BLTWG Meeting 6/26/2008

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  • Time: 10:00 EST
  • Place: EVO and ESNET (with telephone bridge)
  • Connecting: instructions are here
  • Present: Hrachya, Richard, Elke, Sascha, Jim, Tim, Yang, and Elke


  1. Status on the collimator cave background studies - Alex
  2. Status report on tagger microscope prototyping - R. Jones
  3. Discussion of pair spectrometer design document - H. Hakopian
  4. Discussion: strategy for beam tests in Hall B - R. Jones
  5. Agreement for diamond R&D - R. Jones, E. Aschenauer
  6. Magnet design discussion - Yang

Notes by R. Jones

  1. See document by Alex (linked from agenda). Alex simulated the photon beam in the pair spectrometer region, focusing on backgrounds that get out of the collimator cave and emerge from the back of the pair spectrometer. Photon beam halo originates mainly in the primary collimator and has a divergence angle of 100 microradians. This component is mostly soft (less than 1 GeV) and has a total rate 10-3 of that of the primary beam. A lead donut followed by a stack of SEG shielding has been added to the beam line design downstream of the PS vacuum box to stop the background created by the pair converter and interactions in the pair spectrometer walls. The design is based on the BNL 3-m magnet with a 1.64T field. Further simulations that combine backgrounds from inside the collimator cave with the ones associated with the PS complex are underway. Pavel is checking backgrounds in the hall with the new beam line configuration. Preliminary results indicate that the photon beam dump may need more shielding against neutrons.
  2. See document by Richard (linked from agenda). This milestone report updates previous status reports on the ongoing work on the design and construction of a prototype detector for the tagger microscope. The design of the detector-mounted electronics for the SiPM readout of the fibers is advancing to the stage where prototype boards will soon be ordered.
  3. Hrachya presented his revised pair spectrometer design document (linked above under agenda). This design is based on the field and geometry of the BNL 3-m magnet mentioned above that is under consideration for Hall D. His PS focal plane design measures pairs with summed energy from 6.3 to 12.4 GeV at the full field of 1.64 Telsa. Lowering the field gives access to the low end of the spectrum, albeit with somewhat worse fractional energy resolution because of the increased effects of spot size on the energy resolution. Hrachya requests that working group members read through the document and provide him feedback by July 2.
  4. deferred to next meeting
  5. Richard will speak privately with Elke on Friday, time TBD.
  6. Yang presented results of his study of possible magnetic field geometry and focal plane options for a single-magnet design. See the talk linked from the agenda for details. By rotating the magnet to 6.5 degrees from the incident electron direction, he was able to obtain a relatively uniform intrinsic resolution better than 0.05% of E0 across the focal plane from electron energy of 0.5 GeV to 10 GeV, using the same linear focal plane as was adopted for the two-magnet design. Several other scenarios were studied, but there was general agreement that this one is the most interesting. Yang will write up the results for this option in a brief note. The next step will be to enter the geometry for the one-magnet tagger into TOSCA and study the propagation of ray bundles through the detailed 3D magnetic field. He will start working on this immediately. A special tagger working group meeting will take place on evo July 3 at 10:00am devoted to hearing results from that study.