Development of a quartz-based threshold Cherenkov Veto counter with MPPC Readout for a Hadron spectroscopy experiment at J-PARC

We have developed a synthetic fused-silica based Kaon Veto Cherenkov counter (KVC) for the J-PARC E72 experiment, which aims to explore hyperon resonances near the Λ η threshold via K − p reactions. The KVC is required to reject beam-through kaons in the momentum range of 0.65–0.95 GeV/c, corresponding to the velocity region of β ≈ 0.8 – 0.9 , with high efficiency ( > 99 % ) under a 1.0 T magnetic field. A major technical challenge in this velocity region is that the Cherenkov angle is approximately 31 ° – 40 ° for kaons, which falls below the critical angle for total internal reflection (TIR) at the silica–air interface ( θ c r i t ≈ 43.2 ° ), leading to light loss through the radiator side faces. To overcome this limitation, we applied a ground-glass finish to the radiator side faces to induce diffuse scattering and redirect photons toward the sensor planes. In a beam test at the J-PARC K1.8BR beamline, the ground-glass configuration yielded a mean light yield of approximately 86 N p.e. , providing a factor-of-1.8 improvement over the 48 N p.e. obtained with the conventional polished configuration. This enhancement provided a sufficient safety margin for stable operation. The detector was successfully operated throughout the J-PARC E72 physics run, achieving a detection efficiency of > 99 % and enabling stable data collection.