The Hard X-ray Detector (HXD) on board Suzaku covers a wide energy range from 10keV to 600 keV by the combination of silicon PIN diodes and GSO scintillators. The HXD is designed to achieve an extremely low in-orbit background based on a combination of new techniques, including the concept of a well-type active shield counter. With an effective area of 142cm2 at 20keV and 273cm2 at 150keV, the background level at sea level reached ∼ 1 × 10-5 cts s-1 cm-2keV -1 at 30 keV for the PIN diodes, and ∼ 2 × 10-5 cts s-1 cm-2keV-1 at 100keV, and ∼ 7 × 10-6 cts s-1 cm-2keV-1 at 200keV for the phoswich counter. Tight active shielding of the HXD results in a large array of guard counters surrounding the main detector parts. These anti-coincidence counters, made of ∼ 4cm thick BGO crystals, have a large effective area for sub-MeV to MeV x-rays. They work as an excellent y-ray burst monitor with limited angular resolution (∼ 5°). The on-board signal-processing system and the data transmitted to the ground are also described.
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