Abstract
.For the N = 50-56 zirconium (Z = 40) and molybdenum (Z = 42) isotopes, the evolution of subshells is evaluated by extracting the effective single-particle energies from available particle-transfer data. The extracted systematic evolution of neutron subshells and the systematics of the excitation energy of the octupole phonons provide evidence for type-II shape coexistence in the Zr isotopes. Employing a simplistic approach, the relative effective single-particle energies are used to estimate whether the formation of low-lying octupole-isovector excitations is possible at the proposed energies. The results raise doubts about this assignment.
Highlights
One of the most fascinating aspects of the nuclear manybody system is the presence of collective excitations at low energies
Since these excitations emerge in a bound quantum system, which necessarily develops a shell structure, it is of interest to understand how these collective excitations evolve in the framework of the single-particle orbits of the shell structure and the residual interactions
The extracted trends of the effective single-particle energies (ESPE) highlight two features which are important for the octupole excitations in this mass region: first, while for the Zr isotopes the ν1h11/2 subshell migrates away from the other subshells with increasing filling of the ν2d5/2 subshell, this is not the case for the Mo isotopes
Summary
One of the most fascinating aspects of the nuclear manybody system is the presence of collective excitations at low energies. Examples in the mass region of interest are shown in table 1 This assignment was based on observed strong M 1 transitions from higher-lying 3− levels to the first octupole phonon. In the quadrupole sector, such strong M 1 transitions to the first 2+1 quadrupole phonon in combination with a weakly collective E2 ground-state decay serve as the experimental signature of a low-lying isovector (mixed-symmetry) excitation. In particle-scattering experiments such as (p, p ) or (e, e ) enhanced excitation cross sections are observed for the quadrupole isovector levels, as well as for the octupole isovector candidates. The aim of this publication is to extract the excitation energies of the valence shell two-body excitations, which contribute to low-lying octupole excitations, and evaluate their evolution as a function of proton and neutron numbers. A further goal is to test whether the proposed candidates for low-lying octupole isovector excitations in the Mo and Zr region fulfill the condition that at least one of the unperturbed 1p1h or 2p/two-quasiparticle excitations for each subsystem must be situated between the energies of the isoscalar 3−1 and isovector 3−iv one-
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