[Sbs_gems] [EXTERNAL] On APV25 polarity flip

[Gnanvo, Kondo (kg6cq)]

Dear all,
I thought I would share again the slides I showed last October when we noticed the “negative pulses” issues for the first time early during the GMn run.

The slides are about some tests I was performing in 2017 in Hall D with a GEM prototype used as transition radiation detector and this is important because in that case the prototype has 21 mm drift so total primary ionization is 7 times higher than our standard GEM tracker and was causing the polarity flip.
Here I am referring to polarity flip because the data is clean enough that we could actually confirm that APV25 flip polarization without too much speculation and I am going to try to explain the reason why.


  1.  First, these were data taken with APV25-SRS electronics (not MPD) and for that, as a convention, the raw APV25 signal points downside (just a software decoding convention) as you can see on slide 1. Also, because it is GEM-TRD which operates in TPC mode, we needed to operate in the full APV25 data width mode (27 time samples) to be able to catch (in time and space) different ionization clusters and TRD photon signal from the same electron hit traversing the detector



  1.  Slide 2 shows one event with clearly polarity flip in y-strips.
     *   The two top frames are x-strips raw data frames and on the right, you see actually signal from at least 2 ionization clusters (waveforms clearly separated in time)
     *   The bottom 2 frames are the corresponding signal in y-plane where the signal flipped early  after 3 time sample and remained in the flip mode for almost all the 24 remaining time sample
     *   The system here triggered on the Hall D Pair spectrometer electron, so it is a clean trigger with on hit per event virtually no background. This can be confirmed by standard detector so no debate here. So we see hits in x-strips , we expect hit in y ==> what we are seeing is an actual polarity flip
  2.  Slide 3 shows 3 different event with polarity flip in y and more than half of the actual data at the time where showing the same behavior and 95% of the time on y strips
  3.  The explanation on why it is always on y-strips is pretty simple, it it cause by pile-up because of  if you look at the 2D profile of Hall D PS electron beam, it is a narrow strips, with the electron normal to the detector in the vertical direction but coming at a large angle ~10 degree in the horizontal direction ==> for a GEM-TRD which produce an ionization 7 times higher than a standard GEM, the signal collected in x-strips will show several waveform separated in time and also in spaces (several different strips collect the signal form the single electron). But the same signal will be hitting a small set of strips in y causing pile-up and I believe that what cause the polarity to flip.
  4.  When the flip happens, it stays on for several time samples > 20 APV25 time sample ==> It is very likely that the negative pulses we are seeing also are due to similar background hits pile-up that happens way before our real triggered signal but remained in flip state for the time window of the triggered signal and in that case, you will not see when the flip happens, only that it is there.
  5.  You can also clearly see the saturation effect because of the ADC dynamic range for the flip signal, depending on where the baseline is set, the flipped signal amplitude will always be small and the waveform shape will be flat ==> It does not mean that it is not real signal.
  6.  In some case, you see a glimpse of flipping back to the correct polarity (circled in magenta) but the signal is never fully recover and with 6 time sample, it is not clear if e will even have this luck


This is the more serious problem we have other than the effect on corrupting the calculation of CM. It is a long email but I hope these few slides shed a little bit of light on the issue we are trying to understand

Best regards
Kondo
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