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.
The locations of the drip lines on the chart of nuclei are still an open question. Experiments performed in particle accelerators have proved the existence of about 3,000 isotopes2. Up to three times as many are thought to be awaiting discovery3, but the short lifetimes of these bashful nuclei and the difficulty involved in making them makes this a formidable task. Nevertheless, some progress has been made. The location of the proton drip line is known for atoms with up to 90 protons — that is, up to thorium. This has been easier to establish than has the location of the neutron drip line, because electric repulsion between protons restricts the number that can be added to a nucleus with a given number of neutrons.
The situation is very different for neutron-rich nuclei, because no electric repulsion exists to limit the number of neutrons that can be added. The neutron drip line is therefore distant from the stable elements on the chart of nuclei, and so is much harder to reach experimentally. Furthermore, the number of neutrons that can theoretically be added to a nucleus increases as the number of protons increases.
The neutron drip line is therefore firmly established only up to oxygen, which has eight protons and can have a maximum of 16 neutrons. Beyond oxygen, the drip line has been tentatively assigned for elements up to sodium, which has 11 protons and a possible maximum of 26 neutrons. Baumann et al.1 now report the discovery of two more neutron-rich isotopes: a magnesium isotope (40Mg, which has 12 protons and 28 neutrons) and an aluminium isotope (42Al, which has 13 protons and 29 neutrons).
The authors prepared these nuclei by firing the most neutron-rich calcium nuclei (48Ca) at a natural tungsten target. Neutrons transfer from the tungsten to the 48Ca nuclei, which can then break up to form 40Mg or 42Al. Although the number of nuclear interactions that could lead to the desired isotopes was extremely small (approximately 1 in 1015 reactions), Baumann et al. were able to identify them efficiently using state-of-the-art particle accelerator and detection technology. Just three collision events corresponded to the formation of 40Mg, whereas 23 others led to that of 42Al. The authors' results also seem to confirm that several other nuclei (30F, 33Ne and 36Na) do not exist, as they were not observed during the experiment. This is curious, because isotopes of these three elements have been detected4,5 that each contain a further neutron (that is, 31F, 34Ne and 37Na).
The discovery of 42Al and the non-appearance of 36Na are telling results. These isotopes have odd numbers of both neutrons and protons. This means that they lack a stabilizing effect seen in other nuclei: superconductivity. In superconductors, electrons form pairs that differ only by the orientation of their spins. Protons and neutrons share the same tendency to pair up, with the nucleus receiving a boost of stability for each proton or neutron pair formed. An 'odd–odd' situation is a worst-case scenario for nuclei, because they have an unpaired proton and an unpaired neutron that do not contribute to superconductivity.
The idea of proton and neutron pairs helps explain irregularities in the neutron drip line. For example, 35Na nuclei exist, but Baumann and colleagues' results suggest that it isn't possible to add one neutron to make 36Na. But it is possible to add two neutrons to produce 37Na, an isotope with an even number of neutrons — presumably because the additional neutrons form a nucleus-stabilizing pair. Similarly, one can conclude from the stability of the odd–odd nucleus 42Al that it should be possible to add an extra neutron to reach 43Al, because the formation of a neutron pair will stabilize the heavier isotope. In fact, the authors did observe a single collision event that is consistent with the existence of 43Al, but this result requires further confirmation.
So why does 42Al form at all, if it has an odd–odd nucleus? This isotope breaks the established staggered pattern of the neutron drip line, and was predicted not to exist. The answer could be that aluminium nuclei have reached a threshold of stability that minimizes the negative effect of extra neutrons, as has been suggested by some quantum-mechanical calculations. With the existence of 42Al confirmed, there are now good arguments to suggest that the neutron drip line could extend as far as 47Al.
Baumann and colleagues' results stretch the limits of current particle accelerators, so exploration of the neutron drip line at higher proton numbers will have to wait for the next generation of facilities. In the meantime, we must rely on models to predict where the neutron drip line falls — models that currently provide contradictory results. The authors' findings will certainly help to improve the accuracy of these models, so allowing a more confident assessment of the nuclear terra incognita.
Baumann, T. et al. Nature 449, 1022–1024 (2007).
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