Browsing by Author "Pineda-Vargas, Carlos A"

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    Development of a Time of Flight - Energy spectrometer for applications in Heavy Ion - Elastic Recoil Detection thin film analysis
    (2010) Msimanga, Mandlenkosi; Comrie, Craig; Pineda-Vargas, Carlos A
    Two principal aims of this work were firstly, to build and characterise a Time of Fight –Energy spectrometer and demonstrate its usability in Heavy Ion – Elastic Recoil Detection thin film materials analysis, and secondly, to measure stopping powers of heavy ions in thin foil compound materials using the newly built spectrometer. Key to the performance of the assembled spectrometer was its timing resolution. Characterisation measurements using a 27.5 MeV Kr15+ incident beam showed that the best timing resolution could be obtained when measurements are performed without timing preamplifiers in the signal processing chain. The best example was that of a resolution of 190 ps for 18 MeV 40Ca ions. The resultant depth resolution was found to be 6.8 nm for 40Ca recoils ejected from the surface layers of a CaF2 film on a Si substrate. A mass calibration procedure was devised for identification of light impurities in known sample structures, and the mass resolution, also a direct result of time resolution, was found to range between 0.8 u and 1.7 u in the atomic mass range 12 u to 40 u, respectively. Spectrum analysis was implemented using the software code KONZERD, enabling elemental quantification and measurement of film thickness. The lowest detectable atomic concentrations measured were 0.46 at.% for carbon and 0.84 at.% for oxygen. The analytical depth was up to 0.6 μm. Measurements were also carried out to determine stopping powers of ZrO2 and Mylar foils for 12C, 16O, 19F, 24Mg, 27Al and 28Si ions in the 0.1 – 0.6 MeV/u energy range. Of these, 27Al and 24Mg stopping power data generated in this work are the first such measurements for ZrO2, and 24Mg and 19F for Mylar.
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    Optimisation of accelerator optics and the application of nuclear microscopy to the biomedical field
    (2005) Eisa, Mohamed Eltayeb M; Comrie, Craig; Conradie, Lowry; Pineda-Vargas, Carlos A
    With the aim of improving the reliability and stability of the beams delivered to the nuclear microprobe at IThemba LABS, as well as optimization of the beam characteristics along the Van de Graaf acceleratore beamlines in general, relevant modifications were implemented. The design and layout of the beamlines in general, relevant modifications were implemented. The design and layout of the beamlines were revised. The beam-optical characteristics through the accelerator, from the ion source up to the analysing magnet directly after the accelerator, were calculated and the design optimised, using the computer codes TRANSPORT, IGUN and TOSCA. The ion source characteristics and optimal operating conditions were determined on an ion source test bench. The measured optimal emittance for 90% of the beam intensity was bout 50π mm mrad for an extraction voltage of 6kV. These changes allow operation of the Nuclear Microprobe at proton energies in the range 1 MeV to 4 meV with beam intensities of tens of a pA at the target surface. The capabilities of the nuclear micropobe facility were evaluated in the improved beamline, with particular emphasis to the characertisation of hard human calcium-rich tissue materials. The process of demineralization in teeth erosion due to exposure to acidic media was investigated in a group of test and control healthy human molar teeth. Samples were cut in cubes of
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    Thick target pixe analysis
    (1993) Pineda-Vargas, Carlos A; Peisach, Max; Thornton, David A
    The evaluation of the physical parameters governing the X-ray yield production in thick targets by charged particles was investigated and matrix correction factors (MCF) were calculated for a wide variety of materials including values for all pure non-gaseous elements or their compounds for the Kα and Lα X-ray lines. These factors were calculated for 1, 2, 3 and 4 MeV proton bombarding energies. A new methodology named 'common matrix type' (CMT) was developed for the determination of trace element concentrations in carbon-rich materials, such as most biological materials, with unknown matrix composition. A universal set of matrix correction factors (MCF) values for these materials was established for trace elements with Z ≥ 19 and irradiations with proton energies in the range of 1 to 4 MeV. A similar methodology was developed for silicon-rich and calcium-rich materials where the main components, Al, Si and Ca do not vary appreciably in their concentrations. CMT methodologies were applied successfully for the determination of trace elemental concentrations in a wide variety of thick target materials, which included archaeological cultural materials, biological tissues and geological ores. The technique of correspondence analysis was used for the statistical analysis of the extended data matrix generated in most of the applications. This technique of interpretation of multielemental data proved to be a valuable tool. Two modes of PIXE application, macro-mode and micro-mode, were evaluated at different ion bombarding energies ranging from 1 to 85 MeV. Experimental X-ray production cross sections at 66 and 85 MeV were evaluated and found to correlate well with theory based on the plane wave Born approximation (PWBA) for the Kα and Kβ lines. It was found that the irradiation of intermediate thickness samples of geological ores by energetic protons (66 MeV) is a suitable technique for the determination of small traces of rare earth metals with detection limits for analysis expected to be below the μg.g-1 range. The fact that energetic protons can be used means that no need for matrix correction is necessary. Micro-PIXE with low energy protons was found suitable for the determination of small traces of metals in human kidney stones and for the study of interrelationships between trace element concentrations with time of stone formation, in stones excreted from a single patient.