[Sbs_gems] [EXTERNAL] GEM operating voltage

Thu Dec 16 22:46:21 EST 2021

Hi Kondo,

I think it is probably okay to lower most or all of the HVs by -25 V or so (-50 V maybe with more careful study) and I have said that for at least a week now. I also hope both of these runs were “apples to apples” in terms of beam current, target, trigger threshold, etc.

However, with all due respect I must point out the obvious self-contradiction when you say “Using the track based efficiency as only way to decide everything on how to operate the GEMs is a huge mistake” and then you turn around and base your conclusion that we are not losing anything in lowering the HV on… the track-based efficiency, while ignoring the across-the-board ~10-20% drop in the number of tracks found per trigger when operating at lower HV!  (depending on whether you look at numerator or denominator the answer is roughly the same). You are making exactly the same mistake that you accuse others of making because in this particular case it supports your preferred conclusion. I have only ever advocated that decisions be made based on sound evidence and solid analysis about what the data do (and do not) allow us to conclude, and as the author of the analysis software I understand that better than anyone else.

That very real and significant drop in the yield, which you have now confirmed with 10X higher statistics, was why we initially hesitated to permanently reduce the HV for production running (I had no part in that decision process). In fact, if you look at the numerator histogram in the plots that you sent, you see that the number of events in the numerator histogram is exactly the same for all four layers in both cases. This is NOT an accident, it is a byproduct of the fact that we are running with only 4 tracking layers now. The track-based efficiency calculation always requires tracks to have a minimum of four hits to be counted as “efficient”; i.e., to be included in the “numerator” histogram. This is to reduce the bias of the efficiency calculation by requiring hits in at least three layers OTHER than the layer whose efficiency is being measured. However, because we are running with the bare minimum of redundancy in our tracking setup, in the present context, it allows us a fairly robust estimate of the reduction in the yield of 4-hit tracks when lowering the HV.

We know that the fraction of 4-hit tracks that are fake is rather small. The numerators of your efficiency plots for a constant number of BigBite triggers, assuming all other experimental conditions were the same, clearly show that the yield of 4-hit tracks is reduced by about 11% at -25 V. Now this does not translate directly to an 11% reduction of the reconstruction efficiency for real tracks because some of the four-hit tracks at the higher HV would still be reconstructed as 3-hit tracks at the lower HV. But the reduction in the overall yield of 3-hit tracks is similar! I had previously estimated that about 50% of 3-hit tracks were fake. The fake-track fraction could be larger or smaller at reduced HV, but assuming the fraction of 3-hit tracks that are fake stays the same, your efficiency plots imply pretty robustly that you are losing about 10-12% in overall tracking efficiency at 25 V below nominal. But you still need to do an estimate of the fake track fraction (I can help with this) in both cases to get a more robust estimate of the real change in the yield and/or track reconstruction efficiency.

If we have compelling theoretical justification (and some circumstantial empirical evidence from our experience of this run) that the “nominal” HVs we are using are dangerous for the long-term stability and reliability of the GEMs, and that running at -25 V or -50 V meaningfully improves their long-term stability and reliability (I am not yet totally convinced of that), then by all means, we should lower the HVs a bit. But let’s not pretend that you have conclusively demonstrated that we aren’t losing anything, because the plots you circulated very strongly suggest otherwise, and all you’ve done so far is reproduce the previous result with higher statistics. You have your theoretical reasons for believing we aren’t losing anything, but I go by the data, and the data suggest otherwise, in the absence of major efforts in fine-tuning of the analysis to recover some of the lost efficiency. It could be that a 10-12% reduction of the overall tracking efficiency is acceptable for the present circumstances. But in GEP these detectors are supposed to operate at about 30X higher background rate than what we have now. If we can’t safely operate them at full efficiency now, what hope is there for GEP, let alone SOLID or other ambitious high-luminosity experiments? Basically none. If you can only operate your detector safely at ~30-70% efficiency OR at high efficiency but 10-30X lower background rate/luminosity than advertised, then you need to go back and redesign the whole experiment and explain to the PAC why the capabilities of the technology were so dramatically oversold…

Best regards,

Andrew

From: Sbs_gems <sbs_gems-bounces at jlab.org> on behalf of Kondo Gnanvo <kagnanvo at jlab.org>
Date: Thursday, December 16, 2021 at 8:36 PM
To: Bogdan Wojtsekhowski <bogdanw at jlab.org>, Bai, Xinzhan (xb4zp) <xb4zp at virginia.edu>, Liyanage, Nilanga K (nl8n) <nl8n at virginia.edu>
Cc: sbs_gems at jlab.org <Sbs_gems at jlab.org>
Subject: Re: [Sbs_gems] [EXTERNAL] GEM operating voltage
Dear all,
Please find attached the preliminary analysis on 500k replay (Thanks Sean) of two run where we were comparing the efficiency with nominal GEM HV (red) and with the HV lowered 25 V (blue) on all the chambers.

For all layers, the difference in efficiency is within 1% ==> it is even higher for the UV GEM Layer 2 at the lower HV setting (which might be due to the opposing effect of the less current drop in the divider)

Conclusion is that it does not seems to be any significant difference in the performances.
Xinzhan post the same results with 50k replay (less statistic) and I am going to post a log entry for this slides as well

Our plan now is to have a run at -50 V when we move back to LD2 production and to compare once again and after we can make a final decision

Best regards
Kondo

From: Bogdan Wojtsekhowski <bogdanw at jlab.org>
Sent: Thursday, December 16, 2021 11:09 AM
To: Kondo Gnanvo <kagnanvo at jlab.org>; Xinzhan Bai <xb4zp at virginia.edu>; Liyanage, Nilanga K (nl8n) <nl8n at virginia.edu>
Subject: Re: [EXTERNAL] GEM operating voltage

I came to the same idea. We must reduce HV today!
________________________________
From: Kondo Gnanvo <kagnanvo at jlab.org<mailto:kagnanvo at jlab.org>>
Sent: Thursday, December 16, 2021 11:07 AM
To: Xinzhan Bai <xb4zp at virginia.edu<mailto:xb4zp at virginia.edu>>; Liyanage, Nilanga K (nl8n) <nl8n at virginia.edu<mailto:nl8n at virginia.edu>>; Bogdan Wojtsekhowski <bogdanw at jlab.org<mailto:bogdanw at jlab.org>>
Subject: RE: [EXTERNAL] GEM operating voltage

Hi Xinzhan,

I agree with you and I am making this case for quite a while now and I send a slide about it a few days ago.

Using the track based efficiency as only way to decide everything on how to operate the GEMs is a huge mistake

That is why the data taken at -25 and -50 V need to be analyze to see what we loose in efficiency. I am sure we don’t loose anything at -25 V and very small at -50 V.

Best regards

Kondo

From: Xinzhan Bai <xb4zp at virginia.edu<mailto:xb4zp at virginia.edu>>
Sent: Thursday, December 16, 2021 10:47 AM
To: Liyanage, Nilanga K (nl8n) <nl8n at virginia.edu<mailto:nl8n at virginia.edu>>; Kondo Gnanvo <kagnanvo at jlab.org<mailto:kagnanvo at jlab.org>>; Bogdan Wojtsekhowski <bogdanw at jlab.org<mailto:bogdanw at jlab.org>>
Subject: [EXTERNAL] GEM operating voltage

Dear Nilanga and Kondo,

Regarding the newly found dead sector, see HALOG entry:

https://logbooks.jlab.org/entry/3964165

I checked around, I think that it is possible that this is due to discharge from a large ionization.

in the attached two papers (I think similar to our application):

Page 4 of "J_D_Swift_1969....pdf",

and Page 6 of "HV_discharge_acceleration_by_....pdf"

I can see a clear increase of the discharge probability when applying a higher

voltage upon a fixed distance.

Maybe we don't need to lower 50 V for all chambers, but lowering 20 ~ 30 V can also make a big difference in reducing the discharge probability. I think reducing voltage can improve the lift-span of our GEM chambers under high beam current operation.

To me, I am not 100% sure we are losing real tracks considering we have fake tracks. The efficiency from tracking is only around 50%~70% suggests that the fake tracks are more than 20%.

Best,

Xinzhan

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mailman.jlab.org/pipermail/sbs_gems/attachments/20211217/2361dea6/attachment-0001.html>