Things go pear-shaped when you split the atom

Researchers from Australia and Japan have discovered that when atoms split, in the process known as nuclear fission, the nuclei break into pieces the shape of pears: rounded at one end and elongated into a neck at the other. 

Their finding, published in the journal Nature, will help explain how the atoms that make up the world around us were formed in stars and could lead to better designed nuclear power plants.

“For a long time, people thought that fission was mostly influenced by quantum effects for spherical nuclei, but it was not really explaining what was observed,” says lead researcher Cedric Simenel, from the Australian National University.

“Our numerical simulations show that before the nuclei break they deform and form a neck, and break into pear-shaped fragments.”

Fission, the spontaneous breaking-apart of nuclei, is the process that releases energy in nuclear power, and is also a crucial step in the formation of elements in violent cosmic events such as exploding stars or colliding neutron stars.

But some details of how nuclei break apart, especially the relative size of the fragments, have puzzled scientists for years. Large nuclei often do not split into halves with an equal number of protons and neutrons. The new finding helps explain why not. 

It’s known that certain nuclei are especially stable – nuclear physicists call them magic. Examples include oxygen-16, calcium-48, tin-132 or lead-208, and the stability stems from having the right number of neutrons and protons to form a spherically shaped nucleus, which is a low-energy configuration.

These elements are the ones physicists expected might be commonly formed in fission. But often nuclei slightly larger were produced in preference to the magic ones: for example, instead of tin-132, barium-144 is commonly formed.

The research team, led by Guillaume Scamps from the University of Tsukuba in Japan, developed numerical simulations of fission – a process that takes of the order of a few tens of zeptoseconds (10-20 seconds) – and ran them on supercomputers in Australia and Japan.

The simulations showed that nuclei stretched to breaking point naturally formed not spherical fragments, but pear-shaped ones.

Like spherical shapes in magic nuclei, quantum effects can stabilise some nuclei in a pear-shaped configuration. Barium-144 is one of these. The pear shape forms with a different number of neutrons and protons to spherical nuclei, explaining the asymmetric splitting and formation of unexpected elements.

“It’s a breakthrough; we haven’t realised this before,” Simenel says. “It’s satisfying to find a new answer to an 80-year old problem.”

Fission is a major part of the formation of elements, which occurs as stars collide or explode. 

In these extreme environments, nuclei are constantly absorbing neutrons (known as the R-process). There is a limit to how heavy these nuclei can get as they become unstable and then fission.

“Our findings are a major step towards understanding the fission process to be able to simulate its influence on the abundance of the various atoms in the Universe,” Simenel says.

“It’s opened the door to a whole program of new experiments.”