From: Joseph Formaggio (josephf@u.washington.edu)
Date: Wed Jun 01 2005 - 00:33:58 CDT
Dear Matt,
Forgive me if I have sent you this talk before. It is a talk from Moriyama from LRT2004. If you check out his slide 14, he quotes ~0.025 decays/sec/PMT, though these are much smaller PMTs (2"). However, scaling to an 8" tube, that means a rate of ~0.4 Hz/tube (rather than 10). However, the background may not scale simply with area. Hamamatsu does not list the R8778 in their catalog, so it might be what we would expect to see in the future.
Hope it helps,
Joe
On Tue, 31 May 2005, Matthew Worcester wrote:
> 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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