Puzzling Isotonic Odd-Even Staggering of Charge Radii in Deformed Rare Earth Nuclei

TL;DR

This paper presents high-precision measurements of charge radii in deformed rare-earth nuclei, revealing an unexpectedly large odd-even staggering that challenges current nuclear models and enhances understanding of nuclear structure.

Contribution

It introduces a novel high-precision method using highly charged ions to measure inter-element charge radius differences, resolving the Lu inversion anomaly and revealing new nuclear size systematics.

Findings

Revealed a large odd-even staggering in charge radii along the N=94 isotonic chain.

Resolved the longstanding Lu inversion anomaly.

Identified discrepancies between experimental results and theoretical models.

Abstract

The nuclear charge radius is a fundamental observable that encodes key aspects of nuclear structure, deformation, and pairing. Isotonic (constant neutron number) systematics in the deformed rare-earth region have long suggested that odd-$Z$ nuclei are more compact than their even-$Z$ neighbors - except for Lu, whose recommended radius appeared anomalously large relative to Yb and Hf. We report a high-precision determination of the natural-abundance-averaged Lu-Yb charge-radius difference using extreme-ultraviolet spectroscopy of highly charged Na-like and Mg-like ions, supported by high-accuracy relativistic atomic-structure calculations - a recently introduced method with the unique ability to measure inter-element charge radius differences. Combined with muonic-atom and optical isotope-shift data, our result resolves the longstanding Lu inversion anomaly and reestablishes a pronounced odd-even staggering along the $N=94$ isotonic chain. The magnitude of this staggering is unexpectedly large, far exceeding that observed in semi-magic nuclei and in deformed isotopic sequences. State-of-the-art nuclear density functional theory calculations, including quantified uncertainties, fail to reproduce this enhancement, possibly indicating missing structural effects in current models. Our work demonstrates the power of highly charged ions for precise, element-crossing charge-radius measurements and provides stringent new constraints for future theoretical and experimental studies of nuclear-size systematics.