22.06.2022 – 17:12
Darmstadt University of Technology
Long-sought particle made of four neutrons discovered
Research team observes a neutral nucleus for the first time – the tetra neutron
Darmstadt, June 22, 2022.
An international research team with leading participation from the TU Darmstadt has succeeded for the first time in generating an isolated four-neutron system. The researchers overcame the experimental challenge by using a new method.
The experiment was conducted at the Radioactive Beam Accelerator Facility (RIBF) at the RIKEN Research Center in Japan. In addition to the TU Darmstadt, scientists from the TU Munich, the RIKEN Nishina Center and the GSI Helmholtz Center for Heavy Ion Research in Darmstadt were also involved in the large international collaboration. The experiment provided an unequivocal signal for the first observation of the tetra neutron. The result has now been published in the journal “Nature”.
The building blocks of atomic nuclei are the nucleons, which come in two types, the neutral neutrons and the positively charged protons. To the best of our knowledge, there are no stable – or bound – nuclei made up exclusively of neutrons. The only known bound systems that consist almost exclusively of neutrons are the neutron stars in our universe, with a typical diameter of about ten kilometers. These stars are gravitationally stable, which leads to a very high neutron density inside these stars. Atomic nuclei, in turn, are bound by the strong interaction, with the preference to bind the same number of neutrons and protons – this is known from the stable nuclei found on our earth.
However, the research of pure neutron systems is of great importance, since this is the only way to gain experimental knowledge about the interaction of several neutrons with each other and thus about the nuclear interaction itself. Research into the previously hypothetical systems could also help to better understand the properties of neutron stars. Finding out whether such neutron systems exist as unbound nuclear states or even as bound nuclei is therefore a long-standing endeavor in nuclear physics. The team of scientists led by the TU Darmstadt has now made a new attempt and used a new experimental method that differs from all previous attempts. The work was supported by the German Research Foundation (DFG) through the Collaborative Research Center 1245.
“This experimental breakthrough provides a reference value for the theory to understand the interactions of pure neutron clusters and thus also the properties of neutron-rich nuclei,” says Dr. Meytal Duer from the Institute for Nuclear Physics (IKP) at the TU Darmstadt. “The nuclear interaction between more than two neutrons has not yet been tested experimentally, while theoretical predictions lead to very different results. We are now planning a next-generation experiment at the R3B facility at FAIR, which will allow direct measurement of the correlations between the four neutrons with the R3B NeuLAND detector. This will provide new insights into the nature of this four-neutron system.”
The experimental investigation of pure neutron systems represents a major challenge. Because there is no way to produce a neutron target, i.e. the matter that is exposed to the particle beam. In order to generate a multi-neutron system in such a way that the neutrons can interact with each other via the short-range nuclear force (a few femtometers, 10-15 meters), reactions must be used. The great danger that the interaction of the neutrons with other particles involved in the reaction changes the actual signal or makes it invisible was solved here by using a high-energy 8He beam. The 8He nucleus consists of a compact alpha particle surrounded by the four lower-density neutrons. The alpha particle is now shot out of the 8He nucleus in a fast reaction with large momentum transfer by collision with a proton of the liquid hydrogen target: The remaining four neutrons are suddenly free and alone and can interact with each other.
“The key to the successful discovery of the tetra-neutron was the selected reaction and the selected kinematics with high momentum transfer, which immediately separates the neutrons from the charged particles in the momentum space,” says Professor Dr. Thomas Aumann from the IKP at the TU Darmstadt. “This has enabled an almost background-free measurement. We are now planning the same reaction, but with a 6He beam to precisely measure the neutron-neutron interaction at low energies. A suitable neutron detector is currently being built at our university.”
The publication
M. Duer, T. Aumann et al.: „Observation of a correlated free four-neutron system“, in „Nature“ (2022), 22. Juni 2022, DOI: 10.1038/s41586-022-04827-6, https://go.nature.com/3n8mBvu
Contact
Dr. Meytal Duer
Tel.: +49 6151/1623578
E-Mail: [email protected]
About the Technical University of Darmstadt
The TU Darmstadt is one of the leading technical universities in Germany and stands for excellent and relevant science. The TU Darmstadt plays a decisive role in shaping global transformations – from the energy revolution to Industry 4.0 to artificial intelligence – through outstanding findings and future-oriented courses.
TU Darmstadt bundles its cutting-edge research into three fields: Energy and Environment, Information and Intelligence, Matter and Materials. Its problem-focused interdisciplinarity and productive exchange with society, business and politics generate progress for sustainable development worldwide.
Since it was founded in 1877, the TU Darmstadt has been one of the most international universities in Germany; as a European Technical University, it is building in the alliance Unite! a trans-European campus. With its partners from the Rhine-Main universities – the Goethe University in Frankfurt and the Johannes Gutenberg University in Mainz – it is further developing the Frankfurt-Rhine-Main metropolitan region as a globally attractive scientific area.
MI no. 39/2022, Aumann/mih
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