Drip line slips away

Researchers at Michigan State University, US, have squeezed 29 neutrons into an atom of aluminium, bringing into doubt current theories which predicted that it would too unstable to exist. 

Thomas Baumann and colleagues at Michigan State University, US, used a particle accelerator to smash neutron-rich calcium nuclei into a tungsten target and produced ⁴²Al as well as

⁴⁰Mg , the heaviest magnesium yet made, by mixing 28 neutrons with magnesium’s 12 protons. 

Establishing the drip line - the maximum number of neutrons an element can hold - is key to understanding the nuclear reactions in neutron-rich stars - the source of every atom in the universe. But the short-lived isotopes at the drip line are difficult to make, so for most elements scientists rely on models to predict where the drip line lies. 

’Elements reach the drip line because the attractive force that binds the nucleus together seems to saturate,’ explains David Morrissey, one of the Michigan State team. ’Like two magnets, neutrons want to be touching, but there’s a physical limit to how many you can put together.’ 

As experimental techniques improve, heavier and heavier isotopes are made - often requiring the models to be adjusted. 

’There are at least 12 serious models that predict how heavy a nucleus can be before it comes apart,’ said Morrissey. ’Generally, the theories are not right, but if you make adjustments [based on new experimental data] they get a little better.’ 

But the existence of the latest neutron rich isotope of aluminium was not predicted by any of the models - suggesting a bigger overhaul of the models might be necessary. 

Commenting on the results, nuclear scientist Paul-Henri Heenen, of the Free University of Brussels, Belgium, told Chemistry World: ’Nuclei with even numbers of neutrons and protons are more stable, because there is extra stability in pairs.’

Because ⁴²Al has odd numbers of both neutrons and protons, its neighbours, with even numbers, should certainly exist because they would be more stable. In fact, Baumann’s experiments also suggest the presence of ⁴³Al - a result Heenen is confident will be confirmed in the next few years. 

But getting beyond ⁴³Al to the element’s drip line could be beyond even the next generation of particle accelerators, said Morrissey. ’The heavier, more exotic nuclei get rarer and rarer, so might be very difficult to make. Even the new facilities, with beams a factor of 100 stronger, may not be enough.’ 

James Mitchell Crow 

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