[Halld-cal] Light guide shapes

[Zisis Papandreou]

Hi Elton:

very interesting, and I am glad Jim found another 'angle' to this, forgive the pun.  I understood the argument of cost reduction on the light guides, but was never happy with the asymmetric solution.  We are taking a few corners here and there, and I am worried that these my add up in a nasty way as non-uniformities and cause our constant term in the resolution to degrade further.  Following Kei's talk here at the GHP, a person from Julich  asked whether the 5.5%/sqrt(E)+/-2.4% was adequate resultion for multi photon hits.  I explained to her that the 2.4% was from the Hall-B beam test and did not represent a firm grasp on the floor, owing to the small lever arm of the test (up to 650Mev).  However, the recent simulations by Andrei/Irina & David still bother me in the context of the SiPM summing.

Back to the subject, I like the phi symmetry in this design.  First, I would not worry about extending the light guides on the back/outer side.  There is little energy there, and I doubt this would be a problem even in the forward angle region.  I understand the 0.05 gap to mean 25.4mmx0.05=1.27mm.  Is this correct? If yes, even this absence should be hard to notice.  It is in the build up region of the photons (see 'nominal' curve from attached tmax plot from Stamatis' thesis and recall that X0=1.40cm for us), is only ~5% of the thickness of the light guide.  It may have some small effect for 60 MeV photons though, so this should be discussed further at the Tuesday meeting.

Cheers, Zisis...

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On 2011-04-29, at 1:18 PM, Elton Smith wrote:

> Dear Bcal Readout,
> 
> Good ideas never die, even at this late stage. We are re-evaluating the precise shapes of the light guides for the Bcal. Jim Fochtman, who has been laying out the geometry for the readout has resurrected the notion of phi-symmetric light guides. Similar schemes have been proposed previous. See for example GlueX-doc-1536, Figs 7 and 8. Up to know the nominal design has been that given in Fig 8, where there are 20 different light guide types and many of them have rectangular bases, instead of trapezoids. The original concept for the phi-symmetric guides had 40 different shapes and we were searching for ways of minimizing the number and complexity of the shapes.
> 
> The new twist proposed by Jim is to have four columns, each identical, just rotated with respect to the neighbor. In other words, for this geometry, there are only 10 different light guide types. See attached figure. This creates a gaps relative to the bcal physical build at the center of the inner layer and at the outside corners of the outer layer. We will likely want to stretch the height to over-cover part of the aluminum plates in some areas in order to completely cover the entire active surface.
> 
> I wanted to put this out now so people could think about it before our Bcal readout meeting next Tue. Comments and feedback are welcome.
> 
> Cheers, Elton.
> 
> p.s. Note that prototyping for the readout will continue with the present nominal light guide design, even if we might decide to move to the new geometry for the entire detector.
> 
> -- 
> Elton Smith
> Jefferson Lab MS 12H5
> 12000 Jefferson Ave
> Suite #16
> Newport News, VA 23606
> (757) 269-7625
> (757) 269-6331 fax
> 
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