In addition to detecting X-rays, Astro-H will have the ability to detect “soft” gamma rays. Gamma rays are light, just like X-rays, but have even higher energies. “Soft” gamma rays are those with energies just above X-rays. Because of their higher energies, gamma rays are even harder to focus and capture than X-rays. To detect these higher energy photons, Astro-H has the Soft Gamma-ray Detector (SGD), which is a Compton telescope.
The SGD will consist of two separate units, one on each side of the spacecraft, to balance out the mass of the detectors. When a photon enters one of the SGD units, it will Compton scatter in a layer of silicon detectors. This interaction will kick out an electron that will be detected within the silicon detectors. In addition, the interaction will reduce the energy of the incoming gamma ray, which will pass through the detector to be absorbed in a layer of cadmium-telluride detectors.
Using the locations of the two interactions – one in the silicon layer and one in the cadmium-telluride layer – scientists will be able to determine a region of the sky from which the gamma came. The SGD will be able to detect hard X-rays and soft gamma rays in the range of 10 to 600 keV.
