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Performance of an LYSO-Based Active Converter for a Photon Pair-Spectrometer aiming for 52.8 MeV photon detection in Future $μ^+ \to e^+ γ$ Search Experiments

For future $μ^+ \to e^+ γ$ search experiments with a branching-ratio sensitivity of $10^{-15}$, we are developing a photon pair-spectrometer employing an active LYSO converter, aiming at target resolutions of 30 ps in timing and 200 keV in energy measurement for detecting 52.8 MeV photons. The converter generates electron-positron pairs from incident photons while simultaneously measuring their energy deposition and timing. On the basis of simulation studies, we optimized the converter thickness and segment dimensions, and accordingly fabricated prototype LYSO segments. Their single-MIP detection performance was evaluated using an electron beam at the KEK PF-AR test beamline. The prototypes exhibited excellent performance, achieving a time resolution of 25 ps and a light yield of $10^4$ photoelectrons, both substantially surpassing the design requirements.