  Muon beam monitoring is indispensable for indirectly monitoring
accelerator-produced neutrino beams in real time. Though Si photodiodes and
ionization chambers have been successfully used as muon monitors at the T2K
experiment, sensors that are more radiation tolerant are desired for future
operation. We have investigated the electron-multiplier tube (EMT) as a new
sensor for muon monitoring. Secondary electrons produced by the passage of
muons at dynodes are multiplied in the tube and produce signal. Two prototype
detectors were installed at the T2K muon monitor location, and various EMT
properties were studied based on in situ data taken with the T2K muon beam. The
signal size is as expected based on calculation, and the EMTs show a
sufficiently fast time response for bunch-by-bunch beam monitoring. The
spill-by-spill intensity resolution is 0.4%, better than the required value
(1%). Signal linearity within $\pm$1% is achieved at proton beam powers up to
460 kW (with +250 kA focusing horn operation). A gradual signal decrease was
observed during the initial exposure, due to the stabilization of dynode
materials, before the response became stable within $\pm$1%. This work
demonstrates that EMTs are a good candidate for future muon monitoring at T2K,
and may also have other more general applications.
