Abstract
.The neutron-deficient mercury isotopes serve as a classical example of shape coexistence, whereby at low energy near-degenerate nuclear states characterized by different shapes appear. The electromagnetic structure of even-mass 182-188 Hg isotopes was studied using safe-energy Coulomb excitation of neutron-deficient mercury beams delivered by the REX-ISOLDE facility at CERN. The population of 0^{+}_{1,2}, 2^{+}_{1,2} and 4^{+}_{1} states was observed in all nuclei under study. Reduced E2 matrix elements coupling populated yrast and non-yrast states were extracted, including their relative signs. These are a sensitive probe of shape coexistence and may be used to validate nuclear models. The experimental results are discussed in terms of mixing of two different configurations and are compared with three different model calculations: the Beyond Mean Field model, the Interacting Boson Model with configuration mixing and the General Bohr Hamiltonian. Partial agreement with experiment was observed, hinting to missing ingredients in the theoretical descriptions.
Highlights
The neutron-deficient mercury isotopes (Z = 80) serve as an illustrative example of shape coexistence [1,2], whereby at low excitation energies near-degenerate nuclear states are characterized by different shapes
The experimental and theoretical results concerning excitation energies, reduced transition probabilities and spectroscopic quadrupole moments are compared and discussed within the framework of three different models: i) a quadrupole collective model based on the General Bohr Hamiltonian (GBH) [21], ii) a beyondmean-field model (BMF) [16], and iii) an interactingboson model with configuration mixing (IBM-CM) [22]
A set of deformed mean-field states with different axial quadrupole moments is constructed by solving the Hartree-Fock plus Bardeen-Cooper-Schrieffe (HF+BCS) equations with suitably chosen constraints
Summary
Since a large amount of information has been collected for nuclei around the N = 104 midshell between N = 82 and N = 126 using different experimental techniques This resulted, amongst others, in the observation of a large odd-even staggering in the isotope shifts in the mercury isotopes around 181−185Hg [4], which has long been attributed to the intruder structure becoming the ground state in the oddmass isotopes and the observation of shape coexistence at low excitation energy in 185Hg [5]. Mean-field calculations interpret these states as exhibiting a weakly-deformed oblate character [1, 15, 16] This regular pattern is distorted for the lighter mercury isotopes (N ≤ 106) through the intrusion of levels of a very collective rotational band of assumed prolate nature, which decreases in excitation energy reaching a minimum around mass A = 182, 184 [13, 14].
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