Since 2010, the IceCube Observatory at the Amundsen-Scott South Pole Station has been delivering groundbreaking measurements of high-energy cosmic neutrinos. It consists of many detectors embedded in a volume of Antarctic ice measuring approximately one cubic kilometer. IceCube has now been upgraded with new optical modules to enable it to measure lower-energy neutrinos as well. Researchers at the Karlsruhe Institute of Technology (KIT) made a significant contribution to this expansion.
IceCube serves to measure high-energy neutrinos in an ice volume of one cubic kilometer. As neutrinos themselves do not emit any signals, the tracks of muons and other secondary particles are measured precisely. Muons are elementary particles sometimes produced by the interaction of neutrinos with ice. Contrary to neutrinos, muons carry an electric charge. On their way through the ice, they produce a characteristic light cone, which is detected by highly sensitive detectors. Now, 51 researchers from around the world have installed six new strings of novel sensors up to 2,400 meters deep into the eternal ice, thereby expanding the IceCube experiment to also measure low-energy neutrinos.
New optical sensors amplify even weak light signals
"The novelty of the optical sensors in the upgrade is that they are equipped with photoelectron multipliers in all directions, allowing a 360-degree view into the ice. This enables us to observe neutrino interactions at lower energies and thus determine the properties of neutrinos, complementing the KATRIN experiment at KIT," says Dr. Andreas Haungs, scientific director of the IceCube working group at the Institute for Astroparticle Physics at KIT. "In addition, we can better investigate the properties of ice in a large volume, thereby improving measurement accuracy. In combination with the new surface instrumentation, the upgrade also offers new possibilities for measuring high-energy cosmic rays."
The photosensors amplify the weak light signal emitted by charged secondary particles from the interaction of neutrinos during their very rare reactions in the transparent ice. These light amplifiers are installed in 40 cm football-shaped containers, also known as mDOMs (multi-PMT digital optical modules), together with other sensors. Cable harnesses connect these mDOMs and other measuring instruments to form a kind of pearl necklace 1,500 meters long. These chains are lowered into 2,400-meter-deep shafts, which a hot-water drill melts into the ice within two days. Six shafts were created in this way, which freeze again after the instruments have been inserted.
In Germany, the Helmholtz Centers DESY and KIT, as well as the universities RWTH Aachen, Bochum, Dortmund, Erlangen, Mainz, TU Munich, Münster, HU Berlin, and Wuppertal are involved in the upgrade of the IceCube Observatory. The researchers at KIT were responsible for the approximately 10,000 photosensors of the mDOMs that were installed in IceCube. They are also responsible for expanding the instrumentation on the surface of the experiment, which consists of scintillators and radio antennas that the researchers developed and built.
Looking to the future: IceCube Gen2
"The upgrade will extend neutrino astronomy to lower energies. This not only opens a new window onto the universe, but also serves as a meaningful technology and practical test for the proposed expansion to IceCube-Gen2," says Professor Ralph Engel, head of the Institute for Astroparticle Physics at KIT. "Gen2 will then enable neutrino astronomy at the highest energies. The result will be a globally unique observatory capable of measuring neutrinos over an energy range of ten orders of magnitude."
IceCube-Gen2 is proposed as the next expansion stage of IceCube and is intended to increase the measurement volume of the experiment to 8 cubic kilometers. It is a selected project of the German National Roadmap for Research Infrastructures. The full application by the Helmholtz Association, which will be presented in Berlin at the end of February, envisages the two Helmholtz centers, Deutsche Elektronen-Synchrotron DESY and KIT, as equal supporting institutions with a total investment of 55 million euros.
About IceCube
Led by the University of Wisconsin, Madison, USA, around 450 researchers from 58 institutions in 14 countries are working on IceCube. After the USA, Germany is the most important partner. KIT has been a member of the IceCube collaboration since 2016.
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