Broadband Tagger Hodoscope

The coarse fixed-array hodoscope consists of 180 scintillators distributed over a length of 11.2 m at a distance of 30cm from the focal plane of the two-magnet system. The scintillator array detects scattered electrons in the energy range of 0.6 to 9 GeV (for a 12 GeV electron beam).

The device will primarily be used to

• measure the energy spectrum during the calibration of the radiator crystals,
• monitor the photon energy spectrum during data taking with coherent bremsstrahlung photons, and
• the low-energy range of 0.6 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.

In order to allow for operating the hodoscope at the projected high rates, only the high-photon-energy range (above 9.0 GeV) will be fully covered with detectors; the remaining range will be sampled by 50%. In the low-photon-energy range (below 9.0 GeV) the detector positions will correspond to 60 MeV steps in photon energy for a 12 GeV electron beam (~0.8-1.1 MHz per counter at 10⁷ photons/sec on target). In the high-photon-energy range (above 9.0 GeV) the detector positions will correspond to 30 MeV steps in photon energy for a 12 GeV electron beam (~400 kHz per counter at 10⁷ photons/sec on target).

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 adding a second row of PMTs. These additional counters will have 22 cm long (straight) lightguides attached.

Scintillators

     thickness         5mm
     length           30mm
     width (quantity) 30(2), 25(3), 20(6), 15(15), 10(14), 
                      8(13), 7(9), 5(54), 4(24)

Photomultipliers

     1" tubes with magnetic shields

Resolution

     timing:  rms ~100-150 psec
     energy:  20 MeV (above 9.0 GeV photon energy)
              30 MeV (sampled - below 9.0 GeV photon energy)