Zakopane 2026 Conference on Nuclear Physics

The hyperfine anomaly (HFA) originates from finite nuclear-size effects in the hyperfine interaction. It contains contributions from both the finite nuclear magnetization distribution, known as the Bohr-Weisskopf (BW) effect [1], and the finite nuclear charge distribution, known as the Breit-Rosenthal (BR) effect [2,3]. In heavy atoms, the differential BR contribution is expected to be negligible compared to the BW effect (10$^{-4}$ in the region of gold (Z=79)) [4]. In the Au isotopes considered here, the relative HFA is therefore expected to be dominated by the BW contribution. The gold isotopes are a particularly interesting case, since relative hyperfine anomalies between isotopes up to ≈ 10 % have been reported in this chain [5].

We report high-resolution Collinear Resonance Ionization Spectroscopy (CRIS) studies of neutron-deficient $^{181}$Au - $^{197}$Au isotopes, performed at the CRIS experiment at ISOLDE-CERN, using the 6s $^2$S$_{1/2}$ $\rightarrow$ 6p $^2$P$_{3/2}$ atomic transition. The precision on the atomic hyperfine coupling constants has been improved up to 2 orders of magnitude compared to previous studies [5-8]. This allowed us to investigate the electronic-state dependence of the extracted magnetic dipole moments, $\mu$. The magnetic moments measured independently from the lower and upper hyperfine A factors show a pronounced discrepancy, which becomes larger for isotopes with a larger $\mu$, suggesting a sizable hyperfine anomaly in the electronic states of Au. The present measurements confirm the spin assignments for $^{181}$Au-$^{183}$Au isotopes. Together, the spins and extracted g-factors help probe the underlying nuclear configuration and its evolution toward the lightest gold isotopes. These results also extend hyperfine-anomaly studies in gold to the 6p $^2$P$_{3/2}$ atomic level, providing a stringent benchmark for atomic many-body theory and nuclear-structure descriptions near Z = 82.

[1] A. Bohr and V. F. Weisskopf, Phys. Rev. 77, 94 (1950).

[2] J. E. Rosenthal and G. Breit, Phys. Rev. 41, 459 (1932).

[3] M. F. Crawford and A. K. Schawlow, Phys. Rev. 76, 1310 (1949).

[4] H. J. Rosenberg and H. H. Stroke, Phys. Rev. A 5, 1992 (1972).

[5] A.E. Barzakh, et. al, PRC 101, 034308 (2020)

[6] A.E. Barzakh, et. al, PRC 101, 064321 (2020)

[7] J. Cubiss, et. al, PRL 131, 202501 (2023)

[8] R.D. Harding, et. al, PRC 102, 024312 (2020)