[Sbs_gems] [EXTERNAL] Re: Test for GEp

[Andrew Puckett]

CORRECTION: My calculation of pixel size reduction was incorrect: 8,000 extra electronics channels would allow you to reduce the pixel size to 6.5 x 6.5 mm^2, not 5 x 5, and the occupancy would be about 4.3%, not 2.5%. Still, for a pixel chamber, smaller is always better.

Andrew

From: Andrew Puckett <puckett at jlab.org>
Date: Wednesday, May 15, 2024 at 1:51 PM
To: "Liyanage, Nilanga K (nl8n)" <nl8n at virginia.edu>, Bogdan Wojtsekhowski <bogdanw at jlab.org>, "Sbs_gems at jlab.org" <Sbs_gems at jlab.org>, Mark Kevin Jones <jones at jlab.org>
Subject: Re: [Sbs_gems] [EXTERNAL] Re: Test for GEp

Thanks for the detailed explanations. Here are my thoughts:


  1.  While I can see the underlying logic of the 1D U and V strip chambers, and I’m sure they would help on some level, I’m less convinced of their necessity and/or usefulness, given the already high redundancy of the GEP tracking layout.
  2.  On the other hand, the 1 cm^2 pixels, if we put them at the front and the back of the front tracker, would indeed form an extremely powerful combination. They could even, in principle, almost replace the strategy that we plan to use for the front tracker analysis using the ep kinematic constraints from the electron arm; the combination of the two (ep kinematic constraint from electron arm information and search region constraint from pixel detectors) will help not only with the tracking ambiguities but also with the SPEED of track-finding; the lack of vertexing information from ECAL+CDET limits how well we can define the search region in the horizontal direction, we would have to use the correlation between the assumed vertex location and the electron polar scattering angle to narrow the search region in the horizontal dimension.
  3.  The pixel chambers would dramatically reduce the potential combinations we would need to consider and I would LOVE to have pixel chambers, the smaller the better.
  4.  What is the active area of the pixel chambers? Obviously, making a 60x200 cm^2 requires more channels than making a 40x150 cm^2 pixel chamber. So perhaps instead of putting them at the front and back of the back tracker, we put one at the front and one between the 6th and 7th layer in the front tracker; i.e., since the last two layers of the front tracker are XY, this scheme would not reduce the acceptance.
  5.  What if, instead of building these 1D strip chambers (assuming you have the electronics to support them), you instead use those electronics channels to shrink the size of the pixels (other limitations than just electronics might come into play of course…) Just for argument’s sake, suppose each 1D strip chamber has about 4,000 channels. Suppose your pixel chamber is 40x150 cm^2 so 6,000 channels. If I suddenly have 8,000 more channels available, then the pixels could be 5 mm x 5 mm for a 40x150 cm^2 layer, shrinking the occupancy by a factor of 4 (from ~10% to ~2-3%) and imposing an even tighter constraint on the track-finding in the 2D strip chambers?
  6.  Finally, we need to consider space constraints. If we increase the total size of the front tracker along z, then in principle the CH2 analyzer and the back tracker have to be pushed back, reducing the useful acceptance for polarimetry. So unless we want to REPLACE one of the 2D strip chambers (say, an X/Y layer) in the front tracker, we do need to be mindful of the space constraints.

Maybe we should have a phone call to discuss this? I guess I already missed the GEM meeting today.

Cheers,
Andrew

From: Sbs_gems <sbs_gems-bounces at jlab.org> on behalf of "Liyanage, Nilanga K (nl8n) via Sbs_gems" <Sbs_gems at jlab.org>
Reply-To: "Liyanage, Nilanga K (nl8n)" <nl8n at virginia.edu>
Date: Wednesday, May 15, 2024 at 1:08 PM
To: Bogdan Wojtsekhowski <bogdanw at jlab.org>, "Sbs_gems at jlab.org" <Sbs_gems at jlab.org>, Mark Kevin Jones <jones at jlab.org>, "Liyanage, Nilanga K (nl8n)" <nl8n at virginia.edu>
Subject: [Sbs_gems] [EXTERNAL] Re: Test for GEp


The logic behind this new idea is as follows:


  *   The brand new research by Gianni Benchvinni's group at Frascatti has shown that the improved uRwell could operate at hit rates upto 1 MHz/cm^2 (Gianni is physicist who invented uRwell concept; Huong has visited his lab and formed  collaborative connections with his group; Gianni has graciously agreed to collaborate with us on this new develpment aimed at SBS and SoLID.)

  *   At these very high rates, to facilitate stable operation, uRwells need to operate at low gains, about 2000 as opposed to about 8000 for GEMs. These low gains are sufficient to support one readout direction, but not enought for charge to be divided between two directions. As such one 1 detectors, for example U or V are the most optimum in these conditions.

  *   Actually this is much better for us in SBS conditions: most of our random background, low energy photons, only give a random hit on a single layer, but not correlated hits on successive independent detectors.

  *   In our UV or XW case, the total number of channels we have per chamber allows us to build a pixel detector with pixel sizes of 1 cm x 1 cm  to have the same active area and the same total channels as a UV or a XW.

  *   Having two pixel chambers  seperated by some distance and requiring .AND. between hits on both can clean up most of the random hits and select mostly the high energy tracks.

  *   Given the catchment area for these pixels, the occupancy level would be about 1/6th of that of any existing UV or XW chamber; so in the worst case the occupancy would be around 10%; and the .AND. condition would lower this by some factor (beween 1 and 10, and I think closer to 10)

  *   Also we can place one pixel chamber at the front of the stack and one at the end of the stack; they will be built as independent detectors and can be seperated by any  distance.

  *   All this would ensure that we have a pretty narrow (about a factor of 100 smaller in area than right now)  search area for hits on the strip chambers.

  *   As for 1D strip chambers: in the current 2D strip chambers, since the same charge cloud gives both U and V hits,  the random hits and hign energy track hits are indistinguishable: they would both give correlated and correct U/V combinations (as opposed to ghost UV combinations which are filtered out in tracking), in the order of (occupancy); for example, at ~ 40% for GEp

  *   On the other hand, when we have a 1D U chamber and a 1D V chamber (in this case not too far apart), we will only get correlated U/V hits, WITHIN THE NARROW SEARCH AREA POINTED TO BY THE PIXELS., only for the high energy tracks, and only trandom combinations, in the oder of (occupancy^2) for the random hits; for example, ~ 16 % for GEp.

  *   So, this is what makes the combination of two back to back pixel detectors, combined with a back to back 1D U and 1D V detectors so powerful in our situation.



________________________________
From: Sbs_gems <sbs_gems-bounces at jlab.org> on behalf of Liyanage, Nilanga K (nl8n) via Sbs_gems <sbs_gems at jlab.org>
Sent: Wednesday, May 15, 2024 12:33 PM
To: Bogdan Wojtsekhowski <bogdanw at jlab.org>; Sbs_gems at jlab.org <Sbs_gems at jlab.org>; Mark Jones <jones at jlab.org>
Subject: [Sbs_gems] [EXTERNAL] Re: Test for GEp


Also, I am realizing that this information makes what  I have been considering (outlined below)   so much more important and urgent. :

I just got the quotes from Rui at CERN about the pixel uRwell chamber I have been talking about for GEp to make the tracking much easier than robust. The cost of material for a double chamber is about $ 30 k, and Huong and I would be happy to contribute person power from our group to build two double chambers by late fall, hopefully in time for GEp installation; of course this will depend on when we can get the parts from CERN; Huong is traveling there in June and will try to convince Rui to get these foils to us on time.

The ideal and the most powerful combination would be  a double pixel chamber ( two back to back chambers with 1 cm x 1 cm pixels) and a double UV chamber (back to back strip chambers, same pitch as UV, one chamber with U readout and the second with V readout); and we need to find $ 60 k for this set. Given the extremely high importance of this for GEp, and the urgency of the situation, I am happy to go ahead with the orders for the double pixel chamber using my grant funds at UVa. I plan to initiate these orders today.

I will send in a request to Mark and Bogdan soon for the funds for the UV set, if you can find funds from either Jlab or from collaborators, we can place those orders very soon as well and build both double layer detectors at the same time.

Best

Nilanga
________________________________
From: Liyanage, Nilanga K (nl8n) <nl8n at virginia.edu>
Sent: Wednesday, May 15, 2024 12:20 PM
To: Bogdan Wojtsekhowski <bogdanw at jlab.org>; Sbs_gems at jlab.org <Sbs_gems at jlab.org>; Mark Jones <jones at jlab.org>
Subject: Re: Test for GEp

Hi Bogdan

Yes, you are absolutely correct, it is a 30 cm target. I am sorry I missed that. So yes, it would be a factor of 6-7.

Yes, in that case it is good to push higher in the beam current for the test and also test the HV corrections on Sunday.

While I think that from the hardware perspective the GEMs would still be OK at that high luminosity,  I think Andrew might have a really  challenging problem with tracking with that factor of 6-7 increase: currently we are having raw occupancies of ~ 12.5 % , so given the same gain etc. we will have about 80-90% level occupancies for GEp; I do not think we ever expected or prepared for those level of occupancies (Andrew, please correct me if I am mistaken here); in my opinion, it would be next to impossible to run with anything higher than ~ 50-60% occupancy and get useful tracking information.

In that case we will seriously have to think about the hadron filter you have been proposing; and this Sunday could be an idea opportunity to test something like that.

Best

Nilanga


________________________________
From: Bogdan Wojtsekhowski <bogdanw at jlab.org>
Sent: Wednesday, May 15, 2024 12:07 PM
To: Liyanage, Nilanga K (nl8n) <nl8n at virginia.edu>; Sbs_gems at jlab.org <Sbs_gems at jlab.org>; Mark Jones <jones at jlab.org>
Subject: Re: Test for GEp

Hi Nilanga,

We can use BB tracker to monitor the beam stability and conformation from 50k analysis will good.
The difference between GEn-Rp and GEP operation is a bit larger:
GEP plans to use 50 uA on 30 cm long LH2 but GEn-RP used 15 uA on 15 cm LH2 => a factor of 6-7.
In addition, GEP will use SBS at 16 degree angle but now we have 23 degree.
So, it will be useful to see how GEM capable to operate with 30 uA on 15 cm long LH2.

Bogdan

________________________________
From: Liyanage, Nilanga K (nl8n) <nl8n at virginia.edu>
Sent: Wednesday, May 15, 2024 11:29 AM
To: Sbs_gems at jlab.org <Sbs_gems at jlab.org>; Bogdan Wojtsekhowski <bogdanw at jlab.org>; Mark Kevin Jones <jones at jlab.org>
Subject: [EXTERNAL] Re: Test for GEp

Dear All

Looks like thanks to all the hard work put in by Holly, Vimukthi and the team,  all GEM trackers are operating pretty well now, with efficiencies mostly around 75-80% per layer.

The current production running at 15 uA on LD2  is equivalent to about 25 uA on LH2; so about 50% of the GEp goal; this is consistent with the ~ 10-12% occupancies we are seeing in the SBS front tracker, I think Andrew was expecting about 25-30% for GEp.

Given this,  the high efficiencies, good tracking etc. we are seeing now are great signs for GEp.

As I mentioned earlier, what we are doing now  is much harder than the case for GEp: we are now requiring 3 hits out of 4 layers, while GEp will be asking for 4 hits out of 8 layers. This means that the layer efficiencies we are already getting are more than enough for GEp. Given the combinatorials of C(8,4) we should be able to get a very high tracking efficiency with what we already have. In fact, we might be able to lower the gains a bit to achieve more stable performance under higher luminosity, and still be able to get very good tracking efficiency in GEp.

We should be able to demonstrate this with the analyzer removed runs this Sunday

With the individual power supplies, is seems that the gains are pretty linear with beam current. So I think even at twice the current lumonosity we should be able to operate the power supplies like we are doing now, without any additional corrections to account for protective resistor voltage drops.

As for the program on Sunday, I do not think we need to push too high on the beam current, if we can get something like 25 uA on LH2 and LD2, that would be sufficient; with LD2 that would be close to 42 uA on LH2.

One thing we MUST remember for the beam test on Sunday is that the GEM gains are going to go up for both FRONT  AND BACK TRACKERS on SBS with beam current. This means that we may not be able to go to the same high voltage settings we are using for these chambers, especially for the back tracker ones,  for production right now. So, for the beam test,  we should back down all the GEMs voltages  to 3500 V and then carefully go up in voltage in 25 V steps while carefully monitoring the gains.

Also: if the beam is not stable and/or crappy, I think we should not turn on the GEMs on Sunday: given everything we have learned so far, there is nothing much left we must do to prepare for GEp. So if the beam quality is not good, I would rather not risk the GEMs to take some data which is not a must.

I plan to be there on Sunday

Best

Nilanga

________________________________
From: Sbs_gems <sbs_gems-bounces at jlab.org> on behalf of Mark Kevin Jones via Sbs_gems <sbs_gems at jlab.org>
Sent: Thursday, May 9, 2024 5:15 PM
To: Sbs_gems at jlab.org <Sbs_gems at jlab.org>; Bogdan Wojtsekhowski <bogdanw at jlab.org>
Subject: [Sbs_gems] Test for GEp

Hi all,
            Everyone has been working hard to get the GEMs working at this setting. Thanks to everyone.

For the next experiment GEp, we need to verify that the GEMs can be run at that
the luminosity of 50uA on 30cm LH2 target. Nilanga seemed fairly confident
that we can run at the GEp luminosity after the current scan that was early in the experiment.
       Do we need to do additional tests to show the expected performance for the GEp?
I am sure that we will get from Thia and Patrizia questions about the expected performance for GEp.

Cheers,
              Mark
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