New lines added to the French KM3NeT deepsea neutrino detector

23 November 2021

In the Mediterranean Sea near Toulon, four new detector lines were added to neutrino telescope KM3NeT in the deep sea last week. There, a total of ten lines with 180 light sensor globes are now observing neutrinos in the Earth’s dark seawater.

On November 18, the two expedition ships Castor and Janus sailed out of the port of Toulon in southern France. On board were seven new detector lines, to be placed in KM3NeT’s French sector ORCA at 2500 meters depth. The crew kept a blog of the mission.


In total, ORCA will have 115 lines. A second Italian sector ARCA near Sicily will have another 230 lines. Many of the detector bulbs on the lines will be assembled at Nikhef in Amsterdam.

Undersea forest

KM3NeT will be an undersea forest of sensors that pick up light signals from an enormous volume of seawater when particles from the cosmos hit an atom there. The observed volume must be large because neutrinos have no mass and charge and hardly ever collide with other matter.

ORCA mainly studies neutrinos that have traveled through the Earth from below after originating on that side upon the impact of cosmic rays in the atmosphere. The French part revolves around the properties of neutrinos themselves. The Italian part is more of a telescope, looking at sources of neutrinos.

Unwinding Coil

Each line of KM3NeT has 18 detector spheres (DOMs), each with 31 light sensors, and sits on an unwinding coil that is first sunk to the seafloor at a predetermined location. There, an unmanned submarine connects the electronics to a link to shore. Then the line is unwound until it is vertical on the bottom, held up by a float. The coil itself is eventually picked up by the ships for later reuse.

Of the seven lines, four have been installed, tested and unwound. They are now taking readings. Technical problems and rough seas prevented the other three lines from being deployed as of Sunday. That will be done in a future mission. The ships have now returned to the port of Toulon.

Neutrino flavors

The French part of KM3NeT studies the fundamental properties of neutrinos. There are three flavors of neutrinos: electron, muon and tau. Neutrinos do not have an identity, but constantly change flavor. ORCA looks at muon neutrinos in different directions and deduces which part of the neutrino stream they cannot see because the particles have just taken on the invisible flavor.

The research of, among others, the Dutch KM3NeT group at Nikhef mainly revolves around the question of the masses of neutrinos. These are minuscule, but must be slightly different. The main question is how the masses increase from light to something heavier. Logically, the electron neutrino would be the lightest, followed by muon and tau, just like the corresponding mass particles in the standard model. But this has not yet been measured, and the theory does not give a definite answer.