The Compton Spectrometer and Imager (COSI) is a soft gamma-ray telescope, planned to be launched as a NASA Small Explorer program satellite in 2027. The instrument has also been flown on a pathfinding super pressure balloon mission launched from Wanaka, New Zealand on May 17, 2016 which lasted for 46 days.

The namesake instrument is a compact Compton telescope, which measures the position and deposited energy of a gamma ray through an active detector. By determining the order of scattering through the detector medium, the origin of any single photon can be confined to an annulus, known as an event circle. Multiple events from the same source will have overlapping event circles which determines the source location. A Compton telescope, COMPTEL, was successfully used in NASA’s Compton Gamma Ray Observatory (CGRO). COSI replaced the two detecting planes in COMPTEL with a more compact single detecting volume. COSI’s energy resolution will be 20 times better than COMPTEL and capable of observing more than four times as much sky at any instant. COSI will also be able to measure linear polarization.

Mission Characteristics

Lifetime : 2 year planned mission
Spectral resolution: : 6.0 keV at 0.511 MeV; 9.0 keV at 1.157 MeV
Angular resolution: : 4.1° at 0.511 MeV; 2.1° at 1.809 MeV
Field of View: : >25% of whole sky instanteously; all-sky every day in survey mode
Energy Range : 0.2–5 MeV
Special Features : All sky mapping at soft gamma-ray energies (>25% instant
Polarimetry measurements
Payload :

• Compton Spectrometer and Imager (COSI) is the single instrument compact Compton telescope in the COSI satellite (and earlier balloon flight) mission. The detector consists of a 3D-imaging system around 16 Germanium detectors (GeDs) arranged in a 2×2×4 configuration (the last dimension is depth), surrounded on four sides and below by Bismuth Germanate (BGO) scintillator anti-coincidence detectors: these reduce the telescope’s field of view to 25% of the sky while reject off-axis noise, but provide gamma-ray burst detection capability over the entire unocculted sky. Each crystal has 64 aluminum strips deposited with orthogonal orientation to provide 3-D event information within the detector volume (pathfinding balloon flights used fewer wires, but fundamentally similar detection techniques). The GeDs require cooling to operate efficiently: This is achieved with a Stirling cryocooler which requires no consumables (such as liquid nitrogen).

Science Goals:

• Determine the origin of Galactic positrons;
• Study 511 keV (positron/electron collision energy) features;
• Measure polarization of extreme environments;
• Probe the physics of multi-messenger events;
• Characterize the Galactic diffuse gamma-ray emission.

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