On an early morning last week, in the depths of the Mediterranean Sea, a group of technicians installed the first detection unit of a large under-water neutrino telescope: KM3NeT. Leiden physicist Dorothea Samtleben is part of the collaboration of scientists and engineers that make this enormous project happen. Once completed, it will be largest neutrino telescope in the Northern Hemisphere and will be used to study the fundamental properties of neutrinos and map the high-energy cosmic neutrinos emanating from extreme cataclysmic events in the Universe. The Leiden Institute of Physics played already a prominent role in the data analysis of earlier smaller prototypes and the testing of the current detector.

'KM3NeT will start a new era in this field' says Samtleben 'With its unprecedented size and excellent resolution of the neutrino directions it will provide for rich new information to explore our Universe.'

Neutrinos are the most elusive of elementary particles and their detection requires the instrumentation of enormous volumes: the KM3NeT neutrino telescope will occupy more than a cubic kilometre of seawater. It comprises a network of several hundred vertical detection strings, anchored to the seabed and kept taut by a submerged buoy. Each string hosts 18 light sensor modules equally spaced along its length. In the darkness of the abyss, the sensor modules register the faint flashes of Cherenkov light that signal the interaction of neutrinos with the seawater surrounding the telescope. The detector is connected by a 100 km cable to the shore station located in Portopalo di Capo Passero in the south of Sicily.

Samtleben: ‘Of course we had tested the deployment procedure beforehand and also all components of the detector. But this was the first time we installed a full real detector unit, so we waited anxiously for the signal chain to become operational. We followed the news on a live feed over the deployment day. In the evening we could finally evaluate the first data, proving the full functionality. We celebrated the next day at Nikhef (Institute for Subatomic Physics – edit) where a major part of the construction of this detection unit took place.'

Maarten de Jong, director of KM3NeT and professor at Leiden University, is excited as well: ‘This important step in the verification of the design and the technology will allow the KM3NeT Collaboration to proceed with confidence toward the mass production of detection strings and their installation in the Mediterranean Sea’.

A building block of KM3NeT comprises 115 detections strings; the full detector has many building blocks with a total of a few hundred strings (courtesy Marc de Boer/Ori Ginale) (image at http://km3net.org/images/KM3NeT-Telescope-1.jpg)

The neutrino detection string wound around the spherical deployment frame is hoisted down into the Mediterranean Sea.