Broadband Tagger Hodoscope

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Tagger-fixed-array-v3-side.jpg


The coarse fixed-array hodoscope consists of 190 scintillators distributed over a length of 10 m on the focal plane of the two-magnet system. The high-resolution device (Tagger Microscope) will be positioned behind the fixed array. The scintillator array detects scattered electrons in the energy range of 0.3 to 9 GeV (for a 12 GeV electron beam), i.e. it will tag photons between 3.0 GeV and 11.7 GeV.

The device will primarily be used to

  • measure the energy spectrum during the calibration of the radiator crystals,
  • check the alignment of the Tagger Microscope in case that it has been moved to cover a different energy range,
  • monitor the energy range of electrons between 0.3 to 3 GeV will be additionally used during data taking to detect any electrons whose associated bremsstrahlung photons have energies above 9.0 GeV in order to account for possible beam photon candidates with higher energies than covered by the variable microscope, and
  • act as tagging hodoscope for any experiments (other than GlueX) that require a wide tagged energy coverage or require tagging near the end point.


In order to allow for operating the hodoscope at the projected high rates, only the high-photon-energy range (Eγ>9.0 GeV) will be fully covered with detectors; the remaining range will be sampled by 50%.

All counters will be mounted at angles such that they face normal to the path of the scattered electrons. The construction of the hodoscope allows for later addition of counters to fully cover the whole energy range by filling the gaps between the 50% sampling scintillators and filling the second row of PMTs. In order to enable the positioning of the scintillators close to the thin focal-plane window of the vacuum chamber, the PMTs are positioned below the flange of the vacuum window at a distance 30cm below the scattering plane. A second row of PMTs is necessary to allow the full tagging coverage above Eγ>9.0 GeV. Straight lightguides connect the tagger scintillators to the PMTs (lightguides are slighly coned for wide scintillators).

See Specification for broad-band tagging hodoscope for details.

      Tagger-fixed-array-front.jpg

Some specifications on PMTs

Electron trajectories


Hodoscope Construction

Hodoscope Fabrication Readiness Review (June 11, 2013)