visualisation of the Bohr model of an atom. At the centre is the tightly packed nucleus, composed of protons and neutrons.

Atomic nuclei (artist’s impression) contain both protons and neutrons. A newly reported isotope offers the hope of testing fundamental principles of nuclear structure. Credit: Mark Garlick/SPL

Atomic and molecular physics

A peculiar atom shakes up assumptions of nuclear structure

Lopsided potassium isotope survives longer than predicted by theory.

Physicists have found an exotic variant of potassium that is much longer-lived than predicted — hinting at the existence of other more-extreme atoms that stretch the known limits of nuclear structure.

Isotopes are alternative forms of a single chemical element, each with the same number of protons but differing numbers of neutrons. If an isotope has too few neutrons, the nucleus can no longer hold itself together. Such an isotope is said to be ‘beyond the proton dripline’.

Daria Kostyleva at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, and her colleagues formed a beam of argon atoms. The atoms exchanged protons and neutrons, creating potassium-31, which has 12 neutrons and 19 protons. It is four neutrons beyond the dripline — meaning that it is four neutrons short of being stable.

The team’s analysis showed that the half-life of potassium-31 is about five trillionths of a second, much longer than predicted for an isotope so far beyond the dripline. It might be possible to detect atoms that are as many as seven neutrons beyond the dripline, which could help researchers to test fundamental predictions of quantum physics, the authors write.