The detector consists of a 3-dimensional array of optical modules (OMs) immersed in the deep sea, 100 m above the sea bed.
A muon neutrino converts to a muon via a charged current interaction with a nucleon of the rock or the surrounding sea water. Cherenkov light is emitted in the sea water by the muon and then is detected by the OMs. The measurement of the arrival time of the light over at least five OMs allows the reconstruction of the muon direction. The amount of collected light allows to estimate the muon energy which is a lower limit of the neutrino energy.
An OM is made of a photo-multiplier tube (PMT), its electronics and power supply housed in a pressure resistant glass sphere. We have considered 8 and 15 inch PMTs with hemispherical photo-cathode. Each PMT is shielded against the Earth magnetic field by a high permittivity metallic cage.
The OMs are hooked to a mechanical structure with flexible (strings) and rigid elements.
Optical beacons consisting of glass spheres housing blue GaN LEDs allow a local time and amplitude calibration of the OMs. However, to be able to perform a global time calibration of a large scale detector, a light source such as a YAG laser is required, because of its greater light yield.
There are different ways of arranging physically the photo-multipliers (orientation, pairing...). A final choice will result from optimization of efficiency, resolution and cost.
Different schemes for the data transmission can be considered [33,34]. For the readout of the optical modules one can have a cluster of about 10 OMs grouped around a local controller. Most of the signals that an OM will detect will consist of single photoelectrons (SPEs). These SPEs are mostly caused by light emitting background processes such as beta decays of K or bioluminescence. For K the typical counting rates have been measured (see optical background measurements in section ) to be about 60 kHz (20 kHz) at the 0.5 (0.3) PE level with a 15 (8) photo-cathode Hamamatsu PMT. In order to reduce this rate to about 1 kHz, PMT signals can be sent to the controller which elaborates a local coincidence between neighboring PMTs and triggers the digitization of the signals. The digital information is transmitted through several network nodes in the array to a terminal node. The terminal node is connected to an electro-optical cable bringing the power and slow control commands from the shore. In the terminal node the digital information modulates the light output of laser diodes. This light is then transmitted to the shore via optical fibres.