High Speed HPD for Photon Counting

We have succeeded in the development of a highspeed Hybrid-Photodetector (HPD), by using reduced electron lens and newly developed avalanche diode (AD) with very low capacitance. The HPD shows fast time response of less than 400 ps in both rise and fall times, and good timing resolution of 23 ps in sigma for single photons at full illumination on photocathode of Phi 8 mm, after deducting the waveform of the input light pulse. Limiting factor of the timing resolution was further investigated, and clarified to be the transit time difference of electrons in vacuum along the photocathode. The timing resolution was estimated to be less than 10 ps for the spot illumination of Phi 1 mm on the central part of the photocathode, where small transit time difference was expected by the simulation. This resolution is the world highest in the HPD, and matches for those of MCP-PMTs. Both GaAsP photocathode having close to 50% quantum efficiency in visible region and bialkali photocathode having 34% in UV region were fabricated in view of various applications. In this paper, we report the results of the evaluation including discussion about the limiting factors of the timing resolution for the new HPD.