[Halld-cal] Light guide shapes

[Elton Smith]

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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