An atomic nucleus usually has a well-defined shape — elongated like a melon or flattened like a pumpkin. But silicon-30 behaves differently: it seems unable to make up its mind.
In the experiment, beams of silicon-28 and silicon-30 nuclei were directed at a zirconium-90 target at energies near the Coulomb barrier — the threshold where nuclei almost touch but don't fuse. By analyzing how the nuclei scattered, physicists reconstructed their shapes. Silicon-28 turned out to be stably flattened. But silicon-30, with its two extra neutrons, showed something odd: descriptions with elongated, flattened, and even spherical shapes all matched the data equally well.
Further shell-model calculations confirmed that silicon-30 has no single dominant shape. Its ground state is a mixture of different configurations, as if the nucleus constantly 'flickers' between elongated, flattened, and spherical forms. This phenomenon is known as shape fluctuations.
Such behavior is rare for stable nuclei. Usually, the shape is rigidly set by the number of protons and neutrons. But here, at the boundary between magic numbers, quantum effects blur the geometry. Silicon-30 becomes a candidate for a nucleus with a 'floating' shape in its ground state, and that opens a new window into understanding how matter is structured at the subatomic level.
as written by the authors arXiv:2602.06580
Quasi-elastic (QEL) scattering measurements have been performed using the $$^{28, 30}$$Si projectiles off the $$^{90}$$Zr target at energies around the Coulomb barrier. Coupled-channels (CC) calculations were carried out in a large parameter space of quadrupole and hexadecapole deformations for the N=Z, $$^{28}$$Si and N=Z+2, $$^{30}$$Si nuclei. $$^{28}$$Si at the N=Z line is observed to be uniquely oblate shaped in its ground state. In contrast, for $$^{30}$$Si with just two additional neutrons -- oblate, prolate, and spherical CC descriptions are equally compatible with the measurements. To further investigate the nuclear structure evolution with varying neutron number, shell-model calculations were performed. These calculations reveal a sudden change in the nuclear structure aspects at $$^{30}$$Si in going from $$^{28}$$Si to $$^{30}$$Si. Combined reaction and structure analyses consistently indicate that $$^{30}$$Si does not possess a well-defined intrinsic shape, and it is a potential candidate for ``shape fluctuations" in its ground state.
Silicon-28 and silicon-30 are isotopes of the same element, but their nuclei differ in shape more than some different elements do. Two neutrons change everything, just as a single note can change a melody.
In Greg Egan's novel 'Schild's Ladder', matter with unstable geometry is used to create exotic structures — fluctuating silicon-30 nuclei hint at similar physics.