Development of a bunch-width monitor for low-intensity muon beam below a few MeV

A destructive monitor to measure the longitudinal bunch width of a low-energy and low-intensity muon beam was developed. This bunch-width monitor (BWM) employed microchannel plates to detect a single muon with high time resolution. In addition, constant-fraction discriminators were adopted to suppress the time-walk effect. The time resolution was measured to be 65 ps in rms using a picosecond-pulsed laser. This resolution satisfied the requirements of the muon linac of the J-PARC E34 experiment. We measured the bunch width of negative-muonium ions (${\ensuremath{\mu}}^{+}{e}^{\ensuremath{-}}{e}^{\ensuremath{-}}$) accelerated with a radio-frequency quadrupole using the BWM. The bunch width was successfully measured to be $\ensuremath{\sigma}=0.54\ifmmode\pm\else\textpm\fi{}0.11\text{ }\text{ }\mathrm{ns}$, which is consistent with the simulation.