ScienceWeek

Nature 449, 992-993 (25 October 2007) | doi:10.1038/449992a; Published online 24 October 2007

Nuclear physics: A Maximum Number of Atomic Neutrons?

Paul-Henri Heenen

For most atomic nuclei, the maximum number of neutrons that can be bound is unknown. The discovery of two neutron-rich nuclei - and the confirmed absence of others - might help solve this conundrum.

Stars are element factories. Every atom in the Universe is a product of stellar nuclear reactions, but many of the stable isotopes that predominate today have formed from nuclei that existed only fleetingly. To understand the isotopic make-up of the Universe, we must first discover which of these ephemeral nuclei can form. In other words, what are the limits of nuclear composition? Reporting in this issue, Baumann et al.1 describe their discovery of two new nuclei, one of which was thought to be impossible to make (see page 1022). This paves the way for a reassessment of nuclear models.

Atomic nuclei are composed of protons and neutrons, bound together by the strong nuclear force. Because this strong attraction is greater between a neutron and a proton than between two like particles, the most stable isotopes for light nuclei have equal numbers of neutrons and protons. But in larger nuclei - those with more than 20 protons - repulsion between the positively charged protons reduces the binding energy of the nucleus, so that the most stable isotopes have more neutrons than protons. In other words, the attraction between the extra neutrons counterbalances the electric repulsion between the protons.

There are about 300 stable nuclei, but these are only a small fraction of those that have a lifetime long enough to have a role in the formation of elements in stars. A chart of nuclei can be drawn that plots the number of protons against the number of neutrons (Fig. 1). Two lines - known as drip lines - can be plotted on the chart to indicate the limits of possible nuclei. On the proton-rich side of the chart, the proton drip line corresponds to the smallest number of neutrons that can be bound for a given number of protons. If a neutron were removed from a nucleus on the proton drip line, then a proton would be spontaneously emitted. Similarly, the neutron drip line indicates the largest number of neutrons that can accompany a given number of protons; extra neutrons will not bind to nuclei on this line.

atomic physics
neutrons
isotopes
chemical elements