A typical pulse.  Signal in volts, time unit is 0.25 ns.

Taking the area under the curve, we find the typical pulse is 250 mV-ns. Since the manufacturer says a 6.4 mev alpha particle produce 5x10³ photons, assuming 450 nm, one alpha per ns works out to 0.44 nw. With the PMT’s sensitivity (as given by the manufacturer) of 0.5 V/nW, we expect 220 mV for one alpha per ns, essentially what we got.