11th Geant4 Space Users Workshop

In-orbit Performance Estimation of the Hard X-ray Imager onboard ASTRO-H with Monte Carlo simulations

27 Aug 2015, 16:15

25m

Hiroshima Institute of Technology Hiroshima Campus, Hiroshima, Japan

Space missions - instruments and detectors (1)

Speaker

Kouichi Hagino (ISAS/JAXA)

Description

The Hard X-ray Imager (HXI) is one of four instruments onboard 6th Japanese X-ray satellite, ASTRO-H, which is scheduled to be launched in FY 2015. Combined with hard X- ray telescopes, the HXI will realize imaging spectroscopy in hard X-ray band ranging from 5 keV to 80 keV with a sensitivity which is two orders of magnitude better than that of Suzaku/HXD. The HXI is composed of a stacked semiconductor detector module surrounded by BGO (Bi4Ge3O12) scintillators. The BGO scintillators work as the active shields to reduce background events caused by cosmic-ray particles and radio-activation. The main detector of the HXI consists of 4 layers of silicon (Si) detectors and 1 layer of cadmium-telluride (CdTe) detector. Such a stacked structure enables high detection efficiency throughout a wide energy range. The flight model of HXI have already been fabricated, and installed into the satellite. Thus, it is time for selection and prioritization of the targets, which will be observed a few months after the launch. In order to accurately evaluate feasibility of the observations, it is necessary to estimate effective area, in-orbit background rates and detection sensitivity based on the performance of the real detectors. Therefore, we have developed an accurate response function, which relates an output signal of the detector to an input signal from astrophysical objects. To construct the response function for HXI, Monte Carlo simulations based on Geant4 are adopted because Compton scattering and secondary X-ray emissions have non-negligible effects on the detector response in the hard X-ray band. Our simulator implements almost all passive materials as well as the semiconductor detectors and the active shields. In addition, the simulator treats charge transportation in the detector devices and readout noise. By utilizing this simulator, we have successfully reproduced the experimental data. Here, we will present a detail of our Monte Carlo simulator and discuss expected performance of the HXI.