[Halld-cal] FCAL instability and LED?

[Shepherd, Matthew]

Hi Colin,

(cc'ed to Mark and the calorimetry group)

We were discussing instability of pi0 peak that was noticed in the most recent monitoring launch over the 2018 data.

To be concrete, this can be observed by going here:

https://halldweb.jlab.org/data_monitoring/Plot_Browser.html

Selecting:  RunPeriod-2018-01
Version:  Monitoring Launch ver17
Plot:  FCAL Clusters 2
Run Range:  42221 to 42241

In particular, runs 42234 and 42236 seem to show a pi0 peak that is significantly high.

If I roll back to the incoming data plots then I see there are other runs (presumably not "production") that were also taken around that time and show similar behavior.

When I look a little deeper, it seems that the peak shift can be attributed to a pedestal shift.  You can chase this by clicking on the run number to go to the run browser, then clicking on the ROOT file and drilling down the fcal directory to look at the pedestal plots.

You can see the pedestal shifts high from its nominal value of 100 cts. per sample (400 over 4 samples).

I wonder if this is seen in other detectors?  Maybe there is something bad about these runs.

This happened all on/around April 19.  It is interesting that just before the problem run Mark Dalton made a log entry that the LED system was restarted (#3563543).  And Colin was on shift and noted high occupancy in the FCAL, which would happen if the pedestal shifts up.

The problem cures itself by run 42237.  Colin notes shutting off the LED pulser later that night.

Perhaps the rate for the LED's was momentarily too high and this resulted in some LED pileup in this particular run?

Is there any other light you might be able to shed on this? Maybe these runs can be saved if we use an event-by-event pedestal?

Matt

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Matthew Shepherd, Professor
Department of Physics, Indiana University, Swain West 265
727 East Third Street, Bloomington, IN 47405

Office Phone:  +1 812 856 5808

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