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Re: more on BCAL light yield estimates
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/
>>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>>
>