Re: Npe/GeV

["George Lolos" <George.Lolos@uregina.ca>]

Hi Elton:

The fact that we expect quite a higher number of photons delivered to
the ends of the BCAL with the use of BCF-20 has been stated a number of
times, certainly in my presentations to the collaboration and it was the
basis of the decision to go for BCF-20 to begin with.  What is important
in this latest effort - that is summarized in the note Alex posted - is
the "mathematical" justification and the much more robust calculation of
the Npe's based on actual measurements and manufacturers' published
data, including the analytical calculation of wavelength effects.

While we do expect about a factor of two more pe's from SiPM-BCF-20 
compared to BCF-12-PMT combinations, I would be hesitant to relax the
PDE requirement right now because 21% is feasible and nobody had too
many pe's to throw away, this is particularly true for the low photon
energies.  If we know we can push the envelope on the fiber-sensor side,
then we will have a better handle to decide whether we want to improve
the sampling fraction as well to gain us an advantage where we need it
the most.  If 21% PDE cannot be achieved and we accept say 18%, it's
still a gain over settling for 12%.

Cheers,

George

>>> Elton Smith <elton@jlab.org> 12/12/07 8:55 AM >>>

HI Alex,

The implication from this analysis is that we should expect a factor of
2
more light from production 'green' calorimeter modules. If this is
correct, then our requirements on the PDE from the SiPMs can be relaxed
from our current value of 21% back to the original 12% which is less of
a
reach.

George and Zisis: Can you confirm this?

Thanks, Elton.


------------------------------------
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