High energy X-ray spectrometer on Chandrayaan-1

Chandrayaan-1, India's first planetary exploration mission to Moon carries a suite of payloads including a High Energy X-ray spectrometer (HEX) designed to study low-energy (30-270 keV) natural gamma rays emitted from the lunar surface due to decay of uranium and thorium. The primary science objective of HEX is to study transport of volatiles on the lunar surface through the detection of the 46.5 keV line from ²¹⁰Pb decay, which is a decay product of volatile ²²²Rn, both belonging to the ²³⁸U decay series. HEX is designed to have a spatial resolution of ~33 km at energies below 120 keV. The low signal strength of these emissions requires a large area detector with high sensitivity and energy resolution, and a new generation Cd-Zn-Te (CZT) solid state array detector is used in this experiment. Long time integration will be required to detect the emission because of the significant lunar continuum background and weak signal strength. The various sub-systems of the HEX flight payload and test results from ground calibration are described in this article. HEX will be the first experiment aimed at detecting low energy (<300 keV) gamma ray emission from a planetary surface.