Summary 1/23/18

PARTONS AND TIMELIKE COMPTON SCATTERING

• PARTONS is a C++ software framework dedicated to the phenomenology of Generalized Parton Distributions (GPDs). It provides a necessary bridge between models of GPDs and experimental data measured in various exclusive channels, like Deeply Virtual Compton Scattering (DVCS) and Deeply Virtual Meson Production (DVMP).

• Further details and instructions on using the framework can be found here

• For Timelike Compton Scattering (TCS): a code exists at NLO, but has yet to be implemented.

* Next steps:

• Dialogue between experiment and PARTONS regarding implementation of TCS. There is currently no projected timeframe, but it would benefit from experimental efforts
• Dialogue with experiment to develop support for experiment rate projections and impact of measurement on GPD extractions

TIMELIKE COMPTON SCATTERING EXPERIMENT

• Theoretical motivation includes access to the GPD E, in particular the imaginary part that allows for accessing orbital angular momentum

• New development since 2017 collaboration meeting is the addition of the Compact Photon Source - increased figure of merit

• Based on very preliminary studies, the experiment can make an impact on global GPD extractions and is competitive with other envisioned experiments with transverse target

* Next steps:

• Prepare a proposal to the PAC46 (2018)
• Need to make real rate projections
• Refine physics case
• Check impact of radiation due to e-/e+ production when beam interacts with a 1% radiator (target)
• Correlated with the previous point, check if the envisioned X-Y tracker will be operational in the projected radiation environment. During SANE operations, the tracker suffered from secondaries and was not functional as originally intended

PMT AND HV DIVIDER

• PMTs (R4125 Hamamatsu) have borosilicate windows - should be ok even in high radiation environment as tube is attached to crystal and have many photons (G0 detected SPE and had trouble with borosilicate)

• Active HV divider is a good option - regulation on last two dynodes provides stability and linearity at high rates, large dynamic range

• To confirm performance, tests were done over the last year including performance tests with long cables and LED chain tests in Hall D in collaboration with COMCAL/FCAL projects - results confirm performance

• 320 PMTs were procured in 2017. Note that the first ~100 sockets delivered by Hamamatsu were of the wrong size - required removal of a metal ring, this has been done. Subsequent sockets delivered were of the correct size

• PCB assemblies are done through an outside company, casing material (CL2 and fire rating) may not suitable for cable trays, but not an issue here

• Discussion about production status

• HV divider and PCB are in production - COMCAL is in final assembly
• First articles for NPS procured through OU and on the way to JLab. Tests will include cleanliness of the board, assembly, etc. A report will be procided

• Procedures for production tests are being finalized. A special optical darkbox was constructed for tests in Hall D. A similar one could be constructed for NPS. Different options exist for testing all PMT assemblies or selected ones compared to a reference PMT. A set of tests includes:

• HiPot tests planned for all dividers at -1.1 kV (no arcs, trips, supply current < 430uA) - the voltage margin is to be determined
• Characterization of PMTs on a single HV divider against reference (gain, rise time, linearity)
• Comprehensive characterization of all HV dividers individually (QE pulse height, stability)

* Next steps:

• Decide on connectors
• Finalize patch panels
• Production of HV dividers
• Determine voltage margin
• Study temperature stability of the gain
• Find out when Hall D needs the crystals

NPS FRAME

• Discussion about mechanical design and plans for instrumentation tests, construction, and installation at JLab

• Calorimeter frame needs to be integrated into mechanical structure. NOTE: Mike assumed 30 crystals (horizontally) and 36 crystals (vertically)

• Crystal wrapping - teflon (diffusive) vs. VM2000 (specular reflector). Need to determine cost and weight against performance requirements.

• Mechanical structure - carbon structure makes maintenance easier and allows for re-stacking crystals, but introduces material around crystal - to determine the impact on resolution, a simulation is being carried out

• Shielding - goal is to shield PMTs from potential stray fields of the NPS magnet. Assume that mu metal is used around PMTs for now. Can supplement with housing around entire detector. Earlier studies showed that 3mm steel can help mitigate 25 Gauss stray field from NPS magnet

• Cooling - discussion about a water cooling option with copper tubes used in magnet construction, a supplementary option might be to remove heat with fans inside the frame

• Cabling/electronics - discussion about mechanical options to rotate cables, a possible way could be the method used during RCS, photos and samples are available

• Placement of monitoring and curing system

• monitoring will always be on the back
• if the carbon structure is verified not to negatively impact resolution, then curing could also be installed on the back
• if curing not done frequently, then a non-permanent curing from the front is an option

• Timeline: anticipate shipping to JLab in summer 2019

* Next steps: finalize design including, e.g.,

• Decide on carbon structure - feasibility based on simulation
• Determine placement of monitoring and curing systems based on simulation
• Decide on crystal wrapping material
• Investigate cooling option

ADDITIONAL POINTS FROM DISCUSSION ON 1/22/18 WITH MIKE FOWLER:

• NPS frame installation:

• when NPS is installed, the horizontal bend magnet will be removed completely
• at small angle (6 deg), there is ~1/5" clearance between the structure and the existing beam pipe
• tolerance in z positioning is on the order of 1cm, e.g. 3m +- 1cm
• adjustments in angle are done through the SHMS rotation mechanism, the NPS sits on a cantelevered platform on the SHMS structure
• adjustments in horizontal and vertical directions are done on the slider structure, vertical through jacks installed at the front

• NPS frame component details:

• cooling treated as standalone - IPN will take care of this, need input from JLab on fittings, spigots and connectors, note: LCW operating pressure is 250 psi, cooling design should allow for sensors in the box to readout temperature, action item: IPN will make a design and then can iterate on details
• cabling (signal, HV, LED, ..): idea is to send cables through top from back following RCS design, IPN will take care of cables to point where they come out of the box (to patch panel on top) and connectors to use, JLab will do cabling from detector to patch panel at front of SHMS and from there to CH etc., cable motion in hall has to be addressed (use a rotation arm?)
• discussion about using a carbon structure to facilitate crystal installation, and in particular replacement. Drawback is that it adds 1-2mm space between crystals, which impacts resolution and so the physics. Simulations are performed to quantify the impact on the physics. Also need to consider impact on thermal properties - does structure impact temperature gradient in the box and can that be resolved from back?
• Curing system placement: if carbon structure is used, placement on back seems best, wait for simulation result
• Crystal wrapping material: Teflon is a diffusive reflector, cheap, good performance, but is not radiation hard (bad at 150 kGy, not recommended above 1000 kGy), VM2000 is a specular reflector (CUA studies show ESR loses light compared to teflon), but is more radiation hard. VM2000 is more expensive than teflon - need a cost estimate. Gore reflector is diffusive, best reflective properties of all materials, based on CUA studies, was used for SHMS aerogel detector, might be too expensive.

CRYSTALS

• 460 crystals have been procured from SICCAS - 100 have been characterized to date. Anticipate 450 crystals from CRYTUR over the next year

• Dimension measurements: accuracy needed is about 50um, the measuring apparatus can give down to 1um. Full spread measured is about 100um

• Discussion about the crystal performance requirements for Hall D

* Next steps:

• characterize 500 crystals
• send 100 crystals to Hall D - need performance requirements for selection

MAGNET

• Discussion about component status: corrector coil is at JLab, main coil in transit, yoke steel pieces anticipated to arrive by 9 Feb

• Magnet assembly and mapping space is available in test lab

• Discussion about mapping plans:

• central bore: verify Bdl - can be done at reduced current
• beam line: verify Bdl=0 - requires full current

• Magnet mapping tools are available

* Next steps:

• assemble magnet
• map magnet at reduced current