David, There must be a major misprint somewhere. The numbers as I understand them (see microscope and hodoscope contributions to last week's Collaboration Meeting) are Microscope channel width: 8 MeV Fixed array channel width: 30 MeV Beam energy contribution to resolution: 2.5 MeV (see my talk at November 2008 Tagger/Beamline review) The finer segmentation of the microscope is driven more by counting rate than by desired energy resolution. The fixed array can not be used for the coherent peak at normal data-taking rates, because the individual counters would run at 9-14 MHz (see table in hodoscope talk at May 2009 Collaboration meeting). Only the photon energy region above the coherent peak (9-11.7 GeV), where counting rates are lower, is fully covered by detectors Below 9 GeV, there is only 50% sampling for crystal alignment purposes (at low rate), and during full-rate running, these counters can be read out only in current mode. Dan Sober 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 --
Daniel Sober Professor and Chair Physics Department The Catholic University of America Washington, DC 20064 Phone: (202) 319-5856, -5315 E-mail: sober@cua.edu |