A nearly monochromatic beam of 100% linearly polarized $\gamma$ rays has been produced via Compton backscattering inside a free electron laser optical cavity. The beam of 12.2 MeV $\gamma$ rays was obtained by backscattering 379.4 nm free-electron laser photons from 500 MeV electrons circulating in a storage ring. A detailed description of the $\gamma$-ray beam and the outlook for future improvements are presented.

• Received 24 February 1997

DOI:https://doi.org/10.1103/PhysRevLett.78.4569