Verification of Focusing System for Time-of-Propagation Counter

The results of verifying focusing system for Time Of Propagation (TOP) counter is presented. The TOP counter have been developed as a new detector for particle identification at Belle-II experiment, which is a Ring Imaging Cherenkov counter with precise timing information. Performance of the TOP counter is directly depended on the time resolution for single photon detection. Propagated Cherenkov photons have group velocity related to wavelength. It generates fluctuation of propagation time approximately 53 ps/m. Chromatic dispersion provides serious deterioration of time resolution. Against this problem, the focusing mirror is planed to introduce. It focuses Cherenkov light to different channels of MCP-PMT by different wavelength, and so that the deterioration of time resolution is suppressed. We verified focusing mechanism using 120 GeV/c π beam at CERN. Using a prototype TOP counter with the focusing mirror, we could confirm a work of focusing mechanism. The time resolution improved from 147 ps to 95 ps by using focusing mirror.