200629 zeptoseconds

It happened in just zeptoseconds

Australian and US physicists say they have calculated the speed of the most complex nuclear reactions and found that they’re, well, really fast. We’re talking as little as zeptoseconds – a billionth of a trillionth of a second (10-21).

An illustration of two nuclei grazing each other, with fragments of neutrons and protons exchanging to try to even out the charge. Credit: Cedric Simenel, ANU

The finding follows a comprehensive project to calculate detailed models of the energy flow during nuclear collisions.

Cedric Simenel from the Australian National University worked with Kyle Godbey and Sait Umar from Vanderbilt University to model 13 different pairs of nuclei, using supercomputers at ANU and in the US.

In all they calculated 600 different collisions, from head-on to just grazing past each other. In some cases colliding particles stuck, in others they bounced off each other or adhered briefly (zeptosecond briefly) before separating.

Even at such speed, a number of processes take place, the researchers say.

First, the protons and neutrons swap between the newly-united fragments, in order to equalise their neutron-to-proton ratio. Known as charge equilibration, the calculations showed this is the fastest process, taking only one zeptosecond.

On similar time scale, the kinetic (movement) energy of the nuclei and their angular momentum (rotation) get converted to internal heat, a process known as dissipation.

However, a third process was much slower than the others. The mass equilibration process, in which the shape changes, as protons and neutrons flow from the larger fragment to the smaller one, was up to 20 times slower than the other mechanisms.

“This shows that the mass equilibration process is driven by a different mechanism which has little to do with the other processes,” Simenel says.

It was surprising to find, he adds, that the equilibration times are the same no matter the size of the nuclei. “The time is universal; they will be the same if we are colliding a uranium with a carbon, an iron with a lead, or a calcium with a zinc.”

A feature of the calculations is that they can be compared with measurable experimental quantities, and the ANU’s David Hinde, who was not a part of this project, suggests they could help efforts to add new elements to the periodic table.

“These time scales determine how we can picture the sequence of processes as two heavy nuclei move towards equilibration during superheavy element synthesis reactions,” he says

“This understanding will help to predict the best reactions to use to create and study the properties of new, even heavier, synthetic elements.”

The findings are reported in the journal Physical Review Letters.

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