[Halld-cal] FCAL instability and LED?

[Colin Gleason]

Hi Matt,

Good investigative work. To shed some light on LED pulser: it was set to
100x its normal running value (1kHz compared to 10Hz). I don't recall any
odd behavior of the bases, but I have seen at times that changing the rate
of the LED pulser can cause the HV to fluctuate in the base. When I was
trying to figure out what was going on with the large occupancies, I would
manually turn the pulser on/off and adjust the rate. I finally realized
that it was the script causing this to happen when the pulser kept changing
on me. This may be another source of instability. I was then finally able
to kill the cron job that controls the pulser. Also, if I recall correctly,
there was a lot of accelerator downtime during this period. I could imagine
there could be some LED pile up for these runs. For what its worth, the
other monitoring plots for these two runs look like what we see

If it is not possible to use an event-by-event pedestal correction, I may
be able to do a gain calibration or nonlinear correction for these runs
only. This would "correct" the pi0 mass, but would not fix the underlying
issue.

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


The p pi0 plots tell us something about this. We see a decrease in
omegas/trigger in these runs. However, the BCAL pi0 mass looks good, but
the 2 gamma in FCAL and 1 in BCAL and 1 in FCAL do not. It looks like the
BCAL is ok and the problem is coming from the FCAL. For what its worth, we
do see a decrease in the number of rho's/trigger for these two runs (see p
2pi plots). This may be related to the FCAL clogging up the trigger.

Mark is currently out of town so he may not respond soon.

-Colin


On Tue, Sep 11, 2018 at 5:37 PM Shepherd, Matthew <mashephe at indiana.edu>
wrote:

>
> 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
>
> ---------------------------------------------------------------------
> 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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