[Sbs_gems] [EXTERNAL] Negative Pulse Analysis Update

Mon Feb 21 10:40:44 EST 2022

Hi Kondo,

   1. Yes I am removing the highest 54 and lowest 54 ADC channels. So yes I
   agree that this removes the negative pulse effect. Is this not what we want
   in order to correctly calculate the CM?
   2. My apologies, on slide 5 I talk about old and new CM calculation
   methods. The left and right plots are supposed to be the same, but compare
   between the old and new methods. But this slide is already outdated, since
   Andrew has developed a new online CM algorithm that seems even better, and
   he will update about this later.
   3. The only ways I know to definitely prove a baseline shift is to plug
   an oscilloscope into the output and look at it with beam or to correctly
   measure the CM baseline during beam and compare to the pedestal levels. It
   is too late for the oscilloscope option but we can measure the baseline
   from the full readout events. Unless someone has a better suggestion, the
   only way to "prove" the CM is calculated correctly is to visually look and
   see how the result compares to the raw strip data. I have done this using
   the event displays here, https://logbooks.jlab.org/entry/3984280.
   Specifically you can look at this run, which I zoomed in the axis to make
   things even more clear,
   https://logbooks.jlab.org/files/2022/02/3984280/CM_event_display_run13602_zoomed_0.pdf.
   I think from all this data it is clear that the sorting method gives a CM
   result that is within 15 ADC of the true baseline. Then when we compare the
   sorting method results over different beam currents we see the CM offset
   increase up to -35 ADC,
   https://logbooks.jlab.org/files/2022/02/3984417/neg_pulse_study_3.pdf.
   Therefore I see no other explanation than the APV baseline is truly
   shifting downwards while the beam is on.
   4. I selected one MPD from the front most UV layers, which naturally has
   the most background. One MPD on a UV layer covers half of the active area,
   so this does cover the hottest region of the detectors.
   5. You are right, I have not thoroughly investigated the possibility of
   the entire hit being flipped. I have only looked at a hit flipping within
   the 6 time samples. I am still working on using tracking to check for
   negative hits in "expected" areas of the detectors. I will just say that
   from my plots of  "APV Negative Signal Peak Time" we do not see a normal
   pulse shape. But of course this could be due to saturation. or something
   else.
   6. My apologies, my wording was not correct here. My conclusion should
   be more like "There is no evidence of positive signals flipping into
   negative signals inside of the trigger window". As you mentioned there is
   also the scary possibility that the negative strips are not hits, but they
   "occupy" the strip and do not let positive hits in the same location be
   recorded. This can be investigated by checking if positive clusters ever
   have a "break" in the middle from negative strips. Or perhaps anyone else
   has a suggestion.
   7. Wouldn't the negative pulses saturate at 0? Or am I misunderstanding
   something?

Best,
Sean

On Fri, Feb 18, 2022 at 7:04 PM Gnanvo, Kondo (kg6cq) <kg6cq at virginia.edu>
wrote:

> Hi Sean,
>
> Thanks for the studies. It is pretty extensive and a lot of information to
> digest so probably I am still missing a few things, so maybe l can ask a
> couple of few points  that might help clarify a little bit things.
>
>
>
>    1. You use only 20 middles channels to calculate CM using the sorting
>    method, meaning that you remove the 54 highest ADC channels and 54 lowest
>    ADC channels from the calculation è Is that correct? If so, yes this
>    will probably remove the negative pulse effect in almost all cases even
>    though the precision of CM calculation will be strongly affected by
>    definition
>
>
>
>    1. What is the difference between the 2 plots of slide 2 in attached
>    pdf. I see the same title in the y axis è is it typo where you meant
>    Danning CM – baseline CM (on left) and sorting CM – baseline CM (on right),
>    baseline meaning the CM calculated from pedestal data (ideal case?)
>
>
>
>    1. Do you actually have any evidence from the data of the voltage
>    shift downward due to high occupancy and why is it an issue w.r.t to CM
>    correction?
>
>
>
>    1. Even for the 7uA on LD2 data that you are showing, I don’t really
>    see high occupancy data, and since you said that you analyzed data from
>    only 1 MPD, did you select MPD reading out data from the hottest area of
>    the chamber?
>
>
>
>    1. In my view, there are several aspect of what I mean by polarity
>    flip.
>       1. It looks to me like you are looking for a flip for a same strip
>       across the 6 time sample. This is just one way, but very likely the most
>       unlikely
>       2. On slide 1 of the attached pdf, you can clearly see an event
>       with have both positive ADC strips and negative ADC strips on the same APV.
>       The question is are the negative signal flip or not? Unless these strips
>       are always below the baseline (i.e. their pedestal level, which we al agree
>       it is not the case), something makes them flip their ADC level w.r.t the
>       baseline while there was good hit in other strips of the same APV
>
>
>
>    1. Even if there is never an actual signal flip, with your correct
>    conclusion that the negative pulse occupancy  increases in similar fashion
>    than the positive one. I think regardless whether we convince ourselves
>    that signal can flip or not:
>       1. it is not correct to say that negative pulse does not cause the
>       loss of positive pulse
>       2. there is a high probability that a good event hitting these
>       strips might be lost (because of negative occupancy) and I think that is
>       the scariest scenario.
>
>
>
>    1. Most negative pulses have the same amplitude across time sample can
>    be just due to saturation, there are actually some case where they have the
>    typical APV signal waveform (see slide 3)
>
>
>
> Any way these are just a few questions I have after looking at the results
> you present.
>
>
>
> Best regards
>
> Kondo
>
>
>
> *From:* Sbs_gems <sbs_gems-bounces at jlab.org> *On Behalf Of *Sean Jeffas
> *Sent:* Friday, February 18, 2022 4:31 PM
> *To:* Sbs_gems at jlab.org
> *Subject:* [Sbs_gems] [EXTERNAL] Negative Pulse Analysis Update
>
>
>
> Hi All,
>
>
>
> I touched up my negative pulse analysis results that I showed to everyone
> last week. I realized I was making a serious mistake by simply cutting on
> signal > 80 ADC, because the first few strips in the APV always have large
> signals on them from the event header. Therefore this was significantly
> skewing my data. I am now using the correct method of cutting using 5*sigma
> of the pedestal noise, as is done in the tracking analysis.
>
>
>
> Please see my new results in the following slides
> https://logbooks.jlab.org/files/2022/02/3984417/Neg_pulse_study_2_18_22.pdf.
> Also please take the time to look over all the plots in my log entry,
> https://logbooks.jlab.org/entry/3984417. I try my best to pick some event
> examples to show, but I encourage everyone to scroll through the event
> displays I have posted, to get an idea of what the negative pulses look
> like. Please let me know your thoughts and questions.
>
>
>
> Best,
>
> Sean
>
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