Physicists usually create super-heavy elements by slamming heavy nuclei together inside a collider. But if the nuclei are very proton-rich, they often fail to bond because Coulomb repulsion pushes them apart. Instead, mass is transferred between the nuclei, and leads to two separate nuclei with very different masses. This phenomenon - which was previously thought to take place only above a 'charge product' of 1600 - is known as 'quasi-fission', and hinders fusion.

Hinde's team conducted a series of collisions in the 16 megavolt electrostatic accelerator at the Australian National University. They collided pairs of light and heavy nuclei with charge products of between 500 and 1000: carbon-12 and lead-204, fluorine-19 and gold-197, and silicon-30 and tungsten-186. All of these combinations produce a small amount of short-lived radium-216. Hinde and colleagues measured the masses and amounts of decay products from the three samples of radium-216 to compare the reactions.

One might expect the radium-216 produced in each experiment to decay in the same way, but the complex interactions in nuclei containing hundreds of nucleons lead to a range of outcomes. The researchers found that quasi-fission did not take place between carbon and lead. But fragments from the other reactions spanned a wider mass range, which suggested that the process had taken place in the fluorine-gold and silicon-tungsten collisions.

Current theories also predict that quasi-fission should not take place between nuclei with such different masses. But these results suggest that this idea is inaccurate. "This indicates that quasi-fission is competing with fusion in collisions between nuclei with even lower masses, which would otherwise have produced heavy fusion products", Hindes told PhysicsWeb.

The team hopes that improved models of the dynamics of collisions will describe their unexpected data and lead to better predictions of quasi-fission and fusion in reactions that may form super-heavy elements. Together with an analysis of new carbon-lead data gathered at Daresbury in the UK, the researchers plan to extend their studies to heavier nuclei.