Published online 24 October 2007 | Nature | doi:10.1038/news.2007.189

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How many neutrons can an atom hold?

Heavy aluminium and magnesium shed light on atomic question.

Atomic collisions can make supersized elements.M. KULYK/SPL

Atoms can be more overweight than we thought, a team of scientists in the United States has discovered.

They have sent atoms crashing into one another in a particle accelerator to create bloated versions of the elements aluminium and magnesium. The new, artificial forms of these metals have many more neutrons in their atomic nuclei than do the everyday versions1.

Some theories suggest that this excess of neutrons might make the atoms fall apart because of insufficient ‘nuclear glue’ to bind all the subatomic particles together. But the researchers found that the new atoms are stable for the fraction of a second needed to detect them.

The results should help to guide theories of how atomic nuclei are held together — which in turn may tell us how elements are formed in stars, and what controls radioactive decay.

“It’s a benchmark measurement for nuclear theory, a test of how good our understanding is,” says team member Bradley Sherrill of Michigan State University in East Lansing. In particular, the stability of one neutron-laden form of aluminium came as a surprise, showing that there’s plenty still to learn.

Come together

Atomic nuclei are made up of two types of particle: protons, which have a positive electrical charge, and neutrons, which are electrically neutral. Each distinct chemical element is characterized by a specific number of protons, but can have varying numbers of neutrons. These different versions of an element are called isotopes.

Nuclei are held together by the nuclear strong force: a kind of 'glue' that operates between nuclear particles. It is not strong enough to bind protons (which repel one another electrically) or neutrons together on their own. But this 'glue' is slightly stronger between a proton and neutron than between either pair of like particles. As a result, atoms are usually stable so long as the number of protons and neutrons is not too uneven. If this balance isn't right, atoms can split apart through radioactive decay or nuclear fission.

If an atom gets too heavy with neutrons, extra neutrons simply won't stick at all — not even for an instant. Nuclear physicists have long been trying to map out where this boundary of stability lies. They call it the 'neutron drip line', because nuclei larger than this point are like oversized droplets that drip small fragments.

Scientists have measured the drip line for elements up to oxygen, with eight protons. But it's harder to determine for heavier elements, whose neutron-rich isotopes don't hang about for long. And different nuclear physics theories don't agree with each other about where the drip line lies. “For a given number of protons, a nucleus can hold a certain number of neutrons,” says Sherrill. “But we can’t yet predict exactly how many.”

Supersize atoms

Thomas Baumann of the National Superconducting Cyclotron Laboratory at Michigan State University and his collaborators have now pushed these measurements to new extremes.

They fired a beam of high-energy calcium ions into a sheet of tungsten, producing new elements. Among them, neutron-rich versions of aluminium and magnesium could be spotted in the few milliseconds before they decayed.

The researchers found an isotope of magnesium with 28 neutrons (magnesium-40) — more than twice its normal complement. That's bigger than the previous heaviest magnesium isotope found, which had 26 neutrons.

There doesn't seem to be a version of magnesium with 27 neutrons. That fits with the prevailing idea that neutron-rich isotopes with even numbers of both protons and neutrons are more stable than those with odd numbers, because the combinations gain stability from the formation of pairs.

Odd couple

Surprisingly, the team found evidence for isotopes of aluminium (which has 13 protons) with both 29 and 30 neutrons. That takes the drip line at aluminium farther out than was expected, and means that aluminium-42, with odd numbers of both neutrons and protons, is stable.

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This new view of stability implies that the drip line for aluminium may extend all the way to nuclei with 34 neutrons, although these isotopes haven't been seen.

As well as testing nuclear theory, Sherrill says that the results might teach us something about neutron stars: super-dense bodies formed by the collapse of some stars that have exhausted their nuclear fuel. In neutron-rich nuclei, the protons tend to clump in the middle, so that the nuclear surface is almost pure neutrons — just like the surface of a miniature neutron star. 

  • References

    1. Baumann, T. et al. Nature 449, 1022-1024 (2007). | Article |
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