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Sudan Academy of Sciences (SAS) Atomic Energy Council
Investigation o f238 U And 232 Th in Rock Samples Selected From Touria Mountain Omdurman
BY: Nasreen Hassan Mohammed
Thesis submitted for partial fulfillment of the requirements for the degree of M.Sc. in Radiation & Environmental Protection
SUPERVISOR: prof. Isam Salih Mohamed
July. 2013 Sudan Academy of Sciences (SAS) Atomic Energy Research Council
Investigation of 238 U And 232 Th in Rock Samples Selected
From Touria Mountain Western Omdurman University
By:
Nasreen Hassan Mohammed
Examination Committee
— ------1 Title N am e Signature
| Supervisor Prof.Dr. Isam Salih Mohamed
1,1 ■ ...... ■ » * , 1 1
External Examiner Prof.Dr. Mohamed Osman Sid Ahmed — , '
|
July 2013 Acknowledge
Thanks God for helping me is finish this work.
I owe a great many thanks to great many people who helped and supported me during his project.
1 deeply thank my advisor, prof. Isam Salih the guide advice with attention and care. He has taken pain to go through the project and make necessary support as and when needed, and supervision was invaluable.
I am grateful to Sayeb Mohamed Ali Alfki.,Nisreen Hussien Mohammed. Hiaty Hasabelrasoulthose helped me Sample Collection All thanks go to my family ,my father and my mother and my brothers for the ircontinued support to bring this research in this way.
1 am indebted to my many of my colleagues to support me .Acknowledgments thanks to whoever may have helped you in any way.
2 DEDICATION
I dedicate this thesis to:
My father, my mother
My brothers and sisters
All relatives and friends
3 Contents
AC KNOWLEDC,EM ENTS
+ ♦ « ♦ ► • ♦ ♦ DEDICATION 11 TABLE OF ONTENTS m ABSTRACT i\
« • ♦ ♦ ♦ ♦
♦ ♦ CHAPTER ONE: INTRODUCTION 8 1.1 Environmental Radioactivity 8 ! 1.2 Sources of National Radioactivity 8 1.2.1. Primordial radionuclide's 8 1.2.2 Secondarv radionuclide's 8 1.2.3. Cosmosenic radionuclide's 8
• • • 1.3 Non-series Primordial radionuclides 9
>+ ♦ ♦♦ ♦ «+ + 1.4 Objectives 9
♦ ♦ ♦ ! 1.5 Specific objective 9 i CHAPTER TWO: Literature Review 10
• ♦ ♦ «• • «+ «• « ♦♦♦ ♦ V « • « • 2.1 Radioactivity in Natrue 10
♦ # ♦ ♦ ♦ ♦ • 2.2Types1 of Radioactive Decay II ; 2.3 Decav Modes 11
♦ 4 2.3.1 Alpha (a) Decay 11
♦ ♦ ♦ 2.3.1.1 Characteristics of Alpha Decay 12
♦ ♦ ♦ # « 2.3.2Beta Decav 12 2.3.2.1 P- Emission 13 2.3.2 2p+ mission 13
2.3.3. Electron Capture 14
♦♦♦ * « ««M 2.3.4. Gamma Emission and Internal Conversion 15
2.3.4.1.2yemission 15 2.3.4.2. Internal Conversion 16
tmrntm • • • 2.3.5. Spontaneous Fission and Neutrons Emission 16
♦ * % * ♦ ♦ ♦ •+ «•» ♦ «+ ♦ ♦ ♦ ♦ # 2.3.5.1 Spontaneous Fission 16
2.3.5.2Delaved Neutrons 16
• • • 2.4. Natural Decay Series (Uranium. Radium, and Thorium) 17
4 2.5. i he Radio Active Decav l aw 2.6. Series Decav "> *) « r 2.7. Secular Equilibrium > *> 2.8. Transient Equilibrium T "> 2.(). No Equilibrium ") *> 2.10. Rocks 2.11. Radiation Quantities and Units 2.11.1 Absorbed dose : 2.11.2 Equivalent dose 26 2.11.3 Effective dose 26 ! 2.11.4 Committed dose 26 !i « + ^ : 2.11.5 Collective dose 27 ti ; 2.11.6 Committed Equivalent Dose
« CHAPTER THREE: MATERIAL AND METHODS 1 3.1. Gamma-ray Spectroscopy $ ; 3.2Svstem» Calibration 29 ♦ ♦ ♦ « ♦«•
3.3. EneravW * calibration 29 l ! 3.4. Efficiencv calibration 31 & t I ______♦ ♦ ♦ *«•« 3.5. MATERIALS& METHODS M 3.5.1. Sample Collection. Preparation and Measurement 3.6. Calculation of Absorbed Dose Rate in Air ! 3.7. Annual Effective Dose ! CHAPTER FOUR: RESULTS AND DISCUSSION ! ' Conclusion I References « i
5 ABSTRACT
The area of Touria Mountain (located near Omdurman city) has been used for dumpinu different type of wastes, especially medical waste. The main objective of this study is to investigate the radioactivity and assess risks due to presence of radionuclide in the area. > I Investigations of naturally occurring as well as anthropogenic radionuclides are conducted using gamma spectrometry.
No man-made radionuclide's are detected in the area, but natural uranium, thorium an potassium observed with normal levels comparable with the world mean acti\ it\ concentrations in rock. The obtained results showed that the ranges of the activity concentrations of 238U. 232Th and 4(IK are 44.95-24.14. 43.31-29.45 and 1 I 13.96-31.23
Bq.kg'1'- 1 . respectively. Mean concentrations of 238U. ~32Th and 4l)k are 30.9. 38.7 and 538.0 Bq.kg'1' respectively. According to the UNSCBAR report 2000. the worldwide aeli\it\ concentrations of 2l8U, 232Th and 40K are reported to be 1 7-60. 1 I -64 and 140-850 Bq. |\ i J with the mean concentrations of 35. 30 and 400 Bq.kg"1 respectively.
6 اﻟﺨﻼﺻﻪ
د ﻣﺤﻨﻪ ﺟﺒﺐ ﺻﻮرﻳﺔ ﺑﺎﻟﻘﺮب ﻣﻦ ﻣﺪﻳﻨﺔ ام درﻣﺎن اﺳﺘﺨﺪام ﻧﻈﺎم ﻣﺤﻠﻴﺎﻓﻴﺔ ﺟﺎﻣﺎ وﻧﻈﺎم ﻣﻌﺪل اﻟﺠﺮ ﻋﻪ .واﻟﻬﺪف ﻣﻦ ﻫ ﺬه
ا اس؛، ﻫﻮ ﺗﻘﻴﻴﺞ ﻣﻨﻮﻳﺎت اﻟﻨﺸﺎط ا ﻻ ﺷﻌﺎ ﻋ ﻲ واﻟﻤﺨﺎﻃﺮ اﻟﺼﺤﻴﻪ ﺑﺴﺒﺐ اﻟﻨﻮﻳﺪات اﻟﻢ ﺷ ﻌ ﻪ ﻓ ﻲ اﻟﺼﺨﻮر اﻻرﺿﻴﻪ ﻓﻲ
ﺟﺐ.
د ﻳﺘﺐ اﻧﻜﺸﻒ ﺀ ن اﻧﻨﻴﻮﻛﻨﻴﺪا ت ا ﻟ ﺼ ﻨ ﺎ ﻋ ﻴ ﺔ ﻓ ﻰ ﻫ ﺪ ه ا ﻟ ﻤ ﻨ ﻄ ﻘ ﺔ . ﻓ ﻲ ﻫ ﺪ ه ا ﻟ ﺪ ر ا ﺳ ﻪ ﻧﻨﻢ ﻣ ﻘ ﺎ ر ﻧ ﺔ ﻧ ﺘ ﻴ ﺠ ﺔ اﻟ ﻴ ﻮ را ﻧ ﻴ ﻮ م 238 واﻟﻘﻮرﺑﺰب 232 واﻟﺒﻮﺗﺎﺳﻴﻮم 40 ﻣﻊ ﺗ ﺮ ﻛ ﻴ ﺰ ا ت ا ﻟ ﻨ ﺸ ﺎ ط ا ﻟ ﻌ ﺎ ﻟ ﻤ ﻲ ﻓ ﻲ اﻟ ﺼ ﺨ ﻮ ر و ﻓ ﻘ ﺎ ﻟﺘﻘ ﺮﺑﺒ ﺮ ﻟ ﺠ ﻨ ﺔ ا ﻻ ﻣ ﻢ اﻟ ﻤ ﺘ ﺤ ﺪ ه اﻟ ﻌﻠ ﻤﻴ ﻪ ﻋﺰ ﻧﺮ’ ا'ذاﺷﻌﺎع اﻟ ﺬ ر ي ﻏﺎ م ﻻه() 2 ﻧﻨﻢ ا ﻻ ﺑ ﻼ غ ﻋﻦ ﺗ ﺮ ﻛ ﻴ ﺰ ا ت ا ﻟ ﻨ ﺸ ﺎ ط ا ﻻ ﺷ ﻌ ﺎ ﻋ ﻲ ﻓ ﻲ ﺟﻤﻴﻊ ا ﻧ ﺤﺎ ﺀ اﻟ ﻌﺎﻟ ﻢ ﻣﻦ
ﻮم’■ ﻧ ﻴ ﻮ ر ﺗ 238 و ا ﻟ ﺜ ﻮ ر ﻳ ﻮ م 232 واﻟﺒﻮذذاﺑﻮم 40- ا ذ ﻳ ﻜ ﻮ ن ( - )11-140.64-850 - (
ﻣ ﻊ ﻣ ﺘ ﻮ ك ؛ ﻧﻨﺮ اﻛﻴﺮ ( Bq.kg ]400-35-30 ) ﻋﻠ ﻲ اﻟﻨﻨ ﻮاﻟ ﻲ و ا ﻇ ﻬ ﺮ ت ا ﻟ ﻨ ﺘ ﺎ ﻧ ﺞ ا ن اﻟ ﺘ ﺮا ﻛ ﻴ ﺰ ﺗ ﺘ ﺮ ا و ح ﻓ ﻲ ﻧﻔﺜﺎط ﻧﻔﺜﺎ ط ﺗﺬ ﻏ ﻮ رأﺋﺌ ﻮ ج 238 و ا ش * م 232_ و ﺑﺺ١ 40- ﻓ ﻰ ﻟﻤﺪي١ ( (- ٠1 Bq.kg 43.31_29.45. Bq.kg 43.31_29.45. 113.96-31.23 113.96-31.23 1 4 ا 44.95_24. ) ﻋﺌ ﻲ ا ﻟ ﺘ ﻮ اﻟ ﻲ . ﺗ ﺮ ﻛ ﻴ ﺰا ت ﻧ ﺸ ﺎ ط ا ﻟ ﻴ ﻮ ر ا ﻧ ﻴ ﻮ م 238— . و ا ﻟ ﺜ ﻮ ر ﻳ ﻮ م .232— واﻟﺒﻮﻧﻨﺎﺳﻮم 40— ﻫﻲ ﻓ ﻲ ا ﻟ ﺪ CHAPTER ONE INTRDUCTION
1.1 Environmental Radioactivity
The radioactivity due to natural radionuclides in rocks, soil and water generate a significant component of the background radiation exposure to the population. The terrestrial component of the natural background is dependent on the compositions of the rocks, soil and water in which the natural radionuclides are contained 111. Some areas are well-known for their high background radiation like Nubba Mountain in Sudan, the west cost of India and certain beaches in Brazil. Humans are exposed to natural terrestrial radiation that originates predominantly from upper 30 cm of the soil. Humans are also exposed by contamination of the food chain ( I).
Ionizing radiations are ability to excite and ionize atoms of matter with which they interact. Since the energy needed to cause a valence electron to escape an atom is of the order of 4-25 eV, radiations must carry kinetic or quantum energies in excess of this magnitude.
1.2 Sources of National Radioactivity:
Natural radioactivity originates from extraterrestrial sources as well as from radioactive elements in the earth's crust. The radioactivity of the earth includes three major categories:
1.2.1. Primordial radionuclide’s have half-lives sufficiently long that they have survived since their creation.
1.2.2. Secondary radionuclide’s are derived from radioactive decay of the primordial.
1.2.3. Cosmogenic radionuclide's are continuously produced by bombardment of stable nuclides by cosmic rays, primarily in the atmosphere.
Primordial radionuclides
The Primordial radionuclides are divided into two groups:
8 1.3Non-series Primordial radionuclides:
That decay directly to stable nuclide, only 2 of the 17 non series are important contribute
87 IU in background dose, 4t,K (1.28x Kfvears) and X7Rb(4.8*IO'"). Potassium-40 has three naturally occurring isotopes; only 40K is unstable, havirm a half-life of 1.3 \
KTyears. 40K occurs to an extent of 0.0118% in natural potassium. thereb\ imparting a specific activity of approximately 800 pCi g'1 potassium (30 k Bq kg i.
Representative values of the potassium content of rocks, indicate a wide range of values, from 0.3 to 4.5% for various rock types. Certain basalts and sands are low in potassium, whereas granites and other basalts are high. Seawater contains ’ K in a concentration of about 300 pCi (11 KBqm'3).40K account for much of the external background radiation dose from radioactivity to which humans are exposed (2).
The natural radionuclides enter into the soil from the earth's crust where thev have been * present since its creation. The earth's crust is the principal source of natural radionuclides in soils and rocks. Radioactivity levels in various building materials such as soil, sand etc from different geological regions in the world (3).
The specific levels of terrestrial environmental radiation arc related to the geological composition of each area, and to the content in thorium, uranium and potassium of the rock from which the soils originate in each area (4) 1.4 Objectives
The general objectives of the present study are; to evaluate radioactivity content of the
Touria Mountain area and to get general information about the external exposure from
Terrestrial i i-radiation. 1.5 Specific objective: Investigation of 238U-, 232 Th- and 40K- in addition to suspected man-made radionuclides in rock samples of Touria Mountain
9 CHAPTER TWO LITERATURE REVIEW
2.1 Radioactivity in Natrue
The natural radioactivity in the environment aris esprimarily from uranium and thorium, including the series of decay products, and from potassium. Today, then at ural l\ occurring radioactivity is over lain by artificial radioactivity deposited with the debris from the atmospheric nuclear weapon tests during 1950??s and early 1 %()?'.’s. Most of this material, injected into the stratosphere, is deposited on a global scale according to are cognised latitudinal pattern.(12,15)
Possible emissions by the nuclear power industry .from reactors or processing plants and accident al releases can produce constituents similar to the radioactivity of the nuclear weapon fallout. However, the contributions from reprocessing, nuclear power plants or other sources, on a global scale, negligible and in the worst cases are detectable in limited areas in the vicinity of the source itself or on regional scale. Among the radioactive elements produced in atmospheric testing, only those of longer half-lives
(e.g..238PU .239PU, Pu, 37Cs) remain in ecosystem components at the present time. In ^ ' s addition to the weapon fallout, an accident stratospheric injection o f ' ' Pu in 1964 resulted in almost a three-fold increase in the global fallout of this isotope.( 13)
Many studies have been made in west, north and east Sudan. Calculations of the external exposure due to radiation from the ground have been made from the results of the measurements of radionuclide activity concentrations in the soil at various locations in
Sudan(14)
On the average, the exposure was found to be 45nGy If1 corresponded to annual equivalent dose of 278 mSv y'1. With the exception of the Arkuri and Dumper areas in the western part of the country, the calculated exposure falls within the global wide range of outdoor radiation exposure given in the UNSCliAR publications. The average activity concentration of l37Cs determined was4.12 Bq.kg'1. which indicated that the rewash little contamination due to fallout radioactivity at surveyed sites.
Naturally occurring raw building materials and processed products have radionuclides of
the three most commonly known radioactive series :Uranium- senes, thorium series and
10 4Ui/ K isotopes. High concentrations of natural radionuclides in building materials can result in high-dose rates indoors.
Gamma-irradiation from naturally occurring radioactive material (NORM) contributes it ) the whole body dose and in s cases P-irradialion contributes to the skin dose. As (1 particles have higher specific ionization than y-particles. they lose their energv while the> are still in the skin and cannot penetrate further into the body. Apart from |! and y radiations, there are other harmful effects of NORM .(15) 2.2Types of Radioactive Decay: 2.3Decay Modes:
Radioactive decay is a spontaneous nuclear transformation which results in the formation
of new elements. In this process .an unstable “parent" nuclide P is trans formed in to a
more stable “daughter" nuclide D through various processes. Symbolically the process
can be described as follows:!*—»D+dl+d2+...
where the light productsdl+d2+... are the emitted particles. The process is usually
accompanied by the emission of gamma radiation. If the daughter nuclide is also
unstable, the radioactive decay process continues further in a decay chain until a stable
nuclide is reached. Radioactive nuclides decay spontaneously by the following
processes( 16)
2.3.1 Alpha (a) Decay:
In alpha decay, the parent atom emits an alpha particle 4u2 and results in a daughter
nuclide a'4A z-2. Immediately following the alpha particle emission, the daughter atom still
has the Z electrons of the parent - hence the daughter atom has two electrons too man\
and should be denoted by z'4Ai).2.These extra electrons are lost soon after the alpha
particle emission leaving the daughter atom electrically neutral. In addition, the alpha
particle will slow down and lose its kinetic energy .At low energies the alpha particle w ill
acquire two electrons to become a neutral helium atom. The alpha decay process is
described by:
u decay:
11 l z ~ 2 0 ] 2 + jo e —>
Figure (2.1): Emission of the alpha reduces the mass of the nucleus
2.3.1.1 Characteristics of Alpha Decay:
If it is energetically favorable for a nucleus to lose mass, the particle (charge -2e and
mass 4.0026 u= 3.7 GeV) is most commonly emitted because it is a tightly bound system
Alpha decay occurs almost exclusively in heavy nuclei because B/A decreases with A when A is large and so the daughter product of a decay is more stable than the parent.
Mass numbers of nearly all a emitters > 209 and typical a particle kinetic energies Ea = 4
to 6 MeV. Alpha particle energies are well defined as pass through matter, they lose
energy rapidly. Alpha particles of a few MeV are easily stopped by paper or skin.
2.3.2Beta Decay:
Nuclei with either too many protons or too many neutrons will undergo p decav via the
weak nuclear force.
• A neutron-rich nucleus will undergo p— emission.
• A proton-rich nucleus will either emit a P+ particle or be transformed by
capturing an atomic electron in a process called electron capture.
12 2.3.2.1 fT Emission:
In p— decay, the charge on the nucleus increases by one unit. The (1— particle is an electron (mass 0.0005486 u = 511 keV and charge -e). It is considered to be created at the moment of decay. A second light particle, antineutrino(v-) is also created and emitted. It has no charge and very small mass, which generally is assumed to be zero and interact sextremely weakly with matter.
“ decay involves the transformation of a neutron into a proton inside the nucleus: n —> p + [ f + v
Energy must be conserved and so the transformation given in l-'q. (2-7) can occur
only if the daughter nucleus is lighter (more stable) than the parent. This means that the O
value: Qp- = (wp - mu) c 2
Q > 0: Note that the masses rnp an d mu, for the parent and daughter, respectively, are
atomic not nuclear masses and include the masses of the atomic electrons. A beta particle
is much lighter than a particle. Its speed, therefore, for a given energx is much greater
and it is much more penetrating. A few mm of material will stop a I MeV p particle. 2.3.2.2P* mission:
P+ decay changes a proton rich nucleus into a more stable isobar. The nuclear charge is
decreased by one unit. (Figure 2.3)
Figure (2.3): B+ Emission
13 A particle is an electron, called a positron. It is identical to an ordinary electron except that it is positively charged. Effectively, P decay converts (via the weak nucleat force) a proton into a neutron and a positron. A neutrino is also created in the process Note that this is an ordinary neutrino not an antineutrino.
p n + ff + i '
There are 3 bodies in the final state and the positrons are emitted with a continuous range of energies, as are electrons in p- decay. Using energy conservation, the p decay U value, using atomic masses, is given by:
Qp+ = (w P - «?,) - 2m) c '
This must be greater than zero for the process to be able to occur.
2.3.3.Electron Capture:
«•i Electron i • Figure (2.4): Electron capture Electron capture (EC) is an alternative process to P+ decay in that a proton is converted into a neutron. The parent absorbs an atomic electron, usually from the innermost orbit. e - + p-»n + v Energy conservation gives the Q value as: QEC = (mP-mD) c2 Note that QEC is greater than QP- by 2mc2. In electron capture, the mass of an atomic electron is converted into energy, whereas, in p+ decay, energy is required to create a positron. This means that EC can occur when p+ decay cannot. No P particle is emitted in 14 electron capture and so. except for a very small amount of recoil energy of the daughter nucleus, the energy released escapes undetected. 2.3.4. Gamma Emission and Internal Conversion: 2.3.4.1.2y emission: A nucleus in an excited state generally decays rapidly to the ground state by emitting rays. A schematic diagram for gamma emission is showing bellow: 22Na Energy (keV) 22Ne 90% p+ 1274 10% EC ray y emission reduces energy of nucleus 0 22Ne Figure (2.5): Gamma Emission Gamma-ray energies E typically, are in the range O.l to I0 MeV and can be determined very accurately with a modern detector. E is characteristic of the emitting nucleus and are widely used to identify radioactive nuclei. Gamma rays of about an MeV do not interact strongly in matter and will penetrate many cm of moderate-density material. 2.3.4.2.Interna] Conversion: This is an alternative to y emission whereby an excited nucleus de-excites by ejecting an electron from an atomic orbit. Both y emission and internal conversion are due to the action of the electromagnetic force. 15 2.3.5.Spontaneous Fission and Neutrons Emission; 2.3.5.1 Spontaneous Fission: i - :=;* Nucleus splits into two pieces • * ♦ • • 4 ♦ ♦ and releases neutrons # _ * • •X i « Figure (2.6): Spontaneous fission In spontaneous fission a nucleus breaks up into two roughly equal mass fragments 1 he process normally is restricted to heavy nuclei for which BE/A for the fragments >BE A for the initial nucleus. Fission also can be induced by a nuclear reaction, e.g. neutron capture. Spontaneous fission occurs only in very heavy nuclei, such as 252Cf. when the Q value is large enough to overcome the energy needed to deform the parent into two separate pieces. Fission fragments are very neutron rich. Also, several neutrons arc emitted during fission. Neutrons are uncharged and deposit their energy in matter via nuclear interactions. 1 his means that their interaction probability generally is small and their range is not well defined. Neutrons of several MeV will penetrate many I Os of cm of moderately dense materials, such as concrete( 17) 2.3.5.2Delayed Neutrons: Neutron-rich fission products decay by |3— emission. Often, they follow a chain of decavs to stability. Occasionally (in 1 to 2% of cases), a daughter is formed in an excited state that can decay by neutron emission. Compared with (3— decay, neutron decay is ver> rapid and if there is preceding p decay, the neutron emission is delayed hence, the term: delayed neutrons. 16 Ct *• V* ne'.itr c r. Prom pt neutrons Figure (2.7): Delayed neutrons 2.4.Natural Decay Series (Uranium, Radium, and Thorium): Uranium, radium, and thorium occur in three natural decay series, headed by uranium- 238, thorium-232, and uranium-235, respectively. In nature, the radionuclide’s in these three series are approximately in a state of secular equilibrium, in which the activities of all radionuclide’s within each series are nearly equal. Two conditions are necessary for secular equilibrium. First, the parent radionuclide must have a half-life much longer than that of any other radionuclide in the series. Second, a sufficiently long period of time must have elapsed, for example ten half-lives of the decay product having the longest half-life, to allow for in growth of the decav products. Under secular equilibrium, the activity of the parent radionuclide undergoes no appreciable changes during many half-lives of its decay products. The radionuclide’s of the uranium-238, thoriuni-232, and uranium-235 decav series are shown in Figures (2.10),(2.11) and (2.12), along with the major mode of radioactive decay for each. Radioactive decay occurs when an unstable (radioactive) isotope transforms to a more stable isotope, generally by emitting asubatomic particle such as an alpha or beta particle. Radionuclide’s that give rise to alpha and beta particles are shown in these figures, as are those that emit significant gamma radiation .Gamma radiation is not a mode of radioactive decay (such as alpha and beta decay). Rather, it is a mechanism by which excess energy is emitted from certain radionuclide’s, i.e.. as highly energetic electromagnetic radiation emitted from the nucleus of the atom. For simplicity, only significant gamma emissions associated with the major decay modes that is .radionuclide’s listed are those for which the radiation dose associated with uamma ravs W r may pose a health concern. The gamma component is not shown for those radionuclide's whose gamma emissions do not generally represent a concern. 17 Of the two conditions noted above for secular equilibrium, the first is general!) met for the uranium-238.thorium-232 and uranium-235 decav series in naluralK occurring ores. + + W . While the second condition may not be met for all ores or other deposits of uranium and thorium (given the extremely long half-lives for the radionuclide's invoked and the geological changes that occur over similar time scales), it is reasonable to assume secular equilibrium for naturally occurring ores to estimate the concentrations of the various daughter radionuclide's that accompany the parent. The slate of secular equilibrium in natural uranium and thorium ores is significantly altered when they are processed to extract specific radionuclide's. After processing, radionuclides with half-lives less than one year will reestablish equilibrium conditions with their longer-lived parent radionuclides within several vears. For this reason, at processing sites what was once a single, long decay series ( for example the series for uranium-238) may be present as several smaller decay series headed by the longer-lived decay products of the original scries (that is. headed byuranium-238. uranium-234, thorium-230, radium-226, and lead-210 in the case of uranium-238). Haeh of these sub-series can be considered to represent a new. separate decav series. Understanding the physical and chemical processes associated with materials containing uranium, thorium, and radium is important when addressing associated radiological risks( 18) 18 Figure 2.12: Natural Decay Series: Thorium-232 2.5.The Radio Active Decay Law: The activity of any sample of radioactive material decreases or decays at a fixed rate which is a characteristic of that particular radionuclide. No known physical or chemical agents (such as temperature, pressure, dissolution, or combination) may be made to influence this rate. The rate may be characterized by observing the fraction of activity that remains after successive time intervals. 2.6.Series Decay: The subject, "Series Decay" concerns the mathematical relationship of quantities of activity present when two or more radionuclides exist in a decay chain. The relationship between three or more radionuclides is described by H. Bateman. The solution, whil estraight forward, is quite involved. A two-step relationship (parent-daughter) can be readily derived and is reasonably easy to work with. 2.7.Secular Equilibrium: In secular equilibrium the half-life of the parent is very much longer than the half-life of the daughter .When in equilibrium, the activity of the daughter is equal to the activity of the parent. Initially, the majority of the activity will be contributed by the parent. As more and more of the parent nuclide decays, the amount of activity contributed by the daughter will increase. 2.8.Transient Equilibrium: In transient equilibrium the half-life of the parent is longer than that of the daughter, but not very long. In a freshly purified parent fraction, the daughter activity builds up, then decays with the same rate of decay as the parent. 2.9. No Equilibrium: When the half-life of the parent is shorter than that of the daughter, the two never reach a state of equilibrium. Figure 6 illustrates this.(5) 2.10 Rocks The external radiation exposure arises mainly from cosmic rays and from terrestrial radionuclide’s occurring at trace levels in all soils. While absorbed dose rate in air from cosmic radiation outdoors at sea level is about 30 nGy If'for the southern hemisphere(5), the specific levels due to terrestrial background radiation are related to the types of rock 22 from which the soils originate. Therefore, the natural environmental radiation mainlv depends on geological and geographical conditions (Florou and Criticism. IW2). Higher radiation levels are associated with igneous rocks, such as granite, and lower levels with sedimentary rocks. There are exceptions, however, as some shales and phosphate rocks have relatively high content of radionuclide's (6) The island of Cyprus has attracted a lot of geological interest because it contains a maficintrusive complex that is thought to represent an excellent example of an ophiolite slice (7) The Cyprus ophiolile complex, known as Troodos Mass if. consists of pillow basalts overlying a sheeted dyke complex, and then an intrusive complex of gabbro that grades downward into olivine gabbro, then into an in tramafie body of harzburgite and dunite. Some of the harzburgites are serpentinsized. The upper parts of the gabbro and the lower basalts are cut by many closely spaced diabase dykes that for inconspicuous sheeted masses. They are overlaid with line-grained ferruginous, siliceous and sulfide bearing sediments called urn pres. Investigations on terrestrial natural radiation have received particular attention worldwide and led to extensive surveys in many counties (8) Serve as baseline data of natural radioactivity such that man made possible contaminations can be detected and quantitatively determined. They can further be used to assess public dose rates and to perform epidemiological studies .The results obtained in each country can be exploited to enrich the world's data bank, which is highly needed for evaluating worldwide average values of radiometric and dosimetric quantities (9). There are no systematic data on this subject available for Cyprus. Only a few studies on natural radioactivity and on indoor radon concentration measurements are reported by two of the authors (10 23 Table (1.2) absorbed dose in air 1 m above the surface (UNSCEAR 1977) Type of rock Typical activity concentration (Bq Kg'1) A bsorbed “ u dose rate in air (nGy/h) Igneous Acidic 999 5.92 81.4 120 Intermediate 703 2 2 .9 4 32.56 62 (e.g. diorite) Mafic(e.g, basalt) 2 4 0 .5 11.47 11.1 23 Ultrabasic (e.g. 148 0.37 24.42 - durite) Sedimentary Lim estone 88.8 27.75 7.03 20 Carbonate - 2 6 .6 4 7.77 17 Sandstone 370 18.5 11.1 32 1 Shale 703 4 4 .4 4 4 .4 79 2.11.Radiation Quantities and Units: The exposure has been historically defined as the number of electrical charges produced in a unit mass of air and measured in units of roentgens (R). (The international SI unit of exposure is the coulomb per kilogram of air, but this is rarely used in practice). 2.11.1 bsorbed dose: The absorbed dose is relevant to all types o f ionizing radiation fields, whether directly or indirectly ionizing, as well as to any ionizing radiation source distributed within the absorbing medium. 24 d s D Unit: J kg*1. Gray (Gy). dm ( 2. 1) One gray being equal to one joule of energy absorbed per kilogram of matter, and Equivalent tolOORads.The effects of radiation on any material, including biological materials, such as tissue, depend on the magnitude of the absorbed dose. 2.11.2 Equivalent dose: for stochastic risk assessment the ICRP has introduced the quantity equivalent dose, the equivalent dose in a tissue T is given by: R (2.2) Where DT, R is the average absorbed dose to the tissue T from the radiation R and \VR is the radiation weighting factor (3). 2.11.3 Effective dose: The effective dose is thus a measure of the total risk due to any combination of radiations affecting any organs of the body and is expressing stochastic risk to radiation workers I3and to the whole population. To evaluate effective dose, the equivalent dose to the tissue or organ, U is weighted by dimensionless tissue weighting factor \\ . multiplying the equivalent dose II of an organ or tissue by its assigned tissue weighting factor W gives a weighted equivalent dose. The sum of weighted equiv alent doses for a given exposure to radiation is the effective dose. Thus I f IVTX WR D, R ) 25 2.11.4 Committed dose: When radio nuclides are taken into the body, the resulting dose is received throughout the period of time during which they remain in the body. The total dose delivered during this period of time is referred to as the committed dose and is calculated as a specified time integral of the rate of receipt of the dose. Any relevant dose restriction is applied to the committed dose from the intake. The committed dose mav refer to the committed effective dose and the committed equivalent dose. 2.11.5 Collective dose: The collective dose relates to exposed groups or populations and is defined as the summation of the products of the mean dose in the various groups of exposed people and the number of individuals in each group. The unit of collective dose is the man- Sivert 2.11.6 Committed Equivalent Dose: I or radionuclides incorporated into the body, the integrated dose over time.50 years for occupational exposure. 70 years for members of the general public (10) 26 CHAPTER THREE MATERIAL AND METHODS 3.1. Gamma-ray Spectroscopy: Gamma-ray spectroscopy is the quantitative study of the energy spectra of gamma-rav sources. Most radioactive sources produce gamma rays of various energies and intensities. When these emissions are collected and analyzed with a gamma-rav spectroscopy system, a gamma-ray energy spectrum can be produced. A detailed analysis of this spectrum is typically used to determine the identity and quantity of gamma emitters present in the source. The gamma spectrum is characteristic of the gamma-emitting nuclides contained in the source, just as in optical spectroscopy, the optical spectrum is characteristic of the atoms and molecules contained in the sam ple. The equipment set-up used in gamma spectroscopy includes an energy-sensitive radiation detector, a pulse sorter (i.e., multichannel analyzer), and associated amplifiers and readout devices. The instrumentation set-up is given is I ig. 2 .1. Various types of detectors differ in their operating characteristics, but all are based on the same fundamental principle: the transfer of part or all the radiation energy to the detector mass where it is converted into an electrical pulse. The form in which the converted energy appears depends on the detector and its design. In this study a sodium iodide detector (Nal) is used (Scintillation detector). Scintillation detectors use crystals that emit light when gamma rays interact with the atoms in the crystals. The intensity of the light produced is proportional to the energy deposited in the crystal by the gamma ray. The mechanism is similar to that of a thermoluminescent dosimeter. The detectors arc joined to photomultipliers that convert the light into electrons and then amplify the electrical signal provided by those electrons 27 Figure (3.1): Na (TI) detector instrument 3.2 System Calibration If a gamma spectrometer is used for identifying samples o f unknown composition, its energy scale must be calibrated first. Calibration is performed by using the peaks of a known source, such as cesium-137 or cobalt-60. Because the channel number is proportional to energy, the channel scale can then be converted to an energy scale. If the size of the detector crystal is known, one can also perform an intensity calibration, so that not only the energies but also the intensities o f an unknown source or the amount of a certain isotope in the source can be determined. 3.3. Energy calibration: When gamma spectrometer is used for identifying samples of unknown composition, its energy scale must be calibrated first. Calibration is performed by using the peaks of known sources. Because the channel number is proportional to energy, the channel scale can then be converted to an energy scale. Energy calibration was performed by using mixed radionuclide standard (MW625) obtained from the IAEA in the form of 500 ml Marinelli beaker 28 Energy 400 500 600 700 800 Channel Fig3.3: Energy calibration curve Table2.1 .Types o f radionuclides used for system calibration and their corresponding gamma- energies Gamma-ray energy[Kev] Nuclide 60 Americium-241 88 Cadmium-109 122 Cobalt-57 662 Caesium-137 1173 Cobalt-60 1333 Cobalt-60 1460 Potassium-40 29 3.4. Efficiency calibration: An accurate efficiency calibration of the system is necessary to quantify radionuclides present in a sample because the accuracy of all quantitative results will depend on it. i he efficiency of detector refers to the number of radiations actually detected out of total number emitted by the source. There two ways to define detector efficient:}. number of pulses recorded number o f radiation quanta emitted by source Intrinsic efficiency: based on the number radiations that strikes the detector Number of pulses recorded Number of radiation quanta incident on detector The equation below was used to obtain curve of efficiency detector (Na I). From the equation (3.3) the efficiency calculated for each radionuclide Co -60 and Cs-137and relation plotted between efficiency and energy show in fig (3.3). 30 Fig (3.3): Efficiency calibration curve 3.5. MATERIALS & METHODS: 3.5.1 The Study Area: The area under consideration is the Touria Mountain in Khartoum State in Omdurman. Sampling trip was conducted in April 2014 as a continuation for national program on radioactivity monitoring to produce a radiation map for the country. 3.5.2. Sample Collection, Preparation and Measurement; After collection samples The samples breaking in environmental monitoring the laboratories o f the Atomic Energy Commission and weighed in a digital weighing balance with a precision of ± 0.01 g, and were sealed in 500 ml Marinelli beakers and stored for three weeks before counting in order to allow the in-growth of uranium and thorium decay products and achievement of secular equilibrium for “ 'Uand “ 'Th with their short-lived progeny. Each sample was placed onto Nal detector and measured for three hours. The Th concentration was. Samples were approximately 500g-700g 31 weights collected from the mountain rock. Ambient dose measurements at sample locations were recorded using one survey meter (Rados). Samples were taken from the top. medium and bottom o f the mountains and locations were recorded using Global Positioning System (GPS). determined from the concentrations of 2l2Pb (238 keV) in the samples, and that ol t was determined from the average concentrations o f the 2l4Pb (352 keV) and ‘:n "Bi (609 keV) decay products. Whereas 4HK concentration was measured directly using its (1460 keV) gamma-line. Table(3.l) Sampling protocol for twelve samples 1 1 ' l>ose rale • NO Sample i \ GPS reading t♦ • code . By (Rados) * 1 I .ong Lat 4 Alt ! 1. TR1 32.42567 15.56578 391 . 0.11 2. TR2 32.42516 15.56701 382 , 0.10 3. TR3 32.42549 15.56914 389 1 0.12 4. TR4 32.42486 _ 15.57116 385 1 0.07 5 . ' ~TR5 32.42247 15.57246 392 0.12 6. TR6 32.42261 15.57222 395 ; 0.12 7. TR7 32.42266 15.57216 i 389 ! 011 8. TR8 1 32.42308 15.57241 394 • 0.21 9. TR9 32.42205 15.57439 396 |♦ 0.12 . 10. TRIO 32.42191 15.57462 388 0.12 11. T R 1 1 32.42181 15.5745 392 0.05 ; 12. TR 12 32.42196 15.67619 386 0.11 13. TR13 32.42189 15.57623 397 , 0.14 14. TR 14 32.2535 15.3433 396 | 0.10 15. 1R15 32.42136 15.57642 397 i 0.11 16. TR 16 32.2544 15.3433 386 0.12 17. T R 17 __ 32.2564 _ _15.3426 391 , 0.21 ''394'"]' 0.11 \Z. ’ TR 18 ~ 32.2564 15.3423 19. T R 19 I 32.2565 15.342 385 ! 0.06 20. 1R 20 1 32.2552 388 0.05 mm m ♦ ^ mm m _ 15.3407 « • • «• • 32 3.6. Calculation of Absorbed Dose Rate in Air: The radiation effects in the air can be expressed in terms of the exposure rate or the absorbed dose rate in air at I m height. In this study the absorbed dose rate in air at a height of I m above ground surface was calculated from the measured activity concentrations of gamma-emitters using UNSCEAR dose-rate conversion factors (DRCF) The DRCF used here for estimation of dose are adopted from the (20) 0.604. 0.462 nGy h"* per Bq kg'* for ^ K . ^^ T h and ^"^U. respectively). It has to be noted that the calculation of these DRCF has been made on the assumption that all the 2 3 8 2 3 2 decay products of the U and Th series arc in radioactive equilibrium with their precursors, and that the soil density is 1.4 g cm ~ and the activity distribution, natural radioactive nuclides are uniformly distributed in the ground, dose rates D (nG y h '* ) at 1 m above the ground surface arc calculated by the following formula. -i -i D (nGy/h) = (Bq kg'') x Conversion factor (nGy h '/ Bq kg 3.7. Annual Effective Dose: Total radiation risk to an individual organism is measured by annual effective dose (H). thus the estimated absorbed dose rates in air at 1 m height were converted into annualefifective dose using the following formula: -l -i -3 H (pSvy"1) = D (nGylT1) x 24h x 365.25d x 0.2 x 0.7 Sv G y'1 x 10 0.7 SvGy’* is the conversion coefficient from absorbed dose in air to effective dose received by an individual. 0.2 for the outdoor occupancy factor (UNSCF.AR. 1993- 2000) 33 Table: UNSCEAR Conversion factors for different Radionuclide’s N uclide DRCF (nGy h '/ Bq kg ') 232ZT : T h series 0.604 "*U series 0.462 0.0417 34 CHAPTER FOUR RESULTS AND DISCUSSION Activity concentration of “',0U. HUk and I'h in rocks samples collected from -I different locations within Touria mountain in Omdurman reported in lk| kg dr\ weight and presented in Table (4.1). Calculations are relied on establishment of secular equilibrium in the samples, due to the much smaller lifetime of daughter *v • * radionuclides in the decay series of '''T h and ""U. More specillcally. the ' lh concentration was determined from the concentrations of :i“Pb in the samples, and that ol MX 214 214 IJ was determined from the average concentrations of the Pb and Bi d e c a \ products. Thus, accurate determination of '''T h and U radiological concentrations was 4u made and true measurement of K concentration was achieved External exposures outdoors arise from terrestrial radionuclides present at trace levels in all soils. The external gamma dose rate in air is calculated from the measurement of activity•• concentration of the relevant radionuclide in soil. Coefficients for comersion of activity concentration to absorbed dose rale in air are 0.0417. 0.604. 0.462 n(i\ IT * per I3q kg'* for ^ k . “^“'fh and respectively l igure(4-l) concentration o f ''1 IJ within normal level .variation between points location due to types of rock also Figure(4-2) concentration of J0kwithin normal level .variation N ' A between points location due to typos of rock and figure! 4-3) concentration of "''Th within normal level .variation between points location due to types of rock 35 Table (4-1): Activity concentration (Bq kg'1) of 238U, 232Th and 40K in the rock sa m p le s NO Sample Activity concentration (Bq.kg1) for name the sample collected in Touria mountain 238u 232Th 4UK 1. TRl 30.555 38.39 600.54 2. TR2 31.24 40.38 560.19 3 TR3 34.27 43.31 580.29 4. TR4 23.11 29.51 34.32 5. TR5 33.25 42.37 685.14 6. TR6 33.35 37.99 405.36 7. TR7 32.69 40.07 264.9 8. TR8 27.78 43.25 787.53 9. TR9 44.95 41.89 315.81 10. TRIO 29.005 41.57 623.38 11. TR11 28.41 43.92 1113.96 12. TRl 2 39.66 37.99 480.57 13. TRl 3 29.47 39.61 600.71 14. TRl 4 25.17 37.83 360.07 15. TRl 5 25.11 29.45 453.6 16. TRl 6 33.54 38.11 600.71 17. TRl 7 36.38 40.71 680.4 18. TRl 8 30.99 36.67 606.84 19. TRl 9 24.96 39.27 974.62 20. TR20 24.14 30.72 31.23 Mean 30.9 38.7 538.00 std 5.5 4.3 266.2 36 Table (4-2): dose rate and effective dose rates assessment for the 232Th and 238U series and 40K, to the sample collected in Touria mountain NO LABLES effective dose rates Dose rates O iS v y - ') (n G y h " 1) 40K /J /T h --18u /JZT h 40K 1. T R l 17.33 2 8 .4 6 3 0 .7 4 14.12 2 3 .1 9 25.05 2. T R 2 17.7 2 9 .9 3 2 8 .6 7 14.42 2 4 .3 9 2 3 .3 6 3. T R 3 19.43 32.1 2 9 .7 15.83 2 6 .1 6 2 4 .2 4 T R 4 13.1 2 1 .8 7 1.75 10.68 17.82 1.43 5 T R 5 18.85 3 1 .4 2 9 .9 4 15.36 2 5 .5 9 2 4 .4 6 T R 6 18.91 2 8 .1 7 2 0 .7 4 15.41 2 2 .9 5 16.9 7 T R 7 18.53 2 9 .7 13.56 15.1 2 4 .2 11.05 8 T R 8 15.75 3 2 .0 6 4 0 .3 12.83 2 6 .1 2 3 2 .8 4 9 T R 9 2 5 .4 9 3 1 .0 5 16.16 2 0 .7 7 25.3 13.17 10 TRIO 16.45 3 0 .8 2 3 1 .9 13.4 25.11 2 5 .9 9 11 T R l 1 16.1 3 2 .5 6 57 13.12 2 6 .5 3 46.45 12 T R 1 2 2 2 .4 8 2 8 .1 7 2 4 .5 9 18.32 2 2 .9 5 2 0 .0 4 13 T R l 3 16.72 2 9 .3 7 3 0 .7 2 13.62 2 3 .9 3 25.03 14 T R l 4 14.27 2 8 .0 4 18.42 11.63 2 2 .8 5 15.01 15 T R l 5 14.24 2 1 .8 3 2 3 .2 2 11.6 17.79 18.92 16 T R 1 6 19.01 2 8 .2 5 3 0 .7 2 15.49 2 3 .0 2 25.03 17 T R 1 7 2 0 .6 3 3 0 .1 8 3 4 .8 2 16.81 2 4 .5 9 2 8 .3 7 18 T R l 8 17.57 2 7 .1 8 3 1 .0 6 14.32 2 2 .1 5 25.31 19 T R 1 9 14.11 29.11 4 9 .8 8 11.5 2 3 .7 2 4 0 .6 4 2 0 T R 2 0 13.68 2 2 .7 7 1.6 11.15 18.55 1.3 M ean 17.5 2 8 .7 2 7 .3 14.3 2 3 .3 22.25 STD 3 .0 0 3.2 13.5 2.5 2 .6 11.00 Table (4-3): Average dose rate (nGy h *) and Average effective dose rates (pSv y ') assessm ent for the 232Th and 238U series and 40K 238 u 232T h 4°k M e a n ± S td M e a n ± S td M e a n ± S td Dose rates 14.3±2.5 2 3 .3 ± 2 .6 2 2 .2 ± 11.0 (n G y h " 1) Effective dose rates 17.5±3.1 2 8 .6 ± 3 .2 2 7 .3 ± 1 3 .5 ______( n s v y~‘)______ * 37 Figuer (4.1): Activity o f 23*U in samples Figuer (4.2): Activity of ^ in samples 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Smples ♦ ♦ ♦♦♦♦ ♦ ♦♦♦♦♦•♦ ♦♦♦ « Figuer (4.3): Activity of232Th in samples ♦ ♦♦ ♦ ♦♦♦♦ K40 »Th232 ■ U238 1 2 3 4 5 6 7 8 91011121314151617181920 Sam ples ♦ ♦ ♦ ♦ * Figuer (4.4): Activity of ^U, 232Th, 40K in samples 39 Conclusions Based on the results obtained in this study the following concluding remarks can be A A A A A drawn: The activity concentrations of 238U , /JZ Th and 4UK measured in rock of the Touria mountain \ and the associated absorbed dose rate in air at one meter height fall within the world range for normal background area cited in UNSCEAR report 2000. the worldwide activity concentrations. Potassium-40 is the principal contributor to the absorbed dose rate in air relative to contribution from gamma-emitting radionuclides from uranium and thorium series. 40 References 1. Abd El-Hadi El-Kam e', Abdallah Ibrahim Abd El-maaeed1*, Abd El-Bas t Abbadv2. x Shaban Harb“,Imran Iss a Saleh (2012) Natural Radioactivity of Environmental Samples and their Impact on the Population at Assalamia-AIhomira Area in Yemen 2. Eisenbud. M andGesell. T (1997), Environmental radioactivity from natural, industrial and military sources, 4thEd, academic press, USA) 3. Tahir S., Jamil, K, ZaidiJ, A rif .M, Ahmed. N and Ahmad .S (2005): Measurements of activity concentrations of naturally occurring radionuclide’s in soil from Punjab province of Pakistan, Radiation Protection Dosimetry 113,(4): 4. Merdanoglu .Band Altinsoy.N(2006):Radioactivity Concentration and Dose Assessment for soil samples from Kestanbol Granitearea, Turkey, Protection Dosimetry, 121 ,(4):399-405 5. 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Argonno National Laboratory. EVS Human Health Fact Sheet 13. James E. Martin Physics for Radiation Protection. A Handbook. Second Edition. Completely Revised and Enlarged. 14. UNSCEAR(0.0417), United Nations Scientific committee on the effects of atomic Radiation: Sources and Effects of Ionizing Radiation. Report to the General Assembly, with Annexes 42