1

ASSESSMENT OF LEVELS OF NATURALLY OCCURRING RADIONUCLIDES AT

IRON MINING AND PROCESSING SITES IN THE MIDLANDS PROVINCE OF

ZIMBABWE

Dissertation presented to the Department of Chemistry, University of

Zimbabwe in partial fulfilment of the requirements for the degree of

Master of Science in Chemistry

BY

NYENGERAI MANJERU, BSc (Hons) (NUST)

SUPERVISOR: Dr PAUL MUSHONGA

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DECLARATION

I do hereby declare that this research study is my own work towards a Master of

Science degree in Chemistry and to my best knowledge it contains no materials

previously published by another person except where due acknowledgement has

been given

Student signature ……………………………. Date ………………………………

Supervisor signature………………………….. Date…………………………….

ii

ABSTRACT

Mining and mineral operations have been associated with exposures of Naturally occurring radioactive

materials. A study was carried out to assess the activity concentration of naturally occurring radionuclides

in iron mining, processing and waste materials by means of high resolution gamma ray spectrometry

using a 109 HPGe detector. A total of 12 sampling points were sampled for the Buchwa’s Ripple creek

Mine, Buchwa Limestone mine and ZISCO Steelworks damp site. The samples were measured in the

laboratory with respect to their gamma radioactivity for a counting time of 60000 seconds each. From

the obtained spectra, activity concentrations were determined. The mean activity concentrations of 238U,

232Th, 226Ra, 210Pb and 40K in the soil samples were, 220.95, 32.99, 181.60, 112.99 and 276.24 Bq/kg

respectively. The study also highlighted the potential hazards of alpha, beta and gamma emissions on

human body and their mechanisms for inhalation and ingestion. The total effective dose was 0.4275

mSv per annum. The results in this study compared well with the average worldwide average values. The

results indicate an insignificant exposure of public to technologically enhanced NORM.

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ACKNOWLEDGEMENT

I would like to express my sincere gratitude to my supervisor Dr Paul Mushonga for his continuous

support of my Msc study and research, for his patience, motivation, enthusiasm, and immense knowledge.

His guidance helped me in all time of this research.

Beside my supervisor I would like to thank Dr Kugara, the chairman of Chemistry Department, all

lecturers and other staff in the Department for their encouragement, insightful comments and hard

questions.

I thank my fellow Msc students for stimulating discussions, for sleepless nights together working to meet

deadlines and all the fun we have had in the past 18 months. Also I would like to thank Mr R. M Severa

and my fellow workmates at the Radiation Protection Authority of Zimbabwe for the unwavering support

and encouragement.

Finally, I take this opportunity to express the profound gratitude from my deep heart to my wife, Mrs.

Jenitha Manjeru, my daughters Anotidaishe and Anesuishe Manjeru and my son Anold Manjeru and the

rest of the Manjeru family for supporting me spiritually and materially throughout my life.

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TABLE OF CONTENTS

Contents DECLARATION ............................................................................................................................................................... i

ABSTRACT .................................................................................................................................................................... ii

ACKNOWLEDGEMENT ................................................................................................................................................ iii

TABLE OF CONTENTS .................................................................................................................................................. iv

LIST OF FIGURES ......................................................................................................................................................... vi

LIST OF TABLES .......................................................................................................................................................... vii

LIST OF ABBREVIATION .............................................................................................................................................. vii

DEFINITIONS ............................................................................................................................................................... ix

1.0 INTRODUCTION ......................................................................................................................................... 1

1.1 General Introduction ..................................................................................................................................... 1

1.2 Radioactive decay ...................................................................................................................................... 2

1.2.1 Alpha –particle decay ....................................................................................................................... 3

1.2.2 Beta particle decay............................................................................................................................ 3

1.2.3 Gamma Ray emission ....................................................................................................................... 4

1.2.4 Radium Decay ................................................................................................................................... 5

1.3 AIM AND OBJECTIVES ......................................................................................................................... 5

1.4 JUSTIFICATION ...................................................................................................................................... 6

2.0 LITERATURE REVIEW ............................................................................................................................. 8

2.1 Background ................................................................................................................................................ 8

2.2 Sources of NORMs .................................................................................................................................... 9

2.2.1 Cosmic radiation .............................................................................................................................. 10

2.2.2 Terrestrial radiation ........................................................................................................................ 12

2.3 Hazards associated with Radon Exposure ............................................................................................ 18

2.5 Hazards associated with NORMs ........................................................................................................... 21

2.6 Biological effect of radiation ................................................................................................................... 23

2.6.1 Direct Action .................................................................................................................................... 23

2.6.2 Indirect Action ................................................................................................................................. 24

2.7 Instrumentation for measurement of natural radioactivity in environmental samples .................... 25

2.7.1 Principles of Gamma Spectrometry ............................................................................................... 25

2.7.2 High Purity Germanium detectors (HPGe) .................................................................................. 26

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2.7.3 Principles of Thermoluminescent Dosimetry (TLD) .................................................................... 27

3.0 RESEARCH METHODOLOGY ............................................................................................................... 29

3.1 Description of study area ........................................................................................................................ 29

3.2 Materials and procedures ....................................................................................................................... 34

3.2.1 Soil and rock sample collection and preparation.......................................................................... 34

3.2.2 Analysis of samples using direct gamma spectrometry ................................................................ 35

3.2.3 Energy calibration of the gamma ray detector ............................................................................. 36

3.2.4 Efficiency calibration ...................................................................................................................... 36

3.2.5 Determination of minimum detectable activity ............................................................................ 36

3.2.6 Evaluation of activities and index of mass activity ....................................................................... 37

3.2.7 Determination of activity concentrations ...................................................................................... 37

3.2.8 Calculation of annual effective dose from external gamma dose rate measurements ............... 37

3.2.9 Method for Environmental Dosimetry .......................................................................................... 38

4.0 RESULTS ....................................................................................................................................... 39

4.1 DISCUSSION ................................................................................................................................ 48

4.1.1 Average Activity concentration ...................................................................................................... 48

4.1.2 Absorbed dose and total annual effective dose ............................................................................. 50

4.1.3 Average Direct Contamination/ Dose rate measurements ........................................................... 50

5.0 CONCLUSIONS .......................................................................................................................................... 51

6.0 RECOMMENDATIONS ............................................................................................................................ 53

7.0 REFERENCES ............................................................................................................................................ 53

APPENDICES ......................................................................................................................................................... 59

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LIST OF FIGURES

Fig 2.4 Natural decay series of 40K

Fig 2.5 Block diagram for gamma spectrometry setup

Fig 2.6 Block diagram for TLD reader

Fig 3.1 Ripple Creek mine sampling pints

Fig 3.2 Ripple Creek mine sampling map

Fig 3.3 Limestone mine and ZISCO STEEL dump site sampling points

Fig 3.4 Limestone mine and ZISCO STEEL dump site sampling map

Fig 4.1 Trend of activity concentrations of 40K

Fig 4.2 Trend of activity concentrations of 210Pb

Fig 4.3 Trend of activity concentrations of 226Ra

Fig 4.4 Trend of activity concentrations of 232Th

Fig 4.5 Trend of activity concentrations of 238U

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LIST OF TABLES

Table 2.1 Average radiation exposure from natural sources

Table 2.2 Cosmogenic radionuclides

Table 2.3 Population weighted average cosmic dose rates

Table 2.4 Uranium 238 Natural decay series

Table 2.5 Uranium 238 Natural decay series

Table 2.6 Uranium 238 Natural decay series

Table 4.1 Energy calibration of the gamma ray detector

Table 4.2 The minimum detectable activities of K, Pb, Ra, Th and U

Table 4.3 Evaluation of activities and index mass activities

Table 4.4 Sample location and coordinates

Table 4.5 Average activity concentrations

Table 4.6 Average direct contamination / dose rate measurements

Table 4.7 Comparison of environmental dosimetry results to WHO standards

LIST OF ABBREVIATION

IAEA International Atomic Energy Agency

UNSCEAR United Nations Scientific Committee on the Effects of Atomic Radiation

ICRP International Commission for Radiation Protection

WHO World Health Organization

NORM Naturally Occurring Radioactive Materials

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DNA Deoxyribonucleic Acid

ALARA As Low as Reasonably Achievable

ASTM American Society for Testing Materials

GSR General Safety Requirements

TLD Thermoluminescence Dosimeter

MDA Minimum Detectable Activity

MCA Multi Channel Analyser

Bq Becquerel

Bq/Kg Becquerel per kilogram

µSv Micro Sievert

mSv Milli Sievert

nGy Nano Gray

eV Electron Volt

KeV kilo electron Volt

HPGe High Purity Germanium

U Uranium

Th Thorium

K Potassium

Ra Radium

Rn Radon

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DEFINITIONS

Radiation - is energy in the form of particles or electromagnetic waves

Radioactivity - is defined as spontaneous nuclear transformations in unstable atoms that result in

the formation of new elements.

Becquerel (Bq) - a unit of radioactivity equal to 1 disintegration (or transformation) per second

Dose - is a measure of the energy deposited by radiation in a target

Annual dose – the dose from external exposure in a year plus the committed dose from intake of

radionuclides in that year

Effective dose- is a measure of dose designed to reflect the amount of radiation detriment likely

to result from the dose

Environment- the conditions under which people, animals and plants live or develop and which

sustain all life and development, especially such conditions as affected by human activities

Protection of the environment- Protection and conservation of non-human species, both plant

and animal and their biodiversity

Hazard – the potential for harm or other detriment, especially for radiation risks; a factor or

condition that might operate against safety

Deterministic effect – a radiation induced health effect for which a threshold level of dose exists

above which the severity of the effect is greater for a higher dose

Stochastic effect- a radiation induced effect, the probability of occurrence of which is greater for

a higher radiation dose and the severity of which is independent of dose

Dose limit- the value of the effective dose or the equivalent dose to individuals in planned exposure

situations that is not to be exceeded

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1.0 INTRODUCTION

1.1 General Introduction

All living organisms are generally exposed to ionizing radiation from natural sources. These natural

sources include high energy cosmic ray particles and radionuclides that originate from the earth’s crust

and are present everywhere. Human beings are exposed to these natural sources through inhalation and

ingestion causing internal exposure and irradiation from external gamma rays causing external

exposure [1]. The main sources for internal exposures are the alpha and beta radiation.

The new General Safety Requirements (GSR) Part 3 of the International Atomic Energy Agency for

protection against ionizing radiation specify the requirements for protecting the health of people and

the environment against the harmful effects of radiation [2]. Ionizing radiation has two biological

effects on human body. These biological effects are either deterministic or stochastic effects. There are

no acute (short term) health effects associated with levels of radiation from naturally occurring

radioactive materials (NORMs). Stochastic effects generally occur without a threshold level of dose,

whose probability is proportional to the dose and whose severity is independent of dose [3]. Radiation

exposure has been associated with most forms of leukaemia and other types of cancers affecting various

organs such as lungs, breast and thyroid glands. Radiation induced cancer may develop decades after

exposure. The greater the dose (concentration of radiation, frequency and duration of exposure), the

greater the risk of health effects will be. The effects of radiation on the body must be kept as low as

reasonably achievable (ALARA principle) [4].

Naturally occurring radioactive materials (NORMs) are found everywhere on earth. Materials which

may contain the primordial radionuclides such as potassium, radium, thorium uranium and the

radioactive daughter nuclei may cause harm to human beings. Most of the decay products of either the

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thorium or uranium decay series are alpha or beta emitters which can cause internal exposure to delicate

organs of the body. However, the concentration of NORMs is generally low but it can be increased by

human activities which among them includes mining. When undisturbed, the radionuclides in the decay

series are more or less in radiological equilibrium. So human operations like mining and mineral

processes cause disequilibrium. As a result of different properties in the decay series and geochemical

migration and differences in their half-lives, radionuclides can potentially cause harm to biological life

[4, 5, 7].

1.2 Radioactive decay

Radioactive decay is a first –order process described by the equation below

dN/dt = -Nƛ (1)

Where dN/dt is the disintegration rate of a given radionuclide in which dN is the change in the number

of its atoms (N) that undergo nuclear decay in the time (t) interval, dt. The proportionality constant ƛ is

the decay constant with units of inverse time as s-1 . The solution of the differential equation is

Nf =Noe-ƛt (2)

The subscripts f and 0 respectively represent the final and initial number of atoms.

When unstable nucleus of radioactive isotopes undergoes a nuclear transformation it forms a new atomic

species with concomitant emission of one or more forms of radiation. The three basic types of radiation

are alpha, beta and gamma radiation. Three primary modes of nuclear decay are alpha-particle emission,

beta-particle emission, and gamma-ray emission. As a result, mass, charge and energy must be

conserved. Therefore, the sum of mass numbers A and the atomic numbers Z of the products (or

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daughters) must be equal to those of the initial radionuclide (the parent).The mass of the parent must be

equal to the masses of the daughter and emitted particles and mass equivalents of the kinetic energy of

the products .

1.2.1 Alpha –particle decay

Alpha decay is emission of an alpha particle which is equivalent to Helium nucleus. The decay decreases

the parent nucleus by two protons and two neutrons resulting in the total loss of mass of about 4 atomic

mass units (4Da) and a reduction in nuclear charge by two. The mass and charge are carried away by the

alpha particle, as shown by the equation below:

ZAX A-4

Z-2Y +42α (3)

During decay, there is momentum and energy evolved and this must be conserved and distributed

between product nucleus and emitted alpha particle. The alpha particles are low in penetrating power.

They can be stopped by few sheets of paper or by the outermost part of skin. However, these particles

can cause great damage to living tissue within very small distance of the atoms that produced them. The

risk exists only when alpha emitters, like polonium-218 and polonium-214 are inhaled into the lungs.

Some alpha emitters are also dangerous if swallowed, but radon decay products are mostly a problem in

the lungs.

1.2.2 Beta particle decay

Beta particle decay is either negatron emission or positron emission. Negatron emission occurs when

the nucleus has an excess of neutrons with respect to protons. The transition in effect converts a neutron

to a proton. Conversely, positron emission in effect converts a proton to a neutron to attain greater

stability. This occurs when the radionuclide neutron to proton ratio is efficient as compared to stable

nucleus. This can be showed by the equations below,

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AZX A

Z+1Y+v- +β- (4)

AZX A

Z-1Y+v+β+ (5)

Where v- represents the antineutrino and v is the neutrino. Neutrino and antineutrino emissions serve to

balance the energy and rotation before and after decay. Neutrinos have no charge and little mass,

consequently they interact to a vanishingly small degree with matter. The neutrino and antineutrino must

be included in the decay equation to conserve energy, angular momentum and spin. The proton, neutron,

beta-particle, and neutrino have a nuclear spin of ½.The neutrino and antineutrino groups carry more

than half of the decay energy, while beta –particle group carries less than half. This recoil energy, in the

range of few electron volts causes chemical changes such as displacement of an atom from a crystal

lattice.

Beta particles have medium penetrating power. They can penetrate into the skin and also can travel

some distance in the air. Even though beta particles can travel farther into the body, alpha particles cause

more damage inside the lungs than beta particles.

1.2.3 Gamma Ray emission

Gamma ray emission follows alpha and beta emission. The decay leaves the product nucleus in an

excited state. The nucleus is capable of further de-excitation to a lower energy state by the release of

electromagnetic energy. There is no change of mass and charge, only a change in the energy and spin of

the nuclide. So gamma rays are more penetrative. Gamma radiation can travel much more deeply into

objects than alpha or beta particles, and can pass through the body. While gamma radiation can penetrate

all the way through the body, the amount emitted by radon and its progenies is not as detrimental to the

lungs as alpha particles.

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1.2.4 Radium Decay

When a radium atom decays, radon gas is released into the surrounding air or water. Since radon-222

has a half-life of 3.8 days, it has enough time to move from its radium source into buildings, where both

the radon and its decay products can be inhaled, delivering a dose of radiation to the lung tissue. The

decay products: bismuth-214, polonium-218, polonium-214 and lead-214, have very short half-lives.

These decay products account for the major portion of the dose received in most situations and are the

primary source of radon health effects. Polonium-218 and polonium-214 are the alpha emitters that do

most of the damage. Bismuth-214 and lead-214 are beta emitters and also produce most of the gamma

radiation in the decay series.

The radon progenies are different from radon in several ways, including that :

They are short-lived (all less than 30 minutes).

They are left with static electric charges as a result of the radioactive decay that produced them.

They are chemically reactive.

They are solid particles, rather than gases, that act like an invisible aerosol in the air.

These properties show that they easily attach themselves to solid objects such as dust, smoke, walls,

floors, clothing, or any other object. If they attach to dust or smoke particles, then they can be carried

into the lungs, where they can lead to lung cancer

1.3 AIM AND OBJECTIVES

The main aim of this research was to assess the levels of NORMs in the mining, processing of iron and

its waste products focusing on the distribution of the naturally occurring radionuclides of the U/Th decay

series (radon-222 and its progenies, radium-226, thorium-232 and uranium-238 and 235) and

potassium-40.

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The specific objectives were:

To assess the activity concentrations of the radionuclides U/Th series , 40K at the mine and waste

dumpsites

To analyse selected samples by gamma spectrometry and environmental dosimetry

To determine the radiation doses from these activity concentrations and compare with

international recommended dose limits.

To recommend a suitable radiation protection programme for the mine if necessary.

1.4 JUSTIFICATION

There is a worldwide interest and concern expressed for the study of levels of naturally occurring

radionuclides and environmental radioactivity. In Zimbabwe, the industries that produce NORMs have

not been subjected to radiological regulatory control. Data on radionuclide concentrations in raw

materials, residues and waste and data on public exposures are scanty [5]. Consequently, there is lack of

awareness and knowledge of the radiological hazards and exposure levels by regulators, legislators and

operators. The studies on NORMs are useful for both assessment of public exposures and performance

of epidemiological studies. The studies also ascertain possible changes in environmental radioactivity

due to nuclear, industrial and other human activities.

In Zimbabwe, the focus has been on regulation on the use of artificial radionuclides which are used for

medical, industrial and research activities. Since the Zimbabwean economy is dependent on agriculture

and mining, the study on naturally occurring radionuclide concentrations in raw materials, residues and

waste will be vital for awareness of radiological hazards to the people. The data obtained will enable

regulators, legislators and operators to develop and establish guidelines for radiation protection from

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NORM. This will ensure that the health and quality of life to occupationally exposed workers and the

general public is improved and environmental sustainability is increased.

The public and occupationally exposed workers, who are the focus of this study, have little or no

understanding of radiation and its risks. In general, the perceptions about radiation derived from natural

sources, including radon and from artificial sources may be different. There is no understanding of the

biological effects from radiation sources [8]. The data of this study will be disseminated to the public,

thus increasing their knowledge and awareness on the issue of NORMs.

Also, Zimbabwe is in the process of formulating guidelines on setting standards for the regulation of

NORMs. The availability of data from such studies is very vital to all stakeholders involved to ensure

that people and the environment are protected from such harmful effects of radiation. Moreover, from

the stochastic effects of radiation which include cancer in organisms, this project will also supply data

so as to minimise the risk of cancer to the occupationally exposed workers and the surrounding

communities. This will be accomplished by reduction of technologically enhanced NORM waste.

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2.0 LITERATURE REVIEW

2.1 Background

Studies of the presence of naturally occurring radionuclides have been carried out in different industrial

activities such as zircon/zirconium industry, oil and gas extraction, coal and coal fired power

generation, phosphate industries, production of titanium dioxide pigments, mining and mineral

processing of metals such as gold, copper, aluminum. These studies reported that these industrial

activities are potential sources of elevated NORMs [9]. As stated by IAEA researches and publications,

the issue of NORMs and potential hazards were relegated to the background until studies on radon and

its progenies in the mining and processing of uranium ore showed that there were great radiation health

hazards associated with NORMs. As a result of these finding many studies to measure radon and other

naturally occurring radionuclides in mines other than uranium mines have been initiated [9].

Worldwide, there are a number of international meetings and symposia dedicated to discussing the

radiological consequences of NORMs, and these have contributed to worldwide concerns. Despite

these meetings and studies on NORMs, there is still lack of information on the awareness, their

radiological hazards and levels of exposure especially in the developing countries [10]. IAEA Technical

Report Series 419 concluded that most of the data from NORMs is from Europe and North America

and data from developing countries is scarce [6]. The technical series report highlighted some key

issues which concern developing countries with regards to radiation protection from NORMs. The key

issues include that;

Large portion of the world mines are in developed countries

Legislature on environmental radiation protection may be less stringent and the enforcement

may be less strict.

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Less stringent occupational health and safety on artisanal mining operations

Responsibilities of legacy wastes and contamination are unclear.

No or less resources for waste management infrastructures [10]

International Commission for Radiation Protection (ICRP) made several reports with respect to

occupational and public exposure situation on NORMs that have contributed significantly to the

awareness of the radiological consequences and risk associated with the NORMs [11]. The average

annual global effective dose to exposure of NORMs has been estimated to be 2.4 mSv with a typical

range between 1-10 mSv [12]. The main natural sources giving rise to this dose have been identified to

be cosmic rays, terrestrial gamma rays, inhalation of radon gas and ingestion of naturally occurring

radionuclides [12]. However, the IAEA is yet to arrive at an international consensus on the activity

concentration levels that could be used to apply to the concept of exclusion from regulatory control.

The activities considered for exclusion are 1 Bq/g for uranium and thorium radionuclides and 10 Bq/g

for 40K.

2.2 Sources of NORMs

All living creatures are exposed to ionising radiation from natural sources. The level depends on

location and latitude. According to reports from United Nations Scientific Committee on the Effects of

Atomic Radiation (UNSCEAR), the levels vary by factor of about 3 [1]. The main sources are cosmic

rays that come from outer sphere and from the surface of the sun and terrestrial radionuclides that occur

in the earth crust, building materials, water, food and the human body.

Both cosmic and terrestrial sources have been identified as the largest contributors to the collective

dose of the world population. Cosmic radiation has been identified to be intense at higher altitudes

whilst terrestrial concentration of Thorium and uranium in soils are higher in localized areas. The

concentration of 40K in food has been found to be constant everywhere in the world [1]

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Table 2.1 shows the world wide average of annual effective doses for the various sources;

Table 2.1 Average annual effective doses of exposure from natural sources [1]

Source Worldwide average annual effective dose, mSv

Typical

range

External

Cosmic rays 0.4 0.3-1.0

Terrestrial rays 0.5 0.3-0.6

Internal

Inhalation of radon 1.2 0.2-10

Ingestion 0.3 0.2-0.8

TOTAL 2.4 1.0 -10

2.2.1 Cosmic radiation

Cosmic radiation is as a result of the exposure to cosmic ray particles with nitrogen in the atmosphere.

This produces cosmogenic radionuclides which includes 3H, 7Be, 14C, and 22Na. The production of the

cosmogenic radionuclides is highest in the upper stratosphere but some cosmic rays, neutrons and

protons which survive in the lower stratosphere are able to produce the cosmogenic radionuclides as

well. The UNSCEAR has estimated the average annual effective dose worldwide at sea level to be 320

uSv with the directly ionising and non ionising radiation contributing 270 uSv and 48 uSv respectively.

It also states that the dominant component of cosmic ray field at the ground level are muons with

energies between 1 and 20 GeV and they contribute about 80 % of the absorbed dose rate in free air

from the directly ionising radiation. [1]. Further research studies showed that the population weighted

average absorbed dose rate from directly ionising and photon components of cosmic radiation at sea

level is estimated to be 280 uSv/year [1].

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UNSCLEAR reports on the assessment of the cosmogenic radionuclides showed the annual effective

doses of 12 uSv for 14C, 0.15 uSv for 22Na, 0.03 uSv for 7Be and 0.01 uSv for 3H. The cosmogenic

radionuclides are relatively homogenously distributed on the surface of the earth [1]. The cosmogenic

radionuclides, their half-lives and mode of decay are shown on Table 2.2 below. The population

weighted average cosmic ray doses are shown on Table 2.3.

Table 2.2 Cosmogenic radionuclides, half-life and their decay mode [1]

Isotope

Half

life Decay mode

3H 12.33a Beta (100%)

7Be 53.29d EC (100%)

10Be 1.56x106a Beta (100%)

14C 5730a Beta (100%)

22Na 2.602a EC (100%)

26Al 7.41x105 a EC (100%)

32Si 172a Beta (100%)

32P 14,26d Beta (100%)

35P 25.34d Beta (100%)

35S 87.51d Beta (100%)

36Cl 3.01x105a Beta (100%)

37Ar 35.04d EC (100%)

39Ar 269a Beta (100%)

81Kr 2.29x10 5a EC (100%)

Where a is years and d is days

Table 2.3 Population –weighted average Annual cosmic ray dose [1]

Annual Effective dose rates uSv

Conditions Global directly

Global

neutron Global total

ionising components component

Outdoors, at sea level 270 48 320

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Outdoors, altitude

adjusteda 340 120 460

Altitude, shielding and 280 100 380

Occupancy adjustedb

Where a – altitude weighting factors applied at sea level for directly ionising (1.25) and neutron (2.5)

b – building shielding factor of (0.8) and indoor occupancy factor of (0.8)

2.2.2 Terrestrial radiation

Most of the naturally occurring radionuclides have been in existence since the creation of the earth and

are known as primordial radionuclides. These include 238U , 232Th and 40K. 238U has a half-life of 4.47

x 109 years, while 232Th and 40K have half-lives of 1.141 x 1010 and 1.28 x109 years respectively. Other

primordial radionuclides of secondary importance are 235U and 87Rb. Thorium and uranium lead a series

of decay radionuclides which contribute to human radiation exposure [13].

There are three different radioisotopes of uranium atom which are 238U, 235U, and 234U. Naturally

occurring 238U contribute 99.3%, 235U contributes about 0.7% and trace quantities of about 0.005% is

234U. The 238U and 234U belong to the uranium series (4n+2) family, while 235U isotope belongs to the

actinium series (4n+3). Naturally occurring radioisotope 232Th is a member of the thorium series (4n).

The identification numbers are based on the divisibility of the mass numbers of each series by 4 [14].

The naturally occurring radionuclides exist in a secular equilibrium in natural undisturbed

environments. However, due to physicochemical processes in the earth’s crust such as leaching,

weathering, and emanation, the radiological secular equilibrium in each decay series is disturbed. It has

been established by researches that under normal undisturbed secular equilibrium conditions, the mass

and activity ratios of 235U to 238U are about 0.0073 and 0.046 respectively [11]. 40K undergoes beta

decay to stable 40Ca and electron capture to 40Ar. These radionuclides are present in different activity

13

concentrations in soil, water, air and living organisms. Consequently, humans are exposed both to

external and internal irradiations by alpha, beta particles and gamma rays with various ranges of

energies [13, 15, 16].

Natural sources of radiation exposure from inhalation of radon gas, external radiation from the ground

and building materials, ingestion of naturally occurring radionuclides and cosmic radiation contributes

about 80 % to the average effective doses worldwide [ 17] . About 5 % of this has been assessed to be

contributed by ingestion of uranium and thorium series [17]. Uranium exhibits chemical toxicity along

with radioactivity, which is why its concentration is expressed as a mass concentration instead of the

activity concentration. According to UNSCEAR, it expresses that a mean annual intake of 238U and

226Ra from public drinking water supplies in different countries has ranged from 0.3-50 Bq and 0.2-20

Bq, respectively (by employing an annual water ingestion rate of 500 litres) [12]. The mean annual

intake of uranium and radium in diet, including drinking water, has ranged in different countries from

2.9-57 Bq and 9-40 Bq, respectively [12].

The natural radioactivity in rocks depends on their nature with higher concentrations more common in

igneous rocks than in sedimentary and metamorphic, where the main contributing radionuclides are

238U, 232Th and 40K [18]. The rocks show a considerable variation depending on mineralogical,

petrographic, structural and chemical characteristics. The abundance of uranium and thorium in these

rocks, besides the initial concentration, depends on the history of post crystallization of the rocks. [18].

The Buchwa mine rock profile is a long gossanous ore which is made of weathered rock in layers.

Natural sources have become a major environmental and workplace concern to a growing group of

natural resource, medical and manufacturing industries. Natural environmental radioactivity arises

mainly from primordial radionuclides, such as 40K, and the radionuclides from the 232Th and 238U series

14

and their decay products, which occur at trace levels in all ground formations. Primordial radionuclides

are formed by the process of nucleosynthesis in stars and are characterised by half-lives comparable to

the age of the earth. There are different types of radioactive material found in nature but Iron industry

is only concerned with 232Th and 238U, 40K and their daughter nuclei [19]. Radium is of primary concern

not only because it is radioactive, but also because it is chemically toxic. Jobbagy et al reported that

radium may be almost as toxic as plutonium, the most toxic element to man [20].

Ra and Rn are the parent nuclides of decay chains that contain up to 20 radioactive daughters. This

means that Ra and Rn decay into products that are also radioactive and are therefore even more

significant hazards because their daughters will continue to irradiate the body tissues even after Ra and

Rn decayed.

U and Th decay in a series of unique steps known as decay series passing through a number of

transformations until a stable or non-radioactive isotope is reached as shown below:

15

Table 2.4 Uranium 238 Natural decay series [14]

Decay series

Decay Mode Half Life

Uranium -238

Alpha 4.5 x 109 years

Thorium -234

Beta 24 years

Protactinium -234m

Beta 1.2 minutes

Uranium -234

Alpha 240,000 years

Thorium -230

Alpha 77,000 years

Radium -226

Alpha 1,600 years

Radon -222

Alpha 3.8 days

Polonium -218

Alpha 3.1 minutes

Lead -214

Beta 27 minutes

Bismuth -214*

Beta 20 minutes

Polonium -214

Alpha 160 microseconds

Lead -210

Beta 22 years

Bismuth -210

Beta 5.0 days

Polonium -210

Alpha 140 days

Lead -206 (stable)

16

Table 2.5 Uranium 235 Natural decay series [14]

Decay series

Decay Mode Half Life

Uranium -235

Alpha 7.0 x 108 years

Thorium -231

Beta 26 hours

Protactinium -231

Alpha 33,000 years

Actinium -227

Beta 22 years

Thorium -227

Alpha 19 days

Radium -223

Alpha 11 days

Radon -219

Alpha 4.0 seconds

Polonium -215

Alpha 1.8 milliseconds

Lead -211

Beta 36 minutes

Bismuth -211*

Alpha 2.1 minutes

Thallium -207

Beta 4.8 minutes

Lead -207 (Stable)

17

Table 2.6 Thorium 232 Natural decay series [14]

Decay series

Decay Mode Half Life

Thorium -232

Alpha 1.4 x 1010 years

Radium -228

Beta 5.8 years

Actinium -228*

Beta 6.1 hours

Thorium -228

Alpha 1.9 years

Radium -224

Alpha 3.7 days

Radon -220

Alpha 56 seconds

Polonium -216

Alpha 0.15 seconds

Lead -212

Beta 11 hours

Bismuth -212*

Beta 61 minutes

Polonium -212

Alpha 310 nanoseconds

Thallium -208

Beta 3.1 minutes

Lead -208 (stable)

18

Fig 2.4 Natural decay of 40K [14]

2.3 Hazards associated with Radon Exposure

Rn is a radioactive gas. When it is in gaseous state and is breathed in, 75 % to 80 % is exhaled with the

next breath. Radon is believed to be the leading cause of lung cancer after smoking [21]. It is also

considered as one of the most hazardous radioactive gases in the environment. As a result, it is of health

concern in radiological risk assessment [22]. 222Rn with a half-life of 3.82 days is a chemically inert

gas and is produced through the radioactive decay of 226Ra, a member of the 238U decay series [23].

The risk associated with the handling and disposal of materials contaminated with 226Ra are due

primarily to 222Rn progeny (lead-210 and polonium-210), the inhalation of which has been known to

be associated with increased risk of lung cancer. The risks generally depend on the overall rate at which

222Rn is transported to the surrounding atmosphere through diffusion or advection and finally become

released from the material matrix. The risk associated with radon in NORM materials was

underestimated by previous studies [24].

The 232Th and 238U series are the main decay series of interest. The 235U decay series is less important

for radiation protection purposes, except for the radionuclide 227Ac, which can contribute significantly

19

to inhalation exposure. If necessary, the presence of 235U (and, by implication, its decay progeny) can

be taken into account on the basis of the abundances of 235U and 238U in natural uranium (0.711 and

99.284 %, respectively) — the corresponding 235U:238U activity ratio is 0.046 [25].

The primary risk of lung cancer from exposure to radon does not come from exposure to the gas itself,

but from exposure to its decay products. When radon decays, a number of short half-life decay products

are formed, principally polonium-218, lead-214, bismuth-214 and polonium-214. Polonium-218 and

polonium-214 both decay by alpha radiation, emitting alpha particles which deposit their energy over

a very short distance [26]. Beta or gamma radiation, on the other hand, deposits their energy over

greater distances. For radiation protection purposes, alpha particles are considered to be 20 times as

harmful inside the lungs as the same energy of either gamma or beta radiation [26] . Thus, polonium-

218 and polonium-214 contribute most of the dose responsible for lung cancer.

Many of these attached and unattached decay products will not be exhaled, but will instead adhere to

tissues in the lung. Given the short half-lives of the decay products, the remaining alpha particle decays

will occur while in contact with the lung. A portion of the alpha particles emitted will penetrate lung

tissue where damage can occur. The energy released by alpha particles can cause permanent damage

to DNA molecules, either physically or by chemical means [27]. Most of this damage can prevent

further cell division, and eventually the cell will die. Cells also have the capability to repair some

damage. In a very small portion of the irradiated cells, the damaged DNA will be replicated in actively

dividing cells which may induce lung cancer. This is why exposure to radon and progenies does not

mean that you necessarily will contract lung cancer, but exposure increases that risk [28].

20

2.4 NORMs associated with mining activities

Many mineral extraction and processing industries generate wastes that contain concentrated levels of

NORMs. Recent attention has focused on the potential adverse environmental effects and human health

hazards due to NORMs [8, 29].

Processing of minerals often increases concentrations of naturally occurring radioactive materials

(NORM) in mineral concentrates, products and waste streams. This so-called TENORM

(Technologically Enhanced Naturally Occurring Radioactive Materials) phenomenon can result in

usually very small increases of radiation exposures to workers and the public. However, proposed

international radiation protection standards are likely to bring the TENORM issue into the realm of

regulatory concern [30].

As reported by IAEA Safety reports series number 68, the presence of NORMs in the rare earth minerals

in varying concentrations is quite often significant enough to result in occupational and environmental

radiation exposures during mining, milling and chemical processing for the extraction of the rare earth

elements and compounds [10].

Long-lived radioactive elements such as uranium, thorium and potassium and any of their decay

products, such as radium and radon are examples of NORMs. These elements have always been present

in the earth's crust and atmosphere, and are concentrated in some places, such as uranium ore bodies

which may be mined. The term NORM exists also to distinguish ‘natural radioactive material’ from

anthropogenic sources of radioactive material, such as those produced by nuclear power and used in

nuclear medicine, where incidentally the radioactive properties of a material maybe what make it

21

useful. However, from the perspective of radiation doses to people, such a distinction is completely

arbitrary [13].

2.5 Hazards associated with NORMs

Jelena et al reported that mining and tailing are considered to be hazardous steps with respect to

technologically enhanced naturally occurring radioactive material (TENORM) and metal

contamination of the environment [26]. In terrestrial and aquatic ecosystems, radionuclides can be

transferred from the site of origin by air emissions, leaching and by run-off water, from soil into plants,

animals and finally to man. [31]. The radiological and epidemiological impact of former mining

activities, evaluated by IAEA has shown that workers received an annual dose equivalent of 150 mSv

which is much higher than the occupational dose limit of 20 mSv per annum for radiation workers and

even 3 times higher than the allowed maximum dose of 50 mSv in one year in special situations [14].

However, beside investigations considering human radiation doses and risk levels, neither

investigations of contaminants in different environmental compartments nor the ecological risk

assessments for species other than humans have been done at these locations.

NORMs generated by the mining industry can affect the health and safety of both workers in oil fields

and members of the public, in addition to contaminating the environment. The health of workers can

be affected in two ways:

1) External exposure: The precipitation of NORM nuclides in the production equipment can produce

elevated gamma dose rates outside of affected equipment, which can add to the workers’ received

doses. High energy photons emitted from some radium progenies, like 208Tl and 214Bi, can also

penetrate the equipment walls and contribute to the external dose rates [32, 33];

22

2) Internal exposure: Radioactive dust which is generated during maintenance operation or pipe

transport may enter the body by ingestion or inhalation;

Radium is known to accumulate in bones if ingested since they are both in group IIA. About 80-85 %

of ingested radium precipitates in bones where it decays to its progenies [17]. Their alpha, beta and

gamma emissions cause damage to the bone area where they are attached, possibly leading to bone

cancer. Cancer is the major latent harmful effect produced by ionising radiation and the one that most

people exposed to radiation are concerned about. In humans, radiation-induced leukaemia has the

shortest latent period at 2 years. Thyroid cancer after Chernobyl disaster showed up in children about

four years after the accident, while other radiation induced cancers have latent periods greater than 20

years. For the non-radiogenic cancers, it has been hypothesized that either the repair mechanisms

effectively protect the individual or that the latency period exceeds the current human life span [30].

The mechanism by which cancer is induced in living cells is complex and is a topic of intense study.

Exposure to ionizing radiation can produce cancer. However, some sites appear to be more common

than others, such as the breast, lung, stomach, and thyroid.

DNA is a major target molecule during exposure to ionising radiation. Other macromolecules, such as

lipids and proteins, are also at risk of damage when exposed to ionizing radiation. The genotoxicity of

ionizing radiation is an area of intense study, as damage to the DNA is ultimately responsible for many

of the adverse toxicological effects ascribed to ionizing radiation, including cancer [34].

Potassium-40 can present both an external and an internal health hazard. The strong gamma radiation

associated with the electron-capture decay process (which occurs 11% of the time) makes external

exposure to this isotope a concern [35, 36]. While in the body, potassium-40 poses a health hazard from

both the beta particles and gamma rays. Potassium-40 behaves the same way as ordinary potassium,

both in the environment and within the human body – it is an essential element for both. Hence, what

23

is taken in is readily absorbed into the bloodstream and distributed throughout the body, with

homeostatic controls regulating how much is retained or cleared. The health hazard of potassium-40 is

associated with cell damage caused by the ionizing radiation that results from radioactive decay, with

the general potential for subsequent cancer induction.

2.6 Biological effect of radiation

Interaction of radiation with cells in the human body may result in damage, modification or death of

cells and this will affect the function of organs or tissues resulting in deterministic or stochastic effects.

Radiation damages starts at cellular level. Radiation that is absorbed in a cell has the potential to impact

a variety of critical targets in the cell, importantly DNA. The damage of DNA by radiation may cause

long term harm to organs and tissues. Studies have reported that the breakage of double helix structure

of DNA can cause critical change of hereditary materials. Radiation damage could also lead to potential

for progression of cancer induction or hereditary disease [37, 38]. The mechanism of biological effect

arising from exposure to ionizing radiation is a result of either direct or indirect actions:

2.6.1 Direct Action

Direct action occurs when the body is overexposed to ionising radiation resulting in a series of long

and complex events being initiated through ionisation and excitation of relative molecules in the body.

The effects of radiation in which zero threshold doses are postulated could be a result of direct

ionisation and excitation of molecules with the result of dissociation of the molecule [14].

Information for example on the genes of DNA may be prevented from being transmitted to the next

generation due to dissociation by either ionisation or excitation of atoms on the DNA. This point of

24

mutation may occur in germinal cells in which the point mutation is passed onto the next individual or

it may occur in the somatic cells which results in a point mutation in the daughter cell. The point

mutation can be transmitted to succeeding generations showing that the biological effects of radiation

depend on point mutations [14].

2.6.2 Indirect Action

The water in human body constitutes 75% and if irradiated it can form highly reactive free radicals that

may react with DNA. When the human body is irradiated, the following chemical reactions occur:

H2O H2O+ + e- (6)

H2O+ H+ + OHo (7)

The free radical in equation (6) interacts with neutral water as follows

H2O + e- H2O- (8)

Which dissociate immediately

H2O- Ho + OH- (9)

The OH radicals formed may combine with each other leading to production of hydrogen peroxide

(H2O2)

OHo + OHo H2O2 (10)

Hydrogen peroxide is a very powerful oxidizing agent and as a result it can affect molecules or cells

that did not suffer radiation damage directly.

25

2.7 Instrumentation for measurement of natural radioactivity in environmental samples

There are different types of instruments for measuring ionizing radiation. The instruments include: gas

filled detectors (ionization chamber counters, proportional counters, and Geiger-Muller counters);

scintillation counters; and solid state detectors (semi-conductor detectors). The basic mode of detection

is that, the radiation interacts with the detector and the magnitude of the instrument’s response is

proportional to the radiation effect or the radiation property being measured [14].

The result of the interaction in a detector is the appearance of a given amount of electric charge within

the detector’s active volume [14]. Ionising radiation interacts with atoms in the detector to produce

electrons by ionisation. The collection of the electrons leads to an output pulse (signal).

2.7.1 Principles of Gamma Spectrometry

Gamma Spectrometry is a non-destructive technique used for measurement of NORMs in

environmental samples. There are two types of detectors which can be used for gamma spectrometry.

These are Sodium Iodide (NaI) detector and High Purity Germanium (HPGe) detectors. The NaI

detector is less sensitive than the HPGe detector. The energy resolution of the HPGe detector makes it

possible to identify almost all gamma lines emitted by radionuclides in environmental samples. The

detector systems are calibrated using a gamma emitting radioactive source that is incorporated into the

standard sample container representative of the samples to be counted.

A calibration sample can be prepared from inert calcium carbonate or sulphate uniformly mixed with

multi radionuclide solution containing gamma energies across the energy range of the detector. The

calibration sample should have no bias in efficiency due to non-uniform activity or density distribution.

To obtain an accurate detection efficiency for the samples and to minimise errors in the evaluation of

26

the levels of activity, a Monte Carlo particle transport technique is used to model the activity

measurement by gamma spectroscopy system [39].

2.7.2 High Purity Germanium detectors (HPGe)

HPGe detectors are operated at liquid nitrogen temperature of -196 oC to reduce the thermally induced

leakage currents. The detectors have a high energy resolution compared to other detectors such as NaI

or Si detectors, allowing for the differentiation between close gamma lines.

Gamma spectrometry using HPGe detectors is widely used to obtain estimation of natural and artificial

radionuclide concentrations or activities. The detector’s absolute efficiency must be determined

accurately by evaluating the activity of the gamma emitter. This is defined as the ratio of the number

of photons emitted by the source to the number of photons recorded by the detector. [40].The efficiency

depends on photon energy, detector characteristics and measured sample geometry. The estimation of

detector efficiency requires the use of calibration standard samples of the same composition, density,

geometry and radionuclide content as the measured samples [39]. The expected differences in chemical

composition do not affect the measurements of environmental samples [39].

The schematic diagram of a gamma spectrometry setup is shown in fig 2.5

27

Fig 2.5 Block Diagram for gamma spectrometry setup [14]

2.7.3 Principles of Thermoluminescent Dosimetry (TLD)

The principles of thermoluminescence is that when ionising radiation is exposed to some inorganic

phosphors, light is emitted (fluorescence). The fluorescence is either immediate or delayed. The

crystalline forms of inorganic phosphors store some energy imparted to them from ionizing radiation

and release that energy in form of light when the temperature of the crystal is raised. Under controlled

conditions of heating, the amount of light produced is directly proportional to the amount of ionizing

radiation to which the material was exposed. The reproducibility and linearity of thermoluminescent

response are key properties that permit TLD use for measuring ionizing radiation in the environment.

Excitation by ionizing radiation (electromagnetic or charged) raises the energy level of electrons in the

crystalline material from the valence band to the conduction band. At the same time, an electron hole

is created in crystal lattice. In a pure ionic lattice, the electron and hole will quickly recombine, resulting

in fluorescence (emission of light). In thermoluminescent material, the lattice has trace amounts of

impurity ions of slightly different radii. These ions help to create trapping centers where electrons and

28

holes can temporarily be trapped. The energy levels of the trapping centers are below that of the

conduction band. When the temperature of the material is raised, the energy levels of the electrons and

holes are raised. If they are raised sufficiently to reach the conduction band, the electrons and holes can

recombine, resulting in the emission of photons. If the magnitude of the energy difference is about 3 to

4 electron volts (eV), the emitted photon is in the visible region and is the basis of the TLD signal.

Ideally, one photon is emitted per trapped carrier and the total number of emitted photons can be used

as an indication of the original number of electron-hole pairs created by the radiation. All TLDs operate

on this principle, but performance varies between materials and processing methods. The schematic

diagram for a TLD reader is shown on Fig 2.6 below:

Fig 2.6 Block Diagram for TLD reader

29

3.0 RESEARCH METHODOLOGY

3.1 Description of study area

Buchwa Iron Mining Company’s Ripple Creek mine lies 17 km west of the main trunk between Gweru

and Kwekwe, 17 km south west of Ziscosteel. The ore comprises the thickest portion of a long

gossanous ore that strikes northwest- southeast. The deposit has a strike length of 3,5 km, an average

surface width of 155 m and extends to an average of 150 m. The ore consists predominantly of yellow

to brown-grey limonite. The ore is friable, earthy and very porous and hence 70 % of it has to be sintered

before charging into blast furnaces.

Measured geological resources amount to 11 million tonnes at 52,2 % Fe, 2,5 % Mn and 9,9 % SiO2.

Ore extraction is by conventional open pit methods of drilling, blasting, loading and truck haulage with

a capacity to produce 1600 t/month of iron ore. Ore processing at the mine includes primary crushing,

secondary crushing, stacking, blending and reclaiming

The Ripple Creek also has a limestone mine about 2 km west of Redcliff and is adjacent to the

Ziscosteel. The limestone is a crystalline rock of the Bulawayan group. The deposit is lenticular in

shape and has a strike length of 1850 m, a maximum width of 280 m and dips steeply to the Southeast.

Measured geochemical resources of material that varies in MgO content from 1-18 % amounts to some

40million tonnes to a depth of 130 m.

The iron ore and limestone are raw materials for steel production at Ziscosteel. So for this study some

samples were also collected from Ziscosteel dumpsite. The sampling maps and aerial views are shown

in Figs 3.1 to 3.4:

30

Fig 3.1 Ripple Creek Mine Sampling points

31

Fig 3.2 Ripple Creek Sampling Map

32

Fig 3.3 Limestone Mine and ZISCOSTEEL Damp Site Sampling Points

33

Fig 3.4 Limestone mine and Ziscosteel map

34

3.2 Materials and procedures

3.2.1 Soil and rock sample collection and preparation

Soil samples were collected from the following locations within the mines and the surrounding

communities including; Buchwa Ripple Creek mine, Ziscosteel waste dumps and Buchwa Limestone

mine. In order to ensure representative samples were taken from the area for the analysis, an initial

survey was carried out in the area to determine the sampling points. The selection of the sampling

locations was based on the accessibility to the public and proximity to the mine. In addition, the

geological map of the area was used to identify the locations where samples were to be taken. Based

on these criteria, 12 locations were identified for the soil/rock samples sources. Within the mines, soil

samples were collected at 7 mine blocks, dumpsites and open pits. The sampling locations were marked

using a Geographical Positioning System (GPS), Geo Explorer II.

The sampling strategy that was adopted for the soil/rock samples was random [41, 42, 43]. At each

identified location, samples were arbitrarily collected within defined boundaries of the area of concern.

Each sampling point was selected independent of the location of all other sampling points. By this

approach, all locations within the area of concern had equal chance of being selected. The soil samples

were taken using a coring tool to a depth of 5-10 cm. At each sampling location, samples of soil and

rock were taken from at least five different sections of the area into labelled plastic bags. Surface soils

were then taken from different places randomly within the marked and cleared area, and mixed together

thoroughly, in order to obtain a representative sample of that area. Each soil sample was labelled

according to the geographical coordinates of the sampling area, and those coordinates were later used

to indicate the position of the sampling area on the simplified map by open circle points. One kilogram

35

(1 kg) of each sample was collected for analysis. The samples were transported to the laboratory for

preparation and analysis.

The collected soil samples were air-dried, sieved through a fine mesh (~0.5 mm), hermetically sealed

in standard 1000-mL plastic Marinelli beakers, dry-weighed, ground and homogenised within the

beaker volume

.

3.2.2 Analysis of samples using direct gamma spectrometry

Direct instrumental analysis (non-destructive) was used for the measurement of gamma rays of the soil

samples using a semi-conductor detector. The activity concentrations of the radionuclides in the

samples were measured using 109 HPGE. The gamma spectrometry system consists of a p-type Co –

axial HPGE detector coupled to a computer based multi-channel analyser (MCA) for counting period

of 60000 s. The detector was housed inside a 4 mm lead shield to reduce background. The relative

efficiency of the detector was 50 % with energy resolution of 1.8 keV at gamma ray energy of 1332

keV of 60Co. The identification of individual radionuclides was performed using their gamma ray

energies and the quantitative analysis of the radionuclides was performed using gamma ray spectrum

analysis software, ORTEC MAESTRO-32.

The energy and efficiency calibration were done with IAEA standard reference material; a soil standard

of known activity. Standards of known concentrations of radionuclides homogenously distributed in a

1 L Marinelli beaker and a circular plastic foil were used. Background measurements were taken and

subtracted in order to obtain net counts for the samples. The spectrum obtained from the standard was

used to carry out energy and efficiency calibrations which were used in the determination of the activity

concentrations of the radionuclides in Bq/kg for soil samples

36

3.2.3 Energy calibration of the gamma ray detector

The calibration was carried out by counting standard radionuclides of known activities with well-

defined energies within the energy range of interest from 60 keV to 2000 keV. The calibration standard

was counted long enough to produce well defined photo peaks. The channel number that corresponded

to the centroid of each full energy event on the MCA was recorded. The standard was counted on the

gamma detector for 60000 s. The following radionuclides standards were used for the calibration: 57Co,

and 137Cs. The energy calibration results are shown on Table 4.1 .

3.2.4 Efficiency calibration

The efficiency of the detector refers to the ratio of the actual events registered by the detector to the

total number of events emitted by the source of radiation. The efficiency of detection decreases

logarithmically as a function of energy and it is geometric dependent. Appropriate radionuclides were

selected for use as standards in efficiency calibration with the number of calibration points

approximately between 60 keV and 2000 keV [39]. The mixed radionuclides standard used for the

energy calibration was also used for the efficiency calibration. The standard was counted on the detector

for 60000 s. The net counts for each of the full energy events in the spectrum were determined and their

corresponding energies used in the determination of the efficiencies.

3.2.5 Determination of minimum detectable activity

Minimum detectable activity (MDA) is defined as the smallest quantity of radioactivity that could be

measured under specified conditions. The MDA is an important concept in low level counting

37

particularly in environmental level systems where the count rate of a sample is almost the same as the

count rate of the background [39]. Under these conditions, the background is counted with a blank,

such as sample holder, and everything else that may be counted with 12 actual samples. In this work,

1L Marinelli beaker filled with distilled water was counted for 36000 s and the average background

peaks used to determine MDA [Cember, 1996]. For 226Ra (238U decay series), the minimum detectable

activity was determined using average peak areas of the daughter gamma ray lines 295.2, 351.9 keV of

214Pb and 609.31, 1764.5 keV of 214Bi. The daughter gamma ray lines of 238.63 keV of 212Pb, 583.2

and 2614.53 keV of 208TI and 911.21 keV of 228Ac keV were used to determine the MDA of 232Th.

The MDA of 40K was determined using the gamma ray line at 1460.8 keV. The minimum detectable

activities are shown on Table 4.2.

3.2.6 Evaluation of activities and index of mass activity

Evaluation of activities and index of mass activities was done and the data is presented in Table 4.3

3.2.7 Determination of activity concentrations

The activity concentrations of 238U, 232Th and 40K in the soil samples were derived from the

respective peaks in the analysis. (See Appendix:)

3.2.8 Calculation of annual effective dose from external gamma dose rate measurements

At each sampling location, outdoor external gamma dose rates were measured using a digital

environmental radiation survey meter (RADOS, RDS-200, and Finland) and ATOMTEX. The dose

rate meter was calibrated at the Secondary Standard Dosimetry Laboratory (SSDL) of the International

Atomic Energy Agency with a calibration factor provided. At each location, five measurements were

made at 1 m above the ground and the average value taken in μSv/h.

38

The annual effective dose was calculated from the absorbed dose rate by applying the dose conversion

factor of 0.7 Sv/Gy and an outdoor occupancy factor of 0.2 [1].

For the purpose of verifying compliance with dose limits, the total annual effective dose was

determined. The total annual effective dose (ET) to members of the public was calculated using ICRP

dose calculation method [1, 44]. The analytical expression for the total annual effective dose was

determined by summing up all the individual equivalent doses for the exposure pathways considered

in this study.

3.2.9 Method for Environmental Dosimetry

Thermoluminescence dosimeters were placed at selected locations in the mines and dumpsites. The

dosimeters were read using a Harshaw TLD machine for 3 months. The environmental dosimeters were

read with Harshaw TLD reader, model 6600 Plus Automated TLD reader. The instrument performance

was less than 5 KeV for photons and less than 70 KeV for neutrons. The linearity of LiF: Mg, Ti ratio

was less than 5 %. The repeatability was less than 2 % variation based on one standard for 10 repeated

dose measurements of 1 mGy from 137Cs. The capacity of the TLD reader was 200 dosimeters per

loading. Heating was linearly controlled using nitrogen gas. The software used was WinREMS user

software.

Before use, the TLD badges were annealed at 400 oC for 1 hour to reset the trap structures and eliminate

any electrons in residual traps.

39

4.0 RESULTS

Table 4.1 Energy calibration of the gamma ray detector

nuclide name energy (keV)

correction for summing effect

K-40 1460.81 1.0000

Tl-208 583.19 1.0911

Tl-208 2614.53 1.1164

Bi-212 727.17 1.0000

Pb-212 238.63 1.0000

Bi-214 609.31 1.0490

Bi-214 1120.29 1.0849

Bi-214 1764.49 0.9892

Pb-214 295.21 1.0000

Pb-214 351.92 1.0008

Ra-226 186.21 1.0000

Pa-234M 1001.03 1.0000

U-235 163.35 1.0000

U-235 185.71 1.0000

Table 4.2: The minimum detectable activities of K, Pb, Ra, Th and U

Nuclide

MDA

Bq/kg

40K 0.18 210Pb 0.11 226Ra 0.34 232Th 0.21 238U 0.12

40

Table 4.3 Evaluation of activities and index mass activity

nuclide name

energy E ( keV )

measured activity [Bq /kg]

uncertainty of measured

activity [Bq /kg]

corrected activity [Bq/kg]

corrected uncertainty

[Bq/kg]

weighted average [Bq/kg]

weighted average of

series [Bq/kg]

uncertainty of

weighted average of

series [Bq/kg]

K-40 1460.81 1.7 1.0 1.7 1.0 1.7 1.0

Tl-208 583.19 -0.1 0.1 -0.1 - -0.1 1)

Tl-208 2614.53 0.1 0.1 0.1 0.1 not used 0.7 0.3

Bi-212 727.17 -0.1 1.0 -0.1 -

Pb-212 238.63 0.7 0.3 0.7 0.3

Bi-214 609.31 3.6 0.3 3.8 0.3

3.9

4.0 0.2

Bi-214 1120.29 4.7 0.7 5.1 0.8

Bi-214 1764.49 3.8 0.7 3.8 0.7

Pb-214 295.21 3.6 0.6 3.6 0.6 4.0

Pb-214 351.92 4.1 0.3 4.1 0.3

Ra-226 186.21 18.7 2.0 18.7 2.0 57.5% Ak

10.8 1.1

Pa-234M

1001.03 15.9 9.8 15.9 9.8 15.9 9.8

U-235 143.76 0.6 0.4 0.6 0.4 0.6 0.4

U-235 185.71 1.1 0.1 1.1 0.1 42.5% Ak

0.5 0.1

Corrected activity - activity with correction for the difference of density of measured sample and the gauge

and with correction for the summing effect

Ratio

58% of activity Ra-226 2.71

weighted average (Bi-214, Pb-214)

41

Table 4.4 Sample location and coordinates

Sample

Location Latitude

Longitude

1 Block 1 -19.013329 29.750522

2 Block 2 -19.014789 29.750522

3 Block 3 -19.024527 29.753697

4 Block4 -19.018116 29.76022

5 Block 5 -19.010489 29.755671

6 Block 6 -19.0017121 29.745028

7 Block 7 -19.028056 29.758933

8 Limestone mine 1 -19.00356 29.763214

9 Limestone mine 2 -19.003763 29.762763

10 ZISCOSTEEL Dumpsite

1 -19.00986 29.75837

11 ZISCOSTEEL Dumpsite

2 -19.009628 29.75882

12 ZISCOSTEEL Dumpsite

3 -19.009446 29.759141

42

Table 4.5 Average Activity concentration in Bq/kg

Sampling location Radionuclide Activity Concentration in Bq/Kg

K-40 Pb-210 Ra-226 Th-232 U-238

Block 1 280.00

180.00

166.00

10.40

265.00

Block 2 320.00

166.00

290.00

9.65

178.90

Block 3 225.00

191.20

169.20

15.72

156.60

Block 4 299.00

156.60

212.00

24.00

242.24

Block 5 303.00

186.10

207.90

31.05

237.50

Block 6 267.00

165.50

260.00

42.76

107.30

Block 7 234.00

96.00

255.10

73.80

163.50

Limestone mine 1 480.00

48.40

101.90

4.99

108.80

Limestone mine 2 467.80

61.95

81.22

9.23

135.40

ZISCOSTEEL Dumpsite 1 107.06

35.09

93.99

62.46

347.50

ZISCOSTEEL Dumpsite 2 147.10

38.49

145.59

61.99

351.00

ZISCOSTEEL Dumpsite 3 184.90

30.60

193.99

49.90

357.70

Range 107-480 30.60-191.2 81.22-290 4.99-73.80

107.30-

357.70

Mean 276.24

112.99

181.60

32.99

220.95

Standard deviation 112.62

66.67

68.00

24.39

93.57

43

Table 4.6 Average Direct Contamination/ Dose rate measurements

Sample Location Contact

Dose Rate

(uS/hr)

α Surface

Contamination

( Bq/cm2 )

β Surface

contamination

(Bq/cm2)

ϒ Surface

contamination

(Bq/cm2)

1 Block 1 0.13 0 0.05 0.25

2 Block 2 0.11 0.02 0.08 0.54

3 Block 3 0.27 0.01 0.06 0.27

4 Block 4 0.11 0.01 0.02 0.25

5 Block 5 0.17 0.01 0.02 0.32

6 Block 6 0.15 0 0.06 0.15

7 Block 7 0.15 0 0.05 0.26

8 Limestone mine 1 0.39 0 0.27 0.45

9 Limestone mine 2 0.34 0.03 0.03 0.48

10 ZISCOSTEEL

Dumpsite 1

0.32 0.01 0.02 0.74

11 ZISCOSTEEL

Dumpsite 2

0.42 0.03 0.3 0.76

12 ZISCOSTEEL

Dumpsite 3

0.40 0.02 0.3 0.66

44

Table 4.7 Mean comparison of Environmental dosimetry results to WHO standards

Sampling Location

Average Absorbed

dose Annual effective dose in mSv

Block 1 0.0583 0.7

Block 2

0.045 0.54

Block 3

0.019 0.23

Block 4

0.007 0.08

Block 5

0.032 0.38

Block 6

0.013 0.16

Block 7

0.015 0.18

Limestone mine 1

0.020 0.24

Limestone mine 2

0.025 0.3

ZISCOSTEEL Dumpsite 1

0.071 0.85

ZISCOSTEEL Dumpsite 2

0.067 0.8

ZISCOSTEEL Dumpsite 3

0.056 0.67

Range 0.007-0.71 0.08-0.85

Mean 0.0356 0.4275

45

0 2 4 6 8 10 12

100

150

200

250

300

350

400

450

500

Activ

ity c

once

ntra

tion

in B

q/kg

Sample location

Fig 4.1 Trend of activity Concentration of K-40

0 2 4 6 8 10 12

20

40

60

80

100

120

140

160

180

200

Act

ivity

con

cent

ratio

n B

q/kg

Sample Location

Fig 4.2 Trend of activity concentration of Pb-210

46

0 2 4 6 8 10 12

50

100

150

200

250

300

Act

ivity

con

cent

ratio

n B

q/kg

Sample Location

Fig 4.3 Trend of activity concentration of Ra-228

0 2 4 6 8 10 12

0

10

20

30

40

50

60

70

80

Activ

ity C

once

ntra

tion

Bq/k

g

Sample Location

Fig 4.4 Trend of activity concentration of Th-232

47

0 2 4 6 8 10 12

100

150

200

250

300

350

400

Ac

tivity

Con

cent

ratio

n Bq

/kg

Sample Location

Fig 4.5 Trend of activity concentration of U-238

48

4.1 DISCUSSION

The sample location was coordinated using GPS explorer II and the coordinates are as shown on Table

4.4. The sampling points were from Buchwa’s Ripple Creek mine, ZISCO Steelworks and Buchwa

Limestone mine. The sampling points were chosen from the iron raw materials, processing and waste

materials covering the whole chain of activities in the mining and mineral processing.

4.1.1 Average Activity concentration

The average activity concentrations of 238U, 232Th, 226Ra, 210Pb and 40K are shown on Table 4.2 and the

trends are shown on Fig 4.1, 4.2, 4.3, 4.4 and 4.5

The mean activity concentration of 40K is 276.26 Bq/kg and range was from 107-480 Bq/kg. The

highest activity concentration was from the Limestone mining and the lowest was from the waste

dumpsites as shown on Fig 4.1. This shows that limestone mining has a lot of naturally occurring 40K

radionuclides in its geological formation than in iron ore rock and the level of activity concentration

was low in the waste because of the high temperature in the blast furnace which activates chemical

processes leading to formation of more state products of potassium. This compares well with literature

according to Jobbagy et al and Caroli et al [45,46]. Also the activity of 40K was lower than the global

average activity of 400 Bq/kg [1]. The value compares well with other published data from Kenya

which has almost the same geology with Zimbabwe and all the values are below the average world

values [23,47].

The mean activity concentration of 210Pb was 112.99 Bq/kg and the range was 30.60- 191.2 Bq/kg. The

highest value was found at the Iron mine and the levels at the Limestone mine and ZISCO Steelworks

dumpsite were very low as shown by Fig 4.2 This may be because 210Pb , with a half-life of 36 min

would have decayed to stable 208Pb. So there is need to analyse the concentration of stable lead in the

49

environment. Generally the levels of 210Pb were very high as compared to the average global activity

concentration of 30 Bq/kg of the U/Th decay series [1].

The mean activity concentration of 226Ra was 181.60 Bq/kg and the range across the sampling points

was 81.22-290 Bq/kg. 226Ra was highest in the mines due to the secular equilibrium in the undisturbed

environment but in the dumpsites it was very low as shown by Fig 4.3. This may be due to leaching

since 226Ra is very soluble in water and can easily leach into rivers during the rainy season[6, 7, 8].

This is also an indication that at the mine 222Rn concentration may be high during extraction process

but this was not covered under the scope of this research. The average global activity concentration

data on 226Ra has, however not yet been published.

The mean activity of 232Th was 32.99 Bq/kg and the range was 4.99-73.8 Bq/kg. The highest level was

on Block 7 of the mine and in the ZISCO Steel works dumpsites as shown by Fig 4.4. This may be

because at Block 7 there was lot of sand soils. According to Popic et al 232Th is mainly concentrated

in thoriated soils like Monazite and Zirconium soils [26] and also in the dumpsite because silica is a

raw material for iron processing. The mean activity concentration of 32.99 Bq/kg compares well with

the world average concentration of 30 Bq/kg [1].

The mean activity concentration of 238U was 220.95 Bq/kg and the range was 108.80-357.70 Bq/kg.

The highest value was in the ZISCO Steelworks dumpsite as shown by Fig 4.5. This increase may be a

result of technological enhancement during the mineral processing. The mean activity concentration

was higher than the average world concentration of 35 Bq/kg. This may be due to availability of

uranium ore in geological structures of the mine. Also some uranium deposits have been explored in

some parts of Zimbabwe which includes Guruve and the Dande Valley so there is a possibility that

50

Uranium may be rich in soils in Zimbabwe. Neighbouring countries like Malawi, South Africa,

Namibia and Zambia also have uranium deposits [28].

4.1.2 Absorbed dose and total annual effective dose

The total effective dose was calculated from the absorbed dose on the Thermoluminescence

dosimeters. The average annual effective dose was 0.4275 mSv as shown by Table 4.7. This dose is

well below the WHO dose limit of 1 mSv for the public and 20mSv for the occupational workers. Even

though the dose was very low, consideration must be taken that the mine was not in operation during

the time of study and dose may be high during normal situations. The doses compared well with what

Shanthi et al reported on radiological indices [47].

The basic approach to radiation protection is based on the ICRP recommendations. This stipulates that

the doses from ionising radiation must be kept as low as reasonably achievable but below 20 mSv per

year for occupationally exposed workers and 1 mSv per year for the general public.

4.1.3 Average Direct Contamination/ Dose rate measurements

The mean contact dose rate was 0.246 uSv/hr. The mean surface contamination for alpha, beta and

gamma emission was 0.0112, 0.105 and 0.4275 uSv/hr respectively. This was mainly background

radiation.

51

5.0 CONCLUSIONS

The main aim of this research was to assess the levels of NORM in the mining, processing of iron and

its waste products. It focused on the determination of the levels and distribution of the naturally

occurring radionuclides of the U/Th decay series (radon 222 and its progenies, Radium 226, Thorium

232 and Uranium 238 and 235) and Potassium 40. The sampling area covered during this research

include Buchwa Ripple Creek Mine, Buchwa Limestone mine and Ziscosteel dumpsite.

The research was motivated by the fact that naturally occurring radionuclides are a cause of concern in

mining and mineral processes because of human activities that cause disequilibrium of the materials in

the earth. These radionuclides if not properly controlled and managed can cause the risk of cancer to

occupational workers, general public and the environment. The risks may be either radiological or

chemical hazards.

In this research the results on concentration of 238U, 232Th, 226Ra, 210Pb and 40K in different sampling

areas as well as radiation doses were established. The concentrations of these radionuclides in soil

through which workers and the public could be exposed were quantified using gamma spectrometry.

The mean activity concentrations of 238U,232Th, 226Ra, 210Pb and 40K in soil samples were, 220.95,

32.99, 181.60, 112.99 and 276.24 Bq/kg respectively. The results of this study compared well with the

average worldwide activity concentrations except for 238U which is very high.

The total annual effective dose for all route of exposure was 0.4275 mSv which is well below the

recommended annual dose limit of 1 mSv for the public and 20 mSv for the occupationally exposed

workers. However, it is recommended that the mining operations should be periodically monitored if

mining operations resumes.

52

The data from this research will serve as reference data for future studies of NORMs in Zimbabwe and

will assist in setting guidelines for regulatory control to protect people and the environment against

harmful effects of radiation. These results will be published in peer journals to create awareness on

NORMs to the public, academia and policy makers.

53

6.0 RECOMMENDATIONS

The following areas are recommended for further research in the future in order to optimize protection

of people and the environment against effects of ionising radiation:

Assessment of gross alpha and beta activity concentration in drinking water sources

Measurement by radionuclide counting in lungs of occupationally exposed workers.

Determination of heavy metals in drinking water and food in this study area so as to ascertain the

concentration of the stable products from U/Th decay series.

Determination of the whole U/Th decay series in drinking water from the study area using

radiochemical separation, alpha and beta spectrometry.

Radiological assessment of radon gas in the mining and mineral processing areas.

Assessment of the 226Ra and 222Rn emanation coefficient for Technologically enhanced NORM

scales in pipes of the iron treatment plants in Zimbabwe

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APPENDICES

60