Press release


Can a nucleus be made up of neutrons only?

Paris, April 25, 2002


An international team led by physicists from the "Laboratoire de physique corpusculaire de Caen" (Particle Physics Laboratory of Caen, CNRS/IN2P3-ISMRA(1)) , have presented experimental results suggesting the existence of a bound atomic nucleus made up of four neutrons (a "tetraneutron"). These results, to be published soon in Physical Review C, have been obtained by using the exotic beams of the French national large heavy-ion accelerator in Caen (Ganil, CEA-CNRS). If confirmed, this discovery, which would call into question current theoretical models, will have major repercussion in the field of nuclear physics.

One of the major challenges in nuclear physics is to understand how atomic nuclei are built from their component parts, namely nucleons (protons and neutrons). A few simple facts are now established: firstly, all nuclei that are heavier than the hydrogen nucleus (made up of a single proton) make up both protons and neutrons. Furthermore, a system made up of only two neutrons is not bound… but it is very nearly: a slight increase in the attraction between the two particles would result in a bound structure, the "dineutron," being formed. Finally, research into nuclei having more than two neutrons shows that, very often, adding a further neutron increases the stability of the structure.

The question then arises as to whether a neutron system made up of more than two neutrons could exist. On the basis of current knowledge of interactions between nucleons, the theoretical answer is probably not. In fact, for 40 years now, all attempts to show experimentally that such a system could exist have failed. However, over the last ten years, with the advent of high-energy exotic nuclei beams, it has been possible to design new experiments since it is thought that nuclei that are very rich in neutrons could contain clusters made up of neutrons only. Such clusters could be released when very exotic nuclei are broken in collisions with other nuclei. The problem of how to detect and identify such neutral objects remains, since they can easily be confused with single neutrons, which are also released during such collisions. The physicists have developed a method similar in many ways to the method used by James Chadwick when he discovered the neutron in the 1930s. It is based on the fact that a head-on collision between a proton and a four-neutron system transmits much higher energy to the proton than would be passed on to it by an impact with a single neutron.

Careful analysis of the data collected at the French accelerator Ganil, with the British multidetector Charissa and the Franco-Belgian neutron detector Demon, has revealed six events compatible with the characteristics of a tetraneutron that may have been produced when beryllium 14 nuclei break. This number of events is greater than the estimated background noise level, depending on the possible occurrences of other processes. In view of the small number of events observed, it is essential to continue specific experiments to search for the tetraneutron. If such experiments were to confirm the current results, they would call into question the current models of nucleon-nucleon interactions.

1) IN2P3: Institut national de physique nucléaire et de physique des particules (National Institute of Nuclear and Particle Physics);
ISMRA: Institut des sciences de la matière et du rayonnement (Institute of Matter and Radiation Sciences, Caen)

IN2P3 contact:
Geneviève Edelheit
Tel: +33 1 44 96 47 60

CNRS press contact:
Martine Hasler
Tel: +33 1 44 96 46 35