Emulation of the Curiosity rover alpha particle X-ray spectrometer with accelerator-based PIXE: Implications for calibration

A 2.5 MeV broad-beam PIXE system was designed to emulate the alpha particle X-ray spectrometer (APXS) of the Curiosity rover. APXS measurements of the light elements (sodium, magnesium and aluminum) in powdered rock standards showed systematic deviations arising from the rock's violation of the assumption of atomic scale sample homogeneity. These deviations, which are dependent on the iron concentration of the sample, were confirmed with high-accuracy accelerator-based PIXE measurements. Systematically low iron concentrations, not seen in APXS measurements, were present in the PIXE results. These are unlikely to arise due to violation of the homogeneity assumption. Measured particle-size distributions suggest that they are more likely due to the grain size distribution of iron-bearing minerals in the powdered standards. Terrestrial analogs to recently discovered Martian rock types were used to extend the calibration to include high iron and high-alkali samples.