The SGD is a non-focusing soft gamma-ray detector with a 10—600 keV energy range and sensitivity at 300keV, more than 10 times better than the Suzaku HXD (Hard X-ray Detector). It outperforms previous soft-Gamma-ray instruments in background rejection capability by adopting a new concept of narrow-FOV Compton telescope.
In order to lower the background dramatically and thus to improve the sensitivity as compared to the HXD of Suzaku, we combine a stack of Si stripdetectors and CdTe pixel detectors to form a Compton telescope. The telescope is then mounted inside the bottom of a well-type active shield. Above 50 keV, we can require each event to interact twice in the stacked detector, once by Compton scattering in a stack of Si strip detectors, and then by photo-absorption in the CdTe part (Compton mode). Once the locations and energies of the two interactions are measured, the Compton kinematics allows us to calculate the energy and direction (as a cone in the sky) of the incident gamma-ray by following the Compton equation. We only select events that are consistent with gamma-rays coming from the FOV of collimator (Narrow FOV Compton telescope).