Lifetime-issues of MCP-PMTs

Cherenkov detectors of the DIRC principle will be used at the bar PANDA experiment at FAIR. Attractive photo sensor candidates for these devices are micro-channel plate (MCP) photomultipliers (PMT). The reasons are their excellent time resolution and their usability inside magnetic fields, although their lifetime is problematic. Various types were investigated. This paper will concentrate on the performance of the Photonis XP85112 and especially its lifetime behavior. For this MCP-PMT a time resolution of 33ps (σ) was measured and it is stable up to photon rates of about 2 MHz/cm2. The peak quantum efficiency (QE) was determined to 24%. Before the actual lifetime measurement several studies were performed, such as scanning the surface of the detector to get the geometrical detector response function. Additionally the gain and time resolution were investigated in a magnetic field up to 2T. Afterwards the lifetime measurements were done up to an integrated charge of 305 mC/cm2, i.e. the collected charge was 1-5 mC/cm2 per day. The gain and QE were determined in irregular intervals of 1-3 days. Also QE scans as a function of the position at the photo cathode were done every 2-3 weeks