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Re: Back-of-the-envelop photon estimates



Hi Elton,

 Thanks for the message.

 I agree that the 95% absorption is the main reason for the difference.

 I will have to look into it in more detail, but I expect that using the 
exponential form with the 10,000 photons/MeV number is not correct. My 
recollection is that the deep UV part of the emission spectrum has a 
range of order 1 mm in the plastic and so the loss at those wavelengths 
is essentially never measured when people derive the attenuation 
lengths; but the deep UV photons *are* included in the 10,000 
photons/MeV. In any case, I hope that our guys here will soon come up 
with a more quantitative estimate based on the absorption and emission 
spectra. Also, we should try to do a real measurement. (I think it is a 
mistake to ignore the PDG statement, which is based on real experience 
for a detector design that is not very different from BCal.....)

Best regards,

 - Will


Elton Smith wrote:
> Hi Will,
>
> At the last Bcal readout meeting you showed some estimates of photons
> expected in the Bcal that were (factor of 3) lower than the 10,000
> photons/side I have used. I promised to get back on a quick estimate of
> this number. Here it is:
>
> 1. Number of photos/MeV of scintillator = 10,000/MeV (PDG July 2008 Sec
> 28.2)
> 2. 1 GeV photon deposits 120 MeV in scintillator (0.12 sampling fraction)
> 3. Capture fraction/side in fiber is 5.4% double clad, 3.1% single clad,
> corresponding to 26deg and 20 deg total internal reflection angles. I take
> 4% because we have double clad, but these do not seem to yield full
> capture relative to single clad.
> 4. Attenuation from center to one side
> (atten length=280 cm) = exp(-200/280) = 0.5
>
> Putting all this together gives
> Number of photons at end of Bcal = 10^4/MeV x 120 MeV x 0.04 x 0.5 =
> 24,000 photons per side.
>
> The number we get is smaller than this by a factor of 2, but is normalized
> to the measurements of Bcal prototypes. The Montecarlo estimates of the
> shower actually yield somewhat higher estimates (40k/side), but losses at
> different stages reduce the calculated number.
>
> I believe the main difference from your estimate from
> http://www.jlab.org/Hall-D/software/wiki/images/5/58/LightCollection.pdf
> and the one above is due to the assumed loss of light due to attenuation
> (factor of 20 instead of 2). The PDG claims the loss can be "95%... in a
> large collider tracker." This is not consistent with our calorimeter
> design or measurements and I'm not sure why this would be so large, but it
> is for a different application.
>
> Hope this helps.
>
> Cheers, Elton.
>
>
>
> Elton Smith
> Jefferson Lab MS 12H5
> 12000 Jefferson Ave
> Suite # 16
> Newport News, VA 23606
> elton@jlab.org
> (757) 269-7625
> (757) 269-6331 fax
>