[Halld-cal] [EXTERNAL] Re: [Primexd] FCAL non-linear correction

[Shepherd, Matthew]

Hi Colin,

If the primary source of nonlinearity is the cell threshold and not things like leakage, then one wants to characterize the non-linearity in a region of the detector not prone to leakage, hence the exclusion of the inner and outer borders.

This still of course may result in the need for special treatment in regions where leakage is significant.  For GlueX a few percent effect on the inner blocks is likely negligible for any measurement we want to make.

Another source of position-dependent non-linearity is potentially reduction of gain of PMT that comes about because high rate in the inner regions causes high tube current that results in a reduction of the gradient of final dynode stages of the tube.  This would produce reduced gain but only in high-rate regions of the detector.  However, we see in the notes from Paul that this issue is addressed in the base design:  these particular bases have demonstrated linearity with tube currents up to 140 microamps.  I think one would need rates well over 1 MHz to get these types of tube currents, so I don't suspect linearity of the device is an issue.

Matt


On May 15, 2020, at 5:58 AM, Colin Gleason <gleasonc at jlab.org<mailto:gleasonc at jlab.org>> wrote:

In testing out Igal's addition to the FCAL_Pi0HFA, I noticed that in my non linear correction I do not include the inner or outer rings. I apply a fiducial cuts on both photons of less than 20.785cm and greater than 108.4239cm. This effectively cuts out the first and last three rings. It probably explains why the non linear correction does not correct as much for the black and red lines. However, it looks like this lack of no non linear correction translates into a reduced pi0 mass up to ring 10 or 11. We're looking at a sub 1-2% effect here (ignoring block 1, which is sub 3%) for standard GlueX running, so not a big deal with respect to that, but obviously an issue for PrimeX. Maybe I can quickly recalculate the non linear correction removing these fiducial cuts and Igal can rerun his scripts to test the output? My hypothesis is that the inner rings will shift closer to 1 and that the middle and outer rings will shift above 1, which will lead to Igal having a similar conclusion that doing the correction by ring is necessary for PrimeX.

I'm trying to jog my memory on why we did this. If I recall correctly there were maybe a couple reasons: match the fiducial cuts with data, we do not understand leakage in the inner rings and didn't want to add in a correction that would be bias the rest of the detector, and we obviously can not reconstruct pi0s in the outer rings.

-Colin

On Thu, May 14, 2020 at 10:10 AM Igal Jaegle <ijaegle at jlab.org<mailto:ijaegle at jlab.org>> wrote:
All,

I made an entry in the FCAL logbook, https://logbooks.jlab.org/entry/3810625.

It remains to be proven that one function and one set of parameters can be used for all rings.  In the standard GlueX FCAL energy calibration, after a new non-linear correction is applied, in principle the gains should be cross-checked and re-match if needed, and so on, until the gains and non-linear parameters are converging. I suggest that more robust QA/QC be used for the FCAL energy calibration (I will make a pull request with my improvements later today to FCAL_Pi0HFA) and that Colin continues the FCAL energy calibration until he sees no changes in the gains and the non-linear parameters for examples by using the QA that the gain should be updated only if the fabs(new gain-old gain)/old gain > 1-1.5% depending on the standard GlueX QA.

tks ig.

PS: I think, the classical approach while maybe more complicated, requires fewer iterations and is working at least for pi0!
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--
Colin Gleason
Postdoctoral Fellow
Indiana University
Department of Physics
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