Simulations of the HEX payload using Geant4

CHANDRAYAAN-I, India's first mission to the Moon, carries on board, the High Energy X-ray Spectrometer (HEX) payload. This is a spectroscopic experiment, designed to measure the line and continuum flux from the Lunar surface in the 30-250 keV energy range. The main science goal of HEX is to map the strength of the 46.5 keV line, emitted when volatile Rn-222 decays to Pb-210, across the Lunar surface. Mapping the intensity of this line as a function of Lunar latitude, is an indirect tracer of the migration of volatile substances (like water) over the Lunar surface. This paper addresses various design optimizations of the HEX payload, spectral redistribution function of the primary detector, and estimation of the detector background due to Lunar gamma-ray Albedo and Galactic Cosmic Ray protons, using the software toolkit, Geant4.