Development of the HXD anti counters onboard Astro-E2

The hard X-ray detector (HXD) is one of the three instruments onboard Japanese X-ray astronomy satellite Astro-E2 scheduled for launch in 2005. This mission is very unique in a point of having a lower detector background than any other past missions in the 10-600 keV range. In the HXD, the large and thick BGO crystal are used as active shields for reducing the particle and gamma-ray background to the main detector. These anticoincidence shields are called as "anti counters", which have a large geometrical area ∼ 800 cm² and an uncollimated field of view of ∼ 2π. Furthemore, they also have a larger effective area, corresponding to 400 cm² at even 1 MeV due to their thick high-Z materials. This feature enables us to observe the high energy radiation of gamma-ray bursts with a higher sensitivity than previous all-sky monitors. Hence, the anti counters have been developed as all-sky monitors with a broadband coverage of 50-5000 keV. In this paper, we will describe overall design of the HXD anti counters, then report on the results of the pre-flight calibration test on June 2004 using the flight model. By irradiating various radio isotopes with anti counters, we confirmed that they have capability as all-sky monitors. It is striking that the low energy threshold has been archived about 30 keV in spite of large volume of BGO scintillators.