Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Nuclear physics

Four neutrons together momentarily

A system of four neutrons known as the tetraneutron is a hypothetical state in nuclear physics. The report of evidence for the fleeting existence of this state has implications for research into neutron stars.

Atomic nuclei are composed of protons and neutrons, generically known as nucleons. These are not genuine elementary particles because they contain quarks and gluons, which interact with each other through the strong force (one of the four fundamental forces of nature). The strong interaction has subtle properties, with the most unsettling one being that quarks and gluons are never free, only confined within nucleons. Theorists continue to struggle to find exact solutions for various states of the highly complex quark–gluon systems, and to explain the nucleon–nucleon force that extends beyond the confinement region. One long-sought state is the four-neutron system known as the tetraneutron, which has no electric charge. Writing in Physical Review Letters, Kisamori et al.1 present evidence for the existence of such a state.

In the authors' experiment, strongly bound α-particles (composed of two protons and two neutrons, and therefore identical to helium-4 nuclei) in liquid helium-4 (4He) are used as a target for an incident beam of helium-8 (8He, the 'projectile nucleus'). 8He has two protons and six neutrons, and is produced in nuclear-fragmentation reactions in which oxygen-18 hits a beryllium target. The reaction between 8He and 4He is an appropriate choice for generating tetraneutrons, because the four 'extra' neutrons in 8He are weakly bound and can be easily transformed in the interaction with 4He.

The authors observed that the 8He projectile exchanges two units of charge with the 4He target and becomes a beryllium-8 nucleus (8Be, four protons and four neutrons), the energy of which was measured with high precision. Because of charge conservation, the two protons in the target 4He nucleus are substituted by neutrons, momentarily generating a four-neutron system in a quasi-bound state. This lasts only a few multiples of 10−22 seconds, after which it disassembles into free neutrons. This short-lived state appears as a bump in the energy spectrum of the 8Be nucleus that emerges from the reaction.

Nuclear forces are essentially identical between all nucleons, whether they are protons or neutrons. So it might seem strange that the tetraneutron is not bound but that the α-particle of two protons and two neutrons is strongly bound, despite the additional electrical repulsion between protons. The explanation is based on the Pauli exclusion principle, which forbids two identical nucleons from occupying the same quantum state. In the α-particle, all four particles can be in the same state because the two protons have opposite spins, as does the pair of neutrons, so that all four nucleons are different. But for four neutrons, only one pair can be in the lowest-energy state, forcing the second pair into a state of higher energy, thus making the tetraneutron unstable.

By applying the principle of energy conservation to the studied nuclear reaction, Kisamori and colleagues infer that the tetraneutron system has an internal excitation energy of about 0.8 million electronvolts (MeV); the excitation energy is the difference between the tetraneutron mass and the mass of four free neutrons. If this quantity were less than zero, the system would be bound. For the observed tetraneutron it is positive, making it an unbound system that exists for a short time before it decays into free neutrons. The statistical error (± 0.65 MeV) and systematic error (± 1.25 MeV) in the experiment are large, but the case for the existence of the tetraneutron is compelling. The width of the bump in the 8Be energy spectrum is about 2.6 MeV, and this energy uncertainty suggests that the state will eventually decay to another quantum state.

The hunt for the tetraneutron has been going on for more than half a century, and experimentalists have announced the state's discovery before. In 2002, one collaboration claimed to have found a bound tetraneutron2 in an experiment based on the detection of neutron clusters formed by fragmentation of beryllium-14 projectiles. But the result remains unconfirmed, and theorists quickly showed that, based on the best knowledge of the nucleon–nucleon interactions and other arguments3,4, the existence of a bound tetraneutron was nearly impossible.

However, theorists could not rule out the existence of a tetraneutron as a short-lived 'resonant' state on the basis of a dineutron–dineutron structure3,4. The dineutron state is formed by two neutrons, and is not stable. It is known as a virtual state: if its energy were reduced by 66 keV, then the dineutron system would become bound. Decades earlier, it had been proposed5 that dineutrons can become bound in the presence of additional nucleons; this mechanism is responsible for the properties of some bound nuclei that have a neutron excess, such as lithium-11, in which a pair of external neutrons forms a remote halo around the core of lithium-9.

The tetraneutron cannot form an atomic nucleus because it is charge neutral and therefore cannot hold electrons. But there is an intimate relationship between the tetraneutron structure and theoretical studies of neutron stars (Fig. 1), in which neutrons are compressed to densities more than 1014 times that of water6. They are prevented from imploding by an outward pressure that is generated by the nucleon–nucleon interaction and other quantum-mechanical effects.

Figure 1: Neutron systems.
figure 1

a, Neutrons have a radius of about one femtometre (10−15 m), and can be either bound in a nucleus or free (although unbound neutrons decay within about 15 minutes). Dineutrons, composed of two unbound neutrons, are ten times larger and unstable. Kisamori and co-workers1 report evidence for a tetraneutron (a system of four neutrons) that exists in a resonant state for about 10−22 seconds before dissociating into free neutrons. b, If the existence of the tetraneutron state is confirmed, it will help to clarify nuclear interactions in few-nucleon systems, and possibly even in neutron stars.

Nuclear physicists hope to develop a full understanding of how quarks and gluons inside nucleons generate nucleon–nucleon forces, and how many-body objects evolve to form complex structures such as the uranium nucleus and neutron stars. This is a formidable task, with well-understood parts but also many missing links. If Kisamori and co-workers' report of the tetraneutron state is confirmed, even as a short-lived resonance, it will add another structure to the nuclear chart that will help to improve our understanding of the nuclear interaction.Footnote 1

Notes

  1. See all news & views

References

  1. Kisamori, K. et al. Phys. Rev. Lett. 116, 052501 (2016).

    ADS  CAS  Article  Google Scholar 

  2. Marqués, F. M. et al. Phys. Rev. C 65, 044006 (2002).

    ADS  Article  Google Scholar 

  3. Bertulani, C. A. & Zelevinsky, V. J. Phys. G 29, 2431–2437 (2003).

    ADS  CAS  Article  Google Scholar 

  4. Pieper, S. C. Phys. Rev. Lett. 90, 252501 (2003).

    ADS  Article  Google Scholar 

  5. Migdal, A. B. Sov. J. Nucl. Phys. 16, 238–241 (1973).

    Google Scholar 

  6. Shapiro, S. L. & Teukolsky, S. A. Black Holes, White Dwarfs, and Neutron Stars (Wiley, 1983).

    Book  Google Scholar 

Download references

Author information

Authors and Affiliations

Authors

Corresponding authors

Correspondence to Carlos A. Bertulani or Vladimir Zelevinsky.

Related links

Related links

Related links in Nature Research

Nuclear physics: Close encounters of the alpha kind

Nuclear physics: Neutrons with a twist

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Bertulani, C., Zelevinsky, V. Four neutrons together momentarily. Nature 532, 449 (2016). https://doi.org/10.1038/nature17884

Download citation

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/nature17884

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing