Ion and photon beam interactions with atoms and materials: fundamental physics and inter-disciplinary applications including chemical analysis of the Martian surface

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The foundations of our work lie in atomic physics, where we have made many measurements on processes involving inner-electron dynamics for the purpose of testing current theoretical calculations. Our measurements involve particle accelerators, X-ray spectrometers, and sophisticated computer processing of energy-dispersed X-ray spectra. In support, we work on X-ray detector properties and spectrum fitting methodology. Our focus has recently shifted somewhat towards the study of specific fundamental parameters (FPs) which underpin materials analysis techniques that are based on X-ray excitation and emission. These methods, which include X-ray fluorescence, electron microprobe and proton-induced X-ray emission (PIXE), can conduct elemental analysis relative to certified standards or they can take a standard-less approach which then demands accurate knowledge of the necessary FPs. The rate of emergence of new human-made materials is such that standards are not available for many urgent analyses and so the demand for more accurate FPs has grown recently. New experimental approaches are needed. Our present focus is on photo-electric absorption of low-energy X-rays as they traverse matter; the absorption coefficients are crucial to accurate X-ray emission analysis, but current widely-used theoretical and experimental tabulations show large differences for the X-rays of light elements such as silicon. We have introduced a new method based on PIXE and plan to expand this approach. This fundamental work has supported our extensive contributions to the development of PIXE analysis, which employs the same hardware and similar computational methods. Using microbeams (micro-PIXE) gives the option of mapping sample surfaces for element distributions Much of our work has been done with the Guelph scanning nuclear microprobe, which has been the basis for a large number of inter-disciplinary collaborations covering diverse topics such as chemistry of mineral deposits, urban and rural aerosols, and migration of Arctic and Atlantic fish species. Our GUPIX software for PIXE analysis has been supplied to 170 ion beam analysis labs around the world, including the Louvre Museum's AGLAE lab. We plan now to expand GUPIX to support measurement approaches which deploy PIXE simultaneously with other ion beam techniques such as proton-induced gamma-ray emission and proton backscattering. With PIXE and GUPIX as the starting points, we have created new methodology and new software for analysis of X-ray spectra of Martian rocks and soils recorded by the alpha-particle X-ray spectrometers (APXS) on the Mars Rovers Spirit, Opportunity and Curiosity. This enabled us to calibrate the Mars Science Laboratory APXS for measurement of element concentrations, and to understand the degree to which sample mineralogy can affect element analysis in different rock types, e.g. basalts, trachytes etc. This work, at the interface of physics, mineralogy and geochemistry, is truly inter-disciplinary. We have devised a new approach to part of the Martian data – the so-called “X-ray scatter peaks” - which resulted in our making the first quantitative in situ measurement of water within subsurface soils near the Martian equator; this method is undergoing refinement to lower its detection limits. A major part of our research is now devoted to extending the capabilities of the APXS as a unique Canadian instrument for future planetary and lunar missions. For example, we have developed a Monte Carlo method for modeling APXS response to a given sample, and this may form the basis of a new approach which combines APXS data with X-ray diffraction results.