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Nuclear charge radii are sensitive probes of different aspects of thenucleon-nucleon interaction and the bulk properties of nuclear matter; thus,they provide a stringent test and challenge for nuclear theory. The calciumregion has been of particular interest, as experimental evidence has suggesteda new magic number at $N = 32$ [1-3], while the unexpectedly large increases inthe charge radii [4,5] open new questions about the evolution of nuclear sizein neutron-rich systems. By combining the collinear resonance ionizationspectroscopy method with $\beta$-decay detection, we were able to extend thecharge radii measurement of potassium ($Z =19$) isotopes up to the exotic$^{52}$K ($t_{1/2}$ = 110 ms), produced in minute quantities. Our work providesthe first charge radii measurement beyond $N = 32$ in the region, revealing nosignature of the magic character at this neutron number. The results areinterpreted with two state-of-the-art nuclear theories. For the first time, along sequence of isotopes could be calculated with coupled-cluster calculationsbased on newly developed nuclear interactions. The strong increase in thecharge radii beyond $N = 28$ is not well captured by these calculations, but iswell reproduced by Fayans nuclear density functional theory, which, however,overestimates the odd-even staggering effect. These findings highlight ourlimited understanding on the nuclear size of neutron-rich systems, and exposepressing problems that are present in some of the best current models ofnuclear theory.

Charge radii of exotic potassium isotopes challenge nuclear theory and the magic character of $N = 32$