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Dave, The resolution of 0.5% rms (60 MeV) is the design requirement derived from what is needed to optimally reconstruct exclusive final states with the GlueX spectrometer. The actual design resolution is much better than this, as you correctly surmised, driven not by energy resolution requirements but by the segmentation needed to keep the rates per tagger channel within optimal operating range at maximum beam intensity compatible with tagging. The full energy range covered by the 100 channels of the microscope extends a bit above and below the 600 MeV range you refer to in your message. Our current range is approximately 8.3 - 9.1 GeV, with energy channels 8 MeV wide. The actual resolution achieved with this setup in the simulation is about 5 MeV r.m.s., somewhat wider than 8 MeV / 2.3 because of the imperfect focus of the spectrometer and multiple-scattering between channels. I feel like I am explaining this all the time, but apparently it is to different audiences each time. Thanks for asking. -Richard Jones Elton Smith wrote: Pine.LNX.4.58.0905201712040.26027@elton01.jlab.org">---------- Forwarded message ---------- Date: Wed, 20 May 2009 17:09:35 -0400 From: David Lawrence <davidl@jlab.org> To: halld-tagger@jlab.org Subject: Tagger energy resolution? Hi Photon-ists, I'm getting my talk put together for CIPANP next week and I'm a bit confused about the tagger energy resolution numbers I'm seeing. I've spoken with Eugene and Elton and I think I understand them better now, but I would like to get confirmation from the group. I've seen a couple of places (GlueX-doc-1167, GlueX-doc-1127) where the microscope resolution is quoted as 0.5% of the electron beam energy or 60MeV. However, the microscope has 100 detectors covering a 600MeV range so the tagger itself is capable of something closer to 6MeV resolution. As I understand it, the 60MeV comes from the uncertainty in the electron beam energy and not due to any limitation of the tagger design itself. The "over-design" of the resolution is due to rate considerations. What raised a flag for me was the fixed array which seems to always be quoted as having detectors spanning a 30MeV bite which is half as big as the quoted microscope resolution. Though I don't actually see it anywhere, I'm assuming that the fixed array also has an energy resolution of 60MeV limited by the electron beam energy resolution. Can someone confirm that all of this is correct? If this all is correct and the fixed array and the microscope both have the same energy resolution and they both can handle the rate at full luminosity and they both fully cover the same energy range (8.4-9.0 GeV) then why do we need both? Sorry if this is a stupid question, but I've always assumed that the purpose of the microscope was to give much finer energy resolution. Regards, -David |
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