New μs isomers in the neutron-rich 210Hg nucleus

Neutron-rich nuclei in the lead region, beyond N=126N=126, have been studied at the FRS-RISING setup at GSI, exploiting the fragmentation of a primary uranium beam. Two isomeric states have been identified in 210Hg: the 8+8+ isomer expected from the seniority scheme in the νg9/2νg9/2 shell and a second one at low spin and low excitation energy. The decay strength of the 8+8+ isomer confirms the need of effective three-body forces in the case of neutron-rich lead isotopes. The other unexpected low-lying isomer has been tentatively assigned as a 3−3− state, although this is in contrast with theoretical expectations. Atomic nuclei are complex many-body systems with many degrees of freedom; nevertheless their spectral properties often show very regular features due to the symmetries of the nuclear Hamiltonian. A remarkable example of this is offered by the occurrence of the seniority excitation scheme in spherical, semi-magic nuclei [1]. A deviation from this regular behaviour suggests a change in the underlying nuclear structure, as for example a sudden onset of deformation. The neutron-rich regions around double shell closures have been studied for light and medium-mass nuclei, using fission and deep-inelastic reactions. However, the neutron-rich region around 208Pb has not been thoroughly explored so far, due to its high mass and neutron richness. Pioneering work has been reported in Refs. [2] and [3]. A deeper knowledge would be desirable, since 208Pb is a benchmark for the study of nuclear structure thanks to its double-shell closure character. For semi magic neutron-rich 210–216Pb isotopes, a standard seniority structure has been found, as it is expected from neutrons in the 2g9/22g9/2 shell [4]. A 8+8+ isomer was measured in each isotope (with t1/2t1/2 in the range ∼0.1–6 μs∼0.1–6 μs), decaying via the 6+→4+→2+→0+6+→4+→2+→0+ yrast cascade, with the levels spaced with decreasing energies as the spin increases. The analysis of the transition rates from the isomeric states allowed one to assess the role played by the usually neglected effective three-body forces, arising from core excitations outside the valence space [4]. Generally, the seniority scheme provided by the coupling of the valence neutrons in the g9/2g9/2 shell may also hold when few protons are added to the 208Pb core. In fact, they can act as spectators, while the angular momentum of the excited states is given by the coupling of the neutrons. This is indeed the case if two protons are added to the Z=82Z=82 core: the resulting polonium isotopes show a g9/2g9/2 seniority scheme [5], [6] and [7], apart from 214Po whose known level scheme is limited to low-spin states. Similarly, when two protons are removed from the Z=82Z=82 core, leading to mercury isotopes, one would expect to observe the same g9/2g9/2 seniority scheme. The isotope 208Hg has indeed a 8+8+ isomer attributed to the maximally-aligned configuration View the MathML sourceνg9/22[8]. In this nucleus, the two proton holes are in the s1/2s1/2 and d3/2d3/2 orbits, the less bound ones below Z=82Z=82. They appear to be inactive spectators with respect to the neutron valence space. A similar behaviour is, in principle, expected also in the more neutron-rich isotope 210Hg, whose level structure was completely unknown up to now. This Letter reports the first experimental study of excited states in 210Hg providing evidence of two isomers in the μs range. One is interpreted as the 8+8+ isomer expected from the g9/2g9/2 seniority scheme, while the other one, for which we tentatively suggest a 3−3− assignment, is located at an unexpected low energy, remaining therefore a challenge for future experiments and theoretical models.