General Meeting Summary 5/7/20

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Bogdan Wojtsekhowski, Arshak Asaturyan, Hamlet Mkrtchyan, Tanja Horn, Charles Hyde, Vardan Tadevosyan, Alex Camsonne, Paulo Medeiros, Carlos Munoz-Camacho, Ho-San Ko, Rolf Ent, Marie Boer, Steven Lassiter, A Comer, Julie Roche, Vladimir Berdnikov


  • Ongoing


  • Received and checked the DVCS kinematics moved from Hall A (48-J1, 60-J1, 60-J2, 60-J3)
  • Tight fit Q1 and NPS, but no major interferences
  • Action item: post drawings with dimensions
  • Progress with another milestone: Platform and legs drawings
  • structural drawings almost ready, for engineering signature and procurement
  • Corrector magnet - layout ready, but it's 12 months old, need to come back to
  • Position for corrector looks ok
  • Iron shield abandoned in the meantime, need to get together with Bogdan to work out the details
  • Action items:
  • calculate field integral, corrector, shelf positions
  • Is there any existing corrector that can be used?
  • Beamline - back to usual - no need to change it, but need corrector and supports

FRAME (Carlos)

  • With latest projection when work might resume frame delivery might be October/November (~3-4 months delay)


  • Crytur crystals sent in March have been inspected
  • Crytur resumed sending crystals - 52 more arrived at JLab
  • Crytur agreed to the proposed schedule to expedite the 250 crystals already purchased and to make 200 more by Sept 2021

HV DIVIDER - ANODE CURRENT (Carlos, Ho-San, Bogdan, Hamlet, all)

  • High anode current at some kinematics - see Ho-San's presentation from 2020 NPS Collaboration meeting
  • The backgrounds drop a factor of 2-3 in the first few layers, a factor of 5-10 in the last ones (furthest from the beam). There is still a significant problem in all crystals, not only the first ones
  • Possible ways to reduce anode current: reduced PMT gain and external divider, e.g. with x25 like in APEX, and filtering (shielding)
  • Results from Hall A DVCS - anode currents (Carlos)
  • similar PMTs, coupled capacitor to amplifier, no constant level to electronics, was filtered
  • Experiment required angles down to 4 degrees with respect to beamline, 10 uA, 1.5m
  • Anode current up to 20 microAmp
  • Comparison to NPS:
  • NPS has settings 4 deg at 4m, but up to 50microAmp
  • Hall A crystals 3x3cm^3, NPS are 2x2cm^2 (NPS smaller, less integrated noise)
  • Light yield different - PbWO4 is 10-100 factor brighter
  • Hall A PMTs operated at low gain (10^4-10^5)
  • NPS has a sweeping magnet for charged particles
  • Conclusion: If take into account factor of 100, then results seem consistent with what was calculated for NPS, consistent with simulation estimates -- have an issue to deal with NPS anode current
  • Simulations on filtering: shielding to reduce anode current (Ho San)
  • Tried 3cm tungsten and 18cm poly shielding
  • reduces a factor of 8 background dose
  • still have 8mA anode current at high Q2 setting
  • energy resolution decreases to 20% (DVCS/SIDIS need ~2%)
  • position resolution decreases
  • Conclusion: Directly shielding the calorimeter does not seem to work - energies too high

  • Action item: consider options below and/or other options to reduce anode current
  • Switch off 76 small angle blocks (6 block radius). Modest loss of acceptance.
  • Reduction of luminosity for 7.9-8.0 deg. runs with 25 uA instead of 50 uA.
  • Increase of the distance from the target to the calorimeter. MC analysis is required.
  • Different crystals in columns closest to beam
  • Dedicated filter in front of the PMT in several blocks
  • Shielding the beampipe to avoid backgrounds from secondary particles