Design and performance study of the TOP counter

A novel RICH detector called TOP counter has been developed for particle identification in Belle II. It measures the time of propagation (TOP) of Cherenkov photons traveling in the quartz radiator with micro-channel-plate photomultiplier tubes (MCP-PMTs) with a precision of 50 ps. A prototype TOP counter was tested with the 1.2 GeV/c electron beam in the Laser Electron Photon beam line at SPring-8 (LEPS). The TOP distribution was obtained as expected after taking into account the dependence of the MCP-PMT quantum efficiency on light incidence angle and polarization. In this paper, the key features of the TOP counter design and the beam test results are presented.