[Rgc] Today's RG C meeting

[kuhn]

Dear all,

Here are some minutes from today’s meeting:

First a request: If you work on any aspect of the longitudinally polarized target but don’t receive emails from the poltarg12 at jlab.org list, PLEASE SIGN UP TO THAT LIST. Vice versa, if you ARE working on that target and are involved in any way with Run Group C (including the upcoming ERR), please sign up for the rgc at jlab.org list if you haven’t already done so. See https://clasweb.jlab.org/wiki/index.php/Run_Group_C for further info.

Today’s agenda can be found at https://clasweb.jlab.org/wiki/index.php/RGC-March21

We spent considerable time on the question of modeling the electromagnetic background in CLAS12 for various configurations of the Moller shield, FMTs and targets. 
Tony’s most recent results are here:
https://clasweb.jlab.org/wiki/index.php/TF_FTPT_FTconf-GEMC4.3.0 
For the standard LH2 target and the FToff configuration (3rd set of plots) he finds DC occupancies of about 1% in R1. Note the somewhat strange Tomography Plot that shows the outline of the Moller cone - it appears that a lot of background comes from the OUTSIDE surface of the shield! The next row is for a polarized target, using 124000 beam electrons in a 248.5 ns window (this corresponds roughly to 8 nA which is slightly less than the 10-12 nA for the proposed luminosity). Here the occupancy in R1 is 2.3% (before any raster). Note that there are NO FMTs in this set up; also, there is no vacuum beam line between the target and the entrance to the Moller shield. Tony also tried out several modified Moller cones, but without a conclusive result yet.
Angela’s results are at 
https://clasweb.jlab.org/wiki/index.php/Studies_of_the_beam_position,_FT,_Micromega
She finds only 0.8% occupancy in R1 for FToff and no raster, but with FMTs in place - much lower than Tony’s results. She also compared her simulation of LH2 and LD2 targets with actual measurements from RGA, K and B:
https://clasweb.jlab.org/wiki/index.php/Moller_shield_studies
For 11 GeV, 50 nA on LH2 (only 2/3 of standard luminosity) and FToff she finds 0.65% occupancy in R1 (0.8% for LD2). This is actually not inconsistent with Tony’s results. For a direct comparison with data, we only have a 6 GeV run with FToff. The predicted occupancy in that case is 0.45% at 35 nA (1/2 luminosity) while the data show about 2.2% in R1 (actually ranging from below 2 to nearly 4% at forward angles) - a factor 5 higher. All other comparisons are for FTon, which show similarly large discrepancies. 
Tony and Angela will try to compare notes and understand their different results. Our goal is to have realistic predictions of DC occupancy for both the standard FToff configuration (where we can cross-check with RGA and RGB) and different shield configurations as well as standard FTon configuration (cross check with RGB). We also need to decide whether to build a “downstream snout” on the target so the beam traverses vacuum instead of air on the way to the shield. The final step will be to compare with Stepan’s studies of tracking inefficiency vs. occupancy (or beam current as proxy) and to decide what we “can live with”. This will dictate the optimum combination of raster radius and luminosity for both FTon and FToff/new Moller shield.

The other item of discussion are our plans for the ERR. Again, all interested people but especially spokespersons are urged to take a look at
https://clasweb.jlab.org/wiki/index.php/RGC_ERR
and contribute to any items they can. Please look at the Charge, the proposed Agenda and my first draft of a strawman run plan and send your comments. Unless there is a good reason to change that run plan substantially, all simulations and estimates should be based on the listed number of days. For the double cell target, we should assume a raster radius of 1 cm and a luminosity of about 10^35 on ammonia alone (10 nA), while for the “skinny target” and FTon, we probably need to restrict ourselves to a raster radius of 7 mm and a correspondingly reduced beam current (5 nA) which we can partially compensate for with a longer target.

Our next longitudinally polarized target meeting is in 2 weeks - we will concentrate on all target-related aspects of the preparations for the ERR. At the next RGC meeting (in 4 weeks), we should be prepared to provide answers to most if not all questions in the Charge or to identify a path towards such answers.

Best greetings - Sebastian