Germanium Detectors for MeV Gamma-Ray Astrophysics with the Compton Spectrometer and Imager

Germanium semiconductor detectors—particularly high-purity germanium (HPGe) detectors—have been the technology of choice for decades for applications that require excellent energy resolution and good stopping power for MeV gamma rays. Modern advances in contact fabrication enable the fine segmentation of large-volume HPGe detectors to locate individual energy depositions in 3D, not just 2D, which makes possible the accurate reconstruction of Compton-scatter tracks. We describe the state-of-the-art HPGe detectors on the Compton Spectrometer and Imager (COSI), a telescope designed to study MeV gamma rays from astrophysical sources in the Milky Way Galaxy and beyond. These detectors require careful calibration to achieve the simultaneous objectives of spectroscopy with high spectral resolution, imaging of point and diffuse sources, and polarimetry of transient and persistent sources. COSI, previously a successful balloon-borne instrument, is now a funded NASA Small Explorer space mission, slated for launch in 2027 (NASA press release: https://www.nasa.gov/press-release/nasa-selects-gamma-ray-telescope-to-chart-milky-way-evolution ).