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@article{OISHI2026170904,
title = {Development of the Range Counter for the COMET Phase-α Experiment},
journal = {Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment},
volume = {1082},
pages = {170904},
year = {2026},
issn = {0168-9002},
doi = {https://doi.org/10.1016/j.nima.2025.170904},
url = {https://www.sciencedirect.com/science/article/pii/S0168900225007065},
author = {Kou Oishi and Masaharu Aoki and Shion Kuribayashi and Shunya Ueda and Kazuki Ueno and Nicolas Chadeau and Thomas Clouvel and Yuki Fujii and Yoshinori Fukao and Masaaki Higashide and Youichi Igarashi and Satoshi Mihara and Hajime Nishiguchi and Kenya Okabe and Yoshi Uchida},
keywords = {Muon, Thin plastic scintillator, Momentum degrader, Low-momentum measurement},
abstract = {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.}
}