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Hunting for neutrinos with Dyneema®

Synthetic rope made with Dyneema® proved to be the ideal solution

KM3NeT, an ambitious project to learn more about cosmic neutrinos, is moving closer to realization – with crucial help from ropes made with Dyneema® DM20 that provide the perfect tradeoff between strength, drag, and creep-resistance.

A mystery of the universe

Neutrinos are thought to be the second most common particle in the universe after light particles. They may hold the key to explaining supernovae or the black hole at the center of our galaxy. And yet we know virtually nothing about them.

The problem with neutrinos is that they are extremely difficult to detect. They can pass through most materials as if they weren't there, so the only way to spot them is to look for faint splashes of light as the neutrinos hit other particles. And the only way to do that, is to confuse the neutrinos by putting sensors in places where there are no other light sources.

Test of the deployment method of a KM3NeT string

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Building a neutrino telescope underwater

There are already neutrino telescopes in Antarctica, Russia and other parts of the world, but the biggest and most ambitious plan is to build one in the Mediterranean. Testing and research has been going on since 2012, but entered a new phase in 2016 with the start of installation of the first telescope proper, off Sicily.

The idea with the Cubic Kilometer Neutrino Telescope, or Km3NeT for short, is to place three networks of detectors off the coasts of France, Italy and Greece. At around 3,500 meters deep, the network will be at the bottom of the Mediterranean where there are no waves, surface currents or light. KM3NeT needs this total darkness to see the faint amount of light that is given off when neutrinos hit water molecules.

Tracking neutrinos in the Med

  • Environment: Mediterranean seawater
  • Ambient temperature: 12-15 °C
  • Storage temperature: 10-60 °C
  • Seabed depth: 3,500m
  • Sensor depth: 2,500m
  • Length of anchor rope: 1,000m
  • Life expectancy: at least 15 years
  • Vertical displacement: less than 10cm
  • Horizontal displacement: less than 180m at sensor
  • Rope elongation: under 0.01% over 15 years
  • Installation: easy to splice during installation at sea

Among the many challenges raised by the KM3NeT is the issue of how to hold the neutrino telescope’s sensors in place.

Anchoring the KM3NeT neutrino telescope with Lankhorst ropes made with Dyneema®

  • Lankhorst Lanko®force rope
  • Made with Dyneema® DM20 fiber coated black
  • Diameter 4 mm; MBF=1,180kg (proposed)
  • Bending diameter (storage) 20mm (proposed)
  • Horizontal displacement at sensor: 150m, comfortably under 180mm target
  • Vertical displacement: less than 10cm
  • Expected design life: 15 years
  • Projected rope elongation: under 0.01%
  • Static load during mission time: 213 kg/rope
  • Static load if one rope breaks: 425 kg/rope
  • Working depth 2,000m-5,000m

The optimal solution

When completed, KM3NeT will be bigger than both the Antarctic and Russian arrays and monitor a part of the sky the other two don’t cover. The full neutrino telescope will contain around 12,000 pressure-resistant glass spheres attached to about 600 cables – vertical structures with a height of almost one kilometer. Each network will consist of glass spheres housing detectors, with each sphere tethered to others with cables. The cables will be spaced far enough apart to avoid getting tangled with each other, but close enough together to see signs of passing neutrinos.

Precision installation 2.5km down

Among the many challenges has been the question of how to hold in place the sensors that form the telescope. The design calls for them to be positioned 2.5 kilometers down, where they are anchored to the seabed by Detection Units (DUs, also called ‘strings’) 1,000 meters long. Each DU consists of 23 floating bars fixed to the sea bottom and is interspersed with floors, each equipped with two Digital Optical Modules.

All the elements in the DU are connected by four ropes. The maximum horizontal displacement of the sensor must be less than 180m at a survival sea current of 0.3 meters a second. The maximum vertical displacement has to be under 10cm. The strings will be under constant tension for at least 15 years and should be maintenance-free during that time.

A rope that meets the key criteria

Royal NIOZ and Nikhef, the project team responsible for parts of the telescope design, including the deployment system and mooring system, wanted to use synthetic rope because, unlike steel wire rope, its performance at that depth wouldn’t be limited by its own weight. Luckily, there was a rope that could meet all five key criteria. Lankhorst’s Lanko® force rope made with Dyneema® DM20. First, it should be as thin as possible (because the thicker the rope, the higher the drag and the larger the displacement of the sensor). Second, it should be strong enough not to break. Third, it should be rigid enough that the maximum vertical displacement of each 1,000m rope would be less than 10cm. Fourth, the maximum elongation due to creep should be under 0.01% over 15 years. Lastly, it shouldn’t need any maintenance while in service.

Anchoring KM3NeT using synthetic rope with Dyneema®

The first tests were conducted in 2012 and the ropes made with Dyneema® quickly proved to be the ideal solution. At just 4mm in diameter – chosen as the best tradeoff between strength and drag – the displacement at the sensor would be limited to 150m – comfortably within the 180m upper limit. Most importantly, it wasn’t elastic, so wouldn’t stretch, and being made of Dyneema® DM20, the permanent elongation (also known as ‘creep’) would be within spec and could be modeled and so allowed for in the design.

Since overcoming the anchor challenge, a crucial hurdle has been removed and this ambitious project can move forward to unravel more mysteries of our universe.

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