An atomic nucleus isn't always round. It can stretch out like a melon or flatten like a pumpkin. And its size depends on this shape — just as the size of a pie depends on the shape of the dough.
Scientists calculated the shapes and sizes of nuclei of berkelium isotopes — a heavy element produced in reactors. It turned out that flattened nuclei are larger than elongated ones at the same degree of deformation. This is unexpected: previously, it was thought that size depends only on how strongly the nucleus deviates from a sphere, not on the direction of that deviation.
This hollow center is what increases the radius. The nucleus kind of puffs up from within, even though it contains the same number of particles. This connection between shape, level filling, and size helps us better understand how the heaviest nuclei are structured — and why some are more stable than others.
as written by the authors arXiv:2603.07596
The nuclear charge radius provides a fundamental probe of nuclear structure, yet experimental data remain rare in the actinide region. Using the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc) with the PC-PK1 functional, we carry out a systematic investigation of prolate-oblate shape competition in odd-$$A$$ Bk isotopes. Deformation is found to play an important role in the description of charge radii $$r_c$$ by extending the density distribution. Notably, $$r_c$$ exhibits a distinct shape dependence: for a given absolute quadrupole deformation $$|\beta_2|$$, oblate shapes yield larger charge radii than their prolate counterparts in well-deformed nuclei near the mid-shell region, where the empirical formula $$r_c(\beta_2) = \left(1 + \frac{5}{4\pi}|\beta_2|^2\right) r_c(0)$$ fails to capture the observed behavior. This enhancement is attributed to a central depression (or ``bubble" structure) in the proton density, which microscopically originates from the non-occupation of the spherical $$3s_{1/2}$$($$\Omega=1/2$$) orbital in oblate minima. These findings establish a clear microscopic connection between nuclear shape, single-particle occupancy, and nuclear size.
Berkelium is named after the city of Berkeley in California, where it was first produced in 1949. That required a cyclotron and several months of bombarding a target.