Ion and photon beam interactions with atoms and materials: fundamentals and inter-disciplinary applications including chemical analysis on the Martian surface

点击翻译按钮获取中文摘要

My inter-disciplinary program has four linked strategic directions. The first two are the basic physics of X-rays and the applied physics of materials analysis using X-ray detection technology. These have supported our development and application of particle-induced X-ray emission (PIXE) analysis, which uses high-energy proton beams to induce materials of interest to emit X-rays, whose "signatures" and intensities reveal the elements present and their concentrations. We have also developed a software package "GUPIX" for PIXE analysts, supplying it to 180 research labs in 51 countries. It is PIXE that has led my group into its fourth research area - participation in NASA's Mars Science Laboratory (MSL) mission, which seeks evidence for pre-existing life on Mars. The Guelph-based alpha-particle X-ray spectrometer (APXS) on the Curiosity rover uses both PIXE and a second technique (X-ray Fluorescence or XRF) to analyze Martian rocks, sands and soils. Together with the various other instruments on Curiosity, this helps build a picture of the evolution of the geology of the Martian surface over billions of years. Of especial interest is the study of minerals and their element content for evidence that water has been present, since water is a crucial component for life. An example of our current work is the problem of partial dust cover on Martian rocks, as seen in images taken by Curiosity's cameras. This dust, which we have succeeded in analyzing by collecting a wind-blown sample, causes error in the APXS analysis. We have developed a new approach to the data analysis that enables us to separate out the effects of dust cover and thus permit analysis of the substrate rock. This is a significant step towards better accuracy. We are also refining the method so that it can also analyze thin films deposited on rock surfaces by fluids in the distant past. Recently we built and tested a lab-based PIXE system that emulates APXS, giving us the ability to rapidly analyze large suites of rock samples from terrestrial formations that appear similar in geology to formations seen on Mars. Together with computer simulation, this PIXE system is helping us to improve the accuracy of APXS analysis. The issue of accuracy has brought us back to our long-term work on the basic physics of PIXE. Collaboration is now in progress with research institutes in Croatia and Slovenia to improve the database which underpins the method.