Preflight calibration and performance of the astro-E2/HXD-II anti counter as the all sky monitor

The hard X-ray detector (HXD-II) onboard Astro-E2 consists of the main detector which has its energy range from 10 to 600 keV and the active shield detector for background reduction. This shield detector (anti detector) is designed to not only for the background reduction but also for the soft gamma-ray all sky monitor of gamma-ray bursts (GRB) and black hole binaries. The asymmetric shape and large size of BGO scintillators for the anti counter make the gamma-ray response very complicated, and thus we need the careful calibration before the launch. We then performed preflight calibrations of HXD-II in 2003-2004. For the anti detector, we measured the pulse height spectra and stopping power of the unit detector before integration by exposing it to the collimated gamma-ray. After integration of HXD-II, we measured the pulse height spectra and stopping power of the anti detector by irradiating the gamma-ray source from various directions. Furthermore, we performed the same measurement after placing the HXD-II detector on the Astro-E2 spacecraft. Taking into account these experimental results, we construct the gamma-ray response by the Geant4 Monte Carlo simulation. The response developed in this work was found to reproduce the experimental spectra and the count distributions of each unit counter with 10-20% accuracy.