04-07-2025
New study on atomic nuclei overturns 70-year-old theory
The latest study has shown that heavy atomic nuclei are shaped like almonds. (Mutsumi Mitobe)
Some atomic nuclei are not shaped like rugby balls, as a longstanding theory suggests, but are instead somewhat flattened, like almonds, according to a new study by Japanese scientists.
A team from the Riken national research institute and elsewhere published this new theory recently in an international science journal. The scientists said their research results, which overturn the last 70 years of established theory on atomic nuclei shapes, could be useful for discovering new elements.
The nucleus, located in the center of an atom, is composed of protons and neutrons. The shape of a nucleus is an essential characteristic that affects its stability.
Research on atomic nuclei shapes began in the 1930s. At first, nuclei were assumed to be round like soccer balls. However, experimental results were found that could not be explained by spherical nuclei.
During the 1950s, Aage Niels Bohr, Ben Roy Mottelson and Leo James Rainwater theorized that heavy atomic nuclei have ellipsoidal shapes—like rugby balls.
This went on to become an established theory, which earned the trio the Nobel Prize in Physics in 1975.
However, Takaharu Otsuka, a Riken senior visiting scientist who led the Japanese team, said he had his first doubts about the established theory 50 years ago, the same year that Bohr and others were awarded the Nobel Prize in Physics.
Otsuka was a graduate student of nuclear physics at the time.
He said that, no matter how hard he thought, he never understood why atomic nuclei had to be shaped like rugby balls with circular sections.
'It must be like that, since everybody says so,' he said he thought at the time.
But doubts lingered in his mind.
Otsuka began studying the subject in earnest sometime around 2010.
In the latest study, the team of Japanese scientists reworked the theory on the shapes of heavy atomic nuclei by focusing on quantum theory effects and the forces acting between protons and neutrons—neither of which were considered in the 1950s.
Their study showed that an almond-like nucleus shape, with elliptical sections perpendicular to the longest axis, allows the protons and neutrons in the nucleus to be bound together more strongly, making the nucleus more stable than a rugby ball-like nucleus shape with circular sections would.
Simulations using the Fugaku supercomputer also support the new theory, the physicists said.
'I want young people to cherish any doubts that they may have, because technological advances could allow those doubts to be verified one day,' said Otsuka.
About 300 types of atomic nuclei are stable and found naturally on Earth. Some 3,000 more types have been found to exist when created artificially.
Otsuka said he believes that more than 1,000 types of atomic nuclei likely have the deformed almond shape.
