Physicists using the superconducting analyzer for multiple radio isotope beam particles (SAMURAI) in Japan have experimentally observed a resonance-like structure consistent with a tetraneutron state after 60 years of experimental attempts to clarify the its existence.
Schematic illustration of the quasi-elastic reaction investigated by Duer et al. Above: quasi-elastic scattering of the helium-4 (4He) nucleus from a helium-8 (8He) projectile from a proton target in the laboratory frame. The length of the arrows represents the momentum per nucleon (the velocity) of the incoming and outgoing particles. Zbeam is the axis of the beam. Below: the equivalent p-4He elastic dispersion in its frame of the center of mass, where we consider the reactions at backward angles close to 180 °. In this framework, the moment of the proton balances that of 4He, Pp = −P4He, that is, the proton is four times faster than 4He. Image credit: Duer et al., Doi: 10.1038 / s41586-022-04827-6.
A long-standing question in nuclear physics is whether uncharged nuclear systems can exist.
According to current knowledge, only neutron stars represent near-pure neutron systems, where neutrons are approached by gravitational force at very high densities.
The free neutron has a lifespan of just under 15 minutes and decays into a proton, an electron, and an antineutrient.
The two-neutron system, the dineutron, was observed unambiguously in 2012 in the decomposition of beryllium-16 and is known to be bound only by about 100 keV.
The next simpler system of three neutrons is less likely to exist due to the odd number of nucleons and therefore a weaker bond; however, a recent calculation has suggested its existence.
Following these considerations, the four-neutron system, the tetraneutron, is a suitable candidate to address this question.
Numerous attempts have been made to find a clue to its existence as a bound or resonant state.
Most of these experiments were performed with stable nuclei. Towards the 21st century, with the development of radioactive ion beam installations, it was possible to use extremely neutron-rich nuclei in which improved formation of a tetraneutron system can be expected.
“Our experimental breakthrough provides a benchmark for testing nuclear force with a pure system made only of neutrons,” said Dr. Meytal Duer, a physicist at the Institute of Nuclear Physics at the Technische Universität Darmstadt.
“Nuclear interaction between more than two neutrons has not been proven so far, and theoretical predictions give a wide dispersion over the energy and width of a possible tetraneutron state.”
Dr. Duer and colleagues conducted the experimental study using the multi-particle superconducting analyzer of radio isotope beams (SAMURAI) at the radioactive ion beam factory operated by the RIKEN Nishina Center and the ‘Nuclear Studies at the University of Tokyo.
To produce a tetraneutron state, they used the knockout of an alpha particle (helium-4 nucleus) from a high-energy helium-8 projectile induced by a proton target.
“The key to the successful observation of the tetraneutron was the chosen reaction, which isolates the four neutrons in a fast process, compared to the nuclear scale, and the chosen high-momentum kinematics, which separates the neutrons from charged particles in impulse space, ”said Professor Thomas Aumann, a physicist at the Institute of Nuclear Physics at the Technische Universität Darmstadt.
“Extreme kinematics resulted in an almost bottomless measure.”
“We now plan to use the same reaction to make an accurate measurement of the low-energy neutron-neutron interaction. A dedicated neutron detector is currently being built for this experiment.”
An article on the findings appears in the journal Nature.
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M. Duer et al. 2022. Observation of a correlated four free neutron system. Natura 606, 678-682; doi: 10.1038 / s41586-022-04827-6