By Eugenie Samuel Reich

The observation of an unexpected nuclear reaction by an unstable isotope of the element mercury has thrown up a rare puzzle. The enigma is helping theorists to tackle one of the trickiest problems in physics: developing a more complete model of the atomic nucleus.

Nuclear fission, the process in which a nucleus heavier than that of iron breaks into pieces, is generally observed to be symmetric, with the resulting fragments being roughly equal in size. Although instances of asymmetric fission are known, they are usually attributed to the preferential formation of 'magic' nuclei, in which shells in the nuclear structure are filled to capacity.

So when researchers on the ISOLDE experiment at CERN, Europe's particle-physics laboratory in Geneva, Switzerland, set out to study the decay of mercury-180 -- containing 80 protons and 100 neutrons -- they expected it to break into two nuclei of zirconium-90, each containing 40 protons and 50 neutrons. They assumed that outcome would be particularly favoured because 40 and 50 are magic numbers for which shells would be exactly filled.

But the mercury dealt a surprise, splitting instead into ruthenium-100 and krypton-80. "A symmetric split should be dominant and we show that it doesn't happen," says ISOLDE member Andrei Andreyev, presently of the University of the West of Scotland in Paisley. The result is in press at Physical Review Letters.

Pure beam

ISOLDE is unique in being able to create pure beams of unstable heavy elements, the reaction products of which can be collected and studied. Andreyev and his colleagues started with a beam of thallium-180. This mostly decayed by capturing an electron, turning one of its 81 protons into a neutron to form mercury-180, which then performed the unexpected feat of splitting into two pieces of unequal size.

Theorist Peter Möller of Los Alamos National Laboratory, New Mexico, thinks that he has an explanation. He has used a nuclear model that he and his colleagues developed in 20011, and says that the key was to consider not only the stability of the end fragments, but also the stability of the differently shaped nuclei that occur as mercury-180 divides.

Möller had previously explored in detail only the fission of nuclei heavier than mercury, which tend to split symmetrically. But after the ISOLDE result, he applied his model to lighter isotopes and was surprised to find that it predicts asymmetric splits for mercury-180 as well as for a range of other unstable nuclei.

Comparing the masses of the thallium and mercury nuclei predicts that capturing an electron would leave the mercury nucleus with 9.5 mega-electronvolts of excess energy. Möller's calculations show that to split symmetrically, it would have to travel over an energy barrier of 10.5 mega-electronvolts.

An asymmetric split, by contrast, requires far less energy. "Exactly why it's asymmetric we cannot say, but it is a rather delicate balance between surface tension, electrostatic charge and nuclear forces," says Möller. He is now improving and automating his model to be able to predict the splits of nuclei lighter than mercury.

Firming up fission

Nuclear theorist Witold Nazarewicz of the University of Tennessee in Knoxville says that the study demonstrates the extent to which, more than 70 years after the discovery of nuclear fission, we are still learning about the process. "This is very important information for any model of the nucleus," he says.

Nazarewicz says that although engineers' practical knowledge of fission has progressed far enough for us to build nuclear bombs and reactors, "I don't think we have a firm understanding of fission rooted in the interactions of the proton and neutron building blocks." The nuclei that form in a typical reactor core are generally understood, but models are not at the point at which they can be extrapolated to more exotic and unstable isotopes, he says. A better fundamental understanding of the theory may help the design of future generations of reactors.

Experimental facilities scheduled to come online over the next decade should enable further studies of unstable nuclei. Those facilities include the €1-billion (US$1.3 billion) Facility for Antiproton and Ion Research at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, and the $600-million Facility for Rare Isotope Beams at Michigan State University in East Lansing.