Zeptosecond collisions – the remarkable speed of nuclear reactions

Thursday 18 June 2020

Scientists have for the first time calculated the speed of the most complex nuclear reactions and found they’re really fast: mere zeptoseconds.

Being this fast – a zeptosecond is a billionth of a trillionth of a second (10-21 seconds) – makes these nuclear reactions some of the fastest interactions we know of.

A team led by Professor Cedric Simenel modelled a variety of nuclear processes, including collisions that stick together (fusion), collisions in which particles bounce off each other, and those which stick for a few billion-trillionths of a second and then break apart again.

“It’s a fundamental quantum many-body challenge that many people are trying to address,” said Professor Simenel, the head of the Theoretical Physics Department in RSPhys.

“Nuclear interactions are the perfect playground because they happen so fast that the particles don’t have time to interact with the rest of the world.”

Professor Simenel teamed up with colleagues at Vanderbilt University in the United States to calculate detailed models of the energy flow during nuclear collisions. They modelled 13 different pairs of nuclei and published the results in Physical Review Letters [Link https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.212504].

The team used supercomputers at Australia’s National Computational Infrastructure on the ANU campus and in the United States to calculate 600 different collisions – from head-on to just grazing past each other.

Although the collisions happen so quickly, a number of processes take place. Firstly, 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,” Professor Simenel said.

Another surprise to come from the work is 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. It was a surprise when we first saw that,” Professor Simenel said.

A feature of the calculations is that they can be compared with measurable experimental quantities.

Director of the ANU Heavy Ion Accelerator Facility, Professor David Hinde, who was not an author on this work, said the calculations showed better agreement with recent measurements at ANU than with earlier experimental estimates of the mass-equilibration time scale, and 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,” Professor Hinde said.

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

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