PMm²: R&D on triggerless acquisition for next generation neutrino experiments

The next generation of proton decay and neutrino experiments, the post-SuperKamiokande detectors, such as those that will take place in megaton size water tanks, will require very large surfaces of photo-detection and will produce a large volume of data. Even with large hemispherical photomultiplier tubes (PMTs), the expected number of channels should reach hundreds of thousands. An ANR funded R&D program to implement a solution is presented here. The very large surface of photo-detection is segmented in macro pixels consisting of an array (2 × 2 m²) of 16 hemispherical 12-inch PMTs connected to autonomous underwater front-end electronics working in a triggerless data acquisition mode. The array is powered by a common high voltage and only one data cable allows the connection by network to the surface controller. This architecture allows a considerable reduction of the cost and facilitates the industrialization. This paper presents the complete architecture of the prototype system and tests results with 16 8-inch PMTs, validating the whole electronics, the built-in gain adjustment and the calibration principle.