Beam Hole Photon Veto For J-PARC KOTO experiment

The Beam Hole Photon Veto counter (BHPV) for the J-PARC KOTO experiment was designed by MC simulation. This detector is required to operate under extremely large flux of neutrons and low energy γs, as placed in the beamline and directly exposed with such particles in the neutral beam. To realize this detector, we proposed an array of Cerenkov counter modules, each of which has a lead sheet as γ converter, a stack of aerogel tiles as Cerenkov radiator, and light collection system consisting of flat mirrors and Winston Cone funnels. We employ the dual readout to reduce the counting rate for each PMT and optimized the design of lead converter and aerogel. Finally, we succeeded in designing BHPV which can operate in KOTO experiment with ∼ 2MHz maximum single counting rate while keeping ∼ 0.1% inefficiency for γs (Eγ > 1GeV) and 3% acceptance loss by the beam particle such as neutrons.