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Re: more on BCAL light yield estimates
OK - thanks - I was doing a trivial solid angle and there's obviously
more than that - thanks for the reference as well.
Cheers
Alex
At 10:51 AM -0600 11/16/07, Blake Leverington wrote:
>sorry it's (1-cos^2[theta])/2, which includes skew rays as well as
>meridional rays to give a trapping efficiency of ~10.6%., so this
>9.3% which cam from Yianna's simulation is lower than predicted.
>
>-Blake
>
>Blake Leverington wrote:
>>hi Alex,
>>
>>The trapping efficiency for a fiber is (1-cos[theta]^2)/2, not
>>(1-cos[theta])/2 which gives a trapping efficiency closer to 10.6%.
>>See http://arxiv.org/pdf/nucl-ex/0404008 on page 8.
>>
>>-Blake
>>
>>
>>
>>
>>
>>
>>Alex Dzierba wrote:
>>>Thanks to George and Christine
>>>
>>>I appreciate and understand the comments - my goal was to
>>>use other sources to look at the issue and I agree that we
>>>need to quickly converge on putting down on paper what we
>>>know and make sure we can agree.
>>>
>>>Regarding the capture fraction - I may be making a trivial error
>>>but it seems that to first order the fraction of the light
>>>generated at the source, assuming isotropic distribution,
>>>coming out one end within critical angle theta (measured
>>>along the fiber) is (1-cos[theta])/2 or the solid angle
>>>subtended divided by 4pi. For theta = 27.6 degrees
>>>this leads to 5.3% out one end.
>>>
>>>Cheers
>>>Alex
>>>
>>>At 4:27 PM -0600 11/15/07, George Lolos wrote:
>>>>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/
>>>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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/
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~