Influence of the Earth´s Magnetic Field on Large Area Photomultipliers

The influence of the Earth\´s magnetic field on large area photomultipliers proposed for a future deep sea neutrino telescope was studied under the EU-funded KM3NeT design study. The aims were to evaluate variations in PMT performance in the Earth\´s magnetic field and to decide whether the use of magnetic shielding is necessary. Measurements were performed on three Hamamatsu PMTs: two 8-inch R5912 types, one of these with super bialkali photocathode, and a 10-inch R7081 type with a standard bialkali photocathode. The various characteristics of the PMTs were measured while varying the PMT orientations with respect to the Earth\´s magnetic field, both with and without a mu-metal cage as magnetic shield. In the 8-inch PMTs the impact of the magnetic field was found to be smaller than that on the 10-inch PMT. The increased quantum efficiency in the 8 $^{\prime \prime }$ super bialkali PMT almost compensated its smaller detection surface compared to the 10 $^{\prime \prime }$ PMT. No significant effects were measured upon transit time and the fraction of spurious pulses.