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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
e-mail: edelheit@admin.in2p3.fr
CNRS press contact:
Martine Hasler
Tel: +33 1 44 96 46 35
e-mail: martine.hasler@cnrs-dir.fr
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