Nice data sheet, too bad the dark current graph doesn't go to lower
voltages.
figure 9 here shows dark current 'turning on' at 600-700 volts.
www.bo.infn.it/ams/Hamamatsu-PMT.pdf
Did you ever look at the dark current at low voltages?
I never used the RCA 8850 - much too expensive for the sort of work
that I ever got involved with.
I did meet people who had used it, who drew my attention to it's
Achilles heel, which was that if you ever got the final dynodes hot,
you could poison the GaP on the first dynode.
It didn't take prolonged over-heating to do the damage. The tube was
fine for single-photon work - which is what it was designed for - but
if it was ever exposed to lots of photons for period of a second or so
when powered up for single-photon detection, the first dynode could be
seriously and permanently degraded. The intelligent user would design
the resistive parts of the voltage divider to make sure that this
didn't happen, but physicists can't be relied on to be all that
intelligent about electronics, as Review Scientific Instruments
reminds us from time to time - you may recall my most recent
contribution there ...