Re: Back-of-the-envelop photon estimates

[Elton Smith <elton@jlab.org>]

See below.

> 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.

I think you are right and UofR has many measurements of the attenuation of
light as a function of wavelength. There are been many discussions about
what fraction should actully be used [see ref. from the calorimeter
review Feb 2008]. A factor of two is quite consistent, which brings the
numbers closer to measured numbers.

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.

What is closer to a "real measurement" than measurements of photoelectrons
obtained from the Bcal prototype [see GlueX-doc-1069 and the cal NIM
article]? This is what the numbers are normalized to.

(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
> >
>
>