First observation of the $γ$-ray beam production by the backward Compton scattering of extreme ultraviolet light emitted from an undulator

TL;DR

The study demonstrates the first observation of a γ-ray beam produced by backward Compton scattering of extreme ultraviolet light (EUVCS) from a storage-ring undulator, with EUV photons reflected back into the ring by a concave Mo/Si multilayer mirror. The demonstrated setup at NewSUBARU achieves a Compton edge near $E_γ^{max} \approx 0.543$ GeV and records a γ-ray production rate of $1.4 \pm 0.1$ kcps for $E_γ>0.16$ GeV, after correcting for detector efficiency and beamline transmission, aligning with theory within a factor of ~2.3. The results show EUVCS as a viable path to high-energy γ-ray beams approaching the ring energy and outline clear routes to substantial flux enhancements at next-generation storage rings via reduced beam sizes and improved luminosity, enabling hadron photoproduction and other high-energy studies. The work establishes a practical method for in-ring high-energy γ-ray production using seed EUV light, with broad implications for future facilities like SPring-8-II and NanoTerasu. (All equations and key quantities are defined within the text and appendices, with the detailed kinematics and system designs provided to support replication and optimization.)

Abstract

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 light. In this method, $92$ $\mathrm{eV}$ 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 extreme ultraviolet light Compton scattering was observed for the first time in a demonstration experiment at the $1$ $\mathrm{GeV}$ ring, NewSUBARU. The measured energy spectrum was well reproduced by a theoretical calculation with the maximum energy of $0.543$ $\mathrm{GeV}$. The production rate was $1.4 \pm 0.1$ kcps for the energies above $0.160$ $\mathrm{GeV}$. 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.

Figures (7)

• Figure 1:
• Figure 3: Experimental setup for the extreme ultraviolet (EUV) light Compton scattering. The top-left part shows a top view of the NewSUBARU ring. An expanded view of BL07A is displayed in the remaining part.
• Figure 4: (a) Vertical profiles of radiated and reflected EUV light measured by the wire scanner. The vertical and horizontal axes show the signal voltage converted from the measured micro-current and the wire position based on a potentiometer, respectively. (b) Reflectance of the Mo/Si multilayer mirror as a function of the wavelength of EUV light incident on the center of the refractive surface.
• Figure 5: (a) Red open and gray filled (scaled) histograms show the $E_\gamma$ spectra measured in the demonstration experiment with and without EUV light reflection, respectively. (b) A gray filled histogram shows the difference of two spectra in (a). A red thick curve is a fit result of the theoretically calculated spectrum smeared by the calorimeter energy resolution.
• Figure A1: (a) A schematic diagram showing the kinematics of EUV light Compton scattering from high-energy electrons in the laboratory frame. (b) Scattering cone angles ($\theta_1$) as a function of $E_\gamma$ in the demonstration experiment of EUV light Compton scattering. (c) $E_\gamma$ spectra (differential cross sections) for EUV light Compton scattering at SPring-8-II (red solid line) and laser Compton scattering at SPring-8 (black dashed line).