BLTWG Meeting 5/15/2008

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  • Time: 10:00 EST
  • Place: EVO and ESNET
  • Present: Jim Whitlatch, Tim Stewart, Richard Jones, Franz Klein, Dan Sober, Elke Aschenaur, Eugene Chudakov


  1. Discussion of the pair spectrometer specifications
  2. Items required for the Lehmann Review

Notes by R. Jones

Updates to Beamline Specifications [RTJ]

In preparation for discussion of the pair spectrometer specification, I went over the Global Specifications for the Hall D Photon Beam page. A number of these items have come under scrutiny during the last few months, and some of them need to be changed. I have updated the following items in the Global Specifications table on the main beamline specs wiki page. For those who have joined the project during the last year, these changes are only of historic interest. Nice thing about the wiki, you can check if my list below is complete by visiting the specs page and looking at past versions under the History tab.

  1. the upper limit on the range of the tagging hodoscope - changed from 11.4 GeV (old value) to 11.7 GeV (new value). It is a priority for Hall D to provide tagged photons of the highest energy possible at Jefferson Lab. As shown in my talk on Tagging Near the Endpoint at the May 2008 collaboration meeting, the Hall D tagger acceptance extends up to 11.7 GeV. Right now we are modifying the layout of the hodoscope counters above 9 GeV to increase the segmentation, so it is a good time to change the spec on the upper photon energy limit to 11.7 GeV.
  2. energy resolution in the microscope - changed from 60 MeV (old value) to 10 MeV (new value). The 60 MeV value came from studies of its effect on the reconstruction of exclusive final states in the GlueX spectrometer. Basically, one finds that the kinematic constraints coming from knowledge of the initial state energy do not improve significantly once the photon energy is known to a resolution better than 100 MeV. However, rate considerations require the microscope segmentation to be a factor 6 better than this. Improved calorimetry in the forward region may allow us to take advantage of this. In any case, our specification should reflect the capabilities of the actual device we are building. The current microscope design incorporates non-overlapping energy channels of 8 MeV width each, so the actual standard deviation is more like 4-5 MeV. However we may have trouble proving that we can calibrate the absolute energy in the microscope at the 4-5 MeV level, nor has anyone come up yet with argument that we need it at that level. To me, it makes a more coherent picture to specify both at 10 MeV or better, and not worry about the last factor of two.
  3. energy resolution in the tagger hodoscope - changed from 120 MeV (old value) to 20 MeV (new value). The basis for this is that the Primex experiment needs an absolute energy calibration at the 0.2% level (see talk by Gasparian at May 2008 collaboration meeting which is consistent with non-overlapping energy bins of width 30 MeV. Current plans are to instrument the region in photon energy 9.0 - 11.7 GeV with 80 counters subtending equal-width energy bins of 34 MeV, which is consistent with a rms energy resolution somewhat better than 20 MeV.
  4. minimum beam intensity required during setup - changed from 10-4 (old value) of nominal intensity to 10-3 (new value). That was a typo left over from an earlier spec that had 108 Hz polarized photon rate designated as nominal. Our uniform practice now is to designate 107 Hz as the nominal intensity and 108 Hz as "high intensity" running.
  5. absolute tagger energy calibration - changed from 30 MeV (old value) to 10 MeV (new value). The immediate demand is to satisfy the requirement of 0.1% absolute energy calibration for the Primex experiment. More generally, it is a requirement for really understanding the beam line that we can determine the energy of the tagging and pair spectrometers at the scale of their resolution, and that they agree with each other.
  6. precision on absolute polarization determination - changed from 2% (old value) to 1% (new value). The 2% value was based on having only the spectral shape analysis method available. Now that we are backing it up with a direct polarimetry measurement, we will be able to verify our methods and reduce the error to the percent level.
  7. continuous monitor on the post-collimated beam intensity - there was previously no specification on this. The most demanding specification we have on this comes from the Primex experiment, asking for 1% absolute knowledge of the beam intensity. I have penciled in this value for the present. We will probably need their expertise to show how this can be done reliably at the 1% level.

Pair Spectrometer Specification

The discussion centered around the main tasks that the Hall D pair spectrometer must be designed to perform.

  1. monitoring the photon beam spectrum - This is needed for polarized beam running in order to be able to continuously monitor the polarization of the beam. We should measure the spectrum with sufficient resolution to be able to fit the shape and compute the polarization independent of the tagger. Coincidence spectra taken with the tagger will provide a higher energy resolution, and provide checks that the collimation function is what we think it should be.
  2. polarimetry - Direct measurement of the beam polarization is possible using the pair spectrometer in conjunction with a microstrip tracker placed upstream of the pair spectrometer magnet. There was a lot of discussion on the best arrangement for this tracker. In Hall B they use an active target to provide the origin for the pair tracks and then just a double pair of x,y silicon microstrip planes to measure the track directions. Having microstrips in the direct photon beam probably means that they can only be used at reduced beam intensity -- should be checked by Monte Carlo. An alternative design was discussed that would use two tracking detectors separated by a sufficient distance to measure track directions without needing an active target. Concern was expressed that multiple scattering in the first set of planes might smear out the angles to the point where the asymmetry is lost. This should be simulated. Depending on the cost, the microstrips might have to be put into a phase-2 upgrade plan for the beamline design.
  3. beamline energy calibration - This is needed to calibrate the energy scale of the tagger. Because the tagger is a part of the electron beamline, its magnetic field cannot be increased to steer the endpoint into the range of the hodoscope. This means that the energy scale of the tagger needs to be checked using another device. The pair spectrometer can serve that purpose if the detectors are arranged to be able to see the endpoint region with sufficient resolution. This should be incorporated into the design. It will not be used to determine the endpoint energy; we will work out a way with the accelerator folks to do that independently. Once the electron beam energy is known (at least at the 10 MeV level) then the absolute calibration of the pair spectrometer will be made by locating the endpoint. That calibration will then be transferred to the tagger using coincidence spectra.
  4. a relative luminosity measurement - This is needed to provide a continuous monitor of the photon beam intensity at the GlueX target, independent of the tagger. This is needed to monitor the collimated fraction. The intensity measurement should be stable at the 1% level. Absolute normalization will be provided against the total absorption counter.

Action Items

  1. R. Jones - Produce an intensity plot vs x and y of the photon beam downstream of the sweep magnet following the secondary collimator. Produce the same plot showing the polarization at the primary peak.
  2. T. Whitlatch - Review the current design of the collimator cave layout and propose changes that make room for the pair spectrometer and detectors upstream of the keep-out zone for the GlueX target, detectors, racks, etc. Pass the proposal around within the working group for comments.
  3. RadCon - Produce a radiation dose map of the tagger area and collimator cave, indicating regions where rates are high enough to impact the long-term operation of electronics.
  4. H. Hakopian - Produce a modified pair spectrometer design incorporating the feedback received so far. Pass around within the working group for comments.
  5. E. Aschenaur - Propose a date for the next working group meeting that satisfies constraints for travel.