Calibration of the Mars Science Laboratory Alpha Particle X-Ray Spectrometer for Analysis of Visible Elements and Light Invisible Components

The Alpha Particle X-ray Spectrometer (APXS) is a small, lightweight instrument capable of detecting geologically significant elements (Na-Y) in a sample through particle induced X-ray emission and X-ray fluorescence. This makes it an ideal instrument for planetary exploration and it has been on every successful NASA rover mission. This thesis covers the fundamental parameters elemental calibration for the newest APXS instrument onboard the Mars Science Laboratory (MSL) rover, Curiosity. This calibration approach assumes all samples are homogeneous at the sub-micron scale, which is incorrect for geologic materials, the primary target materials of the APXS. Specific elements in geochemical reference materials were discovered to systematically deviate from certificate values; these “mineral phase effects (MPEs)”, have been quantitatively described for the three lightest detectable elements (Na, Mg, Al), which display the largest deviations. Examination of mineral theoretical X-ray yields and bulk elemental theoretical X-ray yields of a geochemical reference material (GRM) showed differences that agree in magnitude with the differences observed in the calibration. This verified that the root cause of the MPEs is the necessary homogeneity assumption. To complete the theoretical yield calculations, accurate mineral abundances and elemental composition must be known. Mineral abundances were determined by X-ray diffraction (XRD) and Rietveld analysis. Mineral elemental composition calculations rely on the bulk chemistry and mineral abundances. These calculations are complex, so a program was developed (APXRD) to compute mineral elemental compositions. APXRD was tested against by-hand calculations and the results agree. APXRD may be used in the future to simplify the study of MPEs in more complex GRMs and Martian APXS targets. MPEs have also been studied using the Guelph proton microprobe, which has replicated MPEs observed in select APXS spectra. These analyses show the value of further MPEs studies with the proton microprobe. Calibration of the L𝛼 scatter peaks method for determining additional light invisible components (ALICs) of MSL APXS targets has been completed. This calibration was tested on GRMs with known ALIC content and it was able to reproduce the known GRM ALIC content where ALICs are greater than 5 oxide wt%. Preliminary analyses of MPE corrections and ALIC content of MSL APXS spectra are presented.