Development of a new photon tagging system for GeV-γ beam line

Photo-production of hadrons is experimentally studied using 0.8 - 1.2 GeV photon beams at the Research Center for Electron Photon Science, Tohoku University, Japan. Photons are produced at the bremsstrahlung process by inserting a carbon fiber into the circulating electrons in the 1.3 GeV electron synchrotron. The energy of each photon is determined from the trajectory of the corresponding recoil electron. The energy resolution and time resolution of the current photon-tagging system is not enough high for the next-generation physics program. We have developed a new system called TaggerIIs. The angular resolution should be higher than 5° to reject the background particles coming from other points than the carbon fiber. The time resolution is required to be less than 200 ps to identify the bunch corresponding to a produced photon. We have adopted a combined system of a multi wire drift chamber and plastic-scintillator hodoscope for the new system. The wire chamber has a frame with length of 1000 mm along pole shape of a bending magnet and consists of 3 sense wire planes (X, X', X) of wire length of 116 mm. A shape of the cell is hexagonal and the spacing of the sense wires is 6 mm. The hodoscope consists of 60 plastic scintillators (EJ212) of 30 mm height and 5mm thickness. The width of each scintillator is designed from 5 mm to 30 mm. A performance of the new system has been tested using positron beams. As a result, we achieved that the detection efficiency of the wire chamber was 99%, the angler resolution was 0.60° and the timing resolution of the hodoscope was 165.8 ps.