Performance of photo-sensors for KM3NeT

The future deep-sea neutrino telescope of multi cubic-km size, KM3NeT, has been designed for an efficient search for high energy neutrinos originating from galactic and extragalactic sources. The detection principle relies on the measurement of Cherenkov light emitted from relativistic charged secondary particles caused by the interaction of neutrinos with matter inside or surrounding the active detection volume. In order to provide a homogeneous photon acceptance and to reduce the environmental background by local coincidences between neighbouring photo sensors, a digital optical module (DOM) containing an array of 31 3-in. diameter photomultiplier tubes (PMTs) has been designed. Optimum performance requires sensitivity to single-photo electrons, high collection efficiency at low dark noise, homogeneous photo-cathode response and excellent timing properties. We have studied the response to single photo electrons of a newly developed 3-in. diameter PMT from ET Enterprises Ltd. A 2D-scanning system with a picosecond laser illuminating various positions on the photo-cathode surface was employed to study the timing and homogeneity of the PMT. Results of these investigations indicate good photo-cathode homogeneity, low dark noise on the sub-kHz level, and an average transit-time spread below 2 ns. Simulations indicate a significantly improved signal-to-background ratio in the multi-PMT DOM as compared to a triplet of optical modules each housing a single 10-in. PMT.