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

[Richard Jones]

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












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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