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Major upgrade for undersea neutrino detector

Building the neutrino detector KM3NeT on the Mediterranean Sea floor has picked up pace. Five detection lines, each carrying eighteen detectors, have been sunk and installed, and the first measurement data are coming in, says neutrino physicist Dorothea Samtleben.

The KM3NeT detector is being built to detect neutrinos coming from outer space. Neutrinos elementary particles which interact with other matter very rarely: every second, millions of them are passing undetected through each square centimeter. 

A track of light in the pitch dark sea

Cosmic neutrinos could bring new insights to astrophysics, but their extreme shyness makes them very hard to detect, and this is exactly what KM3NeT is meant to do, installed on the bottom of the Mediterranean at 3500 under sea level. Very rarely, a neutrino collides with a water molecule, which generates short lived particles tracing a faint trace of light. In the pitch dark water, this so named Cherenkov radiation can be detected by KM3NeT's detectors.

Sites in France and Italy

The extremely light sensitive detectors are installed in pressure resistant glass balls, of which eighteen are mounted on 700 meter long cables. These are installed on the sea bottom like vertical chains of pearls. KM3NeT has two sites, one off the coast at the French Toulon and one close to Capo Passero in Sicily. This last site has been upgraded from one to six detector lines, including a relay station for electricity and data.


Crew in quarantine

'The ship just returned', says Dorothea Samtleben, neutrino physicist at Leiden University, 'they have been working hard, under severe covid restrictions. The crew had to quarantine, but in the end, the installation went flawlessly.' 

How does KM3NeT work?

The detector lines are wound up on a roll, which is sunk to the bottom. Then, the roll unfurls, and the detector balls drift upward. This installation method hase been developed by the Dutch sea research institute NIOZ together with the particle physics institute Nikhef in Amsterdam.

First results in May

Data from the detector lines is coming in, says Leiden neutrino physicist Maarten de Jong, Samtlebens colleague. Now, the detector has to be commissioned. The first neutrino data will probably be available in May. De Jong: 'There is a big conference in our field, and which we hope to show the first statistics.'

'The data show a background signal from muons and neutrinos mostly originating from collisions of cosmic rays within the atmosphere', says Samtleben, 'once we can tell apart the neutrinos well, we're in a good position to detect neutrinos from space.'

Looking for cosmic neutrinos

The Italian KM3NeT-site is designed to be a neutrino telescope, which looks for high energy neutrinos originating from cosmic sources, such as large black holes which emit jets of highly energetic radiation. Eventually, 230 detection lines have been planned, and the next round of installation is planned for May.

New data for students

'The fact that we're not at six detection lines, is also nice for students', says De Jong, 'they can work with fresh data now.' De Jong spent his career developing the neutrino telescope, and developed the neutrino detection software and contributed to a sound pulse based positioning system that the detector balls use. 'I will not rest before we detect neutrinos from astronomical sources', he says.

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