Qweak Polarimetry Conference Call 10/7/2003

October 7, 2003
notes taken by Richard Jones

Participant list: D. Gaskell, R. Jones, S. Kowalski

• D. Gaskell: Report from Jlab
  David has put together a work package for the Hall C Compton chicane design and related beam line modifications, and shown it to the appropriate chiefs in the Jlab accelerator division. The tentative answer seems to be that they will take on the job, meaning that they will take charge of the design of the magnets for the chicane and the mounting structure, reconfigure the existing beam line instrumentation to make room for the chicane, add any additional instrumentation that is required for independent control of the beam at the chicane midpoint and the target, and take care of installation and commissioning of the new beam line. A time scale was suggested in which the design is finished by summer 2004, the magnets contract is signed in autumn 2004, and the installation can begin in winter 2005. This schedule would allow work on the laser and detector systems to begin in 2005, and allow two years for the system to be shaken down before the installation of the Qweak target and detector.
• S. Kowalski: Report from MIT
  The Mainz Compton is now commissioning. The laser has been repaired after its failure earlier this year, and backscatter events have been seen in the detectors. For details about the laser power achieved so far in intracavity operation we should contact Simon. The laser is a c.w. argon-ion design (green). As soon as the system is working reliably, some of us should make a trip to Mainz and see the Compton setup.
• R. Jones: Report from Connecticut
  Since the June workshop, the Compton simulation program has been overhauled to have a more accurate description of the chicane geometry. Now a realistic transverse and longitudinal beam profile is injected into the chicane, and effects from scraping on the lattice elements are taken into account. With help from Rolf Ent, realistic tails on the electron beam are now simulated. The result is that scraping is found to be the dominant source of beam-related background in the detectors, not beam-gas bremsstrahlung that was an earlier suspect. This agrees with experience in Hall A where the Compton performance depended strongly on the quality of the beam and how much work was done in optimizing the orbit through the chicane. A summary of these results will be shown at the MIT meeting.
• Discussion: progress towards a choice for the laser
  The Mainz Compton is currently the closest design in terms of energy and wavelength to what we need for Qweak that we have seen. Results from the commissioning of that device will be available on a time scale that will permit a Qweak decision after we have seen them. Ways that we may improve on this probably involve going to a pulsed laser design. We still want to keep the ability to do coincidences between the electron and photon arms during calibration and setup. This means that the pulse rate should be of order KHz so that appreciable rates can be obtained at reduced intensity in coincidence mode. This may not be necessary if we want to to use the Compton only as a relative measurement, but it should not be given up at this point. David has followed up a lead on a high-power pulsed green laser from Talis Corporation that promised 100W of average power. His visit did not convince him that the company wanted to adapt their product for this application, and since the date of their visit the laser that he discussed with them seems to have disappeared from their product line! Richard has spoken recently with representatives of Lambda Physik, a major producer of lasers for industry and research. They have examined the requirements for an ultraviolet Compton laser, and are working on a design that would adapt one of their 248nm excimer lasers for this purpose. More meetings with Lambda Physik engineers are planned.
• Discussion: progress towards detector choices
  The Mainz design uses a sodium iodide counter to detect the backscattered gammas. The recoil electron counter is a scintillating fiber array. This is thought to be a good choice for Hall C. The option of PbWO crystals was also raised. It would be a more compact design, but the low-energy response may be inferior to NaI.
• Discussion: status report at collaboration meeting
  The report will center on a summary of the workshop and an update on the work described above. David has forwarded a copy of his presentation on the workshop to the Hall C collaboration meeting in September to Richard. He will take a digital photo of the Hall A chicane and send it by email to be included in the report.
• Next meeting: November ?