effective
dose and the outdoor occupancy factor. The effective radiation dose rate outdoors in
units of IlSV per year (u Svy') is calculated by the following formula (Tzortzis et al.,
2003):
(3.9)
where DE is the effective radiation dose rate in IlSvy-l, D is the absorbed radiation
dose rate in nGyh-1 and C; is the conversion coefficient in uSvy' per nGyh-1 given by
equation 3.10:
C, = 24 hours per day x 365.25 days per year x 0.4 (occupancy factor) x 0.7 SvGy"
(conversion coefficient) x 10-3 = 2.46 IlSvy-1per nGyh-1 (3.10)
24
Chapter 4
Materials and Methods
4.1 SAMPLING AND SAMPLE PREPARATION
Samples of surface soil were randomly collected from areas around Nguluku and
Maumba villages (Kwale South and Kwale Central dunes, respectively), which are the
locations of the proposed mining sites (Figure 4.1a). The sampling area was
approximately 15-20 krrr', over a length approximately 10 km. The distance between
neighboring sampling points was approximately 500 metres. At each sampling point,
approximately 30 em by 30 em in size, debris of dead leaves and organisms were first
removed, and then the surface soil dug up from a depth of 5-10 em was thoroughly
mixed before three samples, each of volume about 1000 em:', were collected. The
samples were then sealed in clean and dry polythene bags to avoid cross
contamination and labeled appropriately with sampling point name and number, e.g.,
KTMI which stands for Kwale Titanium Mine sampling point number 1. Altogether
78 samples were collected from 26 sampling points. The samples were collected and}.
analyzed according to the scheme in Figure 4.1b, which shows that three replicate
samples were collected from each of the 26 sampling points and each sample was then
tested three times for radioactivity.
In the laboratory, the samples were oven-dried overnight, ground and sieved through
0.5 mm mesh to homogenize the soil, sealed in 450 ml Marinelli beakers, dry-
weighed and stored for a minimum of 4 weeks before counting in order to allow the
in-growth of radon gas so as to achieve secular equilibrium between 226Ra and its
gamma emitting progenies following loss of radon gas during sample preparation.
25
Mineral Deposits 64°31"S--t----'----'-----'----'----t- 4u31 "
39°30" E
I
I,
/'" ."'"~.1a..?lb~
I
KWALE TITANIUM
MINES (KTM)
Nairobi
Kwale South Dune
(Nguluku)
KEY:
[M
Road
River
Sampling Point
Nt Kwale CentralDune (Maumba)
I
I
I
39°25" E To Shimba Hills
To
Kwale
azi-+~--~----r---~
basa
o
I
1 km
I
Fig 4.1a: Sampling map and profile
26
-----------E ~::~t Test3AKTMI BCKTM21
KTM3 --------------------
3 Repeat------ Measurements
3 Replicate
Samples
KTM26+ 26 SamplingPoints
Fig 4.1h: Sampling and sample analysis scheme
4.2 THE EXPERIMENTAL PROCEDURE
4.2.1 Experimental Gamma-Ray Spectrometry System
The 78 soil samples from the mining area were prepared and analysed at the Institute
of Nuclear Science and Technology at the University of Nairobi. A stand-alone high-
resolution spectroscopic system was used for the measurement of the energy spectrum
of the emitted gamma rays in the energy range between 50 and 3000 keV. The system
consisted of a high-purity germanium (HpGe) detector (coaxial cylinder of 55 mm in
diameter and 73 mm in length) with an efficiency of 20-30%, relative to a 3"x 3"
NaI(Tl) scintillator at the 1.33 MeV gamma line of 60CO. The measured energy
resolution of the detector was 2.1 keV at the same gamma line (i.e., 1.33 MeV gamma
line of 60Co). The detector was mounted on a cryostat which was dipped into a 30 litre
Dewar, filled with liquid nitrogen.
27
E Y T UI~IV RSiTY iSH
Experimental arrangement for spectrometer included the high-voltage bias supply
(1200 V) and signal processing electronics. The latter included a main (linear)
amplifier and an Oxford PCA3 multi-channel analyser (MCA) with IAEA-GANAAS
(IAEA, 1991) PC-based software for data acquisition, storage and display of the
acquired gamma spectra. The detector was surrounded by a cylindrical lead shield of
thickness 12 ern that provided an efficient suppression of background gamma
radiation present in the laboratory environment. A schematic diagram of the HpGe
detection system is given in Figure 4.2.
I Detector Pre-Amp t-+ Linear H MCA ~ Data H Printer IAmp Storagei
HV Power
Supply
Fig. 4.2: Schematic diagram of HpGe detection system (lAEA, 1989)
4.2.2 Energy Calibration of the Spectrometer System
Identification of radionuclides present in a sample is based on matching the energies
of the principal gamma rays in a sample's spectrum to energies of gamma rays in a
standard reference sample. Accurate energy calibration is necessary for the detector to
be able to assign correct energies for each full energy peak (FEP) in a spectrum
(IAEA, 1989; Adams and Dams, 1970). Calibration involves establishment of a
channel number of the multi-channel analyser (MCA) in relation to gamma-ray
energy.
28
Over a wide range of gamma-ray energies, it is adequate to express the relationship
between the photo peak energy (E) and the channel number as a linear function
(IAEA, 1989):
E = constant X Channel number+ Eo (4.1)
In this study, energy calibration and evaluation of energy resolution were done using
the multi-nuclide SRM-l standard reference material obtained from the IAEA. The
calibration source was counted for long periods in order to obtain well-defined
photopeaks (Appendix 1). The gamma lines used for calibration were 59.54 keV
e4IAm), 661.66 keY (!37Cs), and 1173.24 keY and 1332.5 keY (60Co). The energy
resolution (full width at half-maximum or FWHM) achieved by the spectrometer was
2.1 keY at the 1.33 MeV reference gamma line of 60CO.
4.2.3 Sample Counting
The samples were counted on the shielded HpGe detector for long periods, ranging
from 30,000 to 60,000 seconds. For each sample, three measurements were made (see
the sampling .and sample analysis scheme, Figure 4.1b). Prior to the samples'
measurement, the environmental gamma background at the laboratory site was
determined with a Marinelli beaker full of distilled water under identical measurement
conditions. This background was later subtracted from the measured gamma-ray
spectra of each sample.
Each gamma-ray spectrum obtained was analyzed by a dedicated GANAAS computer
software (IAEA, 1991), which performed a simultaneous fit to all the significant
photopeaks appearing in the spectrum, to provide reports with summaries on
information that include centroid energy, net counts, background counts, intensity
29
(count rate) and width (FWHM) of identified and unidentified peaks in the spectrum
(Appendix 4). The naturally occurring radionuclides e32Th, 238U and 40K) were
identified and evaluated for radioactivity by using the spectra of RGTh-1, RGU-1 and
RGK-1 standard reference materials, respectively. These reference materials, obtained
from the IAEA, were selected for this analysis because they are mixtures of
geological materials and are, therefore, suitable for analysis of other geological
materials like soil.
4.2.4 Calculation of Radioactivity
The radioactivity concentration of 232Th was estimated US1l1gthe average of the
radioactivity concentrations of 212Pb (at 238 keY), 208Tl (at 583 keY) and 228Ac (at
911 keY) while the radioactivity concentration of 226Ra was estimated from the
aver-age of the radioactivity concentrations of 214Pb (at 295 keY and 352 keY) and
214Bi(at 609 keV). Radioactivity concentration of 40K was determined directly using
its 1460 keV gamma line. Radioactivity concentration of each radionuclide in each
sample was determined using the method of radioactivity comparison with standard
reference material earlier described in section 3.3.
4.2.5 Detection Limits
The detection limit, LDi of a radionuclide i in a spectrum was evaluated according to
the following expression (Derbertin and Helmer, 1988):
(4.2)
where Ni, is the background count of nuclide i in the laboratory environment.
30
4.3 GAMMA RADIATION DOSE RATES
The likely absorbed radiation dose rate in air at 1 metre above the ground surface was
evaluated according to expression 3.8 (see section 3.4). The absorbed gamma
radiation dose rate in air was mainly determined from the radioactivity concentrations
(Bqkg-1) of 232Th series, 238U series and 4oK, and their respective dose conversion
factors (reGyh" per Bqkq'). In the UNSCEAR recent reports (UNSCfAR, 1993;
\
2000), the Committee used 0.7 Svfry' for the conversion coefficient from absorbed
dose in air to effective dose received by adults, and 0.2 for the outdoor occupancy
factor. In this study, the same value of conversion coefficient (0.7 SvGy-l) was used
but an average value of 0.4 was used for the occupancy factor because of
geographical, climatic and social factors, which allow residents of the proposed
mining areas to stay outdoors for longer periods than the average global period of
outdoor occupancy. Kwale district and Kenya in general, lies within the tropics where
daylights are longer and daily temperatures are higher than in many other areas of the
world. Also, the residents of the area of study generally spend long periods of time
socializing outdoors. Using these values, effective dose rate outdoors in units of
IlSVil was calculated from equation 3.9.
31
Chapter 5
Results and Discussion
5.1 QUALITY CONTROL AND ASSURANCE
To ensure reproducibility of results, the stability of the spectrometer was monitored
regularly by measuring the energy resolution of the detector prior to each sample
counting, This was done throughout the period of study and an average energy
resolution of 2.1±O.3 keV was obtained at the 1.33 MeV reference gamma line of
Background radioactivity in the laboratory environment was measured under identical
conditions using distilled water in a Marinelli beaker. This background radioactivity
was measured prior to each sample measurement and was later subtracted from the
sample spectra.
Also, detection limits of the detector for 232Th, 238U (or 226Ra) and 40K were
determined. The average values of these detection limits are listed in Table 5.1.
Table 5.1: Mean detection limits of the HpGe detector
Radionuclide Detection limit, LD, (Bqkg-l)
232Th 6.23±O.087
5.49±O.034
5.92±O.006
The standard reference materials (RGTh-l, RGU-l, and RGK-l) were analyzed as
unknown samples and their respective radioactivity levels determined. These
measured values were then compared with the certified values of these standards to
32
determine the accuracy and reliability of measurements. The radioactivity levels of
232Th 238U 40 . ,, and K obtamed for the standard reference materials are presented in
Table 5.2.
Table 5.2: Measured and Certified values of radioactivity (Bqkg-1) of the
reference materials RGTh-l, RGV-l, and RGK-l (IAEA, 1987)
Reference Measured Certified Deviation, D
Material Radioactivity, (Am) Radioactivity, (Ae) (%)
RGTh-l e32Th) 3026±251 3280 7.7
RGU-l (238U) 45l2±379 4910 8.1
RGK-l (40K) ll632±1203 13370 8.5
A plot of the measured values against the certified values of radioactivity for these
standard materials is shown in Figure 5.1 below. The plot shows that the measured
values are in good agreement with the certified values. (The correlation coefficient =
0.999). The relative deviations of the measured activities from the certified activities
are between 7.7% and 8.5%. Representing the measured values of radioactivity of the
standard reference material (SRM) with y and the certified values with x, the equation
of the line of best fit for the values in table 5.2 is y = O.85x + 287, which is the
equation of a straight line. These measurements of the reference activities suggest that
the accuracy of the measurements of radioactivity levels in the soil samples is
acceptable and reliable.
33
15
~ 14
OJ. 13~
0co 12'-r; 11.s;:
..0 10uro 90
'"0 8ro~
'"0 7
(\)
I-< 6;::l
U)ro 5(\)
~ .Ij
3
2
2 4 6 8 10 12
Certified Radioactivity (Bqkg-I)
14 16 18
Fig. 5.1: Correlation of measured values with certified values of radioactivity 01
reference materials
5.2 RADIOACTIVITY CONCENTRATIONS OF RADIONUCLIDES
A typical gamma-ray spectrum of the tested soil samples is shown in Figure 5.2
below. As can be seen from the spectral data, only naturally occurring radionuclides
were observed in the soil samples that were analyzed. This is not unexpected since
there are no nuclear industries in Kenya, including in the environment of the study
area.
34
100000
<II§
81000
o\c-228(9ll keY)
Bi-214(609 keY)
K-40(1461 keV)
I
Bi-214
(1764 keY)
Pb212 (238 keV)
10000 :>:'"" ~ Ac-228
~ ~ (338 keV)..Qt!-~
TI-208
(583 keY)
100
100 300 500 700 900 llOO 1300 1500 1700
Energy (ke'0
Fig. 5.2: Typical gamma-ray spectrum of a soil sample
Tables 5.3a, 5.3b and 5.3c present the values of radioactivity concentrations of 232Th,
226Ra e38U) and 40K in the soil samples that were analyzed. The samples have been
named KTMI through KTM26 e.g., KTMI means sampling point number 1 (an
abbreviation of Kwale Titanium Mines sampling point 1) and A, B, and C are the
three replicate samples from the same sampling point. Table 5.4 presents a summary
(minimum, maximum and mean) of these radioactivity concentrations for each
radionuclide in the soil samples analyzed.
35
T bl 53 R di .. . I 232a e . a: a ioactivity concentration (Bqkg ) of Th in soil samples
Sampling Site Sample A Sample B Sample C Mean Value
KTMI 17.2±2.9 17.6±1.5 16.5±3.0 17.1±O.3
KTM2 34.1±3.1 33.2±O.9 33.8±1.9 33.7±O.3
KTM3 7.9±O.6 8.1±1.1 9.2±1.6 8A±OA
KTM4 11.1±2.1 10.5±lA 1O.5±1.3 lO.7±O.2
KTM5 13.2±1.9 13.8±2.0 16.8±2.2 14.6±1.1
KTM6 42.7±2.3 41.6±3.5 46.5±2.8 43.6±1.5
KTM7 37.8±2.1 41.1±3.8 38.7±3.7 39.2±1.0
KTM8 20.3±2.9 22.6±3.0 20.1±1.9 21.0±O.8
KTM9 39.0±2.5 38.5±3A 35.6±2.6 37.7±1.1
KTMIO 32.9±3.0 33.1±2.8 35.7±1.9 33.9±O.9
KTMll 32.l±2.5 34.8±2.0 31.2±2.5 32.7±1.1
KTM12 40.2±2.1 35.9±3.3 33.7±1.7 36.6±1.9
KTM13 37.7±3.2 38.1±2.7 42.1±3.8 39.3±1.4
KTM14 20.3±3.1 24.6±2.0 20.2±1.9 21.7±1.5
KTM15 30.3±2.0 29.9±3.2 34.6±3.5 31.6± 1.5
KTM16 32.1±2A 32.5±2.7 33.8±3.1 32.8±O.5
KTM17 27.9±3A 31.4±1.9 28.3±2.0 29.2±1.1
KTM18 19.3±l.O 17.7±2.1 18.5±1.1 18.5±O.5
KTM19 25.7±1.2 27.0±1.6 30.1±?.2 27.6±1.3
KTM20 25.1±2.5 26.1±2.3 27A±1.9 26.2±O.7
KTM21 21.5±1.8 22.6±2.1 20.1±2A 21.4±O.7
KTM22 17.2±3.1 29.9±2.5 32.3±2.2 29.8±1.5
KTM23 30.2±1.7 27.1±3.3 27.3±1.9 28.2±1.0
KTM24 32.2±3.0 32.8±2.1 30.7±2A 31.9±O.6
KTM25 30.1±2.5 28.1±2.5 26A±3.2 28.2±1.1
KTM26 22.1±1.9 23.0±1.9 19.7±1.8 21.6±l.O
36
Table 5.3b: Radioactivity concentration (Bqkg-1) of 226Ra e38U) in soil samples
Sampling Site Sample A Sample B Sample C Mean Value
KTMI 13.5±l.O 12.9±1.8 15.9±2.1 14.l±O.9
KTM2 25.5±2.1 28.9±3.1 24.2±2.8 26.2±1.4
KTM3 7.9±O.8 6.3±1.9 8.0±1.9 7.4±O.6
KTM4 8.9±1.2 9.l±1.7 10.8±1.9 9.6±O.6
KTM5 12.7±2.0 13.2±3.4 10.4±2.2 12.1±O.9
KTM6 38.3±2.8 41.2±4.3 42.3±3.8 40.6±1.4
KTM7 31.3±3.9 29.9±2.7 29.1±1.9 30.1±O.6
KTM8 14.4±2.0 15.9±1.0 16.5±2.2 15.6±O.6
KTM9 30.6±2.5 31.O±3.4 33.2±3.3 31.6±O.8
KTMI0 25.1±2.7 29.9±2.8 26.0±3.0 27.0±1.5
KTMll 22.1±2.1 20.9±2.8 18.2±1.6 20.4±1.2
KTM12 23.3±1.1 22.4±1.4 22.1±2.0 22.6±O.4
KTM13 33.5±2.2 31.9±3.0 32.7±3.4 32.7±O.5
KTM14 18.3±1.8 16.9±1.0 16.4±2.1 17.2±O.6
XTM15 24.2±2.8 26.7±2.5 25.9±3.O 25.6±O.7
KTM16 22.8±2.2 25.1±2.5 25.0±1.9 24.3±O.8
KTM17 19.0±2.1 17.8±2.0 17.5±1.9 18.1±O.5
KTM18 14.2±O.8 16.3±1.9 16.J±1.8 15.6±O.7
KTM19 20.4±2.7 lS.8±2.2 20.5±2.8 19.9±O.6
KTM20 15.0±1.1 13.9±2.3 14.9±1.9 14.6±O.4
KTM21 16.6±2.0 17.9±2.7 17.4±O.9 17.3±O.4
KTM22 22.9±1.2 24.1±2.4 23.5±2.1 23.5±O.3
KTM23 18.3±1.7 16.9±1.9 lS.2±3.0 17.S±O.5
KTM24 24.l±2.1 22.2±3.1 23.6±2.4 23.3±O.6
KTM25 19.1±2.5 17.6±3.3 17.9±2.4 18.2±O.5
KTM26 lS.3±2.0 17.1±1.1 17.7±1.8 17.7±O.3
37
If r 11V " TT A I 11.11" 1'"" '" •-.. • • - - - - .
Table 5.3c: Radioactivity concentration (Bqkg-1) of40K in soil samples
Sampling Site Sample A Sample B Sample C Mean Value
KTMI 63.2±5.5 66.l±6.0 67.8±4.0 65.7±1.3
KTM2 76.8±2.7 79.4±7.1 75.7±5.7 77.3±1.1
KTM3 32.0±2.4 29.6±2.8 34.l±3.5 31.9±1.3
KTM4 67.7±6.5 66.6±7.0 60.l±3.7 64.8±2.4
KTM5 45.9±3.8 48.9±4.9 39.0±1.9 44.6±2.9
KTM6 54.1±3.0 57.9±2.5 56.0±5.0 56.0±1.1
KTM7 52.l±2.4 53.2±3.3 56.1±1.9 53.8±1.2
KTMS 67.9±2.0 69.l±3.2 62.5±2.7 66.5±2.0
KTM9 55.3±2.1 50.8±2.6 55.0±4.0 53.7±1.5
KTMIO 66.1±6.0 60.9±3.9 66.8±6.4 64.6±1.9
KTMll 73.3±3.l 72.9±5.8 71.3±4.9 72.5±O.6
KTM12 78.9±5.4 75.7±4.6 77.6±4.4 77.4±O.9
KTM13 47.8±4.8 44.6±3.5 49.2±3.3 47.2±1.4
KTM14 77.0±2.9 79.9±4.5 75.9±2.8 77.6±1.2
KTM15 76.l±3.3 79.3±4.5 73.5±4.0 76.3±1.7
KTM16 86.8±5.2 88.l±5.5 89.7±4.5 88.2±O.8
KTM17 85.4±5.8 86.6±6.0 81.2±4.3 84.4±1.6
KTM18 70.7±3.4 72.9±3.9 70.6±4.4 71.4±O.8
KTM19 112.l±3.3 116.7±4.0 113.5±2.2 114.1±1.4
KTM20 95.5±3.4 90.2±4.4 93.6±4.0 93.1±1.6
KTM21 82.2±5.0 79.9±6.1 78.5±4.2 80.2±1.1
KTM22 68.7±2.2 66.4±2.8 63.5±4.1 66.2±1.5
KTM23 75.5±2.2 73.4±5.0 75.2±2.7 74.7±O.7
KTM24 65.5±5.5 66.3±4.3 65.6±3.5 65.8±O.3
KTM25 73.2±3.8 71.9±3.7 70.0±3.0 71.7±O.9
KTM26 67.7±3.9 64.9±2.6 65.7±5.5 66.1±O.8
38
In the present study, the radioactivity of 238Uhas been taken to be the same as that of
226R 'h' h . 238.a, W IC IS a member of the U decay series. Therefore, radioactivity values of
226R d . 238a are presente III place of those of U. The presentation of values of
radioactivity of 226Ra in place of those of 238Uis based on the fact that in the 238U
decay series, 226Rais the chief nuclide of environmental interest as it and its progeny
produce up to 98% of the radiation from the series.
In tables 5.3a, 5.3b and 5.3c it is shown that radioactivity concentrations ranged
between 8.4±O.4 and 43.6±1.5 Bqkg-', with a mean value of 27.6±1.7 Bqkg" for
232Th,between 7.4±0.6 and 40.6±1.4 Bqkg' with a mean value of20.9±1.5 Bqkg-1 for
226Ra and between 31.9±1.3 and 114.1±1.4 Bqkg-1 with a mean value of 69.5±3.2
Bqkg-1 for 40K(Table 5.4).
Table 5.4: Minimum, maximum and mean radioactivity concentrations (Bqkg-1)
of 232Th, 226Ra and 40K in Soil
Nuclide Minimum Maximum Mean
Radioactivity Radioactivity Radioactivity
232Th 8.4±0.4 43.6±1.5 27.6±1.7
226Ra 7.4±0.6 40.6±1.4 20.9±1.5
40K 31.9± 1.3 114.1±1.4 69.5±3.2
The average values of radioactivity of both 232Thand 226Rain the present study (Table
5.4) are similar to those reported for the sediments along the Kenyan coastline
(Hashim et al., 2004) and to the normal global averages (UNSCEAR, 2000).
However, the radioactivity of 40K in the present study is lower than that reported in
the cited literature. The average radioactivity concentration of 40K in the samples
analyzed (69.5±3.2 Bqkg-1) was lower than the global average of 400 Bqkg-1•
Radioactivity concentration of 40K at the Kenyan coast has previously been reported
39
to be 401.7 Bqkg-' (Hashim et al., 2004). One possible cause of the low level of
radioactivity of 40K in the analyzed soil could be leaching of the potassium from
upper layers of soil to lower layers, considering the fact that potassium forms soluble
salts and the sandy soils analyzed in this study easily permit leaching of dissolved
minerals from upper to lower layers of soil.
Figures 5.3a, 5.3b, and 5.3c represent the mean values in Tables 5.3a, 5.3b and 5.3c.
In each case, both a bar chart and a normal probability distribution curve are
presented for comparison. The measured values differ slightly from normal
probability distributions. The values of skewness of the measured values relative to
normal probability distributions are -0.40, 0.62 and 0.25 respectively, for 23Zrh, 226Ra
and 40K. The values of kurtosis of these measured values are -0.42, 0.55 and 1.56
. 232 h 226Ra d 40Krespectively, for T, an .
2
6~----------------------------------,
5
1
7.5 12.5 17.5 22.5 27.5 32.5
10.0 15.0 20.0 25.0 30.0 35.0
Radioactivity of232Th (Bqkg")
o
Fig. 5.3a: 232Th radioactivity levels in soil
40
7~---- ~
6
3
2
1
o
7.5 12.5 17.5 22.5 27.5 32.5 37.5
10.0 15.0 20.0 25.0 30.0 35.0 40.0
Radioactivity of22~ (Bqkg-l)
Fig. S.3b: 226Ra radioactivity levels in soil
10~------------------------------------.
8
4
2
o
30.0 40.0 son 60.0 70.0 80.0 90.0 100.0 110.0
'"X radioactivity (Bqkg-l)
Fig. S.3c: 40K radioactivity levels in analyzed soil
The sample with the highest value of radioactivity of 232Thalso had the highest value
of radioactivity of 226Ra.Similarly, the sample with the lowest radioactivity of 23~h
41
was the one with the lowest radioactivity of 226Ra.This suggests existence of a linear
correlation between the values of radioactivity of 232Th and 226Ra in the samples
analyzed. The ratios of radioactivity concentrations of 232Thto those of 226Rae38U) in
the soil samples varied between 1.1 and 1.8 with a mean of 1.3±0.O1. The
d· " . 232 226ra roactrvity concentrations of Th and Ra obtained in this study actually had a
strong linear correlation between them (correlation coefficient = 0.93). There was
only a weak linear correlation between 232n1and 40K (correlation coefficient = 0.12)
and between 226Ra and 40K (correlation coefficient = -0.08). A strong correlation
between 226Raand 232Thradioactivity means that the presence of 232Thin a sample is
accompanied by the presence of 226Ra in the same sample. A strong correlation
b 226R d 232Th . . di f h f si if fetween a an IS an III rcation 0 t e presence 0 signi icant amounts 0
monazite and zircon sands in the soil (Mohanty et al., 2004). Monazite is a natural
orthophosphate, which can incorporate large quantities of actinides (thorium and
uranium) in its crystal structure during formation (Overstreet, 1967; Gramaccioli and
Segalstad, 1978; Emden et al., 1997).
A weak linear correlation between 232Th and 4oK, and also between 226Raand 4oK,
implies that the presence oe32Th (or 226Ra)in a soil may not indicatepresence of40K.
The weak correlation between 40K and 226Ra and between 40K and 232Th in the
samples also suggests that individual results of 40K and 226Raor 40K and 232Thare not
dependent on each other. Figures S.4a, S.4b, and S.4c show the variations between the
di .. . f 226R d 232Th h f 232Th d 40K d th fra ioactrvity concentrations 0 a an , t ose 0 . an ,an· ose 0
226Raand 4°K, respectively.
42
~ 50
C/j~0"
CO'-" 40ro~
\0
N
N
4-< -}
0 30»......,';;: :.}~'';::: .)oro 20 -}0
~ •..)ro~
10
60
10 20 30 40 50 60
Radioactivity Of232Th (Bqkg-')
Fig. 5.4a: Variation of radioactivity concentration of 232Thwith that of226Ra
.150~
OJ) HO~ .1300"
CO .120<;»
~ ::,'}
0 .1.10-e-
4-< 1000
0 90 {$ ox';;: ,$••..... 80 ~ ;. 'ill.•...• '~,.: ;'i!; '~i<lU .~ ,~ro 70
0 " .;~ ~.:;< 0"0 60ro ".).;) )~ 50 :;>{)
40
30 <)
20
.10
.10 20 30 40
Radioactivity of 232Th (Bqkg-')
50
-
I
60
Fig. 5.4b: Variation of radioactivity concentration of 232Thwith that of 40K
43
150 I-
----- 140 I-"7 130co~ 120cr'
CO 110 J>'-"
~ 100 I-a 90 3""'-H "0 80 ~. , .-:~~>-. 70 > , . t;.•....
" .; O;} .,} "":~ :;.60 I-.•.... .-t' ;.~U 50 I-ro ,;;0 40
'U 30ro0::: 20
10
10 20 30 40 50 60
Radioactivity of 226Ra CBqkg-l)
Fig. 5.4c: Correlation of radioactivity concentration of 226Rawith that of 40K
5.3 HUMAN EXPOSURE TO GAMMA RADIATION
The distribution of radionuclides in surface soil was assumed to be uniform. Absorbed
dose rates in air outdoors due to 232Thand 238Udecay series products and 40K for the
soil samples were then calculated from the radioactivity concentrations of the
respective nuclides using equation 3.8 in section 3.4.
TIle values of absorbed gamma radiation dose rate obtained for each of the 26
sampling points are presented in Table 5.5, which also shows corresponding values of
effective dose rates. As shown in the table, the values of total absorbed gamma
radiation dose rates range between 8.5±O.5 and 36.9±1.1 nGyh-1 with a mean of
25.2±1.4 nGyh'. Corresponding values of total effective gamma radiation dose rates
(obtained using equation 3.9) range between 21.0±1.2 and 90.8±2.6 /lSvy-l with a
mean value of 62.0±3.5 uSvy' (Table 5.6).
44
Table 5.5: Absorbed and effective gamma radiation dose rates in soil
Sample Absorbed dose rate (nGyh-i) Effective dose rate (JlSvy-i)
KTM1 17.l±O.6 42.0±lA
KTM2 31.0±O.7 76.2±1.8
KTM3 8.5±O.5 21.0±1.2
KTM4 11.9±OA 29.2±1.1
KTM5 14.1±1.0 34.8±2.6
KTM6 36.9±1.1 90.8±2.6
KTM7 34.5±O.8 84.9±2.0
KTM8 19.7±O.7 48.5±1.8
KTM9 34.3±O.9 84.3±2.3
KTMI0 30.9±1.1 76.0±2.8
KTM11 28.0±1.0 68.9±2.6
KTM12 31.1±1.2 76.5±2.9
KTM13 35.3±1.0 86.8±2A
KTM14 21.1±1.0 52.0±2.5
KTM15 29.6±1.1 72.8±2.8
KTM16 30.2±O.6 74.3±1.5
KTM17 25.7±O.8 63.3±2.0
KTM18 18.6±O.5 45.7±1.3
KTM19 26.7±1.0 65.7±2A
KTM20 23.1±O.5 56.9±1.3
KTM21 21.1±O.6 52.0±1A
KTM22 27A±1.0 67.5±2A
KTM23 24.7±O.7 60.8±1.8
KTM24 28.5±O.6 70.0±lA
KTM25 24.7±O.8 60.9±1.9
KTM26 20.8±O.7 51.3±1.7
45
Table 5.6: Minimum, maximum and mean absorbed (nGyh-') and effective
(1lSvy-') gamma radiation dose rates in the tested soil samples
Minimum Maximum Mean
Absorbed dose rate
Effective dose rate
8.5±0.5
21.0±1.2
36.9±1.1 25.2±1.4
90.8±2.6 62.0±3.5
In tel111Sof individual radionuclides, 232Th seemed to contribute higher average
absorbed radiation dose rate (14.6 nfiyh') than 226Ra (8.1nGyh-') and 40K (2.7 nGyh-
I) as shown in Figure 5.5, which compares the percentage contributions to gamma
radiation of these nuclides in the analyzed soil with respective global average
contributions. The relative (%) contribution to radiation dose rate by 40K was 11%,
while contributions due to radionuclides 226Ra and 232Th and their decay progeny were
32% and 57%, respectively. It is estimated that 232Th contributes 40%, 238U e26Ra)
25% and 40K '35% to global terrestrial radiation (UNSCEAR, 1988). The relative (%)
contributions of 232Th and 226Ra to radiation dose rate in this study were higher than
their global averages while the relative contribution of 40K to radiation dose rate was
lower than its global average value.
46
- [jKTMSoi1-~
I_~Global Average
c 60 ''-~~----------------~ ~..g 50---
;:l:-3 40 --.:....B 30-oU 20---
~ 10 -t--f ~:\:·;t"':'f
O+-~~--L-,-~~ __L-,-~~ __~
F' 5 5 C t ibuti f 226R 232 4019. .: on rr utions 0 a, Th and K to gamma radiation dose rate
226Ra
Nuclides
According to the UNSCEAR reports (1993, 2000), the worldwide average absorbed
gamma radiation dose rates range between 8 and 93 nGyh-l. The population-weighted
values give an average absorbed dose rate in air outdoors from terrestrial gamma
radiation of 60 nGyh-1, which is higher than that obtained in the present study
(25.2±1.4 nfiyh") by a factor of more than two. Based on this comparison, the
radiation dose rate from the soil samples around the proposed titanium mmes IS
considered low, presenting little or no radiological risk.
The mean effective radiation dose rate (62.0 u.Svy') obtained in this study is below
the normal global background radiation average of 460 uSvy' (UNSCEAR, 1993).
The values of effective radiation dose rate obtained in this study are in close
agreement with those obtained earlier by Hashim et al., (2004) for the Kenyan
coastline but they are far below the range 13.7-106.7 msvy" reported for the nearby
Mrima hill, a monazite mineralized area (Patel, 1991). The values of radiation dose
rates obtained in tins study are also below the ICRP-recommended limits of exposure
of workers and members of the public (ICRP, 1991).
47
'--
The values of absorbed and effective radiation dose l.!+esin Table 5.5 are represented
by the charts in Figures 5.6a and 5.6b. A fitted normal distribution curve indicates that
the radiation dose rates are approximately normally distributed.
6~------------------------------------.
3
2
o
7.5 12.5 17.5 22.5 IT.5 32.5 37.5
10.0 15.0 20.0 25.0 30.0 35.0
Absorbed Dose rate (nGyh-1)
Fig. 5.6a: Distribution of absorbed dose rate (nGyh-1) in the analyzed soil
samples
5
2
o
20.0 30.0
25.0
40.0 SOD SO.O 70.0 80.0 90.0
35.0 ~.o ~.o ~.o ~.o ~.o
Effective Dose rate (J.1Svy·l)
Fig. 5.6b: Distribution of effective dose rate (,.svy-l) in analyzed soil samples
48
llJ:lIVATTA IItJl\l~a~ITV I tDDllD\
The above effective gamma dose rates, DE, was used to estimate the annual number ,
N, of gamma radiation exposure-induced deaths in a population, Ns. A dose-to-risk
conversion factor (DRCF) of 6% per sievert (Jelfs, 1992) applied to the mean
effective dose rate of 62.0 microsievert per year indicates that in a population, Ns, of
3,000 people (equal to the estimated population of the proposed mining area) exposed
to gamma radiation from this soil, the number of likely deaths N, due to cancer caused
by this radiation is:
N = DE x DRCF x Np
N = 62.0 X 10-6 sievert x 6% per sievert x 3000 people ~ 0.01 people.
Thus the estimated average number of people likely to die due to exposure to gamma
radiation from this soil is negligible. However, this number may vary because the
dose-to-risk conversion factor varies with parameters such as sex, age, critical organ
and sensitivity to radiation-induced cancer of the individuals exposed to radiation.
49
Chapter 6
Conclusions and Recommendations
6.1 CONCLUSION
In this study, the radioactivity concentrations of natural radionuclides in surface soil
around the proposed titanium mines in Kwale district are reported. Corresponding
gamma radiation dose rates have been calculated from these activities and are also
reported.
For the soil samples that were investigated, the levels of radioactivity concentrations
vary between 8A±OA and 43.6±1.5 Bqkg-1 with a mean of27.6±1.7 Bqkg-1 for 232Th,
between 7A±0.6 and 40.6±1.4 Bqkg-1 with a mean of 20.9±1.5 Bqkg-1 for 226Ra and
between 31.9±1.3 and 114.1±1.4 Bqkg-1 with a mean of 69.5±3.2 Bqkg-1 for 4oK.
The values of radioactivity of 232Th and 226Ra are comparable with the global
averages of 30 and 35 Bqkg-1, respectively, and with those earlier reported for the
Kenyan coastline. However, the mean radioactivity concentration of 40K in this
investigation was found to be far below the global average of 400 Bqkg-1. Therefore,
this investigation shows that surface soils in the area around the proposed titanium
mines contain low levels of the naturally occurring radionuclides.
The total average effective gamma radiation dose rate from 40K and the progeny of
232Th and 226Ra in the soil has been found to be 62.0±3.5 uSvy', which is below the
dose limit of 1 m'Svy' for the general public. This dose rate is also below the global
normal background level of 460 uSvy'. The average effective radiation dose rate
obtained in this study shows that in the current population of about 3,000 people
living in these areas, the number of radiation-induced deaths should be negligible.
50
6.2 RECOMMENDATIONS
The values of radioactivity of natural radionuclides that have been obtained in this
study will be of great use in assessing the future environmental impact of the
proposed heavy mineral mining in Msambweni division, Kwale district. However,
the study of the radioactivity of soil alone may not result in complete description of
the radiological state of an environment. It is, therefore, recommended that the other
environmental materials, e.g., air and water in these areas be analyzed for
radioactivity and quality parameters prior to the commencement of large-scale mining
operations.
A numerical simulation of spatial and vertical distribution of the radionuclides in the
soil depth profile up to 40 m, where heavy sands are concentrated, is also
recommended.
51
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Appendices
APPENDIX 1: Gamma-ray spectrum ofthe calibration standard (SRM-l)
100000
I137cs(66166 keY) 6060 (;0CQ I0332.S0keV)(J17324keVl
I
10000 24 lAm
1(59.54 keV),
i
1000 .}
~~~.~54~~
100
10
500 1000
Energy- (keV)
1500 2000
56
APPENDIX 2: Uranium-238 e38U) decay series
57
APPENDIX 3: Thorium-232 e32Th) decay series
58
APPENDIX 4: Typical spectrum report of a soil sample analyzed by HpGe
detector-based Gamma ray spectrometer
PEAK ENERGY FWHM Net Background Net %Error
No. keV area Counts/s
01 27.56 2.19 22190 3456 0.417 1.04
02 77.30 4.13 2096 6092 0.039 13.12
03 93.25 1.42 1087 6120 0.020 23.55
04 106.65 1.68 333 3467 0.006 49.25
05 209.94 0.72 371 2180 0.007 30.73
06 239.11 2.05 4092 2059 0.077 4.10
07 295.85 2.24 923 2987 0.017 19.61
08 338.78 1.85 588 2210 0.011 19.61
09 352.56 2.26 1556 1725 0.029 0.23
10 511.77 3.38 1541 1269 0.029 7.72
11 559.16 2.46 267 799 0.005 20.46
12 583.96 2.08 880 1071 0.017 11.25
13 610.24 2.20 1069 1057 0.020 9.17
14 728.01 1.62 175 687 0.003 42.29
15 912.27 2.36 679 517 0.013 10.60
16 1121.74 2.46 245 367 0.004 30.85
17 1462.07 3.09 545 204 0.010 9.17
18 1765.46 2.53 191 127 0.004 17.80
59KENYATTA UNIVERSITY ll'BRARY