The neutrino detectors of the KM3NeT research infrastructure are three-dimensional arrays of light sensor modules or optical modules distributed over large volumes of the transparent water of the deep Mediterranean seas. The optical modules register the faint Cherenkov light from charged particles induced by neutrino interactions in the seawater.
KM3NeT has two neutrino detectors which differ in volume and in the density of optical modules in the detection volume. In the ARCA detector the modules are sparsely distributed in a cubic kilometre sized volume of seawater. ARCA is optimised for the registration of high-energy cosmic neutrinos. The denser distribution of optical modules in the smaller ORCA detector is optimal for the registration of lower energy atmospheric neutrinos necessary for the determination of the neutrino mass hierarchy. Atmospheric neutrinos are neutrinos induced by cosmic ray interactions in the Earth’s atmosphere. Although ARCA and ORCA differ in volume and geometry, the technology used to build the detectors is essentially the same.
The optical modules register the time of arrival of light on each of the photomultiplier tubes inside the modules; the brightness of the light; and the geometrical position of the module at the time of arrival of the light.
The measurements are transmitted to computers in a control room on shore – the shore station – via a network of optical fibres. In the shore station, intelligent software filters the raw data and reconstructs from the measurements the trajectories of the particles traversing the detector. The results are sent over the public internet to the KM3NeT data centres for further study by the KM3NeT scientists.
At the end of the KM3NeT 2.0 construction phase, the ARCA telescope will have a detector volume of about 1 cubic kilometre. In this volume, more than 4000 sensor modules are sparsely distributed supported by in total 230 vertical string-like detection units with a height of about 700 m. The ORCA detector will comprise about 2000 sensors on 115 detection units in about 7 Megatonnes of seawater.
While installing the detectors they are already operational for neutrino research with a smaller though growing number of detection units (see scientific output).
