[Halld-pid] TOF PMT gain and rates

[Beni Zihlmann]

Hi All,
last Friday the Hamamatsu representatives were here at the lab and
we had a short conversation about the TOF PMTs. From this short
meeting I conclude that we need an extensive discussion about the
PMTs.
One important issue is that here at Jlab we do not understand why
you at FSU push so hard for high gain PMTs. Even at the low intensity
beam of 10^7 photons the inner most paddles will see rates of about
1 MHz and consequently the current in the PMT will be substantial if
the gain is high. We can not operate the PMT at a gain that results in
currents that the close to 0.1mA. Such currents will diminish the intrinsic
gain of the PMT substantially in a short time (50% in 1000 hours operation).

Can you estimate how many photo electrons you get for a MIP that passes
through the center of the paddle and deposits the mean energy of a landau
distribution? Having signals of 1000mV for such events will results in too
high currents.

I would like to put this issue on the agenda for next meeting.

cheers,
Beni