Multi-PMT optical module for the KM3NeT neutrino telescope

The future cubic kilometre scale neutrino telescope KM3NeT will employ a novel type of a Digital Optical Module (DOM), developed during the recent FP6 Design Study. A pressure-resistant glass sphere hosts 31 photomultiplier tubes (PMTs) of 3-in. diameter, together with all the electronics for high-voltage generation and signal readout. The optical module forms a complete stand-alone detector that is connected to the outside world via a single optical fibre and two copper conductors providing electrical power. The advantages of using multiple small PMTs in the same DOM are the higher quantum efficiency (>30% expected), smaller transit time spread, better two-photon separation capability and directional sensitivity. Moreover, a longer operating lifetime is expected than for large PMTs due to the accumulation of less charge on the anode. In addition, small PMTs are insensitive to the Earth's magnetic field and do not require μ-metal shielding. In order to maximise the detector sensitivity, each PMT will be surrounded by an expansion cone collecting photons that would normally miss the photocathode. Such an expansion cone consists of an aluminium ring filled with silicone gel. An increase in the overall sensitivity, integrated over all angles of incidence, was estimated to be about 27%. Monte-Carlo simulations have shown that a detector configuration with multi-PMT DOMs requires three times less OMs to achieve the same performance as conventional OMs hosting 10-in. PMTs. Prototype DOMs are currently being built by the KM3NeT consortium.