A GUPIX-based approach to interpreting the PIXE-plus-XRF spectra from the Mars Exploration Rovers: I. Homogeneous standards

A new fitting code has been developed on the basis of GUPIX for application to alpha-particle X-ray spectrometers, which employ simultaneously two excitation mechanisms – XRF and PIXE – for elemental analysis of a single sample, and which are principal analytical instruments on the Mars Exploration Rovers. We present some of the basic aspects of our approaches to spectrum fitting, and we use a subset of the original calibration spectra from the MER mission to test our approach to standardization. This test is limited to results from homogeneous standards such as pure elements, oxides and chlorides, and simple minerals that comprise a single phase. The important, additional issues of multiple mineral phases that arise when geochemical reference materials are used as standards are dealt with in the subsequent paper. It is shown that an accurate standardization can be achieved with only minimal resort to empirical corrections. Attention is paid to fitting of the Compton scatter feature and to the extraction of the Compton–Rayleigh scatter ratio, because the latter quantity is the basis of our method for determining “invisible” light element (water, ice and carbonate) content of Martian samples.