Nature of Youth Unemployment in Kazanchis, Addis Ababa Zelalem Bezabih A Thesis Submitted to ...

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Investigation of natural y-radioactivity of
soil and rock samples
from the Kenticha tantalite processing area of
the Sidamo region
A Thesis Presented to
the School of Graduate Studies
Addis Ababa University
In pmtial fulfillment
of the requirements for the degree
of master of science in physics
By
Selemon Bekele
June 1995
CONTENTS
Acknowledgment ----------------------------------------------------------------------------------iii
List of figures----------~-:--------------------------------------------------------------------------iv
List of tables----------------------------------------------------------------------------------------vi
Abstract --------------------------------------------------------------------------------------------vii
In trod uction -----------------------------------------------------------------------------------------I
1. TYPES OF IONISING RADIATION---------------------------------------------------------~3
I. I Introduction ------------------------------------------------------------------------------------3
1.2 Natural and Induced Radioactivity ----------------------------------------------------------4
1.3 Properties of Nuclear Radiation -------------------------------------------------------------5
1.3.1 a - rays ----------------------------------------------------------------------------------5
1.3.2 P- rays ----------------------------------------------------------------------------------6
1.3.3 Y- rays -----------------------------------------------------------------------------------9
1.4 Natural Radioactivity in Soils and Rocks -------------------------------------------------10
2. ENVIRONMENTAL RADIATION MONITORING ------------------------------------13
2.1 Sources of Radiation in the Environment ------------------------------------------------13
2.2 Pathways in the Environment ---------------------------------------------------------------16
2.3 The Risks ofIonising Radiation ------------------------------------------------------------17
2.4 Environmental Radiation Monitoring -----------------------------------------------------19
3. METHODS OF GAMMA-RAY -SPECTROSCOPY -------------------------------------23
3.1 Interaction of Gamma Rays with Detector Materials ------------------------------------24
3.2 Detection of Gamma Rays -------------------------------------------------------------------26
3.2.1 Ionising effects of gamma rays -----------------------------------------------------27
3.2.2 Energy resolution and detection efficiency ---------------------------------------28
3.2.3 NaI(TJ) scintillation detectors ------------------------------------------------------31
3.2.4 Germanium detectors ----------------------------------------------------------------32
3.3 The Electronics and Data Acquisition System -------------------------------------------33
3J.l Detector bias supply -----------------------------------------------------------------33
3.3.2 Preamplifier ---------------------------------------------------------------------------33
3 J J Amplifier ------------------------------------------------------------------------------34
3 J.4 Multi-channel analyser (MCA) ----------------------------------------------------37
3.4 Background Interference in y-Ray Measurments-----------------------------------------38
4. DATA ACQUISITION AND ANALYSIS ------------------------------------------------ -41
4.1 Geological Features --------------------------------------------------------------------------41
4.2 Sampling Technique s ------------------------------------------------------------------------43
4.3 Measurement with NaI(Tl) Detector -------------------------------------------------------43
4.4 Measurement with HpGe Detector ---------------------------------------------------------47
4.5 Discussion of Results ------------------------------------------------------------------------57
4. 6 Conclusion -------------------------------------------------------------------------------------67
References -------------------------------------------------------------------------------------69
Appendix A -----------------------------------------------------------------------------------72
Appendix B ------------------------------------------------------------------------------------73
Appendix C ------------------------------------------------------------------------------------74
Appendix D ---;-------------------------------------------------------------------------------75
Appendix E ------------------------------------------------------------------------------------77
iii
Acknowledgement
I would like to express my gratefulness to Illy advisor Dr. Siegfried Tesch for his valuable
advice, and continual assistance tluoughout this work. I very much appreciate lLis instmctive
comments and suggestions whlch have played a cmsial role in the completion ofthe project.
I would like to thank the school of graduate studies for the financial support I obtained to
carry out the project, without whlch thls work could not have been done.
I am greatly indepted to the department of Geology and Geophysics for providingg me with
the samples to be studied by y-ray spectroscopy. I am specially very thankful to Dr. Anatoli
who had been very helpful in ssupplying the necessary information about the samples.
I extend Illy warmest gratitude to Ato Mulugeta Desta and Ato Zewdu Bogale of the
National Scientific Equipment Centre, Science and Technology Commission, for their
cooperation and help while I was working in their laboratory.
I am vely thankful to the National Radiation Protection Authority and its staff for the
continual help I got whlle working in their laboratory. My special thanks goes to Ato Hailu
Woldie for hls invaluable advice and support throughout Illy work.
I would also like to take thls opportunity to thank and appreciate the mirListry of education
for providing me with financial support throughout my study.
Last but not least, I am quite grateful to my parents Ato Bekele Gadissa and W/o Mulunesh
Mekonnen for their unrelenting help throughout all the years I have been studying.
IV
List of figures
Fig. 1.1 The atomic number versus neutron number for stable isotopes and longer lived
natural radioactive and transuranic elements. ---------------------------------------------4
Fig. 2.1 A schematic diagram illustrating the relative contribution of various sources to
environmental radiation. --------------------------------------------------------------------14
Fig. 2.2 Exposure pathways in the environment.-------------------------------------------------16
Fig. 3.1 A schematic diagram of a modern y-ray spectroscopy system.-------------------~--34
Fig. 3.2 Signal pulses from a) preamplifier, b) amplifier after base line restoration,
c) amplifier after pulse shaping.-----------------------------------------------------------36
Fig. 3.3 Spectrum of51Co source taken without and with pile-up rejection.-----------------36
Fig. 3.4 y-ray spectrometer with Compton suppression system.------------------------------40
Fig. 3.5 y-ray spectrum of 65Zn taken with and without Compton suppression.-----------40
Fig. 4.1 Geological map of the Adola area. -------------------------------------------------------42
Fig. 4.2 Arial view of the sites for the samples selected for study with HpGe
detector---------------------------------------------------------------------------------------45
Fig. 4.3 Schematic diagram of the NaI(Tl) measuring system.--------------------------------46
Fig. 4.4 Background spectrum as measured with the NaI(Tl) detector.-----------------------46
Fig. 4.5 Spectrum measured with the NaI(Tl) detector for sample KL.----------------------47
Fig. 4.6 Schematic diagram of the HpGe measuring system.----------------------------------48
Fig. 4.7 Spectrum for 6OCO source measured with a) NaI(TI) detector, b) HpGe
detector.--------------------------------------------------------------------------------------49
Fig.4.8 Energy calibration curve obtained by using standard point radiactive sources.---50
Fig.4.9a Spectrum for samples KL----------------------------------------------------------------51
Fig. 4.9b Spectrum for sample L 1S 6 .-------------------------------------------------------------52
Fig.4.9c Spectrum for sample LD6.---------------------------------------------------------------53
v
Fig. 4.10 Some examples of fitting results.-------------------------
Fig. 4.11 The 1764 keY line of214Bi after being refitted.-------
Fig. 4.12 Efficiency versus energy curve for the IAEA referenc
standard source for efficiency calibration.---------------
Fig. 4.13 Background spectrum obtained with the HpGe detect
Fig. 4.14 Histogram showing relative distribution of radioacth
. ,l
,
vi
List of tables
Table 4.1 Setting of the electronics system for NaI(TI) detector.------------------------------44
Table 4.2 Setting of the electronics system for HpGe detector.--------------------------------48
Table 4.3 Values of channel numbers (C) and energies (E) for energy calibration.--------50
Table 4.4 Energy and efficiency values for efficiency calibration.----------------------------56
Table 4.5 Activity calculation results. -------------------------------------------------------------59
Table 4.6 Tatal activity of samples as calculated by MicroSAMPO software---------------61
Table 4.7 Activity values used for the determination of238U.--------------------------------65
Table 4.8 Activity values used for the determination of232Th.-------------------------------66
vii
Abstract
Soil and rock samples from a mining region in southern Ethiopia have been analysed for
natural gamma (y) radioactivity. A NaI(TI) scintillation spectrometer was used to select, out
of 200 samples, those with a marked level of radioactivity above the background radiation.
Then a high resolution p-type germanium detector, inside a passive lead shield lined internally
with copper and aluminium, coupled to a Canberra S I 00 MCA card was employed to measure
the y-radiation from each sample. The y-spectra obtained were then transferred to a Canberra
MicroSampo software for further analysis. The radioactivity content of the samples has been
determined and the results, which may serve as baseline data for future work in environmental
radiation monitoring, are discussed. The uranium and thorium activities have also been
estimated so as to provide preliminary information on which type of soil and rock one should
concentrate for possible prospecting of uranium and thorium minirals.
.
"
I
Introduction
Different kinds of radiations are extensively employed to the benefit of human beings in
industry, agriculture, medicine etc .. They are also used in various research centres around
the world for the advancement of science and technology. Despite their usefulness, nuclear
radiations can be hamlful to man and his environment. The development of the nuclear
industry; the use of radiation and radioisotopes in hospitals, industry, and scientific research;
the radioactivity that prevails io mioing areas such as those for uranium exploration, the
radiation that results fi'om nuclear explosions and reactor accidents have made it necessary
to evaluate the level of radioactivity in order to assess its ioJpacts and ioJplement the
appropriate countenneasures to protect mankind and his enviromllent. As a result, studies
and sUlveys of the natural environmental radiation and radioactivity are of great impOltance
and interest not only for general scientific reasons but also in health physics.
Natural sources of radiation make the largest contribution to the overall exposure of man
through extemal irradiation by cosmic rays, by radioactive substances in the ground and in
buildiog materials, and through internal irradiation as a result of ingestion of naturally
occuning elements in air and diet. The justification for allowiog low radiation doses to the
public as a result of the use of radiation and radionuclides in medicine, agIiculture, industry
and science should be such that the benefit always outweighs the risk of injury. The decision
as to the level of exposure beyond which it is safe to use radiation is a matter of personal
judgement, and by its very nature it will always be controversial. So, in the absence of
conclusive data on the effects of, e.g., very low level radiation exposure, the basic reference
level for the measurement of JloJlulation exposure is the exposure already being incurred by
man from natural sources.
2
Being the main source of human exposure, studies of the dose from natural radioactivity
and its effects on health improve the Ullderstanding of radiation damage and, therefore, are
of great value as a reference when standards and regulatory control actions are established.
TItis requires a knowledge of what radioisotopes exist in the environment together with the
amOlmt of the radiation they emit. But radiation doses that can have serious effects on living
matter are generally so low in energy terms that they cannot be detected by the physical
senses of man. Thus, one has to find a means by which to detect the radiation, measure its
energies and identify them with specific nuclides. One such method is gamma-ray
spectroscopy, since many of the naturally existing radionuclides are ganuna emitters. So, tlte
main objective of this work is to measure the natural gamma radioactivity of soil and rock
samples, brought from Kenticha of the Sidamo region located in southem Ethiopia, using
the methods of gamma-ray-spectroscopy and provide a base line data for future work in
envirOImtental radiation monitOl1ng. In addition to this, the results obtained may provide
prelinrinalY infonnation on which type of soil or rock one has to focus attention for a
possible prospecting of uranium and thorium minerals.
In the first chapter the types of radiation that exist in the enviromnent are briefly desclibed.
The propelties of a-, 13-, and y-rays which are intpOitant fi-om the point of view of
radiation exposure to man are discussed. As the measurement of radioactivity is the statting
point towards the study of radiation hazards, the second chapter deals with enviromnental
radiation in relation to its sources, the risks associated with it, and the pathways leading to
human exposure. In the third chapter the method of gamma-my-spectroscopy for the
measurement of enviromnental radiation is explained. The forth chapter deals with the
experimental set-up for data acquisition and analysis of the results obtained.