The first measurements of the radioactive 135Cs and 134Cs isotopes were made on an accelerator mass spectrometer. The natural Ba interference was suppressed using an isobar separator for anions (ISA) in order to measure the less abundant isobaric 134Cs and 135Cs isotopes. It was found that the Ba interference could be suppressed by a factor of 2 × 10^5 while 25% of Cs was transmitted. Furthermore, through comparing the known natural abundance of Ba isotopes to the measured concentration in a sample it was shown that the ISA does not introduce significant mass dependant fractionation at the level of 0.8%. A slow sequential injection analysis technique was developed to measure 135Cs using 134Cs as a reference isotope. This technique also permitted the monitoring of Ba interference. The ionization efficiency of Cs when analyzed in the molecular anion form, CsF2^- , was on the order of 10^−7 while the total measurement efficiency was 1.7×10^−9. The abundance sensitivity of this system was found to be 135Cs/ 133Cs = (1.3 ± 1.7) × 10^−10 , corresponding to a 3σ detection limit of 132.5 pg of analyte per target. Lastly, using the developed AMS techniques, beta spectroscopy, gamma spectroscopy, and isotope production, a measurement of the half life of 135Cs was made. The two measurements of the half life of 135Cs were 0.72 ± 0.32 Ma and 0.99 ± 0.42 Ma.

No description available.

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. / 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. iv 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. v 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. / GR2018

The existence of singly, doubly, and triply charged diatomic molecular ions was observed by using an Accelerator Mass Spectrometry (AMS) technique. The mean lifetimes of 3 MeV boron diatomic molecular ions were measured. No isotopic effects on the mean lifetimes of boron diatomic molecules were observed for charge state 3+. Also, the mean lifetime of SiF^3+ was measured.

The aim of thesis was to establish a methodology for 210Pb measurements by Accelerator Mass Spectrometry (AMS). The potential application is to measure 210Pb in people who have been exposed to radon. This will better our understanding of radon toxicity, which is not possible now with current radiometric and mass spectrometry techniques. The determination of 210Pb by AMS was done in two major studies 1) Studying Pb chemistry in a Cs+ sputter source used in AMS and 2) Evaluating 204,205 & 208Pb spikes for the quantification of 210Pb by isotope dilution. Pb chemistry was investigated using an 834 SIMS-type and a SO-110 Cs+ sputter source at the IsoTrace Laboratory and A.E Lalonde AMS facility, respectively. Different molecular anions of Pb were studied with the 834 SIMS-type Cs+ sputter source and the strongest molecular anion current of Pb and thus greatest ionization efficiency was achieved form the superhalogen PbF3-. The average 208PbF3- current was unaffected by varying the ratio of the fluorinating compounds (AgF2 and CsF) packed into a target. The average current of 208PbF3- was reproducibly increased by chemically mixing the targets of AgF2, CsF and PbF2 in concentrated HF rather than mechanically mixing them the powders with a stir rod. The count rate of 210Pb reproducibly increased by a factor of 20 when μg quantities of PbF2 were present in mg AMS targets compared to AMS targets that had pg quantities of PbF2. The average current of 208PbF3- for pure PbF2 targets in an SO-100 Cs+ sputter source was reproducibly increased when the Cs+ flux was decreased by a factor of 10. This phase of my work maximized the overall efficiency of PbF3-, to a value of 1.8x10-10 ±8x10-11s-, which was a key first step in the measurement by AMS. Then isotope dilution was tested to quantify 210Pb and the next stage of my work evaluated the use of 204,205 & 208Pb spikes. 210Pb was measured in the +3 charge state by isotope dilution assays using 204,205 & 208Pb spikes. 204Pb+3 reproducibly suffered from the molecular interference from 68Zn3+3, which could not be easily removed without negatively impacting the detection limit for 210Pb. 205Pb+3 continually suffered from 205Tl+3 interference which could be readily be removed but not without negatively affecting the II detection limit for 210Pb. 208Pb+3 suffers from no molecular interferences but if a large amount of 208Pb is needed to swamp the Pb in a sample, this could limit the detection limit for 210Pb as the abundance sensitivity is 210Pb/208Pb=1.3×10-12. A calibration curve is required when 208Pb is used as a spike due to a difference in collection efficiency of a Faraday cup, where 208Pb+3 is detected and the gas ionization chamber, where 210Pb+3 is detected. The quantification of 210Pb with 208Pb as a spike yielded a detection limit of 4.4mBq at the IsoTrace facility. A theoretical detection limit of ≤0.11mBq is expected at the A.E Lalonde AMS facility. The expected detection limit at the A.E Lalonde AMS facility is on par with α-spectroscopy but AMS samples can be counted in less than 1 hour whereas alpha spectrometry samples must be counted for about 1 day.

Accelerator Mass Spectrometry (AMS) is a sensitive technique for the analysis of rare isotopes. Optimizing the output of the cesium-sputter ion source is a fundamental method for improving measurement precision, efficiency, and reliability. Several strategies for improving the ion source are discussed and lead to an understanding of the electrodynamics within the ion source to inform further improvement in design and operating parameters. At the Andr´e E. Lalonde Accelerator Mass Spectrometry Laboratory (Lalonde AMS), the High Voltage Engineering Europa (HVEE) SO-110C ion source was modelled using Integrated Engineering Software (IES)’s Lorentz-2E ion trajectory simulation software. Lorentz-2E incorporates the mutual space-charge interaction between the positively charged cesium ion beam and the sputtered negative ion beam. A critical component of this work was the development of the Rijke code. Rijke communicates with Lorentz-2E to initiate, generate, and run varied sequences of simulations, as well as analyze and record the input and output data in formats convenient for timely analysis. This software and its interconnection with Lorentz- 2E is described in extensive detail for a prospective user. Initial simulation work examined the effects of modifying various electrode geometries within the source such as the extraction cone, the target aperture, a simple cratered sample model as well as examining the effects of varying the cesium ion current. The self-repulsion of cesium was found to be important at currents of 250 µA and above. At high enough cesium currents, the expansion of the cesium beam is such that parts of it impinge outside the extents of the sample material. Through both simulation and experiment, it was demonstrated that this effect can be mitigated by either recessing (translating along the axis of symmetry away from the ionizer) the target holding the sample or by adjusting the potential difference between the target and ionizer. Experimentally, at routine settings (6 kV target to ionizer potential, 115 ◦C cesium oven temperature, and 35 keV output energy), a target recess of 1 mm gave the most stable and sustained output of 12C from graphite blanks. While the peak current was less than the unrecessed case, the total measured charge from the recessed target was higher. Cesium currents at these routine settings were found to be below the theoretical space-charge limited maximum. Using 10Be standards, a multi-dimensional experimental study examined the effects of increasing the cesium current, adjusting the target-ionizer potential from 4 to 11 kV, while also examining target recesses of 0 to 4 mm. Multiple combinations of these settings produced enhanced currents of 9Be2+, measured at the high-energy offset Faraday cup, as high as 13.5 µA. This was higher than previously observed, resulting in the most precise measurement of 10Be performed to date at Lalonde AMS. The electrodynamics within the ion source can be characterized as three competing processes: a) a strong locus of positive space charge located at the centre of the sample, depending primarily on the focusing of the cesium beam, which draws negative ions across the axis of symmetry; b) a bulk positive space charge external to the negative ion beam, depending primarily on the magnitude of the cesium current, draws the outer-most negative ions away from the axis; and c) the raw field from the electrode potentials and geometry which is mainly defocusing for negative ions. These effects are mitigated the most when the cesium beam is distributed across the entire sample surface with the additional critical benefit of maximizing the sample material accessed for sputtering. This thesis work has demonstrated that both the mutual and self space-charge interaction of the cesium and negative ion beams were critically important and that the use of the simulation software can inform both improved design and operation settings of the ion source.

The goal of this research was to devise a new analytical technique, using Accelerator Mass Spectrometry (AMS), to measure Platinum Group Element (PGE) concentrations to the sup-ppb levels in natural, insulating, samples. The challenges were threefold. First, a method of sputtering an insulating sample to successfully produce a stable beam of anions needed to be devised. Second, a suitable standard of known PGE concentrations had to be found and third, spectral analysis of the beam had to verify any claims of PGE abundance. The first challenge was met by employing a modified high intensity negative ion source flooded with neutral caesium that successfully sputtered insulators to produce a beam of negative ions. The second challenge, that of finding a suitable standard, was fraught with difficulties, as no synthesized standards available were found to be appropriate for this work. As a result, direction is provided for future production of standards by ion implantation. The third challenge, successful spectral analysis, was accomplished using a newly designed gas ionization detector which allowed for resolution of the interfering molecular fragment from the PGE ions. Coupled with the use of the SRIM computer programme, positive identification of all peaks in the spectra of the analyzed samples was accomplished. The success of the first and third challenges lead to the qualitative analyses of geological samples for sub-ppb levels of PGE by AMS. Quantitative analyses await only for the appropriate standards and with those will come a whole new range of research possibilities for measuring sub-ppb levels of PGE in insulating samples by AMS.

Archaeological bones are usually dated by radiocarbon measurement of extracted collagen. However, low collagen content, contamination from the burial environment or museum conservation work have previously lead to inaccurate results, especially for old bones, compromising the ability to reconstruct reliable past chronologies. It is reported, for example, that up to 70% of Palaeolithic radiocarbon dates on bones are likely to be underestimates of the real age, blurring the picture of modern human dispersals and Neanderthal extinction. In this thesis, a method for isolating and radiocarbon dating the collagen amino acid hydroxyproline is described. Hydroxyproline consists of about 10% of bone collagen but is not found in significant amounts elsewhere in nature. The hydroxyproline dating method uses a mixed-mode (i.e. ion-exchange combined with hydrophobic chemistry), semi preparative HPLC methodology. The amino acids do not require derivatisation, and no organic solvents are used, thereby avoiding addition of carbon. The hypothesis of this thesis is that the hydroxyproline can be used as a bone specific biomarker, improving dating accuracy and making it possible to obtain radiocarbon determinations where previously it has been impossible. It was calculated that on average 3.3±1.4μg of contaminant carbon are added to each sample in the process of isolating the hydroxyproline, a low level suitable for 14C dating. It was investigated whether a deliberately contaminated bone and 'naturally' contaminated archaeological bones, yielding erroneous dates when dated using the normal pretreatment method, could be dated accurately using this method. In addition, a hydroxyproline date was obtained for a bone with too little surviving collagen to be dateable by the bulk collagen method. Finally, using the hydroxyproline dating method, the earliest direct ages for the presence of anatomically modern humans on the Russian Plain were obtained. The method proved to be a powerful tool that can help resolve longstanding archaeological questions.

Understanding the link between the Sun and climate is vital in the current incidence of global climate change, and 10Be in natural archives constitutes an excellent tracer for this purpose. As cosmic rays enter the atmosphere, cosmogenic isotopes like 10Be and 14C are formed. Variations in solar activity modulate the amount of incoming cosmic rays, and thereby cosmogenic isotope production. Atmospherically produced 10Be enters natural archives such as sediments and glaciers by wet and dry deposition within about a year of production. 10Be from natural archives therefore provides information on past solar activity, and because these archives also contain climate information, solar activity and climate can be linked. One remaining question is to what degree 10Be in natural archives reflects production, and to what extent the local and regional environment overprints the production signal. To explore this, 10Be was measured at annual resolution over the last 600 years in a Greenland ice core. Measurement potentials for these samples benefited from the development of a new laboratory method of co-precipitating 10Be with niobium. To diversify geographic location and archive media type, a pioneer study of measuring 10Be with annual resolution in varved lake sediments from Finland was conducted, with samples from the entire 20th century. Pathways of 10Be into lake sediments are more complex than into glacial ice, inferring that contemporary atmospheric conditions may not be recorded. Here, it is shown for the first time that tracing the 11-year solar cycle through lake sediment 10Be variations is possible. Results also show that on an annual basis, 10Be deposition in ice and sediment archives is affected by local environmental conditions. On a slightly longer timescale, however, diverse 10Be records exhibit similar trends and a negative correlation with solar activity. Cyclic variability of 10Be deposition persisted throughout past grand solar minima, when little or no sunspot activity was recorded. 10Be levels indicate that although solar activity has been high during the 20th century, levels are not unprecedented in the investigated 600 years. Aerosol 10Be/7Be values indicate possible influence of stratosphere-troposphere exchange on isotope abundance and the production signal.

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