[Halld-pid] Start Counter Meeting Minutes, June 25, 2020

[Mark Ito]

Folks,

Please find the minutes here 
<https://halldweb.jlab.org/wiki/index.php/GlueX_Start_Counter_Meeting,_June_25,_2020#Minutes> 
and below.

   -- Mark


    GlueX Start Counter Meeting, June 25, 2020, Minutes

Present: Eugene Chudakov, Rupesh Dotel, Tolga Erbora, Mark Ito (chair), 
Simon Taylor, Beni Zihlmann

There is a recording of this meeting <https://bluejeans.com/s/FNL0_/> on 
the BlueJeans site. Use your JLab credentials to gain access.


      Review of minutes from the last meeting

We went over the minutes from May 28 
<https://halldweb.jlab.org/wiki/index.php/GlueX_Start_Counter_Meeting,_May_28,_2020#Minutes>. 



        Time Resolution Studies

Rupesh showed plots of time resolution for all thirty counters for a run 
late in the last run period, run 71840. Find his plots here 
<https://halldweb.jlab.org/wiki/images/1/19/SCmeeting_June25.pdf> 
(starting with slide 3). Recall that last time he showed evidence for 
worsening of the resolution as a function of time as the run progressed 
when using calibration constants obtained early in the run. This time 
constants were obtained for run 71840 and applied to the same run. For 
this run, results were not much different than those with the 
earlier-in-the-run constants. He also tried a run even later in the run, 
71???, which slowed slightly better resolution. To see if there was 
anything special about run 71840, he looked other runs in the same 
run-number neighborhood. Those were all roughly the same as for 71840.

Eugene asked about the source of the counter-to-counter variation in 
resolution, noting that the pattern repeats for all runs shown. Beni and 
Simon attribute that to the rate variation from counter-to-counter due 
to a slight tilt of the assembly and the concentration of the rate at 
the nose of the counters where small variations in distance from the 
beam make a big difference in rate.


        Gain Calibration

After the last meeting Sean Dobbs reported via email that an initial SC 
gain calibration was done by Simon back in 2015, and that has been what 
we are using so far. Beni also pointed out that the Start Counter NIM 
paper had dE/dx plots both before and after "corrections." Rupesh and 
Beni will look at some recent monitoring plots to see if they give an 
indication about the current adequacy of the calibration.


        Improving Speed of Performing Calibrations

Rupesh consulted Beni after the last meeting and found that he could 
reduce the number of files analyzed for a calibration pass from the full 
run (over 200 raw data files) to 25 and still get reliable results. That 
turns a month into a few days.


      SiPM Replacement?

Beni raised the issue of how we might approach replacement of the SiPMs 
if we conclude, either now or in the future, that radiation damage is 
too severe to continue to use the current units. We need to talk to 
Fernando about whether it would be easier to replace the SiPM on the 
current boards or just make new boards with new SiPMs. We would have to 
research the availability of the current units. Also, we would need a 
plan to do the replacement mechanically including locating any necessary 
support jigs or structures.


      Preparations for the Upcoming Run

Beni proposes that we leave the start counter alone until after the 
Fourth of July. After turn-on, he wants to take dark-current 
measurements (non-sparsified raw mode data) before beam arrives in the 
Hall. He recalled for us that that attempt before the previous run was 
mis-configured, DAQ-wise, and the resulting data unusable.


      Action Item Review

 1. Document calibration procedure (Rupesh)
 2. Reproduce Beni's dark current results. (Tolga)
 3. Check status of the gain calibrations using monitoring histograms.
    (Rupesh, Beni)
 4. Revisit Yi's study of SiPM annealing. (Mark, Beni)
 5. Ask about the current recommendation from SciComp on copying files
    to the local disk from cache before analyzing them. (Mark)
 6. Ask Fernando about strategy for SiPM replacement (Mark, Beni)

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