KM3NeT: Deep under the Mediterranean Sea

A scientific mystery

Deep down in the darkness of the Mediterranean Sea stand hundreds of mysterious vertical strings, each carrying large spherical glass frames stretching far into the distance.

This improbable infrastructure is built at 3,500 m below the waves and covers a cubic kilometer of seawater. Such an undertaking can only be driven by the pursuit of something equally as extraordinary: understanding the beginning of our universe through the observation of neutrinos.

Neutrinos are incredibly difficult to observe due to their elusive nature. Their weak interaction plays a crucial role in our learning of how matter evolved from simple particles into more complex composites, creating the world we live in. Only in the darkness nearing the seabed can detectors capture the faintest amount of light from neutrinos striking the water molecules.

The European research infrastructure KM3NeT houses the next generation of neutrino telescopes underwater. Named after its instrumented cubic kilometer volume, institutions from across the world are collaborating in the development of these neutrino studies. Their first objective is the discovery and subsequent observation of high-energy neutrino sources in the Universe; their second is the determination of the mass hierarchy of neutrinos¹.

Faced with the difficulty of observing these subatomic particles and the titanic technical challenge of their research environment, only incredibly sophisticated technology can successfully shine a light on this scientific mystery.

Photomultiplier tubes (PMTs) - product lineup

An enormous endeavor

KM3NeT is an exceptionally intricate submarine operation. Not only does it require a large panel of skills and expertise to build such an infrastructure, but the right technology is crucial to its success. One of the key technologies is the detectors used for capturing the faint level of light indicating the neutrino interaction. This information is then transferred back to the data centers for scientific analysis.

These ultra-fast photosensors are known as photomultiplier tubes (PMTs). As one of the world's leading manufacturers of PMTs, Hamamatsu Photonics was approached to participate in this venture. However, this was no small challenge.

The solution needed to be cost-effective, with a good response time, and provide a wide energy range, high-energy resolution, high sensitivity, and low noise. As neutrinos pass through matter as though it is not there, increasing the number of detectors meant increasing the chances of catching a neutrino interaction. At first, the idea of using a large-sized PMT made sense especially as Hamamatsu is known for offering the world's largest PMT (20 inches). However, the KM3NeT Collaboration thought of a more efficient design: large glass spheres called Digital Optical Modules, which included 31 PMTs of only 3 inches. This was the ideal cost-effective and highly performant solution as the PMTs were smaller, and easier to produce yet their spatial resolution was greater given the quantity. However, this meant that around 192,000 of these 3-inch PMTs were needed.

The challenge faced by Hamamatsu Photonics was to produce this large quantity of PMTs while maintaining a high level of quality and repeatability. PMTs are made of glass and are often even handmade; their production is not automated like other products, so realizing thousands at such high standards in a short timeframe was an enormous endeavor.

A new era

Thankfully, Hamamatsu Photonics already had experience working with many of the world’s leading neutrino observation projects, including the University of Tokyo’s Super-Kamiokande in Japan. Although this was a tough task, the Hamamatsu spirit never wavered.

To meet the research institutes’ demands, Hamamatsu decided to invest in doubling its manufacturing capacity. This major enterprise led them to produce much higher volumes of PMTs managed not only through an increase in the factory workforce but also through the introduction of a much more complex logistical network.

Hamamatsu manufacturing experts also achieved remarkable consistency in the manufacture of PMTs through non-stop repeatable production, which still goes on to this day. In-house testing complemented this approach and ensured the best possible quality of sensors – crucial for the accuracy of the data studied. So far, Hamamatsu has already delivered all the PMTs which are needed to integrate more than 2,000 DOMs, enough for putting together more than 100 detection strings.

Since the start of this adventure in December 2015² arrays of thousands of these optical sensors have been working relentlessly to capture the faint light in the deep sea. Yet KM3NeT is only just getting started, more infrastructures are already being built, meaning even more collected data, aiming at building the largest detector of neutrinos in the Northern Hemisphere.

“Most telescopes look upwards at the light coming from the sky. The KM3NeT telescope searches for neutrinos and prefers to look downwards through the Earth to study the austral sky.” from Dr. Paschal Coyle, Spokesperson of KM3NeT Collaboration.

REFERENCES

1: https://jphysplus.iop.org/2016/06/24/km3net-the-next-big-thing-in-neutrino-physics/

2: https://www.km3net.org/the-construction-of-km3net-the-next-generation-neutrino-telescope-has-begun/

KM3NeT: Deep under the Mediterranean Sea

A scientific mystery

An enormous endeavor

A new era

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