[Sbs_gems] [Revised Logentry] Results of the GEM HV scan from 7/22

[adr at jlab.org]

Logentry Text:
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The data were replayed by the new "back-to-front" tracking method implemented by Andrew recently. Due to the low amount of statistics and background being a quite a smooth function of 'dpp', a straight-line fit was applied for background estimation and subtraction via the side-band fitting method. Please see the plots in attached pictures labelled for each FT HV setting. The normalization of the extracted elastic yield was performed by dividing by the number of events replayed as the data were take from a single CODA run with HV changes performed within certain recorded split regions as mentioned in the referenced log entry. Therefore, what you see is basically a plot of 'Elastic events per event vs GEM HV'. 

We can see a plateauing behavior starting around 3625 V. Which means, at 3650 V, we should be running at already passed the optimal HV for most of the FT GEM layers. We know that it is around this region these GEM modules hit the efficiency plateau. But the motivation for this effort was to understand the combined effect of GEM hardware, APV CM sagging and broadening at high rate, and tracking and to learn where should we run the GEMs considering all these factors to maximize our overall elastic yield/statistics. The exact location of the plateau could be slightly altered by the offline software threshold applied; but we seems to have isolated the effect on elastic yield as a function of GEM HV, when all the other variables are held constant. 

PS: It might be of interest to take a few more data points near the plateau region for us to show the performance / characteristics of the GEMs. Especially under these high-rate conditions. Up to now, all we have are the track-based efficiency studies which becomes harder to interpret with due to the increased number of false tracks under these conditions and this type of study allows us to do a much cleaner separation. 

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This is a plain text email for clients that cannot display HTML.  The full logentry can be found online at https://logbooks.jlab.org/entry/4436404
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