Development of a Monte Carlo simulator for the Astro-E2 Hard X-ray Detector (HXD-II)

The Hard X-ray Detector (HXD-II) is one of the scientific payloads on board the fifth Japanese cosmic X-ray satellite Asrto-E2, scheduled for launch in 2005. The HXD-II is designed to cover a wide energy range of 10-600 keV with a high sensitivity of /spl sim/ 10/sup -5/ cnt/s/cm/sup 2//keV, using 16 identical GSO and BGO phoswich counters combined with 2 mm-thick silicon PIN diodes. In order to investigate the in-orbit performance of HXD-II in cosmic radiation environment, a Monte Carlo simulator based on the Geant4 toolkit is currently developed. There are two main goals of this simulator, which is directly connected to the detector's performance. One is to derive energy response to photons within the acceptance energy range, with 5% accuracy, after several types of standard event-selection of the HXD-II. The other is to estimate detector background with 10% accuracy. In addition to the background caused directly by the primary and secondary cosmic-rays, of particular importance is the radio-activation background induced by MeV protons trapped in the South Atlantic Anomaly. The simulator is also used in the pre-launch verifications of the HXD-II hardware. This paper describes the design concept of the Monte Carlo simulator, and its verification through comparison with the actual data of pre-flight radio-isotope irradiation experiments, together with calculated outputs that can demonstrate the in-orbit performance of the HXD-II.