Development of the Range Counter for the COMET Phase- α Experiment

The COMET Phase-α experiment aims to evaluate the novel muon transport beamline for the muon-to-electron conversion search at J-PARC, Japan. A dedicated Range Counter (RC) was developed to measure the momentum spectrum of transported negative muons with momenta of 30–100 MeV/c. The RC consists of graphite momentum degraders, a muon absorber, and plastic scintillation counters (T0, T1, and T2) to detect decay-in-orbit (DIO) electrons from stopped muons. The number of muons stopped in the absorber is reconstructed from the decay time distribution. A copper absorber was selected due to the short lifetime of muonic atoms in copper, which enhances signal separation. The counters’ performance was evaluated experimentally. The T0 Counter, made of a 200×200× 0.5 mm3 scintillator plate, achieved a muon-trigger efficiency exceeding 99.9%. The T1 and T2 Counters also demonstrated high electron-detection efficiencies of >99%. Based on these results, simulation studies estimate the acceptance for reconstructing the number of DIO electrons from the absorber to be approximately 47% with a corresponding signal purity of 60% against muon capture-induced backgrounds.