Ground calibration of Solar X-ray Monitor on board the Chandrayaan-2 orbiter

Chandrayaan-2, the second Indian mission to the Moon, carries a spectrometer called the Solar X-ray Monitor (XSM) to perform soft X-ray spectral measurements of the Sun while a companion payload, CLASS, measures the fluorescence emission from the Moon. Together these two payloads will provide quantitative estimates of elemental abundances on the lunar surface. The XSM with its high time cadence and high energy resolution spectral measurements, is also expected to provide significant contributions to solar X-ray studies. For this purpose, the XSM employs a Silicon Drift Detector and carries out energy measurements of incident photons in the 1 – 15 keV range with a resolution of < 180 eV at 5.9 keV, over a wide range of solar X-ray intensities. Extensive ground calibration experiments have been carried out with the XSM using laboratory X-ray sources as well as X-ray beam-line facilities to determine the instrument response matrix parameters required to carry out quantitative spectral analysis. This includes measurements, under various observing conditions, of gain, spectral redistribution function, and effective area. The capability of the XSM to maintain its spectral performance at high incident flux as well as its dead-time and pile-up characteristics have also been investigated. The results of these ground calibration experiments of the XSM payload are presented in this article.