Re:

[Alex Dzierba <dzierba@indiana.edu>]

Hi Elton

Indeed, one of the reasons George, Zisis and I did the spectral
analysis was to understand that factor of four based on Npe estimates
obtained from the 4-m module (blue fibers) with cosmic rays.
We believe that a factor of two contribution to that factor of
four is illustrated in Fig. 2 of our note.  The bulk attenuation length
would lead one to expect a factor of 2 reduction for  the source
at the center of the 4m module (2 m of light travel) but the analysis
shows a factor of 4 reduction (loss of the blue light).  Another
factor is the capture fraction which had been estimated as
9.3% from GUIDEit.  Simple solid angle estimate based on the
critical angle is more like 5% (what the manufacturers quote
as "at least").  Finally, there are uncertainties in transmission through
the interfaces and prism light guide.  So the estimates of approx 25 Npe/cell
for the 4-m blue module with cosmics, may not be inconsistent with
the 8000 or so photons/MeV at the source. 

But we'll write this all down, with numbers and errors, as you suggest.

Cheers
Alex

At 8:20 AM -0500 12/12/07, Elton Smith wrote:
>Hi Alex,
>
>Thanks for putting together all the information on the fiber spectra and
>photoelectrons. The one key parameter that should be able to be derived
>from this is the number of photons/GeV (Eq. 2 in GlueX-doc-795). The old
>original number was 84000, but based on recent information we revised the
>calculations using four times less photons. The photon yield directly
>affects our specification for the PDE of SiPMs, which is now under
>discussion for a statment of work with further SiPM development.
>
>Based on your calculations, it would be useful to estimate the Npe/GeV for
>various configurations: the two Bcal prototypes and the best estimate for
>a production green fiber (this latter value would drive our design
>specifications).
>
>Thanks, Elton.
>
>
>--------------------------------
>Dear Colleagues
>
>During the past six days I have been visiting U of Regina where
>George, Zisis and I worked on issues related to the introduction
>and calorimeter sections of the CDR for CD-3.  We also co-authored
>a GlueX-doc-924-v1 (just posted) on an analysis of fiber emission spectra.
>
>Cheers
>Alex
>--
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>Alex R. Dzierba
>Chancellor's Professor of Physics (Emeritus)
>Department of Physics / Indiana U / Bloomington IN 47405 / 812-855-9421
>JLab Visiting Fellow
>Jefferson Lab / 12000 Jefferson Ave / Newport News, VA 23606 /
>757-269-7577
>Home Phone: 812-825-4063  Cell:  812-327-1881  Fax: 866-541-1263
>http://dustbunny.physics.indiana.edu/~dzierba/


-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Alex R. Dzierba
Chancellor's Professor of Physics (Emeritus)
Department of Physics / Indiana U / Bloomington IN 47405 / 812-855-9421
JLab Visiting Fellow
Jefferson Lab / 12000 Jefferson Ave / Newport News, VA 23606 / 757-269-7577
Home Phone: 812-825-4063  Cell:  812-327-1881  Fax: 866-541-1263
http://dustbunny.physics.indiana.edu/~dzierba/
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~