Lifetime of MCP-PMTs

The main particle identification detector of the PANDA experiment will be of the DIRC type. For various reasons the favored sensors are micro-channel plate photo multipliers (MCP-PMTs). Despite many advantageous properties MCP-PMTs until recently had serious problems with aging issues at high photon rates. In this paper we present a simultaneous aging measurement for the latest MCP-PMTs of BINP, Hamamatsu (R10754X) and PHOTONIS (XP85112). In these devices specific countermeasures have been applied to reduce the loss of quantum efficiency (QE) of the photo cathode (PC) with increasing anode charge, which is the main feature of aging. We find that the new techniques show a significantly increased lifetime of the concerning MCP-PMTs. With several surface scans across the active area of the MCP-PMTs we find evidence that the loss of QE develops from the rims and corners of the PC. In addition, the spectral QE of the MCP-PMTs is given as a function of the integrated anode charge revealing that for the R10754X the QE drops faster for long wavelenghts than for short ones. At this point of the ongoing measurements the XP85112 does not show any significant aging signs after 5.6 C/cm2 which is a more than an order of magnitude improvement compared to former MCP-PMTs and corresponds to more than 10 years of running time for the PANDA Barrel DIRC. In this PMT the micro-channels are coated by an atomic layer deposition (ALD) technique which currently appears to be the most promising countermeasure against the aging of MCP-PMTs.