Ground Calibration of Alpha Particle X-ray Spectrometer (APXS) On-Board Chandrayaan-2 Pragyaan Rover: an Empirical Approach

Alpha Particle X-ray Spectrometer (APXS) was one of the scientific experiments on-board the Pragyaan rover of the Chandrayaan-2 mission to Moon, with the objective to determine the elemental composition of the lunar surface near the landing site in the south polar region of the Moon. APXS estimates the abundances by measuring the characteristic X-ray lines of the constituent elements present in the target surface, when irradiated by the onboard Cm-244 radioactive source. X-ray spectral measurement in the energy range of 0.8-25 keV with an energy resolution of (similar to)135 eV at 5.89 keV enables APXS to detect all the major rock-forming elements, such as Na, Mg, Al, Si, Ca, Ti, and Fe. This paper describes the calibration of APXS as an X-ray spectrometer, provides a procedure for quantitative estimation of elemental concentrations based on the measurements of standard geochemical reference materials and provides the detection limits of the major elements. It is shown that, following this procedure, the concentration of the major elements in any sample can be determined with typical accuracy of better than 10% of the actual value.