Development of a scintillation counter with MPPC readout for the internal tagging system

We have been developing an array of scintillation counters as an electron detector for a photon tagging spectrometer at the Research Center for Electron Photon Science, Tohoku University. Since it is installed in the limited space of the bending magnet of the accelerator, a compact size and tolerance to the magnetic field are required. We also aim at the improvement of the timing resolution of the counter in order to set a strict event selection. For the photosensor of the scintillation counter, we chose the multi-pixel photon counter (MPPC) which fulfills those requirements. We made a prototype scintillation counter with a plastic scintillator in a shape of a small rectangular prism directly coupled to the MPPC. We developed a dedicated compact amplifier circuit board on which two MPPC's and two sets of amplifier circuits are mounted. We fabricated a prototype of the electron detector of the photon tagging system with four scintillation counters with MPPC readouts. We performed a test with the electrons and positrons converted from a high energy photon beam. We selected single-electron-passing events and measured their coincidence timing. The timing resolutions of the counters were obtained from the fluctuations of the coincidence timing. The best intrinsic timing resolution of the counter was 80 ps (RMS). The best timing resolutions of all four scintillation counters ranged from 80 ps to 110 ps. The timing resolution and the range of their indivisual difference are appropriate for the use as the electron detector for the photon tagging system.