Abstract
Neutron-deficient $^{177-185}$Hg isotopes were studied using in-source laser resonance-ionization spectroscopy at the CERN-ISOLDE radioactive ion-beam facility, in an experiment combining different detection methods tailored to the studied isotopes. These include either alpha-decay tagging or Multi-reflection Time-of-Flight gating to identify the isotopes of interest. The endpoint of the odd-even nuclear shape staggering in mercury was observed directly by measuring for the first time the isotope shifts and hyperfine structures of $^{177-180}$Hg. Changes in the mean-square charge radii for all mentioned isotopes, magnetic dipole and electric quadrupole moments of the odd-A isotopes and arguments in favor of $I = 7/2$ spin assignment for $^{177,179}$Hg were deduced. Experimental results are compared with Density Functional Theory (DFT) and Monte-Carlo Shell Model (MCSM) calculations. DFT calculations with several Skyrme parameterizations predict a large jump in the charge radius around the neutron $N = 104$ mid shell, with an odd-even staggering pattern related to the coexistence of nearly-degenerate oblate and prolate minima. This near-degeneracy is highly sensitive to many aspects of the effective interaction, a fact that renders perfect agreement with experiment out of reach for current functionals. Despite this inherent diffculty, the SLy5s1 and a modified UNEDF1^{SO} parameterization predict a qualitatively correct staggering that is off by two neutron numbers. MCSM calculations of states with the experimental spins and parities show good agreement for both electromagnetic moments and the observed charge radii. A clear mechanism for the origin of shape staggering within this context is identified: a substantial change in occupancy of the proton $\pi h_{9/2}$ and neutron $\nu i_{13/2}$ orbitals.
Highlights
Since the mid-1970s, an unexpected large difference in the mean-square charge radius between 187Hg and 185Hg was observed by measuring the isotope shift in a radiation detection of optical pumping (RADOP) experiment performed at ISOLDE [1,2]
Information on the difference in mean-square charge radius δ r2 A,A between two nuclei with mass A and A of the same isotopic chain is extracted from the difference in the positions of the centers of gravity of their respective hyperfine structures (HFS), ν0, i.e., their isotope shift of a certain transition
Fitting of the spectra was performed with the open-source Python package SATLAS [39] and cross-checked with a similar fitting routine in ROOT [37] and the fitting procedure that was used in our previous HFS studies as, for instance, in Ref. [27]
Summary
Since the mid-1970s, an unexpected large difference in the mean-square charge radius between 187Hg and 185Hg was observed by measuring the isotope shift in a radiation detection of optical pumping (RADOP) experiment performed at ISOLDE [1,2]. To 185Hg, the 181,183Hg isotopes were found to exhibit a large isotope shift from their evenmass neighbors 182,184,186Hg [3,4]. Ever since these measurements, the observed pattern became known as “shape staggering.”. Studying the levels at low excitation energy in more detail, different shapes were identified in close vicinity to the ground state and the mercury isotopes are one of the most illustrative examples of shape coexistence [5]. The large radius staggering was interpreted as transitions between weakly deformed, oblate ground states and strongly deformed, prolate ground states [6]
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