Photon Beam Design Requirements

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Global Specifications for the Hall D Photon Beam

The following top-level specifications for the Hall D photon beam are determined by the physics requirements of the GlueX experiment and the anticipated capabilities of the CEBAF accelerator at 12 GeV. The specifications for Hall D beamline components and related instrumentation are derived, in turn, from these. References added to the end of each item in the list below indicate which subsystem is affected.

  1. choice of photon energy in the range 8 - 12 GeV [1]
  2. duty factor >90% at 500 MHz beam pulse repetition rate [1]
  3. orientable diamond crystal bremsstrahlung radiator [2,3]
  4. peak linear polarization of 40% at 9 GeV with collimation [2,3,4]
  5. average polarization 36% in coherent peak 8.4 - 9.0 GeV [2,3,4]
  6. photon beam collimator with half-angle 22 μr [4]
  7. photon beam centered on collimator to within 3 μr [1,4]
  8. photons tagged with energy resolution better than 60 MeV within coherent peak [1,5,6]
  9. beam tagged with reduced resolution over extended range 3.0 - 11.7 GeV [5,7]
  10. complete tagger coverage in range 9.0 - 11.7 GeV [5,7]
  11. sampled tagger coverage in range 3.0 - 9.0 GeV [5,7]
  12. nominal operation at 100 MHz tagged photons in coherent peak after collimator [1,5,6]
  13. tagging rate 250 MHz within coherent peak at nominal intensity [6]
  14. beam adjustable down to threshold of 10-4 nominal intensity for absolute normalization measurements [1,8]
  15. average tagging efficiency at least 70% within coherent peak under nominal conditions [1,5,6]
  16. continuous real-time monitoring of the post-collimator beam intensity spectrum with energy resolution better than 60 MeV over range 6.0 - 12.4 GeV [9,10]
  17. average polarization within coherent peak determined with accuracy ±1% absolute [9,10]
  18. absolute tagged photon energy calibration better than ±30 MeV [9,10]
  19. post-collimated beam shielding sufficient to keep detector backgrounds originating outside the experimental target below 1% of those coming from the target seen by all detectors, for each significant particle species considered separately [11]

Photon Beam Subsystems

  1. 12 GeV electron beam and associated instrumentation
  2. diamond crystal radiator
  3. goniometer mount for radiators
  4. active photon collimator system
  5. tagging magnetic spectrometer
  6. high-resolution tagging microscope
  7. broad-band tagging hodoscope
  8. total absorption counter
  9. pair spectrometer and target
  10. pair spectrometer hodoscopes
  11. post-collimator beamline and shielding