Solar X-ray Monitor (XSM) on-board Chandrayaan-2 orbiter

The remote X-ray fluorescence spectroscopy is a powerful technique to investigate the elemental abundances in the atmosphere-less planetary bodies. The experiment involves measuring spectra of fluorescent X-rays from lunar surface using a low energy X-ray detector onboard an orbiting satellite. Since the flux of fluorescent X-ray lines critically depend on the flux and spectrum of the incident solar X-rays, it is essential to have simultaneous and accurate measurement of X-ray from both Moon and Sun. In the context of Moon, this technique has been employed since early days of space exploration to determine elemental composition of lunar surface. However, so far it has not been possible to exploit it to its full potential due to various reasons. Therefore it is planned to continue the remote X-ray fluorescence spectroscopy experiment on-board Chandrayaan-2 which includes both lunar X-ray observations and solar X-ray observations as two separate payloads. The lunar X-ray observations will be carried out by Chandra Large Area Soft x-ray Spectrometer (CLASS) experiment; whereas the solar X-ray observations will be carried out by a separate payload, Solar X-ray Monitor (XSM). Here we present the overall design of the XSM instrument, the present development status as well as preliminary results of the laboratory model testing. XSM instrument will have two packages namely – XSM sensor package and XSM electronics package. XSM will accurately measure spectrum of Solar X-rays in the energy range of 1–15 keV with energy resolution ∼200 eV @ 5.9 keV. This will be achieved by using state-of-the-art Silicon Drift Detector (SDD), which has a unique capability of maintaining high energy resolution at very high incident count rate expected from Solar X-rays. XSM onboard Chandrayaan-2 will be the first experiment to use such detector for Solar X-ray monitoring.