Notes from Phone Meeting October 19, 2007

• Compton Polarimeter

• Compton Photodetector.
  • Gregg raised the question of whether to use PbWO₄ or BGO for the January beam tests. Advantage of BGO is that it would provide a light level more comparable to the GSO crystal which is on order. It is agreed that, since it both are available, it is reasonable to have both on hand. The more challenging test (PBWO₄) is probaby the most important.
  • The January Compton detector test will need to be done at 3 pass, at least, so that the larger photon energies and analyzing power should make the test easier.
  • The large light output of the new GSO crystal should allow self-triggering on signals from single incident electrons down to very low photon energies (~<1 MeV). If we understand the situation at HIGZ correctly, they have a well measured photon beam energy, but relatively poor total intensity determination. This should be a very useful facility for calibrating the linearity of the GSO crystal (and a study might even be of use to HIGZ as an intensity calibration). We need to develop a plan for calibrating this detector at HIGZ.
• Kent points out the schedule for Compton installation. elog:1216
• The status of the Compton cavity upgrade was discussed. The upshot is that progress is evident but the schedule is obviously challenging. There are currently to alternative, parallel development tracks: the original high-gain cavity with a low-power laser source (the Prometheus), or a relatively high-power green laser (Lightwave to fiber amp to PPLN) driving a relatively low gain cavity.
• We need information on the Compton beampipe size and the present collimating aperture. See elog:1213.

• Lumi Monitor

• Discussion of debris in lumi lightguide. Brian and Dustin charged with posting photographs for further discussion
• All PMTs were to be removed from installation, for eventual removal from the hall (pending radcon approval).
• The necessary steps for testing the rad-con tagged PMTs should be planned.
• The one light guide already deinstalled should be removed from the hall, when possible, both to test cleaning approaches and to test-fit new filter box design.
• Bryan reported that the PMT bases were fragile due to apparent radiation damage. We should acquire spares.

• January Beam Tests

• New schedule of beam tests is reported. Dedicated PREX tests will probably cover January 25-30.
• Kent reports that, given the pass energy and polarization requirements for Hall C, we will likely run with a Wien configuration providing near longitudinal polarization at 3 pass, and significantly transverse at 1 pass.
• Kent reports that the beam modulation magnets used for dithering have been removed from the beam line, along with some of the their cabling. The system will be revived for the January tests.

• PMT linearity studies

• Reviewed recent results from Luis ( elog:1215 and elog:1217). Recent improvements in measurement technique, most significantly a larger asymmetry (~30%) to make slow drifts less important and a non-ordered sequence for advancing through the filters. New plots showing the maximum fractional range of the asymmetry over the filter settings (instead of a poorly-fit slope) are now used for presentation.
• A clear minimum of the asymmetry is seen at approximately the same HV, for (maximum) light levels corresponding to cathode currents of 5nA, 10 nA, and 20 nA.
• The measurement improvements appear to have improved data quality, so that slopes are typically well-fit on a given scan over the ND filters. Luis reports that the minimum range correponds to a sign-flip of the slope of the asymmetry vs. light level, that is, a sign change of the expected "beta" non-linearity term.
• The following improvements are suggested:
  • Repeat scans at 1nA for comparison
  • Since slopes now appear to be reliable, supplement plots of range with plots of slope (as a function of HV).
  • Show the scans (asymmetry vs. filter setting) for the points settled in at 1.5% range on the 20nA light level
  • Tweak the measurement technique to visit each filter multiple times, to further cancel/randomize effects of slow drifts. The runs can remain approximately the same length.