The Compton Spectrometer and Imager (COSI) is a balloon-borne soft gamma-ray (0.2-10 MeV) telescope designed to study astrophysical sources of nuclear line emission and gamma-ray polarization. It employs a novel Compton telescope design, utilizing twelve 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. The Compton imaging serves three purposes: imaging the sky, measuring polarization, and very effectively reducing background. This program is based off the previous Nuclear Compton Telescope (NCT) program. We are currently preparing for the second flight of the COSI telescope on a Super Pressure Balloon. See our Design page to learn more about the telescope.

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 hope to answer with COSI.