[Sbs_gems] [EXTERNAL] GEM Pedestal Comparisons

[Sean Jeffas]

Hi All,

There was some discussion last week about using the GEM pedestals to
determine if any APVs were partially disconnected. Thir and I have
performed a test on a single backplane in EEL.

Run 2320 - Backplane completely unplugged from readout
Run 2321 - All APVs plugged into readout except APV 0
Run 2322 - Half of APV 0 unplugged from the readout
Run 2323 - All APVs plugged in properly
Run 2324 - Half of APV 0 unplugged again to test repeatability.
Run 13637 - Pedestal data from the BigBite GEMs.

In the data attached below, on the second page, each histogram is a single
APV, and the y-axis is the channel RMS, while the x-axis is the strip
number in the APV chip order.

Just like Kondo said, when the APVs go from unplugged to plugged, the
pedestal RMS changes from ~4 ADC to ~8 ADC. In run 2322 we clearly can tell
from the data that APV 0 is unplugged. Interestingly in the two runs, where
I half plugged in APV 0 we see that the channels become noisy. Again, note
that this plot is in APV chip channel order, so we expect strips
physically next to each other to be spread out like this. However it is
notable that we see this increased noise, instead of the disconnected
strips dropping down to  ~4 ADC like we expected.

Therefore if any APVs are partially disconnected I think it would not be
easy to tell from looking at pedestal plots. The only sign is some
increased noise, but even with that some strips are noisier even if
everything is plugged in fine (see APVs 2 and 4). For example, in the BB
pedestal data, I know for a fact that half of the strips in MPD 18 APV 13
are physically ripped off of the readout board. But looking at the pedestal
data this APV seems completely fine.

By looking through run 13637, there are no obvious APVs with low RMS values
that show disconnected strips. Additionally strips that are missing on
hitmaps, do not look any different than strips that are not missing.
Overall my conclusion is that the BB pedestal plots do not give further
insight into APVs missing strips, and I will have to use the hit maps to
determine which APVs to replace.

Best,
Sean
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