Fingerprints of triaxiality in the charge radii of neutron-rich Ruthenium

TL;DR

This work provides the first high-precision charge-radii measurements for nine neutron-rich ruthenium isotopes using a new ATLANTIS setup at CARIBU, demonstrating that triaxial deformation leaves a detectable imprint on $R_c$ through shell effects. By comparing the data to Brussels-Skyrme-on-a-Grid (BSkG) energy-density functionals that incorporate triaxial shapes, the authors show that triaxiality yields a markedly better description of the radii than axial-symmetric configurations. The findings indicate that neglecting triaxial degrees of freedom can lead to systematic biases in radii predictions in regions where triaxial shapes are energetically favored, and that charge radii can serve as an additional constraint on nuclear shape in global EDF models. The combined experimental-theoretical result supports a coherent picture in which spontaneous symmetry breaking due to shell effects influences a broad set of observables, including masses, spectroscopic properties, and charge radii in this region of the nuclear chart.

Abstract

We present the first measurements with a new collinear laser spectroscopy setup at the Argonne Tandem Linac Accelerator System utilizing its unique capability to deliver neutron-rich refractory metal isotopes produced by the spontaneous fission of 252Cf. We measured isotope shifts from optical spectra for nine radioactive ruthenium isotopes 106-114Ru, reaching deep into the mid-shell region. The extracted charge radii are in excellent agreement with predictions from the Brussels-Skyrme-on-a-Grid models that account for the triaxial deformation of nuclear ground states in this region. We show that triaxial deformation impacts charge radii in models that feature shell effects, in contrast to what could be concluded from a liquid drop analysis. This indicates that this exotic type of deformation should not be neglected in regions where it is known to occur, even if its presence cannot be unambiguously inferred through laser spectroscopy.

Figures (8)

• Figure 1: A sample of the recorded data for the stable reference isotope $^{102}$Ru (top row) and all other recorded radioactive isotopes. The solid black lines show a fit to the data. The inset shows a time--frequency profile of a $^{110}$Ru resonance.
• Figure 2: Comparison between the experimental charge radii for the Ru, Mo Charlwood.2009, and Pd geldhof2022Renth.2025 isotopic chains, and BSkG$x$ calculations with the inclusion of triaxial deformation (yellow) and without (blue, see text for details). The colored bands show the spread in predictions between all BSkG$x$, and the solid lines indicate the calculated BSkG4 values.
• Figure 3: Normalized total energy of $^{112}$Ru obtained with BSkG4 as a function of $\beta_2$ and gamma. The black line indicates the minimal energy as a function of $\gamma$; the global energy minimum is indicated by a black star.
• Figure 4: Root-mean-square charge radius $R_c = \langle r_c^2 \rangle^{1/2}$ of $^{112}$Ru across the $\beta-\gamma$ plane, as calculated for BSkG4. The black line and black star are as in Fig. \ref{['fig:pes']}.
• Figure 5: A King fit of the (modified) nuclear parameter $\Lambda^{A,A'}_\mu$ and (modified) isotope shifts in stable nuclei, shown in black. The new $\Lambda^{102,106-114}$, shown in yellow, can be extracted by using the isotope shifts presented in this letter.