Characterization of high count rate capability of Solar X-ray Monitor on-board Chandrayaan-2 — the second Indian mission to the Moon

Chandrayaan-2 is the second Indian mission to the Moon, planned to be launched during 2015. One of the scientific experiments onboard Chandrayaan-2 is the remote X-ray Fluorescence spectroscopy to measure abundances of the major rock forming elements of the lunar surface. The experiment involves measuring spectra of fluorescent X-rays from lunar surface which are generated due to the incident solar X-rays. Since the flux of fluorescent X-ray lines critically depend on the flux and spectrum of the incident solar X-rays which are highly variable, it is essential to have simultaneous measurement of X-ray from the Sun in order to have quantitative interpretation of the lunar X-ray fluorescence spectra. The Solar X-ray Monitor (XSM) onboard Chandrayaan-2 orbiter will accurate and real time measurement of the solar X-ray spectrum using state-of-the-art Silicon Drift Detector (SDD). Here we present the overall design of the XSM instrument as well as initial results of the XSM laboratory model tests at different incident count rate. The X-ray spectrometer provides energy resolution of ~200 eV at 5.9 keV for the pulse peaking time of 0.8 μs by cooling the detector to -35°C. We have characterized the engineering model of XSM for high count rate measurement and shown that the energy resolution is stable at ~200 eV for the count rates up to 70K couts/s and the change in peak position is <;0.5%.