Speaker
Description
Modern nuclear structure studies in the heavy-element region combine state-of-the-art experimental techniques with advanced theoretical models [1,2]. This interplay between experiment and theory not only enhances the interpretation of experimental data but also drives the refinement of theoretical approaches, underscoring the importance of benchmarking models against independent experimental observables.
The $K^\pi=8^-$ state in $^{254}$No has been investigated extensively over the past two decades [3-5], yet its configuration remained unresolved. Owing to the production mechanisms employed in previous studies, which do not populate the rotational band built on the isomer, in-beam and decay spectroscopy measurements have not been sufficient to determine its $g$-factor, precluding a definitive configuration assignment. Consequently, both neutron–neutron [4] and proton–proton [3,5] two-quasiparticle configurations have been proposed based on indirect evidence, such as decay patterns and comparisons between measured excitation energies and theoretical predictions.
In this work, we resolve this ambiguity by presenting nuclear model-independent measurements of the electromagnetic multipole moments of the $K^\pi=8^-$ state in $^{254}$No. Using the in-gas-jet laser ionisation spectroscopy setup JetRIS [6,7], we have recorded the hyperfine spectrum of the short-lived isomer ($T_{1/2}={259(7)}~{ms}$ [5]), enabling the extraction of its $g$-factor. In addition, we provide information on the quadrupole deformation of the isomer and on the change in mean-square charge radius between the ground and excited states of $^{254}$No.
Previous studies in the $N=150$ region have identified and characterised $K^\pi=8^-$ states in $^{244}$Pu, $^{246}$Cm, $^{250}$Fm, and $^{252}$No, all consistently assigned a neutron–neutron two-quasiparticle configuration [8,9]. Our results for $^{254}$No demonstrate that this structure persists across the $N=152$ sub-shell gap, challenging the predictions of most theoretical models.
[1] M. Block et al. Prog. Part. Nucl. Phys., 116, 2021.
[2] J. Dobaczewski et al. Nucl. Phys. A, 944, 2015.
[3] R-D Herzberg et al. Nature, 442(7105), 2006.
[4] R.M. Clark et al. Phys. Lett. B, 690(1), 2010.
[5] S. G. Wahid et al. Phys. Rev. C, 111, 2025.
[6] S. Raeder et al. Nucl. Instrum. Methods Phys. Res. B, 463, 2020.
[7] J. Lantis et al. Phys. Rev. Res., 6, 2024.
[8] F.P. Heßberger. arXiv:2309.10468, 2023.
[9] F.G. Kondev et al. Atomic Data and Nuclear Data Tables, 103-104,2015.