First observation of the $\gamma$-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 $\gamma$-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, $92$ $\mathrm{e}\mathrm{V}$ photons obtained from an undulator in a storage ring were reflected back to the original ring using a Mo/Si multilayer mirror. Consequently, $\gamma$-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 $1$ $\mathrm{G}\mathrm{e}\mathrm{V}$ ring, NewSUBARU. The measured energy spectrum was well reproduced by a theoretical calculation with the maximum energy of $0.543$ $\mathrm{G}\mathrm{e}\mathrm{V}$. The production rate was $1.4\pm 0.1$ kcps for the energies above $0.160$ $\mathrm{G}\mathrm{e}\mathrm{V}$. This rate was quantitatively explained by the luminosity and the scattering cross section. The present work paved the way to create a new $\gamma$-ray beam source for future applications such as hadron photoproduction experiments.