[Sbs_gems] [New Logentry] GEM operating gain and detection efficiency during GEP

[puckett at jlab.org]

Logentry Text:
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Bogdan and I were just discussing the GEM operating gain and HV and detection efficiency goals, and the question of what is the minimum acceptable per-layer detection efficiency. Fortunately, by rearranging the polarimeter layout for more redundancy, the tracking system is more robust. 

Based on the experience from GEn-RP and from simulating GEP, my goal would be to have a high (>~95%) efficiency for a minimum hit requirement of 5; i.e., 5 out of 8 layers with good hits on the track. In the expected background environment, tracking with 4/8 or 3/8 minimum hit requirements will be extremely contaminated. 

At the same time, we know that our high-energy protons are close to minimum-ionizing and will have smaller signals and lower efficiencies to begin with. We want to operate at the lowest gain consistent with minimally acceptable detection efficiency, so the GEMs will be more stable at high luminosity. At the same time, we need the gain high enough for acceptable detection efficiency. 

Based on these considerations and basic binomial statistics, this suggests a "minimum acceptable" per-layer detection efficiency of 80%; i.e., at 80% per-layer detection efficiency we will 94% overall 5/8 tracking efficiency. 

An "optimal" detection efficiency would be 85-90%, particularly since not all protons of interest will pass through all 8 layers in the front and/or back trackers, etc. 

At 85% per-layer efficiency, the 5/8 tracking efficiency increases to 98% (ignoring rate-dependent efficiency losses due to background contamination/occupancy), and at 90% per-layer efficiency, the 5/8 tracking efficiency increases to 99.5%. 

So in my opinion, our HV tuning should have a goal of reaching 85-90% per-layer detection efficiency (as defined by the track-based efficiency obtained from low-luminosity runs), with ~80% as a minimally acceptable level. If we are able to reach ~85-90% "local" efficiency (excluding dead areas) in working regions of the detector, then there should be no need to try to push higher than that. 



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