  A muon-flux measurement behind the beam dump of the J-PARC Hadron
Experimental Facility was performed with a compact muon detector that can be
inserted into a vertical observing hole which was dug underground with 81 mm in
diameter. The flux of the muons penetrating the beam dump was scanned
vertically at intervals of 0.5 m, showing a wide distribution with a maximum at
the beam level. The muon flux was consistent with the expectation from a
Monte-Carlo simulation at more than 1 m away from the beam axis, which is
expected to be used for signal-loss evaluation in the future KOTO II experiment
for measuring rare kaon decays. The data can also be used in improving the
accuracy of shielding calculations in the radiation protection.
