Optics Meeting May 30 2023 200PM ET

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Agenda

1. low energy gammas in GEMs - update related to SBS
2. answering more questions on sieve design
3. reconstructing matrix elements from sieve hole locations

Attendance

David, Kent, Kate, Vassu, Bill, Tyler, Michael D, Ciprian, Andrew, Juliette.

Minutes

1. (David) Discussion with Eric Fuchey regarding g4sbs simulation for SBS. The latest results (shown by Andrew Puckett at ERR for GEP experiment, April 24 2023), slides 20 and 49, shows that g4sbs reproduces observed BigBite GEM rate/occupancy at “low” beam current (3 uA) to within about 20%. I have confirmed with Eric that their GEANT 4 simulation (g4sbs) does not require special Livermore libraries to get the GEM singles rates correct. He has shared the list of libraries used with me. So, we should pursue using modified version of remoll, but with standard libraries, to estimate singles rates from photons. SBS does the following: in g4sbs they make the Ar/CO2 gas of the (3 mm thick) drift region of each GEM into an active detector (I'll check their GDML implementation). GEANT 4 records the energy deposition by any incident particle that hits that region, and stores these as "hits". They then use a post-processor code (libsbsdig libraries) to add together the deposited energy from each track, and creates avalanches from each cluster of deposited energy, projects the created charge onto the x/y or u/v strips, and converts that this to a charge vs. time pulse for each strip. See slide 29 et seq. from Eric's talk here:[1]. We need to decide if we wish to do something similar for our simulations.
2. (David) Request that Bill agrees to take on: implement our drift volumes GEMs as active detectors, check that the deposited energy is stored in the remoll tree. We can then use post-simulation root scripts to look at the rate of hits that come from photons which make deposited energy near or above that of minimum ionizing (i.e. Moller electrons) tracks. This should answer our desire to estimate the singles background rate from photons.
3. (Vassu) Looked at which holes allow the two 12C foils to be cleanly separated using r,r'. There are a fair number that cannot be clearly separated; and we have to remember that we have to worry about GEM resolution - the r resolution may be as poor as one mm sigma, while the phi resolution expected to be much better (100 micron scale). With a roughly 1.2 m separation between first and last GEM modules a 1 mm r resolution translates to a 0.8 mrad r' resolution. Should decide if we are properly able to separate the foils for the most important holes (the ones in the Moller acceptance for the three beam energies). We may have to back away from the idea of simultaneously measuring with two foils, but instead have three optics targets one at the target center, one at upstream end, one t downstream end of the LH2 target.
4. (all) Discussion of advantages of smaller holes and/or tilting of holes. We are likely to be able to do better on foil separation with smaller diameter holes. Engineering tells us that the minimum diameter for drilled holes with the present sieve thickness (10 cm) is 5 mm diameter. If we go to smaller holes than the theta,phi angle for a given hole is better determined, of course (may be important for extracting optics from the hole centers rather than from the Monte Carlo truth values), however with smaller diameters might need to revisit the hole tilting question. Engineering has told us that if we desire titled holes, will likely have about a 1 mrad uncertainty on the tilt angle. Should try simulation of 12C elastics with 5 mm diameter holes everywhere with and without tilts and try to quantify effect on "smearing" out of holes images on GEM plane. Not sure what best criterion to use might be. Smaller holes might allow us to sneak in a few more holes, maybe at smaller radius.
5. (Juliette) Is working on making magnetic fields for simulation in which we can turn off one or more coil set - will load separate fields for each coil set (segmented by z), which one can turn on or off in remoll. Also looking into whether the power supplies can be run outside of the +- 10% of nominal current that we are assured will be possible.
6. (Tyler) Now has Vassu's fitting code running (matter of getting correct Python and Root versions). Ready to start trying to do the machine learning fit using the hole centers rather than the GEANT 4 "truth values" for track theta and phi.

Meeting link information

See email invitation, or contact David Armstrong, Kate Evans or Jennifer Finch for Zoom link