Notes from Phone Meeting November 2, 2007

• Compton Polarimeter

• Gregg announced that the schedule for delivery of the new GSO crystal is now mid-December. It may be possible to use the GSO during beam tests. Paul points out that we should be cautious to not rush the GSO into the beam, if we could be putting this expensive crystal at risk.
• Kent reports that the Hall A photodetector prototype has been instrumented with a positive-HV PMT. As we've learned in the past, one cannot integrate a positive HV PMT, so this is useless for our tests.
• Gregg has been trying to acquire a piece of PRIMEX PbWO₄. One has been located, but it is already instruemented with an as-yet unidentified PMT with is likely not compatible with our need for high gain. Dustin will follow up with Ashot to try to find an uninstrumented crystal for the January tests from the PRIMEX stash.
• The CMU bench tests of integration have, so far, been stymied by problems in setting up their DAQ. Bob will help them get going through phone and remote work. Separately, Bob will send an FADC with accumulator to CMU, as a provisional loan while they wait for their own modified FADCs to arrive.
• Kent suggests that we plan a test of the Compton DAQ to try to shorten the 5 msec dead period by more than an order of magnitude. Kent will consult with David L. for his opinion. Bob was coerced into agreeing to try to find time for this test.
• Tharanga reports that they have improved the conversion efficiency and greatly improved the stability of the PPLN doubling system. However, instabilities still arise at the highest power outputs, presently about 30mW from 500 mW IR. The primary technical challenge is to improve the thermal stability. They have developed a thermal control system based on thermoelectric couplers (TECs). They are planning further improvements for this system.
• Thraranga also reports that work on the cavity development is again focusing on the home-built locking electronics, which may have some failure.
• The photo-detector collimator aperture is discussed. Gregg posted haplog:1213 with a pointer to Diana's photos of the Compton photodetector housing and this odd "U" shaped collimation. The opening in the semi-circular plate is approximately 1cm radius, which matches the size of the existing photodetector crystals. This is not an issue for the January tests, but the effective energy cut off due to angular acceptance will ultimately need to be considered in normalizing the integrating measurement.

• PMT linearity studies

• Reviewed recent results from Luis's studies. Continued problems were seen in non-statistical noise over changing the filter settings. Effects are potentially large enough to confuse results, even on the level of a few percent. A recent study showed the the observed PMT signal varied at the 3% level over repeated settings at the "open" (100% transmission) filter setting.
• A small aperture was installed to remove any geometric effects of small deviations in the filter wheel position. This removed the intensity deviations over multiple settings to the 100% filter, and initial indications are that the noise is reduced on the asymmetry as well.

• Lumi Monitor

• Brian is consulting the machinists at the W&M shop to complete the filter box design. Kent will discuss the system with him at W&M on Tuesday.
• Brian shows a quote for new light guide material, which is reasonably inexpensive. A question was raised as to whether this is the material that was actually used, since the present lightguides may be thicker than 50 mils. KK remembers suggesting a different vendor for a similar material. Possibly we should check with Riad on what was actually used, and/or call Piotr for info on similar material. If we have a quote for the correct material, then it is rather inexpensive and we should build new lightguides.
• Discussion of the metallic filters. It is unclear whether such filters can be stacked to vary the attentuation. Kent will look into the question.
• Kent will also look into having a few glass, absorptive ND filters available at run time as a back up to the metallic filters.

• Other issues

• It was pointed out that the new meeting time (11am Friday) interferes with the JLab physics seminar. Kent will organize a new meeting time.
• Kent reports that new beam modulation system magnets have been returned to the beamline. The system is now waiting on trim cards for completion. The system checkout has been scheduled for November 20.
• A few ideas have been developed for 10x/12x analysis. Dustin will look into those.
• The injector DAQ will need to be fired up soon.
• Kent will solicit individual contributions to the beam test run plan over the next couple of weeks.
• We discuss dates for the next collaboration meeting at JLab. Kent will coordinate preferences on the date. The following dates are on the table: Nov 16, Nov 30, and Dec 7.