Fusion research

The results of an Australian PhD questions current theory about nuclear fusion and fission, paving the way for future experiments involving 'superheavy' elements.

The research shows that the formation of very heavy elements by fusion is inhibited by a process called 'quasi-fission' at a much lower threshold than previously thought. "This work has implications for research into making superheavy elements," said team leader Dr David Hinde, from the Department of Nuclear Physics at the Australian National University. 'Superheavy' elements are those with atomic numbers above 82. They can be created momentarily by fusing two other stable highly charged nuclei. But they are unstable because of quasi-fission, which occurs when the colliding elements hold together briefly, then pull apart, with transfer of mass from the heavier of the two nuclei to the lighter one.

Theories about what is going on during this process have been restricted, however, by a lack of empirical research. ANU team member, Annette Berriman, tested what happened in nuclear reactions involving three different combinations of elements as part of her PhD research. She and her colleagues were interested in what happened when heavy projectiles were collided with an even heavier target. They used the Van-de Graaff Accelerator at ANU, which can achieve 16 million volts, to reach sufficient acceleration for the fusion process.

"Nuclei have a positive charge. The heavier the nuclei, the more protons, the stronger the positive charge in each and the harder it is to get the two repelling positive forces to touch," Dr Hinde explained. The only way to overcome these repelling forces is to accelerate the nuclei past their electrostatic repulsion and collide them at high speed. The researchers fired isotopes of silicon at the much heavier tungsten, fluorine at gold, and carbon at lead. If fusion occurred, this combination of elements would form radium 216. The team then measured the amount of Radium produced, and compared it to how much would have been produced if full fusion had occurred.

"Our expectations were not met - the heavier the projectiles we used, the less there was, which meant quasi-fission was competing with the fusion process," said Dr Hinde. Overall, the researchers found that quasi-fission can prevent fusion if the product of the nuclear charges of the two nuclei you are bringing together is as low as 700. This is considerably lower than the threshold of 1500 theoretically predicted. The finding eliminates one of the unknown factors in nuclear experimentation.