A pressurized gas Cherenkov detector

We report the performance of a pressurized gas Cherenkov detector with small effective material tested by pion beams at KEK. With its small material (9.1% r.l.), the detector is suitable for particle identification placed at the upstream of electromagnetic calorimeters. The radiator is sulfur hexafluoride, SF₆ (2.0 MPa), contained in a cylindrical aluminum vessel, 2 m long, 10 cm in diameter, and 2 mm thick. A flat elliptical mirror (45 degrees) was tested for transporting Cherenkov photons to photomultiplier tubes for the tracks in the central region. The measured transmission of ultra-pure sulfur hexafluoride is 72% (200 nm) and 97% (300 nm) per 1 m path at 2.0 MPa. Using the pion beams (0.76-4.0 GeV/c), the number of photoelectrons were measured for different incident positions, angles and momenta. Thresholds of pion momentum were measured as a function of pressures (0.5-1.66 MPa), front which the refractive indexes of SF₆ were derived at different pressures. The lowest threshold momentum measured was 0.73 GeV/c at 1.66 MPa, corresponding to the refractive index (n-1) of 18.1×10^-3