Performance of particle identification by an upgraded back-anti counter in the KEK-E391a experiment

Development of particle-identification (PID) method between neutrons and photons by a back-anti counter (BA) is a key issue to reduce a signal-acceptance loss in the KEK-E391a experiment. For this purpose, we have upgraded the BA by replacing lead/plastic-scintillator sandwiches with segmented PbWO4 crystals. The PID method has been evaluated by using test-beam data of protons and positrons and Monte-Carlo simulations, and applied for neutrons and photons. In this paper, we report on the performance of the PID using shower-shape differences in the BA.