General Meeting Summary 5/7/20

PARTICIPANTS: 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

MAGNET STUDIES (Charles)

• Ongoing

MECHANICAL DESIGN (Paulo, Steven)

• 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)

CRYSTALS

• 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

NEXT MEETING: THUR 28 MAY AT 9:00AM (ET)