Gamma-Ray Polarimetry

While the scientific potential of high-energy X-ray and gamma-ray polarimetry has long been recognized, measuring the polarization of high-energy photons is challenging. To date, there have been very few significant detections from an astrophysical source. However, recent technological developments raise the possibility that this may change in the not-too-distant future. Significant progress has been made in the development of gamma-ray burst (GRB) polarimeters and polarization-sensitive Compton telescopes. A second-generation dedicated GRB polarimeter, POLAR-2, is under development for launch in 2024, and COSI, a second-generation polarization sensitive Compton telescope, has been selected by NASA for launch in 2025. This chapter reviews basic concepts and experimental approaches to scattering polarimetry of hard X-rays to MeV γ-rays and pair production polarimetry of higher-energy photons.