Re: no of photoelectron analysis

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

Hi Richard:

I had made the point during our last meeting at JLab that while the 8000
photons/MeV is the "universally" accepted number of photons created by
one MeV of energy deposition in plastic scintillator, this number is the
total (integrated) yield over the whole spectrum, say 420-600 nm.  
However, as we have reported by actual spectroscopic measurements of
SciFi's, blue fibers lose essentially all their photons with wavelength
below ~ 460 nm and their peak emission - after say one meter - is around
480 nm instead of the source at ~ 425 nm.  Therefore, from the 8000
photons/MeV, more than half are lost by absorption and the rest are
attenuated by the bulk attenuation of the plastic.  My rough spectral
integration gives around 40% yield surviving the first few cm's or 3200
photons/Mev but we will get a better number soon since we have the data.

This loss of photons accounts partially for the loss of pe's and it may
explain (in principle) the low numbers.   Here are some more realistic
calculations to argue about:

3200 ph/MeV x 2.0 MeV/cm x 1.9 cm x 0.093 x 0.5 x 0.1 pe/ph x 0.75 = 42
pe's.

The factors are:

1.9 cm = equivalent SciFi thickness in 3.8 cm of read-out cell height

0.093 = fiber capture ratio

0.5 = reduction factor due to bulk attenuation for 2m light propagation

0.1 pe/ph = QE (includes collection efficiency)

0.75 = the transmission efficiency of light guides as it came out of
Blake's simulations.

So now we have a difference between 42 and 22 pe's.  Some adjustments
can be made in the QE because I took the average from Burle curves but
collection efficiency  was assumed 100% and in reality it's around 85%
for an average PMT of the types we used.  Also, loses from BCAL to light
guide contacts and light guide to PMT further degrade the number of
photons reaching the photocathode film.  

My conclusion is that the 22 pe's from the Hall B beam tests are lower
what we would expect but upon more detailed study the difference is not
that much.  In fact, some old measurements on plastic scintillators were
consistent with 2000 photons/MeV as the effective number of photons
reaching the photo-sensors.  If this is correct for our SciFi's as well,
the difference is essentially eliminated.

The bottom line is that we must extract the actual photon survival yield
from the SciFi data we have obtained in Regina.  This will determine
with great accuracy the photon yield as a fraction of the 8000 ph./MeV
that survives the absorption within the first few cm's.  All other
factors play a role but this number dominates the final number of pe's.

I hope this clarifies somewhat the observations.

George

>>> Richard Jones <richard.t.jones@uconn.edu> 07/21/07 11:56 AM >>>