Re: more on BCAL light yield estimates

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

Hi Christina:

As you noticed with Alex's messages, we have been in frequent
communication on the issue of number of photo-electrons from the BCAL. 
I have a couple of comments on your message and Alex's analysis.  My
comments do not alter any conclusions but offer a bit more refinements
for incorporation if work we need to pursue further on this matter.
 

>>> "Christina Kourkoumeli" <hkourkou@phys.uoa.gr> 11/15/07 10:26 AM >>>
>>> Dear all,
>>> Let me also add some input to these arguments:few years ago I have
done
>>> exactly what Alex did:I took the transmission spectra measured in
Regina
>>> and available in the portal for different lengths of fibers and for
blue
>>> and green fibers digitized them after normalizing to the tails and
>>> furthermore I have multiplied by a typical Bialkali QE (all as a
function
>>> of lambda ). After the product was done then I observed the
following:
>>> at 5cm the green gave as much light as the blue
>>> but at 175cm (the longest length available) the green was only 75%
of the
>>> blue. This was also verfied by us with a rough experiment using UV
light
>>> and green/blue fibers.So our conclusion was that the green fiber is
not
>>> justifiable for a PM read-out.


A bialkali PMT has a Q.E. curve shown in Alex's message of yesterday. 
It's fairly flat between ~400-440 nm at around 23%.  Even as far as ~
500 nm, Q.E. is around 15%.  A blue fiber with peak emission at ~ 430
nm, at a distance from the source of 100 cm the peak emission is at ~
470 nm with very little strength left below 450 nm.   A green SciFi, on
the other hand, has no emission whatsoever below 470 nm at 100 cm from
the source.  So, I am surprised that the combination of green SciFi and
blue PMT will give as much light as the same PMT with blue SciFi, UNLESS
the latter has indeed lost so much of the "primary" wavelengths due to
absorption right at the source while the green SciFi has not.  

One also has to be careful to compare emission spectra of identical or
very similar types of SciFi's.  For example, Alex used the curves from
Kuraray for SCSF-78 to base his calculations, however, he normalized to
the curve shown by Bicron for BCF-12 to extrapolate to the source.   For
"proof of principle", this is fine, however, there are some differences
that will alter the final numbers.  SCSF-78 has a peak wavelength of 450
nm and high photon yield (Kuraray's claim without numbers attached). 
BCF-12, on the other hand has a peak wavelength of 435 nm.  Normalizing
the latter to the former is good for showing the effect but will not
give as accurate numbers as one will get with SciFi's of the same kind.


>>> Concerning the absolute numbers ,with different MC we always got
>>> collection efficiency from both sides of the fiber higher than 9% so
we
>>> always assumed that it was 9% per side.
>>> But I do agree that the spectrum after 2m of (175cm) has to be
normalized
>>> to the 2cm one-absolute measurement-and then multiplied by the
functional
>>> SiPM or PM efficiency to arrive at the right numbers.
                             Cheers,Christine

Both GEANT-based and GUIDEIT - based simulations show 9.3% per side as
well.  We will figure out what the "true" number is and document it by
calculations for all of us to agree upon.

Cheers,

George



 > Dear Colleagues
>
> It is important for us to understand the apparent factor of 4
discrepancy
> between the photoelectron yield estimates and measurements for
> cosmic rays using the BCAL test module.  I find the same factor of
> 4 (roughly) as George -- but I get there by a somewhat different path.
>
> Please read the attached and comment.
>
> Our goal should be to produce a note on our understanding of light
> yields since setting specifications for the fibers and SiPM's depend
> on this.   In my role as Design Report editor I will generate this
note
> with help and input from others.   I will also be spending the first
week
> of December in Regina working with my good friends there on the BCAL
> section of the Design Report.
>
> 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/
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~