Monte carlo simulations of the high energy X-ray spectrometer (HEX) on Chandrayaan-I

Abstract

HEX (High Energy X-ray Spectrometer) was one of the instruments on Chandrayaan- I, designed to detect and measure the intensity of high energy x-rays from 30 keV to 270 keV. The study of planetary radiation in this energy range is very challenging due to low signal strength and intense detector and planetary background emission. HEX consists of a CdZnTe compound semiconductor as the primary detector, and a CsI(Tl) scintillator as the secondary detector. The purpose of the secondary detector is to act as an anti-coincidence detector (ACD) to facilitate Compton suppression and background rejection in the primary detector. There were in total eleven experiments onboard the satellite, and the lifetime of the mission was planned to be two years; so there were constraints of size and weight of the various experimental payloads. This thesis addresses design optimization of the ACD in terms of the number of detectors and shield thickness. It also includes estimation of the background that would be seen by the CdZnTe detector of HEX, and the Compton suppression and background rejection e_ciency of the ACD. The predicted background includes contributions from high energy lunar albedo emission, and from the interaction of galactic cosmic rays with the payload and spacecraft. As a part of the ground work, the simulation codes and logic are validated against experimental and published results. The instrumental response of the CdZnTe and CsI(Tl) detectors are also simulated, using input from experimental data collected during calibration of the ight detectors. The Monte Carlo simulation toolkit, Geant4 is used extensively in this work. The methodology and results of the simulations, as well as experimental results and cross comparisons are presented in this work.