Mass measurements of $^{179-184}$Yb identify an anomalous proton-neutron interaction

TL;DR

This study reports six new mass measurements of neutron-rich ytterbium isotopes, revealing an unexpected strong proton-neutron interaction below lead-208, which challenges current theoretical models and enhances understanding of nuclear structure near r-process waiting points.

Contribution

First-time mass measurements of neutron-rich ytterbium isotopes identify an anomalous proton-neutron interaction, providing new data to test and improve nuclear models in the 'hole-hole' regime.

Findings

Identified an anomalously strong proton-neutron interaction below $^{208}$Pb.

Experimental results challenge existing mean-field model predictions.

Data benchmarks models for better r-process nucleosynthesis predictions.

Abstract

Mass measurements of nuclei can identify structurally-driven trends in binding energy across isotopic chains, and can also isolate specific nucleon-nucleon interactions, such as the $\delta V_{\mathrm{pn}}$ interaction of the last two valence protons with the last two valence neutrons. Below $^{208}$Pb, investigation of the local binding energy and $\delta V_{\mathrm{pn}}$ systematics can facilitate a better understanding of the behaviour of the proton-neutron interaction in the 'hole-hole' regime (where valence interactions can be modelled in hole-space rather than particle-space) and provide insight on the potential onset of a prolate-to-oblate shape transition. However, measurement of the necessary nuclei has been exceptionally challenging. Here we present six first-time measurements of neutron-rich ytterbium, using advanced rare isotope production and mass spectrometry techniques, leading to the identification of an anomalously strong proton-neutron interaction in the 'hole-hole' quadrant below $^{208}$Pb. The scale of this interaction, at $^{186}$Hf, is comparable to that of similar signals at doubly-magic nuclei and shape transitions. The experimental results are compared with contemporary mean-field model predictions, that do not accurately reproduce the anomaly. The results are also used to benchmark predictions from several models, to facilitate more accurate descriptions towards a key r-process waiting point at $N = 126$.