  The Soft Gamma-ray Detector (SGD) is one of the instrument payloads onboard
ASTRO-H, and will cover a wide energy band (60--600 keV) at a background level
10 times better than instruments currently in orbit. The SGD achieves low
background by combining a Compton camera scheme with a narrow field-of-view
active shield. The Compton camera in the SGD is realized as a hybrid
semiconductor detector system which consists of silicon and cadmium telluride
(CdTe) sensors. The design of the SGD Compton camera has been finalized and the
final prototype, which has the same configuration as the flight model, has been
fabricated for performance evaluation. The Compton camera has overall
dimensions of 12 cm x 12 cm x 12 cm, consisting of 32 layers of Si pixel
sensors and 8 layers of CdTe pixel sensors surrounded by 2 layers of CdTe pixel
sensors. The detection efficiency of the Compton camera reaches about 15% and
3% for 100 keV and 511 keV gamma rays, respectively. The pixel pitch of the Si
and CdTe sensors is 3.2 mm, and the signals from all 13312 pixels are processed
by 208 ASICs developed for the SGD. Good energy resolution is afforded by
semiconductor sensors and low noise ASICs, and the obtained energy resolutions
with the prototype Si and CdTe pixel sensors are 1.0--2.0 keV (FWHM) at 60 keV
and 1.6--2.5 keV (FWHM) at 122 keV, respectively. This results in good
background rejection capability due to better constraints on Compton
kinematics. Compton camera energy resolutions achieved with the final prototype
are 6.3 keV (FWHM) at 356 keV and 10.5 keV (FWHM) at 662 keV, respectively,
which satisfy the instrument requirements for the SGD Compton camera (better
than 2%). Moreover, a low intrinsic background has been confirmed by the
background measurement with the final prototype.
