MeV γγ-ray generation from backward Compton scattering at SPring-8

A test experimental line at SPring-8 has been constructed for the generation of backward Compton scattered (BCS) γγ-rays by head-on collision between an 8 GeV electron beam and a far-infrared (FIR) laser. For the FIR laser, we use a continuous-wave methanol laser optically pumped by a carbon dioxide (CO2)(CO2) laser. The FIR laser output power achieved is 1.6 W at maximum for 118.8μm lasing; this value is the highest in the case of this type of laser. As a result of the direct measurement, the generated BCS γγ-ray flux was found to be about 103103 photons/s. This value is consistent with the estimation calculated by taking into account the input laser power and the transmittance of the laser transport system.