Atomic inner-shell excitation and de-excitation processes; X-ray spectrometry; fundamental parameters for X-ray emission analysis of materials; ion beam analysis of materials using small accelerators; development of proton-induced X-ray emission (PIXE) and PIXE software for elemental analysis and imaging; development of fundamental parameters approach for in situ X-ray emission analysis of Martian rocks, soils and aeolian dust for elements and water.
I am a co-investigator in the 500-strong Science Team on NASA’s Mars Science Laboratory mission. In this context my immediate collaborators are R. Gellert (U. Guelph). M. E. Schmidt (Brock U.), J. A. Berger (Johnson Space Centre). I initiated collaborations on multiple ionization X-ray satellites, now in progress, with the Ruder Bosković Institute in Croatia and the Josef Stefan Institute in Slovenia. The accelerator beam time for this work is funded by the European Union’s RADIATE Trans-National Access Program. Another collaboration is with the Universidad Federal del Sol in Brazil, this one involving development and testing of PIXE software.
I have presented two-day training schools on the use of our PIXE software package GUPIX at several sites in Europe. The sponsors were (i) the AGLAE Laboratory of the Louvre Museum in Paris, (ii) the Istituto Nazionale di Fisica Nucleare with the University of Florence, (iii) the Max Planck Institüt fur Kernphysik in Heidelberg, and (iv) the Universities of Seville and Madrid. I am a regular speaker at international conferences on applications of accelerators in pure and applied physics and in X-ray spectrometry: the proceedings of two such conferences were dedicated to me in recognition of my contributions to PIXE. I was an early member of the X-ray Fundamental Parameters Initiative launched by the French and German National Standards labs and the European X-ray Spectrometry Association. For a decade I was chair of the international advisory committee for the triennial international conferences on PXE and its analytical applications
Major Research Achievements
Two examples are noted:
The computer software package GUPIX developed by my group on the basis of our experience in PIXE and its underlying physics has been supplied to 180 ion beam analysis laboratories in over 50 countries. This “fundamental parameters” approach to PIXE analysis has become the most widely adopted PIXE software. A recent Google search for GUPIX and GUPIXWIN showed over 26,000 hits. Our first three “Guelph PIXE Software Package” publications attracted 1728 citations in Google Scholar.
The Mars alpha particle X-ray spectrometers analyze Martian rocks and soils with a combination of PIXE and X-ray florescence analysis XRF. To handle these spectra, we developed the unique GUAPX code which combines the physics of the two techniques. We also created “Martian” versions of various ancillary PIXE codes; these have enabled analysis of collected Martian atmospheric dust and correction of rock analysis for the ubiquitous dust cover.
Research Activities: Overview
Fundamental research focuses on creation and de-excitation of core vacancies in atoms, using particle and photon beams, radionuclides and X-ray spectroscopy. Interests include natural widths of core levels, X-ray transition rates, the Auger process, multiple ionization, satellites chemical bonding effects, fluorescence and Coster-Kronig probabilities. Our recent work on a PIXE approach to testing X-ray attenuation coefficient databases was in part stimulated by the workshops of the X-ray Fundamental Parameters Initiative.
Applied research focuses on the properties and response of silicon- and germanium-based energy-dispersive X-ray detectors. Accurate PIXE analysis demands a quantitative understanding of the X-ray detectors involved.
Ion beam Analysis of Materials is based on the Guelph PIXE and micro-PIXE facilities, which provide elemental analysis and imaging on a wide variety of sample types. We continue to refine, test and develop our GUPIX software, which is a powerful tool used worldwide to fit PIXE spectra and derive element concentrations. A new tool GUMAP was released in 2017 and a second development phase is underway.
In situ analysis of Martian rocks and soils with the ‘alpha particle X-ray spectrometer’ is done in collaboration with Prof. Ralf Gellert who is the lead scientist for the Mars Science Laboratory APXS on board the Curiosity rover. APXS employs two X-ray excitation methods - PIXE and XRF – to excite the elements in these samples. Based on GUPIX [see (2) above] we have created a new fundamental parameters-based code GUAPX to process the rock and soil spectra recorded on Mars by the APXS. Our 2012 paper on calibration of the MSL APXS using terrestrial geochemical reference materials revealed anomalies in the results for the light elements examined by the PIXE branch. Quantitative modelling of the interaction of physics, mineralogy and geochemistry within geo-standards has now enabled us to predict the effects of the mineral structure of rocks upon APXS results and achieve agreement with experimental data. With our new accelerator-based PIXE system we can emulate APXS using terrestrial emulants of Martian rocks.