The Compton Spectrometer and Imager (COSI) is a soft gamma-ray (0.2-5 MeV) survey telescope designed to perform imaging, spectroscopy, and polarization of astrophysical gamma-ray sources. It employs a novel Compton Telescope design, utilizing sixteen 3D-imaging, high spectral resolution germanium detectors (GeDs), enclosed on the sides and bottom by an active CsI well, and with an overall field-of-view of 25% of the sky.

COSI’s guiding principle is that high efficiency and excellent background reduction are critical for advances in soft gamma-ray sensitivity. The compact geometry achieves high photopeak efficiencies – COSI increases the effective area per unit detector volume by a factor of 100 over COMPTEL. The combination of Compton imaging, active shielding, and analysis techniques made possible with our 3D position sensitive GeDs serve to dramatically decrease the background – a factor of 30-100 per unit volume over INTEGRAL/SPI. See our Science page to learn about the questions we can answer with COSI and see our Compton Telescope design page to learn more about the telescope operation.