Development of BGO active shield for the ASTRO-H soft Gamma-ray Detector
2010
Soft Gamma-ray Detector (SGD:40-600 keV) will be mounted on the 6th Japanese X-ray observatory ASTROH
to be launched in 2014. The main part of the SGD is a Compton camera with a narrow field of view and
surrounded by BGO active shields (SGD-BGO). Via this combination, the SGD can achieve sensitivity more than
ten times superior to the Suzaku/HXD. The BGO active shield will also function as a gamma-ray burst monitor
as proven by the wide-band all-sky monitor (WAM) of the Suzaku/HXD. Avalanche Photodiodes (APDs) are
used to read out scintillation lights from the BGO. The size of the former also means we need to focus on
collecting light from large, complex-shaped BGO blocks. The significant leakage current of the APD means a
lower temperature is preferred to minimize the noise while a higher temperature is preferred to simplify the
cooling system. To optimize the BGO shape and the operating temperature, we tested the performance of the
BGO readout system with various BGO shapes under different operating temperatures. We also apply waveform
sampling by flash-ADC and digital filter instead of a conventional analog filter and ADC scheme to reduce the
space and power of the circuit with increased flexibilities. As an active shield, we need to achieve a threshold
level of 50-100 keV. Here, we report on the studies of threshold energy of active shield under various conditions
and signal processings.
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