From: Matthew Worcester (mworcest@hep.uchicago.edu)
Date: Wed Jun 01 2005 - 09:40:33 CDT
Hi Hans,
Jon and I had some confusion over this awhile ago, and I think the answer
is that miniboone uses homemade, flat bases to reduce the length of their
tubes. The 290mm on the Hamamatsu spec includes 70mm for the 20-pin base.
We can do the same trick at braidwood to get more buffer if we think it is
critical.
Cheers,
Matt
On Wed, 1 Jun 2005, Hans Jostlein wrote:
> 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?
>
> Hans
>
>
>
> ----- Original Message -----
> From: "Matthew Worcester" <mworcest@hep.uchicago.edu>
> To: <braidwood@hep.uchicago.edu>
> 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.
>>
>> Cheers,
>> Matt
>>
>
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