From: Hans Jostlein (email@example.com)
Date: Wed Jun 01 2005 - 09:17:56 CDT
Thanks for the detailed and useful description, Matthew
Meanwhile I have come across a much more mundane riddle.
The display of the Miniboone tubes here on Wilson hall 10 has, indeed, the
cathode at about 10 inches (254 mm) from the steel wall.
However, if I look up the R5912 PMT in the Hamamatsu catalog,
the drawing says that the tube itself is 290 mm tall, i.e. about 50 mm
taller than the Miniboone tubes.
What is the real story here?
----- Original Message -----
From: "Matthew Worcester" <firstname.lastname@example.org>
Sent: Tuesday, May 31, 2005 7:58 PM
Subject: pmt glass radiation
> 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.
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