A large spherical HPD for a novel deep-sea neutrino experiment

An underwater neutrino experiment has been proposed which provides precise measurements of the neutrino mixing parameters θ23 and Δm232 and permits an increase of sensitivity for the small angle θ13 by more than one order of magnitude. A Cherenkov detector of about 1.5 Mt active mass, deployed in the Gulf of Taranto, utilizes the CNGS beam in off-axis configuration which represents an essentially mono-energetic source of muon neutrinos. A unique feature of the experiment is the possibility to move the detector and therefore exploit different baselines around 1200 km where the oscillation pattern is fully developed. The conceptual detector design consists of O(30,000) large area and acceptance photosensors arranged in a matrix of ∼300×300 m2 size. Hybrid photon detectors are considered as promising candidates as they provide clean signal characteristics and uniform collection efficiency. We discuss the design and expected performance of a large spherical HPD with 380 mm diameter, which is housed in a high-pressure glass container. A scaled prototype HPD of 208 mm diameter is currently under development using the existing CERN HPD facility.