From: Matthew Worcester (mworcest@hep.uchicago.edu)
Date: Tue May 31 2005 - 19:58:00 CDT
Hi Mike et al,
Here is what I've learned about the pmt glass radiation from my MC
studies, in no particular order. With a 90 cm buffer the pmt face is
actually 25-30 cm inside the oil, so the reduction is not as much as you
might expect. The most important parents of high-energy gammas are
208,210Tl and 212,214Bi, which are in the U/Th chain. Hamamatsu quotes 10
decays/pmt/sec but won't give any further information.
So the simulation generates gammas from all 4 isotopes above with proper
BR and multiplicity, with one decay per event. It assumes pointlike PMTs
to generate from, and places them 30 cm inside the buffer. It has 935
pmts. The gammas compton scatter through the oil. Attached is a plot of
rate vs. thickness, which fits an exponential nicely. With the 90 cm
buffer you see about 5% of the events have at least 1 gamma compton
scatter inside the scint, for a rate of about 400 Hz.
The key to understanding the radiation is that most gammas that hit the
scint have already scattered away most of their energy. So the energy
from those gammas is buried in the pedestal noise. So the question,
which I don't have an answer to, is how often out of that 400 Hz does pmt
radiation fake a positron? Looking at the energy from the gammas (those
plots are in my May 13 software talk) I'm guessing it will only be about
1% of the time, for a 4 Hz fake e+ rate (almost all from the Tl).
We've measured the spectrum from a Hamamatsu R5912 tube at Chicago with a
Ge detector and seen the K,U, and Th peaks but have not converted that
into g/g yet. We are also getting a sample 8" pmt from Electron Tubes to
check its radiaiton.
I would not put any of these plots in your talk, but I think if you are
asked about it it's fair to say that we have a working simulation that we
are refining with pmt samples that we have received from several vendors.
Cheers,
Matt
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