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 (mu(+)e(-)e(-)) accelerated with a radio-frequency quadrupole using the BWM. The bunch width was successfully measured to be sigma = 0.54 +/- 0.11 ns, which is consistent with the simulation.