Dave, see my responses below. Are you planning to post your talk somewhere for us to look over? -Richard Jones David Lawrence wrote: > I saw the 2.5 MeV number in Jim's tagger note (GlueX-doc-1127 table 4.1 > pg. 42) > GlueX-doc-1127 table 4.1 pg. 42 is electron beam properties. The electron beam width is what comes out of the accelerator. We request optics choices, but fundamental beam properties like the longitudinal emittance and transverse emittance are what the accelerator can give us. We base our design of the photon beam on what they say they can give us for the electron beam. Table 4.1 summarizes what we took for the electron beam properties as a starting point for our design. > The only place I saw the 8MeV number was in Igor's talk from the beamline review (GlueX-doc-1167 slide 6) where it has a bullet that says "resulting ~8 > MeV resolution more than satisfies GlueX requirement". However, on slide 4 of the same document, it gives 60MeV as the "GlueX detector capability". The GlueX detector is the GlueX spectrometer, right? The tagger is not the GlueX detector, nor is the microscope. > I'm guessing this is a mis-statement and the detector capability is ~8MeV (or 8MeV/sqrt(12)) but the tagged photon energy resolution is 60MeV, driven by the beam energy resolution. > No, it is the photon beam energy resolution based on the GlueX detector final state resolution. The initial photon energy enters as a constraint in a kinematic fit together with energies and momenta measured in the tracker and calorimeters. There is a limit beyond which better beam energy resolution does little further improvement. That is where the 0.5% energy resolution requirement number came from. > Thanks Sascha for the correction on the microscope range. I will include > the correct range in my talk. I'm not getting something about what the 2% of 12GeV (=240MeV) represents. Is this the actual tagged photon resolution? If so, what happened to the 60MeV? > He meant 2 MeV from the intrinsic tagger magnet optics. It is about that in the region of the microscope. > I'm not sure though that I follow the fixed array argument. It would seem that the limitation of the electronics would be applied on a per channel > basis. If the single channel rate is 5MHz, then that would be a hit once every 200ns which the scintillator and our electronics should easily handle. Is the 20 counters coming from the solid angle subtended by a bundle of 20 fiber scintillators into the magnet being about the same as that of a single fixed array scintillator? I guess that would make sense (5 scintillators per > energy bin times 4 (8MeV) energy bins would cover 32 ~= 30MeV). However, > I'm not sure how well the argument holds even then since more than half of the rate is contained in the center fiber of a row of 5 in the microscope (see slide 6 of Igor's talk GlueX-doc-1167). > His statement was about the rates in the fixed array, for tagging above the range of the microscope. It is not related to the microscope rates or ability to tag with the microscope. If we want to take data specifically with end-point photons then we would replace the collimator with a larger one and turn down the beam intensity to alleviate some of the congestion in those end-point counters. When running with polarized beam, the rates above the coherent peak are too high to use effectively as a veto to eliminate high-energy events because of accidentals. > Sorry if I'm being a pest. > When giving an overview talk, it's worthwhile to make the effort to get it right. -Richard Jones
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