KM3NeT - Telescope

The Online Data Filter for the KM3NeT Neutrino Telescopes

10 June 2025 – Recently, we submitted a new paper with the title ‘The Online Data Filter for the KM3NeT Neutrino Telescopes’

In this paper, we present the design and performance of the software that is used to filter the data recorded by the photo-sensors of the KM3NeT detectors.

The KM3NeT telescopes

The KM3NeT collaboration is constructing a large research infrastructure at the bottom of the Mediterranean Sea comprising two telescopes named ARCA and ORCA. Unlike conventional telescopes, the KM3NeT telescopes detect neutrinos and not light from the cosmos.

Neutrinos are notoriously difficult to detect. To overcome this difficulty, KM3NeT uses water in the deep sea. Neutrinos are detected indirectly using three-dimensional arrays of photo-sensors which detect the Cherenkov light that is produced when relativistic charged particles emerge from a neutrino interaction. The density of the water in the deep sea provides for the necessary mass for neutrinos to interact and its transparency for a sufficiently large detection volume.

The online data filter

To filter the data recorded by the photo-sensors in the deep sea, we have implemented a custom designed software system. First, the analogue pulses from the photo-sensors are digitised offshore in the deep sea. Then, all digital data are sent to a control station onshore where they are processed in real time using a farm of commodity servers and custom software.

The filter quality

We have evaluated the performance of the data filter in three terms: its purity, its capacity and its efficiency. The purity – or signal-to-noise ratio – is measured by a comparison of the event rate caused by muons produced by cosmic ray interactions in the Earth’s atmosphere with the event rate caused by the background from decays of radioactive elements in the sea water and bioluminescence. The capacity of the filter is measured by the minimal number of computer servers that is needed to sustain the rate of incoming data. The efficiency is measured by the effective detection volumes of the sensor arrays.

Different event topologies

In nature three different flavours of neutrinos exist, namely electron, muon and tau. They are named after the charged particle that emerges from the neutrino interaction with matter. In the KM3NeT detectors these charged particles are recognised by the different topologies of photo-sensors hit by the Cherenkov light. In particular, a muon produces a long linear track of sensors hit, while the electron reveals itself as a ‘shower’ of sensors hit in multiple directions. Each type of neutrino yields a different effective detection volume.

The effective detection volumes in the figures

As an example, we present in the figures below the effective detection volumes of the ARCA telescope as a function of the neutrino energy for muon- and electron-neutrinos using two designated software algorithms. They show that in both cases the largest volume is obtained with the algorithm that matches the neutrino flavour.

They also show that for neutrinos of about 1 TeV the effective volume reaches the geometrial volume of the detector (dashed line). Below this threshold, the effective volume is smaller due to limited visible energy. Beyond the threshold, the growth of the effective volume can be attributed to neutrino interactions in the vicinity of the detector.

The paper has been submitted to section A of the journal on Nuclear Instruments and Methods in Physics Research – (NIM-A) and is available as a preprint at arXiv 2506.05881.

2nd DOM integration workshop

27 May 2025 – Do you know how the optical modules of KM3NeT are built? This has been shown and practised in the second edition of the Digital Optical Module (DOM) integration workshop which took place last week.

In total 30 people participated in the workshop. They came from the eight KM3NeT integration labs, including the new lab in Salerno, Italy. Experts from the KM3NeT steering committee and the central and local quality management were also present.

The workshop was purely hands-on! Each step in mounting a DOM was scrutinised, while the participants shared their experience and the procedures were discussed.

The workshop took place in the CAPACITY laboratory in Caserta, Italy, taking advantage of the new facilities which were recently inaugurated.

The construction of the KM3NeT optical modules consists of many steps, comprising several delicate operations. The final product is a pressure-resistant glass sphere which contains 31 photomultipliers and various electronics devices for the power supply and acquisition and transmission of data.

In addition, the optical modules contain important calibration devices, such as a compass, a piezoacoustic sensor for positioning the modules and a fast LED pulser, the nanobeacon, for calibrating the photomultipliers. They are fundamental for pushing the performance of the KM3NeT neutrino detectors.

The two hemispheres which compose an optical module are assembled and tested separately. When everything is installed and all functional tests are passed, it is time to proceed to non-reversible steps of integration, such as pouring optical gel in the interface between the photomultipliers and the glass of the hemispheres. Finally, the optical module can be closed and sealed. After undergoing a last acceptance test, the module is ready for being integrated in a detection line.

All integration and test procedures strictly comply with the high quality standards of KM3NeT.

And what if a problem occurs in a completed and sealed optical module? Is it possible to open it? The answer is yes! But with a very very delicate procedure.

Below are some pictures taken during the workshop.

Sneak peek into the KM3NeT labs: Nikhef

17 January 2025 – Today we start a series of items highlighting the work of our technical staff in the labs of KM3NeT. Numerous technicians and engineers are working on the construction of the ARCA and ORCA detectors of the KM3NeT neutrino telescope. Together, but in distributed labs, they design and build the many detector components, assemble them into thousands of optical modules and integrate them into hundreds of deployment-ready detection units. It requires high standards of quality control and logistics between the labs.

Spotlights on the Nikhef lab

In this first item we set the spotlights on the KM3NeT production lab of the Nikhef institute in Amsterdam, The Netherlands. It is the lab where our multi-PMT optical module was born and it is one of the first production labs in KM3NeT.

Video’s in this item: Courtesy Nikhef – Marco Kraan. He has filmed the full process of assembling an optical module at Nikhef. Below we use his collection of short video clips.

“You just have to get the hang of it” according to Menno de Graaff, electronics engineer featuring in the clips.

The KM3NeT multi-PMT optical module is the key component of the neutrino telescope. It is a complex sensor module that registers the Cherenkov light from the charged particles from neutrino interactions with the seawater and monitors the position of the module in the deep sea. All sensors and the electronic boards for their power and readout are densely packed in a pressure resistant glass sphere. Therefore, assembly of the module requires highly skilled technicians.

(Compilation video (3min))

Step 1 – The assembly of an optical module starts with the empty pressure resistant glass hemispheres.

Step 2 – In the top hemisphere an aluminum cooling ‘mushroom’ is glued to the glass using a gel, which is applied in layers to avoid bubble forming in the gel. Once in the deep sea the cooling mushroom will keep the temperature in the optical module below 30 degrees.

Step 3 – In the bottom hemisphere an acoustic sensor is glued to the glass using a hard adhesive for good contact with the glass. The sensor is part of the acoustic system used to monitor the position of the optical module in the deep sea.

Step 4 – Two electronic boards are installed in the cooling mushroom in the top hemisphere. One provides the electrical power to the sensors in the module. The other is the heart of the module – the central logic board that collects and digitises the signals of all sensors in the module and transmit the digitised data via optical fibres to the backbone cable of the detection unit that is connected to the electro-optical cable network toward the control station on the shore. The connection is made via a feedthrough in a hole in the glass hemisphere. It is carefully checked that the hole is leaktight after installation of the feedthrough.

Step 5 – On top of the central logic board a fibre tray is installed with the fibres that connect the board with the backbone cable of the detection unit.

Step 6 – Switch from the glass spheres to stacking the 31 photomultiplier tubes (PMTs). In the structure that later fills the top glass hemisphere, 12 PMTs are stacked leaving room for the cooling mushroom to pass. A reflective metal ring is put around each PMT to improve their light collection.

Step 7 – In the structure that later will be placed in the bottom glass hemisphere, 19 PMTs are stacked. Together the 31 PMTs form a ‘fly’s eye’ looking in almost all directions. The PMTs will ‘see’ the faint Cherenkov light of charged particles induced by neutrino interactions in the deep sea.

Step 8 – The structures filled with PMTs are placed in the prepared glass hemispheres and connected to the ‘octopus’ electronics boards which are placed in the space left between the PMTs. The hemispheres are ready for a functional test in the test room.

The two structures are connected through a test cable to verify the functional behaviour of the optical module before closing it. During the functional test, the PMTs, acoustic piezo sensor and a LED are verified to work according to specification. In addition, fibre and power connections are checked, as well as the temperature of the electronic boards.

Step 9 – Once the functional test is passed, the PMTs are fixed on their position in the glass sphere and the space between the PMTs and the glass is filled with a special highly transparent gel. This is necessary in order not to loose light during operation in the deep sea. The filling must be done with care to avoid forming of air bubbles in the gel. Air bubbles would distort the path of the light.

Step 10 – Time to close the two hemispheres. The trick is that the readout electronics boards of the PMTs in the lower hemisphere of the module must be mounted on the stem of the cooling ‘mushroom’ before closing the optical module. When everything is connected correctly, the module is closed and some air is removed from it to create a small underpression. Then the module is sealed with a sticky strip to prevent the two hemispheres getting loose during transportation or during deployment to the bottom of the Mediterranean Sea. Once in operation the two remain tightly close due to the high pressure in the deep sea. Finally, a collar for attachment to the supporting ropes of the detection unit is mounted.

Step 11 – The module is ready for connection to the electro-optical backbone cable of a detection unit. The backbone cable runs the full length of hundreds of metres along the detection unit. It comprises copper wires for electrical power and optical fibres for data transmission. The video clip presents a birds-eye view of two sets of the final product of 18 optical modules connected with a backbone cable.

Step 12 –  The 18 optical modules connected to a backbone cable are shipped to the labs in France or Italy, which will build a ready-to-deploy detection unit and load it onto a launching vehicle for deployment in the deep sea.

This marks the end of our highlight of the work of technicians and engineers constructing KM3NeT multi-PMT optical modules and detection units at Nikhef.

In a next edition we will report on the work of technical staff in other KM3NeT labs.

Interested in the technical details of the KM3NeT multi-PMT optical module? We published a paper here.

KM3NeT@neutrino2022

Fifty years ago, in 1972, the first neutrino conference was held in Hungary, because the organisers were not satisfied with the subdued position of neutrino interactions at the international conferences at that time. Nowadays, the neutrino conference is one of the major conferences in neutrino (astro)particle physics. Neutrino2022 took place in virtual Seoul, 30 May-4 June, 2022 and of course KM3NeT was there to show the results of data taking with the first detection units of ARCA and ORCA.

Aart Heijboer, physics coordinator of KM3NeT, showed the results of one year of data taking with ORCA6 and 100 days with ARCA6. The ‘6’  refers to the number of detection units in a detector used in the data analysis.

The ORCA detector is optimised to measure the oscillation parameters of neutrinos travelling through the Earth. Neutrino oscillation is a quantum mechanical phenomenon in which a neutrino created with a particular flavour – electron, muon or tau neutrino – can be later measured to have changed its flavour. In figure 1 below, it is evident that the data does NOT follow the flat blue horizontal line indicating the absence of neutrino oscillations. In other words, already with only six detection units, the ORCA6 detector ‘sees’ oscillations. In the second figure two oscillation parameters are plotted against each other. Clearly, the contour of ORCA6 is still wider than that of other experiments. More data with more detection units will make it narrower.

 

Also the ARCA detector, optimised for the search of high energy neutrinos from sources in the Universe, is well underway pushing the limits of the potential to discover sources of neutrinos down towards the expected limits of the full detector.

 

In the poster sessions KM3NeT physicists presented the details of many analyses being performed with the ARCA and ORCA detectors.

Aart Heijboer concluded at the plenary session that ARCA and ORCA will span eight decades in energy, that there is a rich variety of data analyses going on in the collaboration and that construction of the detectors is ramping up. Promising conclusions.

Very nice to have been invited to share the progress of KM3NeT with the community of neutrino (astro)physicists!

The KM3NeT Collaboration thanks the organisers of Neutrino2022 for an excellent edition of the conference. See you in two years time in Milano.