Preflight calibration and performance of the Astro-E2/HXD-II wide-band all-sky monitor

The hard X-ray detector (HXD-II) onboard Astro-E2 consists of a main detector with energy range 10-600 keV and a BGO active shield detector for background reduction. The shield detector wide-band all-sky monitor (WAM) has been designed not only for background reduction but also for all sky monitoring of gamma-ray bursts (GRB) and bright soft gamma-ray sources. WAM has a large geometrical structure of BGO scintillators for reinforcing its stopping power and has an asymmetric shape for reducing its weight. This particular structure of WAM makes the gamma-ray response very complicated, and thus we need careful calibrations before the launch. We then performed preflight calibrations of WAM as a part of the HXD-II calibrations in 2003-2004. We measured the pulse height spectra and stopping power of each unit counter individually before the HXD-II integration by exposing them to collimated gamma-rays. After integration of the HXD-II detector and installing it to the spacecraft, we measured the same issues as above for WAM by irradiating the gamma-ray source from various directions. Taking into account these experimental results, we constructed the gamma-ray response matrix of WAM using the Geant4 Monte Carlo simulation. The response developed in this work was found to have reproduced the experimental data within 10-20% accuracy.