8pi spectrometer in TRIUMF Lab.

Data collected by the 8π spectrometer (pictured) at the TRIUMF particle-accelerator centre in Vancouver, Canada, suggest that some cadmium nuclei can take on shapes similar to those of rugby balls. Credit: TRIUMF

Atomic and molecular physics

Squashed nuclei undermine long-held doctrine of nuclear structure

Stable cadmium nuclei can change shape even when their energies are low.

A cadmium atom’s nucleus shape-shifts even when its energy is low — a finding that overturns a widely held conviction about the structure of atomic nuclei.

According to the standard theory of nuclear structure, the protons and neutrons that make up an atomic nucleus are arranged in structures called shells. Scientists have long thought that a stable nucleus whose shells are almost full remains spherical unless it becomes highly energized, or ‘excited’.

Paul Garrett at the University of Guelph in Canada and his colleagues investigated this theory using stable forms of the element cadmium. When cadmium nuclei relaxed from excited states, they emitted γ-rays. This radiation held clues to the properties of these excited nuclear states.

The authors compared these properties with the results of innovative calculations, and suggest that the nuclei take on a variety of non-spherical rugby-ball shapes despite the fact their nuclear shells are nearly full. If the squashed nuclei can be observed directly, textbooks describing nuclear structure might need to be rewritten, the team suggests.