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Why a Pear-Shaped Nucleus Has Almost No Electric Charge

Original: "Independent Validation of Octupole Collectivity in radium-224 through lifetime measurements of low-lying negative-parity states"
· D. White, D. O'Donnell, A. Avaa, J. R. Murias, S. Murillo Morales, R. Umashankar, V. Vedia, C. Andreoiu, D. W. Annen, A. D. Ayangeakaa, G. C. Ball, V. Bildstein, M. Bowry, I. Dillmann, E. G. Fuakye, L. P. Gaffney, A. B. Garnsworthy, P. E. Garret, E. D. Geerlof, S. Georges, A. L. Grimes, G. F. Grinyer, G. Hackman, P. M. Jones, J. Liu, B. Olaizola, S. D. Olorunfunmi, F. Rowntree, N. K. Syeda, D. Shah, P. Spagnoletti, C. E. Svensson, F. Wu
arXiv:2609.07556v1 · 2026-09-07 · CC BY 4.0 · 1 min
Nuclear Experiment
The nucleus of radium-224 is shaped like a pear, but its electric charge is distributed so that it is barely visible from the outside.

Take a pear and rub it against wool: one end picks up a charge of one sign, the other end the opposite sign. That creates an electric dipole moment. Many atomic nuclei that are pear-shaped have such a moment. But radium-224 has almost none, even though its shape is about as pear-like as it gets.

The intrinsic dipole moment is the shift between the center of mass and the center of charge in a nucleus. If protons and neutrons are distributed unevenly, a potential difference arises, like in a battery.

Scientists measured how quickly the excited states of this nucleus decay. To do that, they produced francium-224, which turned into radium-224, and tracked the gamma rays emitted by the nucleus. The lifetime turned out to be about 450 picoseconds. That is incredibly short: one picosecond is one trillionth of a second. If a second lasted 30,000 years, a picosecond would be a single instant.

From these measurements, they calculated the dipole moment. It turned out to be tens of times smaller than in other pear-shaped nuclei. This means a delicate cancellation is happening inside the nucleus: the contribution from the shape is almost completely canceled by the contribution from the motion of the particles. This test matters because pear-shaped nuclei are used to search for new physics beyond the Standard Model. Now the models that describe this have gained a solid footing.

The 224Ra nucleus is shaped like a pear, yet its electric dipole moment is almost zero, even though pear-shaped nuclei usually possess a noticeable dipole moment. It is as if the pear had perfectly balanced electric polarization despite its obvious shape asymmetry.