Scintillator-based hybrid photon detector development for the KM3NeT (km3km3-scale) deep sea neutrino telescope

Scintillating crystal-based hybrid photon detectors (X-HPDs) have been demonstrated as viable single photon detectors since 1996 in the Lake Baikal neutrino telescope. Prior to this, the Philips XP2600 was developed under the DUMAND program, while more recently developments at CERN have demonstrated the advantages of a true concentric geometry with a scintillator at the geometric centre of a spherical photocathode, giving almost 100% electrostatic collection efficiency over 3π3π solid angle coverage. We have started to develop a new series of quasi-spherical X-HPDs starting at 8 in. and progressing toward the maximum that can be fitted in a standard 17 in. optical pressure sphere for a future large deep sea neutrino telescope. The thrust of this R&D will be to investigate the industrialisation of the X-HPD to the point where it represents a significant cost reduction per cubic kilometre of instrumented volume compared to conventional PMTs, thereby allowing for extremely large telescope target volumes. Such gains will arise through an all-glass envelope, internal processing of a standard or enhanced bialkali photocathode, and either from cost reductions in the central scintillating crystal or from the use of a deposited phosphor viewed by a small PMT. Details of the development program and recent progress in the characterisation of prototypes will be presented.