The increase in the electron beam energy, with the upgrade of the CEBAF accelerator, means that the current JLab tagger system must be redesigned. The current tagger magnets would be unable to bend an electron beam with double the energy in the space currently available. This means that one of the space available or the magnet strength must be increased, both of which would incur great cost. To circumvent these problems a different concept to produce tagged almost-real photons was developed. This involves tagging electrons scattered at very forward angles, in kinematics where the exchanged virtual photon is quasi-real.
To detect the initial photon and cleanly separate the events from the large photon background, the FT is required to detect at very forward angles (<5∘). The detection of scattered electrons between 2.5∘−4∘. and an energy of 0.5-4.5 GeV is required to give access to the regions where hybrid mesons are predicted. The FT is a conglomeration of three sub-detectors:
- A Tracker to determine the scattering angle and plane.
- A Hodoscope to separate electrons and photons.
- A Calorimeter to determine the electron energy.
The Hodoscope Detector
The hodoscope is a segmented array of plastic scintillator tiles, embedded with WaveLength Shifting (WLS) fibres (Kuraray Y11) and read out by Silicon PhotoMultipliers (SiPMs) via optical fibres.
The plastic scintillator used (EJ-204) provides fast timing and good radiation resistance for use in high rate environments. Due to the high radiation flux and large magnetic field in the vicinity of the hodoscope, it is required that the scintillation light is taken to a far more controlled environment for detection.
WLS fibres are used to shift the frequency of the scintillation light and transport it out of the area of high magnetic field. The WLS fibres absorb the UV light produced in the plastic scintillator and emit at a longer wavelength, which matches with the optimal quantum efficiency of a typical SiPM (green). Kuraray Y11 was chosen because of its established radiation hardness and good timing properties. The fibre has been used successfully in other detectors such as the hodoscope used with the inner calorimeter of CLAS12.