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Optical spectroscopy is able to measure subtle shifts in the energy of the atomic electron levels, arising from changes in the charge distribution of the nucleus [1]. For a given isotopic chain, this effect, known as the isotope shift (IS), arises due to changes in the nuclear mass and size. From this, the change in mean-square charge radius ($\delta\langle r^2\rangle$) can be extracted in a nuclear-model independent way. Similarly, spin ($I$), magnetic dipole ($\mu$) and electric quadrupole ($Q$) moments can be deduced from the hyperfine splitting of optical lines. The optical spectroscopy is therefore a sensitive and direct method of probing the nuclear ground and metastable states that enable to obtain a wealth of new information about shape evolution across the nuclear landscape.
In this contribution, we present the results of the optical spectroscopy measurements for neutron deficient Au isotopes performed at the ISOLDE facility (CERN). In order to study very neutron deficient isotopes with low yield, it was necessary to use the most sensitive laser spectroscopy method: in-source resonance-ionization laser spectroscopy [2]. The advanced atomic calculations of the factors needed for extraction of the nuclear observables from the measured IS’s, enable us to decrease substantially the uncertainties of the $\delta\langle r^2\rangle$ values.
Evolution of deformation in the gold isotopic chain proves to be different from that found earlier in the adjacent chains: shape staggering for Hg and Bi [3] isotopes, gradual increase of deformation in Pt or Po isotopes, retention of the near spherical shape in Pb and Tl nuclei. Thus, the small changes in $N$ and/or $Z$ in this region lead to the dramatic variations in the pattern of the shape evolution which make these data a stringent test of the theory.
The experimental results are compared to mean-field calculations [4], that reproduce the unusual behavior of $\delta\langle r^2\rangle$ fairly well only when the nuclear ground states are chosen in accordance with experimental spin and magnetic moments rather than in accordance with the energy of the corresponding levels. This observation reveals the fundamental deficiency of the current mean-field approaches.
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- A. Barzakh et al., Phys. Rev. Lett. $\bf{127}$, 192501 (2021).
- J.G. Cubiss, et al., Phys. Rev. Lett. $\bf{131}$, 202501 (2023).
| Section | Nuclear structure: theory and experiment |
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