Fig. 15.

Event rate in the neutron veto as a function of time for 4 (red), 6 (green), and 10-fold (blue) PMT coincidence after filling the water shield with demineralized water. The initial high rate due to contamination with natural $_{}^{222}$Rn decreases consistent with its half-life, after which the rates stabilize to below 100 Hz even for the 4-fold coincidence requirement