[Halld-tagger] [EXTERNAL] let's cancel the Photon Beam wg mtg today

[Richard Jones]

Hello,

I do not know of anything urgent for the photon beam working group as a
whole to consider today, so I propose that we skip this one, and consider
meeting again in two weeks. There is work going on behind the scenes by
Sasha to gather what he has learned about beam flux systematics from the
Primex runs into a summary document. This document, which he is working on
in parallel with the FCAL2 upgrade work he is overseeing, will be the
starting point for estimating flux systematic errors for the GlueX run
periods.

Other than this priority, there is just one other conversation we might
have that was brought up to me by Dave Mack, regarding the possible use of
positrons in the Hall D photon source. I have done a fairly comprehensive
search over the kinematics of radiative e+e- annihilation in a diamond
crystal, and there does not seem to be any accessible kinematics that have
significant coherent gain. That leaves just the regular coherent
bremsstrahlung process, except with the positron beam which has
significantly worse emittance than the electron beam. The amorphous target
annihilation process produces a beam spot that is larger than our
collimator can handle, and the annihilation photons sit on top of the
dominant bremsstrahlung spectrum, and are unpolarized. Running with
positrons would entail additional background in the tagger, which I could
quantify in simulation. Before making that effort, I would like to hear an
argument for why making an ordinary bremsstrahlung beam would be a good use
of a CEBAF positron beam.

-Richard J.
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