Nuclear charge radii globally scale with atomic mass number A as A^1∕3, and isotopes with an odd number of neutrons are usually slightly smaller in size than their even-neutron neighbours. This odd–even staggering, ubiquitous throughout the nuclear landscape¹, varies with the number of protons and neutrons, and poses a substantial challenge for nuclear theory^2–4. Here, we report measurements of the charge radii of short-lived copper isotopes up to the very exotic ⁷⁸Cu (with proton number Z = 29 and neutron number N = 49), produced at only 20 ions s^–1, using the collinear resonance ionization spectroscopy method at the Isotope Mass Separator On-Line Device facility (ISOLDE) at CERN. We observe an unexpected reduction in the odd–even staggering for isotopes approaching the N = 50 shell gap. To describe the data, we applied models based on nuclear density functional theory^5,6 and A-body valence-space in-medium similarity renormalization group theory^7,8. Through these comparisons, we demonstrate a relation between the global behaviour of charge radii and the saturation density of nuclear matter, and show that the local charge radii variations, which reflect the many-body polarization effects, naturally emerge from A-body calculations fitted to properties of A ≤ 4 nuclei.

Molecular spectroscopy offers opportunities for the exploration of the fundamental laws of nature and the search for new particle physics beyond the standard model^1–4. Radioactive molecules—in which one or more of the atoms possesses a radioactive nucleus—can contain heavy and deformed nuclei, offering high sensitivity for investigating parity- and time-reversal-violation effects^5,6. Radium monofluoride, RaF, is of particular interest because it is predicted to have an electronic structure appropriate for laser cooling⁶, thus paving the way for its use in high-precision spectroscopic studies. Furthermore, the effects of symmetry-violating nuclear moments are strongly enhanced^5,7–9 in molecules containing octupole-deformed radium isotopes^10,11. However, the study of RaF has been impeded by the lack of stable isotopes of radium. Here we present an experimental approach to studying short-lived radioactive molecules, which allows us to measure molecules with lifetimes of just tens of milliseconds. Energetically low-lying electronic states were measured for different isotopically pure RaF molecules using collinear resonance ionisation at the ISOLDE ion-beam facility at CERN. Our results provide evidence of the existence of a suitable laser-cooling scheme for these molecules and represent a key step towards high-precision studies in these systems. Our findings will enable further studies of short-lived radioactive molecules for fundamental physics research.

Resonant laser ionization and spectroscopy are widely used techniques at radioactive ion beam facilities to produce pure beams of exotic nuclei and measure the shape, size, spin and electromagnetic multipole moments of these nuclei. However, in such measurements it is difficult to combine a high efficiency with a high spectral resolution. Here we demonstrate the on-line application of atomic laser ionization spectroscopy in a supersonic gas jet, a technique suited for high-precision studies of the ground- and isomeric-state properties of nuclei located at the extremes of stability. The technique is characterized in a measurement on actinium isotopes around theN=126 neutron shell closure. A significant improvement in the spectral resolution by more than one order of magnitude is achieved in these experiments without loss in efficiency.

The magnetic moments and isotope shifts of the neutron-deficient francium isotopes $^{202\mathrm{\text{-}}205}\mathrm{Fr}$ were measured at ISOLDE-CERN with use of collinear resonance ionization spectroscopy. A production-to-detection efficiency of 1% was measured for $^{202}\mathrm{Fr}$. The background from nonresonant and collisional ionization was maintained below one ion in ${10}^{5}$ beam particles. Through a comparison of the measured charge radii with predictions from the spherical droplet model, it is concluded that the ground-state wave function remains spherical down to $^{205}\mathrm{Fr}$, with a departure observed in $^{203}\mathrm{Fr}$ ($N=116$).

Nuclear spins and precise values of the magnetic dipole and electric quadrupole moments of the ground states of neutron-rich ${}^{73\text{–}78}\mathrm{Cu}$ isotopes were measured using the Collinear Resonance Ionization Spectroscopy (CRIS) experiment at the CERN On-Line Isotope Mass Separator (ISOLDE) facility. The nuclear moments of the less exotic ${}^{73,75}\mathrm{Cu}$ isotopes were remeasured with similar precision, yielding values that are consistent with earlier measurements. The moments of the odd-odd isotopes, and ${}_{29}^{78}\mathrm{Cu}$ ($N=49$) in particular, are used to investigate excitations of the assumed doubly magic ${}^{78}\mathrm{Ni}$ core through comparisons with large-scale shell-model calculations. Despite the narrowing of the $Z=28$ shell gap between $N\sim 45$ and $N=50$, the magicity of $Z=28$ and $N=50$ is restored toward ${}^{78}\mathrm{Ni}$. This is due to weakened dynamical correlations, as clearly probed by the present moment measurements.