An Iowa State University-led team of physicists has demonstrated the quasi-stable existence of a tetraneutron, a subatomic structure once thought unlikely to exist.

On their own, neutrons are very unstable and will convert into protons after several minutes.

Systems of two or three neutrons do not form a stable structure, but the sophisticated supercomputer simulations in this research demonstrate that four neutrons together can form a resonance, a structure stable for a period of time before decaying.

For the tetraneutron, this lifetime is only 5*10-22 seconds. Though this time seems very short, it is long enough to study, and provides a new avenue for exploring the strong forces between neutrons.

“This opens up a whole new line of research. Studying the tetraneutron will help us understand interneutron forces including previously unexplored features of the unstable two-neutron and three-neutron systems,” said senior author Prof. James Vary, from the Department of Physics and Astronomy at Iowa State University.

The advanced simulations demonstrating the tetraneutron corroborate the first observational evidence of the structure earlier this year in an experiment performed at the RIKEN Radioactive Ion Beam Factory in Saitama, Japan, by Prof. Susumu Shimoura of the University of Tokyo and co-authors.

The tetraneutron has been sought for several decades with little evidence supporting its existence, until now.

The properties predicted by the calculations in the simulations were consistent with the observed properties from the experiment in Japan.

Prof. Shimoura’s team used a beam of Helium-8, Helium with four extra neutrons, colliding with a regular Helium-4 atom.

The collision breaks up the Helium-8 into another Helium-4 and a tetraneutron in its brief resonance state, before it, too, breaks apart, forming four lone neutrons.

“The experiment in Japan found a candidate resonant tetraneutron state with an energy of 0.83 MeV above the tetraneutron disintegration threshold and with an upper limit of 2.6 MeV for the width,” the scientists explained.

The existence of the tetraneutron, once confirmed and refined, will add an interesting new entry and gap to the chart of nuclides, a graph representing all known nuclei and their isotopes, or nuclei with a different number of neutrons.

Similar to the periodic table, which organizes the chemical behavior of elements, the nuclide chart represents the radioactive behavior of elements and their isotopes.

While most nuclei add or subtract neutrons one at a time, this research shows that a neutron itself will have a gap between a single neutron and a tetraneutron.

The only other known neutron structure is a neutron star, small but dense stars thought to be made almost entirely of neutrons.

These stars have neutrons on the order 1057. They may be only about seven miles in radius but have a mass similar to that of our Sun.

Further research may explore if there are other numbers of neutrons that form a stable resonance along the path to reaching the size of a neutron star.

The team finding was published in the Oct. 28, 2016 issue of the journal Physical Review Letters.

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A. M. Shirokov et al. 2016. Prediction for a Four-Neutron Resonance. Phys. Rev. Lett. 117 (18): 182502; doi: 10.1103/PhysRevLett.117.182502

K. Kisamori et al. 2016. Candidate Resonant Tetraneutron State Populated by the 4He (8He,8Be) Reaction. Phys. Rev. Lett. 116 (5): 052501; doi: 10.1103/PhysRevLett.116.052501