[Halld-pid] TOF PMT signal hight estimation

[Beni Zihlmann]

Hi All,
in

https://halldweb1.jlab.org/wiki/index.php/Detector_Rates_and_Lifetime#Operating_PMT_at_0.02mA

I summarized the estimations of signal amplitudes for PMTs on full 
paddles that are close to the beam
line assuming a current limit of 0.02mA at low intensity running with 
10^7 photons/s in the coherent
peak. Note that such a current limit is about 5 times lower the maximum 
recommended current of
0.1mA. So this is not a very conservative limit!
It basically means after 5000 hours a gain reduction of a factor of 2. 
5000 hours are 208 days of
operation which for us means about 2 years of running. That is probably 
equivalent to our initial
low intensity running period.
I find that only the innermost paddle will have a marginal signal hight 
at these conditions. In order to
avoid amplifiers one could relax the current limit for this paddle and 
run it at double the current.

At this point I want to add one more issue. Since we seem to play with 
the idea of replacing PMTs after
some operation time we might consider coupling the PMTs to the light 
guide using optical grease only
and servicing such joints every shutdown by removing the PMTs cleaning 
the surface and reattach the
PMTs back with a new fresh grease joint.

cheers,
Beni