Gamma-ray Production in a Storage Ring Free-Electron Laser

A beam of 100% linearly polarized γ-rays has been produced via intracavity Compton backscattering inside of a free-electron laser optical cavity. A beam of 12.2 MeV γ-rays having a flux of 3.2×10^5 γ-rays/sec, as expected for a 3.2 mA ring current, was obtained when 379.4 nm FEL photons were backscattered from a 500 MeV electron beam. Collimating the γ-ray beam provided a flux of 1.2×10^3 γ's/sec with an energy resolution FWHM of 120 keV, in agreement with predictions. The polarization of the γ-rays ( ~100%) was determined by measuring the neutron scattering asymmetry from the d(γ,n)p reaction. The present system should allow for production of beams with energies up to 50-60 MeV and fluxes in excess of 10^8 γ's/sec. Partially supported by the US DOE, ONR and AFOIR.