Their mass is extremely low, but how light are neutrinos really? A collaboration comprising German and international research groups has optimized its experiments to determine the mass of these "ghost particles". In doing so, they succeeded in further pushing down the upper limit on the neutrino mass scale. As part of the “Electron Capture in Ho-163 Experiment” (ECHo), the researchers use the isotope holmium-163 (Ho-163). Its decay processes allow for drawing conclusions on the neutrino mass. According to Professor Loredana Gastaldo, a scientist at the Kirchhoff Institute for Physics at Heidelberg University and spokesperson of the ECHo collaboration, the current results also verify that even larger-scale investigations will be feasible in future to get even closer to the neutrino mass and to ultimately determine it precisely.
A detector module for the ECHo experiments developed and built at the Kirchhoff Institute for Physics. The detector chip is located in the middle; the four surrounding chips contain the Superconducting Quantum Interference Devices (DC-SQUIDs) that read out the signals. Image Credit: ECHo Collaboration
Neutrinos are elementary particles with extremely low mass that have no electrical charge. Because their interaction with matter is very weak, the properties of these "ghost particles" are very difficult to determine. This is especially true for the neutrino mass, which has yet to be precisely measured, with only an upper limit being known. According to Loredana Gastaldo, determining the mass could pave the way for new theoretical models beyond the Standard Model of particle physics and thereby contribute to a better understanding of the evolution of our universe.
Determining the Neutrino Mass Based on the Decay Energy of Holmium-163
Several research groups worldwide are working on the determination of the neutrino mass scale through the analysis of radioactive decays. The lowest upper limit thus far has been obtained by the so-called Karlsruhe Tritium Neutrino Experiment (KATRIN), which is, however, approaching its final sensitivity. The ECHo experiment has been designed to complement the KATRIN results and eventually reach an even better sensitivity. The collaboration includes research teams from Heidelberg, Mainz, Darmstadt, Tübingen, and Karlsruhe, as well as Geneva (Switzerland) and Grenoble (France).
As part of the ECHo experiments to determine the neutrino mass, the researchers are studying the energy released during the decay of Holmium-163. In this decay process, a proton in the atomic nucleus of this radioactive isotope captures an electron, yielding a daughter nucleus in an excited state. The interaction between proton and electron produces a neutron and a "ghost-like" neutrino, which is ejected with a specific energy. The mass of the neutrino causes a slight change in the energy distribution of the atomic excitations. "We can draw conclusions about the mass of the neutrino from the slight changes in the measured energy spectrum," stated Gastaldo. According to the experimental physicist, the isotope holmium-163 is especially well suited for these measurements, because very little energy is released during its decay. That means that even tiny fluctuations in the spectral shape can be registered with appropriate detectors.
Scientific Collaboration of ECHo Collaboration Partners
However, the holmium-163 isotope does not occur naturally. It was laboriously produced artificially for the experiments. To this end, the research group of Professor Christoph Düllmann in the Department of Chemistry at Johannes Gutenberg University Mainz (JGU) produced special purified erbium samples. These were then irradiated with neutrons over several weeks at the research reactor of the Institut Laue-Langevin in Grenoble, France, which led to their conversion into holmium-163. The produced holmium was then transported to Mainz, where it was chemically separated from the remaining original erbium.
The ECHo experiments use metallic magnetic calorimeters. These detectors were developed and built at the Kirchhoff Institute for Physics under the direction of Prof. Loredana Gastaldo. They are approximately 200 micrometers in size and operated at extremely low temperatures of 20 millikelvins, so even tiniest energy differences in the form of temperature fluctuations can be detected.
Embedding of holmium-163 into the calorimeters was also carried out at JGU, in the research group led by Professor Klaus Wendt at the Institute of Physics. To do this, the holmium atoms were vaporized at temperatures exceeding 1,500 degrees Celsius and ionized by the absorption of laser light. The resulting ions were accelerated in the RISIKO mass separator so that they were implanted into the detectors mounted behind the separator. The detectors were then transported back to Heidelberg. Thanks to an improved detector design, the experiment conducted now at Heidelberg University made it possible for the first time to observe approximately 200 million such holmium-163 decay processes.
This allowed the researchers to push the upper limit of the neutrino mass further down by approximately one order of magnitude compared to previous ECHo measurements – and by a factor two compared to the results of the HOLMES Collaboration, which also uses Holmium-163 to determine the neutrino mass. "This result reinforces the significance of the ECHo experiments and demonstrates that even larger-scale experiments using Holmium-163 will be possible in future," stressed Gastaldo. To this end, she plans to increase the number of detectors from currently 100 to 20,000. For the "Electron Capture in Ho-163 – Large Experiment" (ECHo-LE) project, she has obtained an ERC Advanced Grant from the European Research Council (ERC).
Teams from Heidelberg University, the Max-Planck Institute for Nuclear Physics in Heidelberg, Johannes Gutenberg University Mainz, the Helmholtz Institute Mainz, the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, the University of Tübingen, and the Karlsruhe Institute of Technology have all contributed to the current research. Other contributors include researchers from the CERN European research center in Geneva (Switzerland) and the Institut Laue-Langevin in Grenoble (France). The German Research Foundation funded the work. The results have recently been published in Physical Review Letters.