BLTWG Meeting 5/15/2008

• Time: 10:00 EST
• Place: EVO and ESNET
• Present: Jim Whitlatch, Tim Stewart, Richard Jones, Franz Klein, Dan Sober, Elke Aschenaur, Eugene Chudakov

Agenda

1. Discussion of the pair spectrometer specifications
2. Items required for the Lehmann Review

Notes by R. Jones

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Updates to Beamline Specifications [RTJ]

In preparation for discussion of the pair spectrometer specification, I went over the Global Specifications for the Hall D Photon Beam page. A number of these items have come under scrutiny during the last few months, and some of them need to be changed. I have updated the following items in the Global Specifications table on the main beamline specs wiki page.

1. the upper limit on the range of the tagging hodoscope - changed from 11.4 GeV (old value) to 11.7 GeV (new value). It is a priority for Hall D to provide tagged photons of the highest energy possible at Jefferson Lab. As shown in my talk on Tagging Near the Endpoint at the May 2008 collaboration meeting, the Hall D tagger acceptance extends up to 11.7 GeV. Right now we are modifying the layout of the hodoscope counters above 9 GeV to increase the segmentation, so it is a good time to change the spec on the upper photon energy limit to 11.7 GeV.
2. energy resolution in the microscope - changed from 60 MeV (old value) to 10 MeV (new value). The 60 MeV value came from studies of its effect on the reconstruction of exclusive final states in the GlueX spectrometer. Basically, one finds that the kinematic constraints coming from knowledge of the initial state energy do not improve significantly once the photon energy is known to a resolution better than 100 MeV. However, rate considerations require the microscope segmentation to be a factor 6 better than this. Improved calorimetry in the forward region may allow us to take advantage of this. In any case, our specification should reflect the capabilities of the actual device we are building. The current microscope design incorporates non-overlapping energy channels of 8 MeV width each, so the actual standard deviation is more like 4-5 MeV. However we may have trouble proving that we can calibrate the absolute energy in the microscope at the 4-5 MeV level, nor has anyone come up yet with argument that we need it at that level. To me, it makes a more coherent picture to specify both at 10 MeV or better, and not worry about the last factor of two.
3. energy resolution in the tagger hodoscope - changed from 120 MeV (old value) to 20 MeV (new value). The basis for this is that the Primex experiment needs an absolute energy calibration at the 0.2% level (see talk by Gasparian at May 2008 collaboration meeting which is consistent with non-overlapping energy bins of width 30 MeV. Current plans are to instrument the region in photon energy 9.0 - 11.7 GeV with 80 counters subtending equal-width energy bins of 34 MeV, which is consistent with a rms energy resolution somewhat better than 20 MeV.
4. minimum beam intensity required during setup - changed from 10^-4 of nominal intensity to 10^-3. That was a typo left over from an earlier spec that had 10⁸ Hz polarized photon rate designated as nominal. Our uniform practice now is to designate 10⁷ Hz as the nominal intensity and 10⁸ Hz as "high intensity" running.

Pair Spectrometer Specification

The discussion centered around the main tasks that the Hall D pair spectrometer must be designed to perform.

1. monitoring the photon beam spectrum This is needed for polarized beam running in order to be able to continuously monitor the polarization of the beam.