Published online 10 February 2010 | Nature | doi:10.1038/news.2010.58

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Superheavy atoms weigh in

Precise mass measurement aids the hunt for heavy elements that decay slowly.

SHIPTRAPNobelium has been weighed after being captured in a Penning trap.GSI Helmholtzzentrum für Schwerionenforschung

Using a special trap, researchers have captured and weighed three isotopes of the superheavy element nobelium — the heaviest element so far to have its mass measured directly.

The measurements are an important step towards discovering the 'island of stability', a small class of yet-to-be discovered heavy elements that physicists think are likely to remain stable for minutes, days or even years. The study, published this week in Nature1, will also help to refine descriptions of the heaviest atoms ever made.

The heaviest element that occurs naturally on Earth is uranium, but scientists have synthesized 25 heavier elements using particle accelerators. A few of these elements are stable for years — such as plutonium — but most last only a few seconds or less before decaying by nuclear fission into something lighter.

Determining the precise atomic masses of superheavy elements is no mean feat, says study author Michael Block, a nuclear physicist at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany. Until now, physicists could only estimate the mass of heavy elements indirectly, by adding up the masses and energies of the more stable 'daughter' and 'granddaughter' nuclei that are created when a heavy element decays.

But the mass of a heavy nucleus is more than just the sum of its parts. That's because the energy that binds an element's protons and neutrons together also contributes to its mass, as predicted by Albert Einstein's famous formula E = mc2. Indirect estimations of mass don't always get this binding energy right.

Shotgun approach

To measure the mass of these superheavy elements directly, Block and his colleagues first had to make them, using an accelerator to fire calcium atoms onto a target made of lead. In rare cases, these atomic nuclei collided and fused to form heavier nuclei. About once every second, the accelerator spat out an isotope of nobelium, a synthetic element that can live for just milliseconds or for several minutes, depending on the number of neutrons it has.

Once researchers had made the nobelium, they quickly had to separate it from the trillions of other atoms that passed through the lead target. To do that, the team used a special combination of magnetic and electric fields, which allowed nobelium to pass through unperturbed while scattering the lighter, fast-moving nuclei. The heavy nobelium was then slowed down by feeding it through cells filled with inert helium gas. Finally, the mass of nobelium was measured inside a Penning trap, a device that uses electric and magnetic fields to make the nobelium atoms spin in a circle. Measuring the speed of the spinning provides a direct measurement of the mass.

Massive problem

Using this technique, Block says that "we have been able to improve the accuracy by which the mass is known quite a lot". The trap can measure an atom with an accuracy equivalent to that of weighing a 100-kilogram person down to the milligram scale, he says. In the case of one isotope, nobelium-253, the new measurement was a factor of five better than previous estimates.

Rolf-Dietmar Herzberg, a nuclear physicist at the University of Liverpool, UK, was so impressed by the measurement that he has hung the results on the wall of his lab. "It's very, very good work indeed," he says.

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Accurately measuring the mass of known elements such as nobelium will allow scientists to improve their searches for still heavier elements, including those that are thought to be part of the island of stability. This region on the nuclide chart (which plots neutron number against proton number) is home to elements that are far heavier than anything yet seen. The latest work will allow scientists to measure such masses more accurately, without the problems of weighing daughter or granddaughter nuclei. Some nuclei on the island of stability are believed to be stable for years or more, meaning that they can be stored for long periods of time — suggesting a possible future use as hyper-efficient nuclear fuels for deep-space travel.

But the work is also valuable in the near term, says Herzberg. Current nuclear theory cannot precisely predict masses or nuclear structures for the heaviest elements, he says. The direct measurements will help with that and could, for example, lead to more efficient methods of nuclear-waste disposal by helping to pin down the structure of the decaying material. 

  • References

    1. Block, M. et al. Nature 463, 785-788 (2010). | Article
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