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CHEW BOON KIAT

Jan 02, 2017

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  • Optimization and Implementation of

    low-background Gamma Spectrometry using

    HPGe Detector in Environmental

    Radioactivity Research

    Author : Boon Kiat Chew

    Supervisor: Dr Taw Kuei Chan

    Co-supervisor : A/P Thomas Osipowicz

    A Honours Thesis submitted in

    partial fulfilment of the requirements for the

    Degree of Bachelor of Science with Honours

    Department of Physics

    Faculty of Science

    National University of Singapore

    Academic Year 2014/2015

  • Abstract

    We are interested in the ability of the high-purity Germanium (HPGe) de-

    tector in detecting low-level gamma energies. We first did a energy calibra-

    tion for the HPGe detector using a Eu-152 sample. Using this calibration,

    we found the efficiency, energy resolution, minimum detectable activity and

    minimum detectable mass for the detector. We then did a back-calculation

    to find the activity of another Eu-152 sample. We also took a reading of rice,

    flour, milk powder and soil.

  • Acknowledgement

    I would like to thank Dr Chan and A/P Thomas for their invaluable help for

    this project, where they would often draw time out from their busy sched-

    ules for project meetings. I would also like to thank them for their patient

    guidance and advice, for without which this project would not have been

    possible.

    1

  • Contents

    Contents 2

    1 Motivation 5

    1.1 Environmental Effects . . . . . . . . . . . . . . . . . . . . . . 5

    1.1.1 Chernobyl Nuclear Disaster . . . . . . . . . . . . . . . 5

    1.1.2 Fukushima Daiichi Nuclear Disaster . . . . . . . . . . . 9

    1.2 Radionuclei . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

    1.3 Nuclear Plans In Asia . . . . . . . . . . . . . . . . . . . . . . 13

    2 Theory 16

    2.1 Gamma radiation . . . . . . . . . . . . . . . . . . . . . . . . . 16

    2.1.1 Compton Effect . . . . . . . . . . . . . . . . . . . . . . 17

    2.1.2 Photoelectric Effect . . . . . . . . . . . . . . . . . . . . 18

    2.1.3 Pair Production . . . . . . . . . . . . . . . . . . . . . . 18

    2.1.4 Processes In Detector . . . . . . . . . . . . . . . . . . . 20

    2.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

    2.3 Experimental Set-up . . . . . . . . . . . . . . . . . . . . . . . 25

    2

  • CONTENTS CONTENTS

    2.3.1 Semiconductor Detectors . . . . . . . . . . . . . . . . . 25

    2.3.2 High Purity Ge Detector . . . . . . . . . . . . . . . . . 29

    2.3.3 Pre-amplifier . . . . . . . . . . . . . . . . . . . . . . . 33

    2.3.4 Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . 33

    2.3.5 Multichannel Analyser . . . . . . . . . . . . . . . . . . 36

    3 Data Analysis 38

    3.1 Energy Calibration . . . . . . . . . . . . . . . . . . . . . . . . 38

    3.2 Background Spectrum . . . . . . . . . . . . . . . . . . . . . . 43

    3.3 Energy Resolution . . . . . . . . . . . . . . . . . . . . . . . . 44

    3.4 Energy Peak Efficiency . . . . . . . . . . . . . . . . . . . . . . 47

    3.5 Limit Of Detection . . . . . . . . . . . . . . . . . . . . . . . . 51

    3.6 Back Calculation . . . . . . . . . . . . . . . . . . . . . . . . . 55

    4 Samples 57

    4.1 Food Products . . . . . . . . . . . . . . . . . . . . . . . . . . 57

    4.2 Soil Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

    5 Conclusion 63

    Appendix A Gamma Vision 65

    Appendix B Tables 70

    B.1 Energy Calibration . . . . . . . . . . . . . . . . . . . . . . . . 70

    B.2 Energy Resolution . . . . . . . . . . . . . . . . . . . . . . . . 71

    3

  • CONTENTS CONTENTS

    B.3 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

    B.4 Minimum Detectable Activity . . . . . . . . . . . . . . . . . . 72

    Bibliography 73

    4

  • Chapter 1

    Motivation

    1.1 Environmental Effects

    With the usage of nuclear energy, there is a chance that radionuclides can be

    accidentally released into the environment. Studies have been made to study

    the effect of nuclear accidents on the environment, and more importantly, to

    humans. There have been two notable nuclear accidents that had resulted in

    the release of large amounts of radionuclide into the environment, Chernobyl

    and Fukushima Daiichi.

    1.1.1 Chernobyl Nuclear Disaster

    The Chernobyl nuclear disaster in 1986 resulted in large quantities of radioac-

    tive particles being released into the atmosphere. resulting in radioactive con-

    tamination of the surrounding environment. Environmental contamination

    5

  • 1.1. ENVIRONMENTAL EFFECTS CHAPTER 1. MOTIVATION

    can result in direct exposure of radioactivity to humans, or indirect exposure

    through contaminated food. Studies have been done after the accident to

    study the effect on the environment and humans.

    One study by the IAEA in 2005 found that most of the radionuclide re-

    leased during nuclear accidents have short half-lives.1 Smaller amounts of

    the long-lived radionuclide were released. Out of the short-lived radionu-

    clide, radioactive iodine is a cause for concern as it will accumulate in the

    thyroid after ingestion. For radionuclide with long half-lives, Cs-134 and

    Cs-137 are important contributors to radioactive contamination. Other ra-

    dionuclide have deposition levels were too low to cause a problem, or have

    low transfer ratio of soil-to-plant to cause real problems in agriculture.

    In urban areas, open surfaces such as roads and roofs became contaminated

    with radionuclide. However, the water solubility of caesium resulted in high

    Cs-137 activity around houses, where the rain had transported the Cs-137

    from the roofs to the ground. Additionally, cleaning process lead to the sec-

    ondary contamination of sewage systems. The nuclear incident also lead to

    contamination of food products. Initially, milk was the main contributor to

    internal dose due to large amounts of I-131 being released. The radioiodine

    deposited on plant surfaces were grazed by dairy cow. Radioiodine ingested

    is absorbed in the gut and is then transferred to the animals thyroid and

    milk within a day. During this period, the I-131 activity concentration in

    6

  • 1.1. ENVIRONMENTAL EFFECTS CHAPTER 1. MOTIVATION

    milk in affected regions exceeded regional action levels by a few hundred to

    a few thousand Becquerel per litre. In Russia and Ukraine this lead to sig-

    nificant thyroid dosage to those who consumed milk, especially children. In

    the long run, milk was contaminated with radiocaesium.

    Contamination of plant products happen over two phases. Initial contami-

    nation was due to the direct deposition of radionuclei onto the plants. After

    direct contamination, plants uptake radionuclei through contaminated soil,

    hence continuing to pose a health issue. Cs-137 and Cs-134 where especially

    problematic due to its solubility in water, was well as it being used by the

    plant in place of other minerals such as potassium. The highest levels of

    contamination with radiocaesium was observed in mushrooms, due to their

    tendency to accumulate mineral nutrients, such radiocaesium. For animals,

    the radionuclides circulate in the blood after ingestion. Some accumulate in

    specific organs, for instance, radioiodine accumulates in the thyroid, whereas

    radiocaesium is distributed throughout the soft tissues.

    Hence it was found that I-131 and Cs-137 in meat, milk and plant products

    are the most important contributors to human internal dose. However, due to

    the long half-life of Cs-137, the activity concentration in these food products

    have been decreasing slowly. The decrease in activity concentration is about

    3 to 7 percent per year. This means that Cs-137 will continue to contribute

    to human dose for years to come.

    7

  • 1.1. ENVIRONMENTAL EFFECTS CHAPTER 1. MOTIVATION

    Another study of foodstuff in Poland has found that I-131, Cs-134 and Cs-

    137 were the main contributors to activity in foodstuffs.2 The contamination

    with I-131 decreased quickly after June 1986. The only considerable con-

    centrations observed were for Cs-134 and Cs-137. It was found that the

    radiation contamination of fruits and vegetable remains the same after a

    few years. However, the higher radioactivity still remains in milk and forest

    mushrooms. This agrees with the finding of the previous report.

    Figure 1.1: Cs-137 activity in milk. It can be seen that activity is decreasingslowly, and has not returned to pre-accident levels.

    For cases of transference of radionuclei from soil to grass to animals, milk

    is an ideal liquid to dissolve the radionuclei.3 This is because milk contains

    fat, while also existing as an aqueous solution, Therefore, both fat-soluble

    and water-soluble contaminant can be found in milk as it offers both envi-

    ronments. This is important as milk is a fundamental food for infants and

    children, and is consumed by all the age groups. Hence from this we can

    8

  • 1.1. ENVIRONMENTAL EFFECTS CHAPTER 1. MOTIVATION

    see that there is a need to identify food that are vulnerable to radioactive

    contamination, and to have a reference set of data we can refer to in any

    event of a contamination. Hence preliminary data is necessary for such food

    products.

    1.1.2 Fukushima Daiichi Nuclear Disaster

    The Fukushima Daiichi Nuclear Disaster in 2011 resulted in radionuclides in

    the form