Designing the future of the IceCube Neutrino Observatory

The IceCube Neutrino Observatory is a successful and large scientific facility located near the Amundsen-Scott South Pole station in Antarctica. This observatory hosts IceCube, a cubic-kilometer deep-ice particle detector that is, so far, the largest ever built – and on the surface, IceTop, an extended air shower array.

Completed in 2010, IceCube has recently discovered astrophysical neutrinos, revealing their potential to explore our universe at energies at the PeV scale and above, where most of the universe is opaque to high-energy photons. But the big questions remain unsolved: where do these neutrinos come from? How does nature accelerate particles to such extreme energies?

Prof. Olga Botner, IceCube spokesperson and a physics professor at the University of Uppsala, and Prof. Francis Halzen, IceCube principal investigator and a professor at the University of Wisconsin–Madison, tell us about the plans for an upgrade to the IceCube Neutrino Observatory. As an extension of the current detector, it can be built in a few years and within an affordable budget, thanks to expertise acquired with IceCube.

Artistic view of the Antarctic surface around the South Pole station, showing the position of the 86 strings of sensors in IceCube and the possible grid of the next-generation detector. Image: J.Yang/IceCube Collaboration

Q: What has IceCube accomplished so far?

Olga Botner (O): IceCube is the world’s foremost neutrino observatory, which, after just two years of running in its final configuration, discovered neutrinos from outer space that have energies a billion times larger than those of neutrinos produced by our Sun and a thousand times larger than any produced on Earth with man-made accelerators. The discovery of this high-energy neutrino flux is a turning point for neutrino astronomy: a dream of 50 years ago on the verge of becoming reality.

Francis Halzen (F): The high level of the observed neutrino flux implies that a significant fraction of the energy in the non-thermal universe, powered by the gravitational energy of compact objects from neutron stars to supermassive black holes, is generated in hadronic accelerators. This tells us that we are approaching exciting times when high-energy neutrinos will reveal new sources or provide new insight on the energy generation in known sources.

But IceCube has also been a successful detector with respect to its technical development. We developed highly successful designs for transforming natural ice into a particle detector. The optimized methods for deploying and commissioning large volume detectors in ice can be used for a next-generation detector; minimal modifications will target improvements focused on modernization, efficiency, and cost savings.

O: This is a very important point. The detector was built within the expected time frame, within budget, and with a performance at least a factor of two better than anticipated.

Going back to physics, I should also add that IceCube has yielded many interesting results beyond neutrino astronomy. We are studying cosmic rays, looking for signatures of the annihilations of dark matter particles into neutrinos, and investigating the properties of the neutrinos themselves. We have published competitive results in all these areas.

Q: Why do we need a next-generation IceCube detector?

F: We all agree on the observed spectrum of neutrinos, there’s no doubt about the discovery, but independent analyses of IceCube data have produced only on the order of 100 astrophysical neutrino events in several years. These modest numbers of cosmic neutrinos limit the ability of IceCube to be an efficient tool for neutrino astronomy over the next decade. A next-generation detector will provide an unprecedented view of the high-energy universe, taking neutrino astronomy to new levels of discovery. It is likely to resolve the question of the origin of the cosmic neutrinos recently discovered.

O: That’s right! IceCube’s discovery of extraterrestrial neutrinos has shown us that even a cubic-kilometer detector is not enough. To fully exploit the potential for neutrino astronomy, a much larger observatory is needed. We are already working on its design. The new detector has been named IceCube-Gen2.

Q: Is it feasible and cost-effective to build an even bigger detector at the Pole?

O: It sure is. The good news is that the successful deployment and running of IceCube demonstrates that we have mastered the technologies to construct and operate a detector in the deep ice. The drilling systems and the optical modules for the next-generation detector will closely follow the designs that have been proven to work well—with certain modifications to improve the overall performance. This makes us confident that a next-generation detector is not only feasible but can be built in a cost-effective manner, just like IceCube.

F: We didn’t know this before IceCube, but now we have measured the extremely long photon absorption lengths in ice. This will allow the spacing between strings of light sensors to exceed 250 m in a future IceCube extension; i.e., the instrumented volume can rapidly grow without increasing the costs much. In fact, we can build a ten-cubic-kilometer IceCube-Gen2 telescope by roughly doubling the instrumentation already deployed. Thus, a tenfold increase in astrophysical neutrino detection rates could be achieved with a cost comparable to the current IceCube detector.

Q: And what about the time scale of this project? Will we need to wait a long time to see new results?

O: We are aiming at an expanded array instrumenting a volume of 10 km3 for the detection of high-energy neutrinos—but also at improving the low-energy performance through deployment of a densely instrumented infill detector, PINGU, targeting neutrino mass hierarchy as its prime goal. We believe that this new IceCube-Gen2 observatory can be built within seven years of obtaining funding.

Q: Sounds like a plan. Who is leading this next-generation IceCube?

F: The present plan is to build IceCube following a management strategy that was successful in delivering IceCube on time and on budget. The collaboration is rapidly expanding, both in the US and in Europe and Canada. We expect that a larger fraction of the cost will be carried by significant contributions from our foreign collaborators.

O: Exactly. The high-energy array and PINGU are both envisioned as parts of an IceCube-Gen2 observatory. A new collaboration, including IceCube members and additional institutions, is now being formed. This IceCube-Gen2 collaboration will work to develop proposals in the US and abroad to secure funding. We hope that IceCube-Gen2 will become a flagship scientific project for NSF as well as for funding agencies abroad.

This image shows a simulated high-energy event of about 60 PeV in the proposed IceCube Gen2 detector. Image: IceCube Collaboration

Q: Can other current or in-design experiments do better than IceCube-Gen2?

F: Well, we have strong competitors. Early efforts for cubic-kilometer neutrino detectors focused on deep-water-based detectors, including DUMAND, Lake Baikal, and ANTARES. So far, there is no cubic-kilometer neutrino detector in deep water, but these experiments have paved the way toward the proposed construction of KM3NeT in the Mediterranean Sea and GVD in Lake Baikal.

O: These new projects, GVD in Lake Baikal and KM3NeT in the Mediterranean, are presently in the prototyping or early construction phase. They will eventually provide a complementary view of the sky to that of an Antarctic observatory.

**Q: Should we expect IceCube-Gen2 to be as successful as IceCube?

That may be the desire, but are there objective reasons to think so?**

O: The main one is that we already have established the existence of a flux of high-energy neutrinos. What we now need are substantial number of events to further characterize this flux in terms of energy spectrum, a possible energy cut-off, flavor composition, and provenance. We just need a larger detector to do this in a reasonable time. The higher event rates in a larger array will also improve the chances of correlating our neutrino events with observations by the new generation of high-energy gamma-ray telescopes and gravitational wave detectors, together charting the non-thermal universe.

F: The larger samples of high-energy neutrinos with improved angular resolution and energy measurement will give us a detailed understanding of the source distribution. This sample will reveal an unobstructed view of the universe at energies at PeV and above. Those are unexplored wavelengths where most of the universe is opaque to high-energy photons. As Olga was mentioning, the operation of IceCube-Gen2 in coincidence with other telescopes and detectors will present totally novel opportunities for multimessenger astronomy and multiwavelength follow-up campaigns to obtain a truly complete picture of astrophysical sources.

Info “IceCube-Gen2: A Vision for the Future of Neutrino Astronomy in Antarctica,” IceCube Collaboration: M.G. Aartsen et al.

This white paper presents early studies toward a next-generation IceCube detector with the aim of instrumenting a 10 km3 volume of clear glacial ice at the South Pole and delivering an order of magnitude increase in astrophysical neutrino samples of all flavors.

Read also a short description of IceCube-Gen2 on the IceCube website.