[Sbs_gems] [EXTERNAL] GEM Negative Pulse Overview

[Sean Jeffas]

Hi All,

I am mostly sending this out because Kondo, Andrew, and Bogdan were not in
the GEM meeting today and I overviewed the issue of the negative GEM
signals seen in the hall. I have attached my slides below.


https://hallaweb.jlab.org/wiki/images/f/fd/Neg_signal_1_26_22.pdf


At the meeting Ben suggested one interesting reason for why the CM would
decrease at higher currents, but the fraction of negative pulses would also
decrease. He suggested that this may happen if the positive signal
"overrides" a negative signal on the same strip. So at higher currents,
with higher occupancy, there are less strips with negative signals (because
they are already positive), decreasing the fraction, but the negative
signals are still used in the CM calculation, causing the decrease in the
CM. (Ben, please correct me if I have not described your idea properly).

Overall we need more data and analysis to understand this problem. We
should decide on a set of quick runs to study the negative pulses more.
This will also be simple because Kondo is RC this week. Kondo has already
suggested a test with HV ~1500V and beam on with online zero suppression
enabled. I think we should also take some data at different beam currents
with nominal HV values and the online zero suppression disabled. This can
be accomplished with 30 minutes of beam time. Please suggest any other
ideas for tests with beam.


For analysis, there are not many options in the analyzer right now to study
the negative signals. We only have a few predefined histograms saved for
full readout events, but most of the data is discarded because it is not of
interest for the tracking. We will need to add some analysis to store all
the strip information to study this properly. Then we can fully see what
type of events the negative signals affect on average. We can check if they
show up in patterns of strips on all the APVs, their occupancy, and how
they correlate to real signals. Please suggest any other analysis ideas for
distributions to check.

Best,
Sean
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