From DESY German Electron Synchrotron [Deütsches Elektronen-Synchrotron] (DE) : “Chasing cosmic particles with radio antennas in Greenland’s ice”

From DESY German Electron Synchrotron [Deütsches Elektronen-Synchrotron] (DE)


Pioneering project listens for neutrinos from outer space.

The first station of the network on the Greenland ice. The red flags mark underground antennas powered by solar panels (dark rectangles). Credit: Cosmin Deaconu/Radio Neutrino Observatory in Greenland-U Chicago (US). The Radio Neutrino Observatory – RNO-G – will be built in Greenland to search for ultra-high energy neutrinos.

The phased array as deployed in the Askaryan Radio Array-U Wisconsin (US) at the South Pole. The main trigger of RNO-G will be an updated version of this technology.

The Aero6gen unit was temporarily put up on a 20′ tower; afterward the turbine was to be retrograded, and the tower sections were reused on the Whisper turbine at Site 2. In the foreground on the box is a 235W PV panel, not yet mounted.

The layout for each of the 37 planned clusters.

A schematic diagram of how ARA detects and analyzes neutrino interactions.

Briefly, ARA uses radio antennas to detect nanosecond-long radio pulses from high-energy neutrinos. These are believed to be produced by ultra-high-energy cosmic rays, perhaps emanating from supermassive black holes in nearby active galactic nuclei. ARA uses the Askaryan effect, whereby charged particles can similarly emit radio or microwave radiation. By comparison, IceCube utilizes the Cherenkov effect, where charged particles moving faster than the phase velocity of light can emit light radiation. Because ARA looks for high-energy particles, a larger array is required than that of IceCube.

Schematic layout of the equipment.

The revised site layout for the Askaryan Radio Array (ARA) for the 2011-12 season.

In Greenland’s ice sheet, a set-up unlike any other in the world will in future be listening for extremely elusive particles from space. The Radio Neutrino Observatory in Greenland-U Chicago (US) is a pioneering project that relies on a new method of detecting very high-energy cosmic neutrinos using radio antennas. The scientists involved in the project have now installed the first antenna stations in the ice at the Summit Station research facility.

“Neutrinos are extremely elusive, ultralight elementary particles,” explains DESY physicist Anna Nelles, one of the initiators of the project. “These particles are created in vast quantities in space, especially during high-energy processes like those that take place in cosmic particle accelerators. But they are very difficult to detect because they hardly ever react with matter. From the Sun alone, some 60 billion neutrinos pass completely unnoticed through a speck on Earth the size of a fingernail – every second.”

The ultralight elementary particles are sometimes called ghost particles because they have no trouble passing straight through walls, the Earth and even entire stars. “This property makes them interesting for astrophysicists because they can be used to look inside exploding stars or merging neutron stars, for example, from which no light can reach us,” explains Nelles, who is also a professor at Friedrich–Alexander University Erlangen–Nürnberg [Friedrich-Alexander-Universität Erlangen-Nürnberg] (DE). “Also, neutrinos can be used to track down natural cosmic particle accelerators.”

On extremely rare occasions, however, a neutrino does in fact interact with matter when it happens to bump into an atom as it passes through – the Greenland ice sheet, for instance. Such rare collisions produce an avalanche of secondary particles, many of which are electrically charged, unlike the neutrino. This cascade of charged secondary particles emits radio waves that can be picked up by the antennas.

Summit Station is situated in the middle of the ice sheet. Credit: Cosmin Deaconu/ RNO-G.

“The advantage of using radio waves is that ice is fairly transparent to them,” explains DESY physicist Christoph Welling, who is currently in Greenland as part of the project team. “This means we can detect radio signals over distances of several kilometres.” The greater the range, the larger the volume of ice that can be monitored, and the greater the chances of detecting one of the rare neutrino collisions. “RNO-G will be the first large-scale radio neutrino detector,” says Welling. Previous smaller-scale experiments had already shown that it is possible to use radio waves to detect cosmic particles.

Overall, the scientists plan to install 35 antenna stations, each 1.25 kilometres apart, around Summit Station on the mighty Greenland ice sheet. Nevertheless, it could take months or even years before the observatory records a signal. “Neutrino research calls for patience,” explains Nelles. “Capturing high-energy neutrinos is an incredibly rare event. But when you do catch one, it reveals an enormous amount of information.” The researchers are also already thinking ahead to the next step, because the next radio neutrino observatory is planned literally at the other end of the world, augmenting the IceCube neutrino telescope at the South Pole.

IceCube neutrino detector interior.

There, an international consortium, which includes DESY, has installed some 5000 sensitive optical detectors to depths of several kilometres inside the Antarctic ice. These photomultipliers are looking out for a faint bluish flash of light, which is also produced by the energetic secondary particles from one of the rare neutrino collisions as they race through the subterranean ice. Using this technique, IceCube has already succeeded in making some spectacular observations of neutrinos arriving from the vicinity of a gigantic black hole or shattered star, for example. The visible light from the subterranean secondary particles cannot be tracked over such long distances in the ice as radio waves. However, the photomultipliers make up for this by responding to cosmic neutrinos with lower energies.

“The higher the energy, the rarer the neutrinos become, which means you need larger detectors,” explains DESY scientist Ilse Plaisier, who also is part of the installation team in Greenland. “The two systems complement each other perfectly: IceCube’s grid of optical detectors registers neutrinos with energies of up to about a quadrillion electron volts, while the array of radio antennas will be sensitive to energies from about ten quadrillion to a hundred quintillion electron volts.” The electron volt is widely used as an energy unit in particle physics. One hundred quintillion electron volts roughly corresponds to the energy of a squash ball travelling at 130 kilometres per hour – but in the case of a neutrino, that energy is concentrated in a single subatomic particle that is a quintillion quintillion times lighter than a squash ball.

The first stage of installing the equipment for this pioneering project is due to continue until mid-August, and carrying this out during the pandemic has been a huge logistical challenge: teams have had to spend several weeks quarantined at various locations before arriving at Summit Station, to avoid introducing the coronavirus. RNO-G will remain on the Greenland ice sheet for at least five years. The individual stations can operate autonomously, powered by solar panels, and will be connected with each other via a wireless network. Based on their operation, radio antennas are planned to be added to the IceCube neutrino detector at the South Pole as part of its Generation 2 expansion (IceCube-Gen2).

“Detecting radio signals from high-energy neutrinos is a very promising way of significantly increasing the energy range we can access, and thus opening this new window to the cosmos even further,” says Christian Stegmann, DESY’s Director of Astroparticle Physics. “We are pursuing this path via initial test structures in Greenland, and will then go on to install radio antennas at the South Pole as part of IceCube-Gen2.”

More than a dozen partners are involved in the pioneering project, including the University of Chicago (US), Free University of Amsterdam [Vrije Universiteit Amsterdam] (NL), Pennsylvania State University (US), the University of Wisconsin-Madison (US) and DESY.

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DESY German Electron Synchrotron [Deütsches Elektronen-Synchrotron] (DE) is one of the world’s leading accelerator centres. Researchers use the large-scale facilities at DESY to explore the microcosm in all its variety – from the interactions of tiny elementary particles and the behaviour of new types of nanomaterials to biomolecular processes that are essential to life. The accelerators and detectors that DESY develops and builds are unique research tools. The facilities generate the world’s most intense X-ray light, accelerate particles to record energies and open completely new windows onto the universe. 
That makes DESY not only a magnet for more than 3000 guest researchers from over 40 countries every year, but also a coveted partner for national and international cooperations. Committed young researchers find an exciting interdisciplinary setting at DESY. The research centre offers specialized training for a large number of professions. DESY cooperates with industry and business to promote new technologies that will benefit society and encourage innovations. This also benefits the metropolitan regions of the two DESY locations, Hamburg and Zeuthen near Berlin.