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

我们工作的基础在于原子物理学，在原子物理学中，我们对涉及内部电子动力学的过程进行了许多测量，目的是测试当前的理论计算。我们的测量包括粒子加速器、x射线光谱仪和复杂的能量分散x射线光谱计算机处理。为了提供支持，我们研究x射线探测器的特性和光谱拟合方法。最近，我们的重点在一定程度上转向了特定基本参数（FPs）的研究，这些参数是基于x射线激发和发射的材料分析技术的基础。这些方法包括x射线荧光、电子显微探针和质子诱导x射线发射（PIXE），可以进行相对于认证标准的元素分析，也可以采用无标准的方法，然后需要准确了解必要的FPs。新人造材料的出现速度如此之快，以至于许多紧急分析无法获得标准，因此对更精确的FPs的需求最近有所增长。需要新的实验方法。我们目前的重点是低能x射线穿过物质时的光电吸收；吸收系数对精确的x射线发射分析至关重要，但目前广泛使用的理论和实验表格显示，硅等轻元素的x射线存在很大差异。我们已经介绍了一种基于pxie的新方法，并计划扩展这种方法。这项基础工作支持了我们对使用相同硬件和类似计算方法的pxie分析开发的广泛贡献。我们的大部分工作都是用圭尔夫扫描核微探针完成的，它已经成为大量跨学科合作的基础，涵盖了不同的主题，如矿藏化学，城市和农村气溶胶，以及北极和大西洋鱼类的迁徙。我们的GUPIX软件用于PIXE分析已提供给全球170个离子束分析实验室，包括卢浮宫博物馆的AGLAE实验室。我们现在计划扩展GUPIX，以支持与其他离子束技术（如质子诱导伽马射线发射和质子后向散射）同时部署pxie的测量方法。以PIXE和GUPIX为起点，我们创造了新的方法和新的软件，用于分析火星岩石和土壤的x射线光谱，这些光谱是由勇气号、机遇号和好奇号火星漫游者上的α粒子x射线光谱仪（APXS）记录的。这使我们能够校准火星科学实验室的APXS来测量元素浓度，并了解样品矿物学对不同岩石类型（如玄武岩、粗叶岩等）元素分析的影响程度。这项工作，在物理，矿物学和地球化学的界面，是真正的跨学科。我们设计了一种处理火星部分数据的新方法- -所谓的“x射线散射峰”- -这使我们第一次对火星赤道附近地下土壤中的水分进行了实地定量测量；该方法正在改进以降低其检出限。我们研究的一个主要部分现在致力于扩展APXS的能力，作为加拿大未来行星和月球任务的独特仪器。例如，我们已经开发了一种蒙特卡罗方法来模拟APXS对给定样品的响应，这可能成为将APXS数据与x射线衍射结果相结合的新方法的基础。

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.