Photomultipliers for the KM3NeT optical modules

The KM3NeT project aims to construct a multi-cubic-kilometre scale neutrino telescope in the Mediterranean Sea. The telescope's detection units, deployed in the deep sea, will be instrumented with optical modules, each housing 31 three-in. photomultiplier tubes (PMTs). Three companies are developing new types of 3 in. PMTs for the KM3NeT project. The first PMT samples of type R6233mod have been delivered from Hamamatsu and tested at the Erlangen Centre for Astroparticle Physics and at Nikhef. The results of these tests are presented. Many of the parameters of the existing versions of these PMTs already meet the KM3NeT requirements and production versions will be used to build the first developmental optical modules. Hamamatsu have started the development of new versions of PMTs of current interest, with better timing parameters and a slightly larger photocathode. The first two examples of a new PMT from ET Enterprises are under tests at Nikhef. The delivery of first 82 mm diameter PMTs from MELZ is also expected in October 2011. To increase photodetection efficiency in the multi-PMT optical module, PMTs will be surrounded by reflective light gathering rings ('expansion cones'). Test results of such an assembly are also presented.