development of a 6 mv tandem accelerator mass spectrometry facility and its applications.

212 Pages·2017·6.11 MB·English

DEVELOPMENT OF A 6 MV TANDEM ACCELERATOR
MASS SPECTROMETRY FACILITY AND ITS
APPLICATIONS.

Kamela Godwin Sekonya

Student number: 9811024J

A thesis submitted to the Faculty of Science,
University of the Witwatersrand, Johannesburg,
in fulfilment of the requirements for the degree of

Doctor of Philosophy

School of Physics

Johannesburg, 2017
I declare that this thesis is my own, unaided work. It was submitted for the
Degree of Doctoral of Philosophy at the University of the Witwatersrand. It has not been submitted
before for any degree or examination in any other university.

Kamela Godwin Sekonya

04 June 2017

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ABSTRACT
Accelerator Mass Spectrometry (AMS) is an ultra-sensitive isotopic analysis technique
that allows for the determination of isotopic ratios of rare long-lived radionuclides such as
radiocarbon. AMS has become an important tool in many scientific disciplines, due to its
sensitivity of detecting isotopic ratios at the level of 10-15 by making use of nuclear physics
techniques and methods.
The objective of the present work was to design and implement a new AMS system at
iThemba LABS, the first of its kind on the African continent. The system is described in detail
along with the relevant ion optics simulations using TRACE-3D. Beam optics calculations
were performed for carbon isotopes, using the TRACE-3D code, in order to optimize the design
of the new spectrometer and assess its overall performance.
The AMS technique was applied in two unique South African research projects in
relation to archaeology and environmental air pollution studies. The AMS technique, combined
with the Proton-Induced X-Ray Emission (PIXE) technique, was also applied in an
environmental study with respect to the contribution of contemporary and fossil carbon in air
pollution in the Lephalale District, close to both the newly built Medupi coal-fired power
station (~5 GW, the largest ever build in South Africa), and the existing Matimba coal-fired
power station.
The discrimination of contemporary carbon and fossil carbon is accomplished by using
the AMS technique in measurements of the 14C/C ratios of aerosol particulate matter. The
absence of 14C in fossil carbon material and the known 14C/C ratio levels in contemporary
carbon material allows us to distinguish between contemporary carbon and fossil carbon and
decipher in this manner different anthropogenic contributions.
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The contemporary carbon throughout our sampling campaign in the Lephalale District
has been measured to be approximately 53% of carbon aerosol. As many studies have been
performed of contemporary carbon and fossil carbon, no other contemporary and fossil carbon
source assessment method provides the definitive results that can be obtained from radiocarbon
measurements.
PIXE analysis for the determination of the elemental composition of particulate matter
in samples near the Medupi coal-fired power station in the Lephalale District was also
performed for 6 elements, namely, K, Ca, Ti, Mn, Fe, and Zn. In the samples that were analyzed
the particulate matter concentrations did not exceed the air quality standards regulation at
Lephalale. The recommended daily limit air quality standard by South African legislation is 75
µg/m3.
Enrichment Factor (EF) analysis of soil with respect to Fe shows anomalously high
values for Zn.
AMS was also applied to archaeological studies of early herding camps of the khoe-
khoe people at Kasteelberg, situated on the southwest coast in South Africa, and are among the
best preserved sites of their kind in the world. Sea-shell samples from the Kasteelberg B (KBB)
site have been dated with AMS at Lawrence Livermore National Laboratory (LLNL) in an
effort to elucidate the relationship between the herder-foragers of the inland and shoreline sites
in terms of migration patterns. The radiocarbon dates obtained are in general agreement with
the other studies that have been performed on the site, and show that the ages of artifacts are
less than 2000 years. The samples for this study originate from various well defined
stratigraphic-levels at square A3 at KBB. It was evident from excavation that the artefacts seem
to be of the same period and there is no evidence of mixing from different stratigraphic layers.
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Radiocarbon dates were calibrated using Calib 6.1 and each was corrected for marine
reservoir effect. The date range between the earliest and most recent dates that were obtained
span gap is approximately 400 years from AD 825 to AD 1209. The majority of the radiocarbon
dates of the KBB site belong to dates of 1002-1100 AD, the other few belong to 825-958 AD,
and the last single date of 1209 AD. The new AMS dates from this work suggest the high
probability that indeed there was a hiatus between the two occupations designated as lower and
the upper KBB. The significant changes seen in material culture styles as well as in the nature
of occupation and change in accumulation rate of deposits therefore do not necessarily indicate
a cultural replacement caused by the arrival of a new population. This implies that the
occupants of lower KBB may also have been Khoe-speakers, and not local San.

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ACKNOWLEDGEMENT
I would like to thank Dr. Tom Brown of Centre of Accelerator Mass Spectrometry at Lawrence
Livermore National Laboratory for fruitful scientific discussing on beam optics calculations
for the Accelerator Mass Spectrometry (AMS) system. I wish to thank Professor Elias Sideras-
Haddad for giving me opportunity to work in the field of accelerator mass spectrometry at the
Physics Department, University of the Witwatersrand. Professor Stuart Piketh of the University
of North West (Potchefstroom Campus) is being acknowledged for allowing me to use his
equipment such as the Partisol-plus and filters for air quality in Lephalale projects for PIXE
and environmental samples for AMS. I would also like to thank Professor Karim Sadir of the
Archaeology Department at the University of the Witwatersrand for shell samples from
Kasteelberg B. Many thanks goes to Paula Zermeno and Dr Tom Brown at Lawrence
Livermore National Laboratory for introducing me to sample preparation of graphite targets
for AMS and for helping me to prepare my quartz filter samples for the environmental air
pollution studies and shell samples for the archaeology project. I also thank Dr. Tom Brown
for scientific discussions on AMS and proof-reading of my thesis. I would like to thanks Mr.
Hans Delsiki of iThemba LABS for the drawing of the lower energy and higher energy AMS
systems of the iThemba LABS facility. I am indebted to the iThemba LABS Accelerator Group
for providing me with TRACE-3D beam optics software and I do not forget Mr. Peter van
Schalkwyk who helped with setting up the ion source test bench and testing the LLNL/CAMS,
and Dr. Mark Sundquist of National Electrostatics Corp (NEC) for providing some information
on some of the equipment in the AMS system.
Warmest thanks are expressed to my wife Mmamokete and my three children Maeleletje and
Tumelo Sekonya and Tumelo Nogemane for supporting me all these years. I also thank my
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parents Maeleletje and Ramaisela Sekonya for supporting me for so many years and allowing
me to further my studies.
Financial supports from IAEA for allowing me undertake a scientific visit to LLNL and from
NRF for study fees of my PhD are acknowledged.
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CONTENTS
ABSTRACT ................................................................................................................... III
ACKNOWLEDGEMENT .............................................................................................. VI
CONTENTS ................................................................................................................ VIII
LIST OF FIGURES ........................................................................................................ XI
LIST OF TABLES ......................................................................................................... XV
ABSTRACT III
ACKNOWLEDGEMENT .............................................................................................. VI
CONTENTS VIII
LIST OF FIGURES ........................................................................................................ XI
LIST OF TABLES ......................................................................................................... XV
CHAPTER 1 INTRODUCTION ..................................................................................... 1
1.1 Accelerator Mass Spectrometry ..................................................... 1
1.2 Objective s of present PhD research study. .................................... 9
CHAPTER 2 RADIOCARBON DATING AND CALIBRATION ............................ 14
2.1 Radiocarbon in the environment .................................................. 14
2.2 Dating and calibration .................................................................. 18
CHAPTER 3 ACCELERATOR MASS SPECTROMETRY AT ITHEMBA LABS23
3.1 The New 6 MV EN Tandem AMS Facility at iThemba LABS-
South Africa ................................................................................ 23
3.1.1 iThemba LABS Tandem accelerator and modified AMS system25
3.1.1.1 Negative Heavy Ion Cs-Sputtering Source ................................. 27
3.1.1.2 Lower energy injection systems for AMS .................................. 33
3.1.1.3 The 6 MV EN Tandem accelerator for AMS .............................. 44
3.1.1.4 HE AMS system .......................................................................... 55
3.1.1.5 Detectors for AMS ...................................................................... 64
CHAPTER 4 BEAM OPTICS OF THE TANDEM ACCELERATOR AT
ITHEMBA LABS .................................................................................... 66
4.1 Introduction to Ion Beam Optics ..................................................... 66
4.2 Beam Optics Simulations for the Low Energy Injection System.... 84
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4.3 Beam Optics Simulations of the HE AMS System ......................... 96
4.4 Results and discussion ................................................................... 105
CHAPTER 5 RADIOCARBON AMS SAMPLE PREPARATION ........................ 109
5.1 Sample preparation for radiocarbon ......................................... 109
5.2 Sample preparation of quartz filter for environmental air pollution110
5.3 Sample preparation for shells samples for AMS measurements113
5.3 Sample preparation for Standards ............................................. 114
5.4 Cathode Carbon Pellets ............................................................ 114
CHAPTER 6 ENVIRONMENTAL ASSESSMENT OF THE AREA AROUND
THE LEPHALALE COAL-FIRED POWER STATION USING
AMS AND PIXE TECHNIQUES ........................................................ 116
6.1 Application to Environmental samples ..................................... 116
6.1.1 Lephalale-Marapong Sampling ................................................. 116
6.1.2 Measurements and methods ............................................... 120
6.2 Proton Induced X-ray Emission (PIXE) Analysis .................... 129
6.2.1 Basic principles of the PIXE technique .................................... 129
6.2.2 Application on aerosol samples ................................................ 133
6.3 PIXE analysis in the Lephalale district ..................................... 134
6.3.1 Sampling collections ................................................................. 134
6.3.2 PIXE Analysis and Results ....................................................... 135
6.3.3 Enrichment factor analysis ........................................................ 139
6.3.4 Source Profiles by Unique ratios (SPUR) ................................. 141
6.4 Results and discussion .............................................................. 144
CHAPTER 7 AMS 14C DATING OF ARCHAEOLOGICAL SITES OF KHOI-
KHOI PEOPLE ..................................................................................... 147
7.1 Application of AMS on archaeology ........................................ 147
7.1.1 Introduction to Kasteelberg B site............................................. 147
7.1.2 Sampling ................................................................................... 154
7.1.2 Measurements and methods ...................................................... 155
7.2 Results and discussion .............................................................. 161
CHAPTER 8 CONCLUSIONS ........................................................................ 165
REFERENCES .............................................................................................................. 170
APPENDIX A SHORT DESCRIPTION OF ACCELERATOR
IMPROVEMENTS ............................................................................... 184
APPENDIX B PIXE ANALYSIS AT LEPHALALE ............................................... 187
APPENDIX C BEAM OPTICS ............................................. 190
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