"On the Effects of Resolution on APXS-GUAPX Analytical Results: From Spectrum to Peak Areas to Concentration Estimate"
The present work is an in-depth investigation into the effects of spectral resolution on Alpha Particle X-ray Spectrometer (APXS) measurements processed using the GUAPX fitting code. The APXS is a contact instrument that uses X-ray spectroscopy to determine the elemental composition of soils and rocks, ranging from Na to Y. It is one of the competitively selected instruments carried on the Mars Science Laboratory (MSL) rover, Curiosity, to contribute to understanding rock and soil types on the red planet and the processes by which they formed. The continuous instrumental degradation due to neutron damage, in conjunction with the limited temperature control capabilities for the APXS and other mission constraints, make it such that resolution is not always the same and cannot always be optimized. Thus, it was necessary to investigate the relationship between the precision of the results given by the fitting program and the spread of results due to variations in FWHM.
Preliminary work showed relationships between elemental concentration estimates and energy resolution attained at different temperatures. Further examination was indicative of a differential effect for heavier element concentrations relative to those for the light element. The concentrations estimated for the light elements (Na - Al) decreased with worsening resolution. Heavier elements, with the exception of Mn, showed an increase in estimated concentration with worsening resolution. A methodology was developed using ratios of relevant analytical results (count rates, concentration estimates, FWHM) to characterize the relationship between estimated peak areas, concentration estimates, and resolution. The end result is a description of the mechanism behind the observed differential effect favouring the heavier element concentration estimates with worsening resolution, as well as a quantitative estimate of the link between relative change in FWHM and concentration estimates for ten of the most geochemically abundant elements. Lastly, variations in estimated elemental concentration were compared with the precision errors. It was found that Na, Mg, Al, Mn, Si, Ca, and Fe concentration estimate variations fall outside precision errors, over resolution and concentration ranges similar to those seen on Mars.
Dr. Robert Wickham, Chair
Dr. J. L. (Iain) Campbell, Advisor
Dr. Ralf Gellert
Dr. Joanne O'Meara
Honors B.Sc. in Physics from the University of Guelph (2015)
- Dean’s Scholarship 2013 - 2015
- Dean’s List 2011, 2012 - 2013
- In-Course Bursary 2010 - 2013
- Queen Elizabeth II: Aiming for the Top Scholarship 2008
- Entrance Scholarship 2008- 2015
Publications – Papers in Referred Journals.
Quantitative determination of mineral phase effects observed in APXS analyses of geochemical reference materials.
Glynis M. Perrett, John L. Campbell, Susan Glasauer and Renato Pardo
X-Ray Spectrometry 43, 359 (2014)