[Halld-cal] newest revision of FCAL beam test NIM article

[Matthew Shepherd]

Richard,

It seems that the story is not as clear as it seemed to me.  

I'm in favor of not drawing a such a strong comparison with RadPhi -- it is really not a central or necessary component of the article.  i think we just want to show that we can achieve the design goals.  For me, this means that the addition of light guides (necessitated by magnetic shielding considerations) hasn't done anything to adversely affect the performance.  It may have improved it, but it seems difficult to make an apples-to-apples comparison with E852 or RadPhi and the conditions for tests were different in each case.

Kei has modified the draft accordingly.

Matt

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Matthew Shepherd, Associate Professor
Department of Physics, Indiana University, Swain West 265
727 East Third Street, Bloomington, IN 47405

Office Phone:  +1 812 856 5808

On Apr 9, 2013, at 2:06 PM, Richard Jones <richard.t.jones at uconn.edu> wrote:

> Matt, I have nothing against referring to Radphi in this article!  If we are going back and rebuilding something, why not do it better?  I just want to get the story right.  Maybe the misunderstanding is all mine, that will be up to you to decide.
> 
> Thank you for reminding me of this technical note 850.  In reference #7 that is cited in the draft paper, which is reference #1 in doc 850,  it states (Optical Coupling Study section):
> Two calibrations were 
> performed, one with an air gap of order 1 mm between 
> the lead glass and photomultipliers, and one with optical 
> grease. For each condition the electron energy peak was 
> calibrated to peak at 5 GeV. and a Gaussian fit done to 
> give an estimate of the total energy resolution. The 
> quantity sigma/E was found to be 0.05 with and without 
> optical grease. A significant increase in the number of
> photoelectrons collected would have decreased the stati- 
> stical term in the shower energy resolution. narrowing 
> the ratio sigma/E. No such narrowing was observed. 
> Is it your view that the stated conclusion is not warranted by the observations?  A 5 GeV shower might not be statistics dominated.  If not, no observable narrowing might occur as a result of an increase of photostatistics by a factor of ~3.
> 
> I have no problem that a favorable comparison is made with Radphi, provided that the factors contributing to the improvement are properly identified.  What I see claimed in this paper is that the statistical term improvement from 7.3%/sqrt(E) to 5.x%/sqrt(E) is entirely explained by the insertion of the optical coupling between the block and the phototube.  I am wondering outloud whether is it really plausible that the difference comes entirely from photostatistics.  Keep in mind that the E852 calorimeter in test beam -- that had the same glass-air-glass coupling scheme as Radphi did -- reported 6.0%/sqrt(E) for the statistical term, (see footnote 19 in the Crittenden et.al. article and the associated reference).  If you took that at face value, wouldn't you conclude that about half of the improvement comes from the light coupling scheme and half from other sources?
> 
> -Richard Jones
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> On 4/9/2013 11:45 AM, Matthew Shepherd wrote:
>> Richard,
>> 
>> We attempted to study this in detail several years ago as we were designing the light guides.  
>> 
>> You may examine:
>> 
>> GlueX-doc 850
>> 
>> Table 1 is the table of interest.  There we indicate that a cylindrical light guide and cookie provides a factor of three improvement in photon collection efficiency over the air gap used in E852 and RadPhi.  When we wrote this, we consulted you on what the contribution of photo-statistics was to 7.3% RadPhi statistical term and then tried to assume that we just improve that component of uncertainty.
>> 
>> In my opinion we could remove all direct comparison with RadPhi from the NIM article and simply state that we are attempting to demonstrate that we can achieve the GlueX design resolution (which is really based on experience from E852 and RadPhi, but we don't need to make the connection so explicit in the paper).
>> 
>> Matt
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>