First observation of the γ-ray beam production by the backward Compton scattering of reflected synchrotron radiation in the extreme ultraviolet range

Compton scattering of photons off high-energy electrons is a fundamental quantum mechanical process widely utilized to produce a γ-ray beam for scientific research. Instead of injecting laser light into a storage ring as a conventional way, we have developed an innovative method to achieve drastically higher energies approaching the ring energy by the backward Compton scattering of extreme ultraviolet (EUV) light. In this method, 92 eV photons obtained from an undulator in a storage ring were reflected back to the original ring using a Mo/Si multilayer mirror. Consequently, γ-ray beam production through the EUV light Compton scattering using reflected synchrotron radiation was observed for the first time in a demonstration experiment conducted at the 1 GeV ring, NewSUBARU. The measured energy spectrum was well reproduced by a theoretical calculation with the maximum energy of 0.543 GeV. The production rate was 1.4 ± 0.1 kcps for the energies above 0.160 GeV. This rate was quantitatively explained by the luminosity and the scattering cross section. The present work paved the way to create a new γ-ray beam source for future applications such as hadron photoproduction experiments.