Professor Nick van Eijndhoven from Vrije Universiteit Brussel (VUB-ELEM) speaks to The Innovation Platform’s International Editor, Clifford Holt, about the Radio Neutrino Observatory, a proposed next-generation ultra-high energy neutrino observatory to be built in Greenland.
Neutrinos are unique messengers. They point back to their sources and can reach us from the most distant corners of the universe because they travel undeflected by magnetic fields and unimpeded by interactions with matter or radiation. Unlike γ-rays, which can be explained by inverse Compton scattering, the observation of high-energy neutrinos from these objects provides incontrovertible evidence for cosmic ray acceleration, since both neutrinos and γ-rays are produced when cosmic rays interact with ambient photons or matter within their source. Resolving the sources of cosmic rays and the acceleration mechanisms will require a comprehensive multi-messenger programme involving observations of cosmic rays, γ-rays, and neutrinos across many decades of energy.
With the discovery of a diffuse flux of astrophysical neutrinos and the identification of a multi-messenger source candidate, the success of IceCube has established neutrinos as a powerful messenger in the exploration of the high-energy universe.
The Radio Neutrino Observatory (RNO-G), a proposed next-generation ultra-high energy neutrino observatory which will be built in Greenland, will extend multi-messenger neutrino astronomy to energies above 10 PeV. RNO-G is designed around a broad multi-messenger astrophysics programme to be an instrument that measures of order 10 neutrinos at the highest energies, possibly including the first discovery. The RNO-G collaboration will develop the techniques required to rapidly produce and respond to alerts of astrophysical transients.
Clifford Holt, International Editor at The Innovation Platform, spoke to Professor Nick van Eijndhoven from Vrije Universiteit Brussels (VUB-ELEM) Inter-university Institute for High Energies, about how the RNO-G will build on and complement other neutrino observatories and some of the challenges involved.
Could you begin by briefly outlining the Radio Neutrino Observatory in Greenland, its background and what you hope it will achieve?
As you probably know, in 2013 the IceCube Neutrino Observatory at the South Pole discovered a flux of high-energy neutrinos (which are a species of elementary particles) originating from cosmic sources.
So far, we have not been able to trace back the sources of these neutrinos, due to the fact that until now we have observed only about 100 of these particles. To identify the sources and to study the characteristics of the energy spectrum of these neutrinos, we need to record many more of them.
However, at these high energies, the flux is very low. As such, even in a cubic kilometre detector such as IceCube we record on average just one event per month. Thus, to significantly increase the statistics, we need a detector which is about 100 times larger than IceCube. However, to do this using the IceCube technology would be too expensive, as IceCube is based on the detection of light flashes induced by neutrinos interacting in the Antarctic ice or underlying bedrock. Since light only travels for about 200m through ice, the IceCube sensors can be placed at a maximum distance of around 200m to 250m apart, otherwise the light cannot reach the sensors.
Radio waves (which are also produced when a neutrino interacts in the ice) on the other hand, can travel over a distance of about 1km in ice, which means that with a sparse network of radio antennas one can cover a volume much larger than that covered by IceCube.
As such, we are now constructing a first large scale radio detector array in Greenland. This is foreseen to comprise 35 stations, spaced about 1.5km apart, covering about 50km2. In addition to increasing the number of recorded high-energy neutrinos, this detector will also allow us to explore even higher energies at which neutrinos are expected but have not yet been observed.
With the Greenland detector array, we will gain experience with this new radio detection technique on a large scale, which will help us in the design of a large (500km2) radio extension of the current IceCube detector in the near future.
How will the new observatory build on previous (and, indeed, current) neutrino detectors such as ARA and ARIANNA? What will it do that others cannot?
As indicated above, the new observatory will enable us to explore a high-energy regime that is not accessible with current detectors and we will indeed use the experiences gained from the first attempts at radio detection of neutrino interactions in ice (the RICE, ARA, and ARIANNA experiments).
The ARIANNA experiment uses the same simple antennas which are used by amateur radios, placing them at the surface of the Antarctic ice sheet. The ARA experiment uses more complicated, custom made, antennas located at a depth of about 200m beneath the Antarctic ice sheet. These experiments are sensitive to different signatures of the interacting neutrinos and cosmic rays impinging the detector from above.
In the new Greenland detector, we will combine both techniques to achieve sensitivity to both signatures. Furthermore, we will also locate antennas at various depths and couple some of them with specific phase differences. This will allow us to sum the signals of these ‘coupled antennas’ (called a ‘phased array’) which will strengthen the recording of a neutrino-induced signal in certain directions. This will allow us to lower the detection threshold (which is equivalent to becoming sensitive to neutrinos of lower energy) and, as such, close into the energy domain where IceCube has so far seen only a handful of events. This would enable us to significantly increase the number of recordings at these (and higher) energies.
What is the main science case for RNO-G? What do you hope the main outcomes will be, particularly in terms of fundamental physics and/or multi-messenger astronomy?
As mentioned, the RNO-G observatory is expected to increase the number of recordings of high-energy neutrinos to allow a systematic study of their energy spectrum and identification of their cosmic sources. Furthermore, it will open the exploration of an even higher energy regime, where neutrinos from a different origin (so called cosmogenic or GZK neutrinos) are predicted due to the interaction of ultra-high energy cosmic rays with the cosmic background radiation. This would unambiguously probe the explanation of the sudden drop in cosmic ray flux at the highest energies, as observed with the Pierre Auger and Telescope Array detectors.
We also foresee the development of an alert system for RNO-G, which can either react to alerts from other observatories, or send an alert to other observatories when RNO-G detects an interesting event. This would be in the same spirit as IceCube’s current alert system.
In this way, we would open the way for multi-messenger studies via follow-up observations. As RNO-G is located at the Northern hemisphere, while IceCube is located at the South Pole, there is also a large degree of complementarity. In the case of RNO-G observing an event from a certain location in the Northern sky (it cannot see high-energy neutrinos from the Southern sky, because at these high energies the Earth is opaque to neutrinos), for instance, IceCube can investigate that same location for neutrinos of lower energy (which can indeed travel through the Earth).
The Northern sky is where IceCube is the most sensitive, due to the Earth shielding it from background events; which would make a very versatile synergy.
Is the required technology already available, or do further hurdles exist? What do you anticipate will be the main challenges in terms of making the observatory operational?
In principle, the required technology is already available, but we are currently optimising the systems. One of the main hurdles is the power supply. In light of the large distances involved, our stations have to work autonomously. For the time being, this is being achieved via the well-proven technique of solar panels.
However, since we are so far North, there are some periods in which the sunlight is too weak to provide sufficient power. To enable data gathering during these ‘dark ages’, we are investigating an alternative power supply that involves a small-scale wind turbine coupled to each detector station. However, wind turbines, of course, consist of moving parts, and these may introduce electromagnetic interference into our detector systems. Currently, we have some very promising wind turbine prototypes, however, and the intention is to install one at the first deployment at one of our stations to provide a test bed.
The other challenge is the development of antennas that are sensitive to horizontally polarised radio waves and still fit in our 11” borehole. Measurement of the polarisation of the radio signals is needed for a good reconstruction of the arrival direction of the neutrino. For this, both the vertical and horizontal component of the radio signal has to be recorded. The vertical component is not a problem, as one can make a vertical antenna as long as one wants in our (vertical) borehole. However, for a horizontal antenna it is another story due to the fact that the borehole has a diameter of only 11”. To overcome this problem, we have developed a quite complicated design for antennas that are sensitive for the horizontal signal component.
In an RNO-G station, we will deploy separate antennas with vertical or horizontal sensitivity. This will allow the recording of both polarisation components of the signal.
Of course, the ongoing COVID-19 situation is also presenting challenges of its own.
What are your hopes for the RNO-G in both the short (as construction is expected to start next year) and the longer terms?
In the short term (that is, the first one to two years after the first deployment) we would like to record atmospheric cosmic ray showers with the surface antennas and see whether we can also ‘see’ these in the deep detectors. That would be a proof of principle and allow us to further optimise the trigger and data acquisition system.
Once the observatory is completed (i.e. 35 (or more) stations, which is expected to be achieved following three deployment seasons), we will exploit it to search for the signals as outlined above and join the various multi-messenger campaigns.
Nick van Eijndhoven
Professor of Physics
Vrije Universiteit Brussel (VUB-ELEM)
Inter-university Institute for High Energies (IIHE)
+32 (0)2 629 3212