Lifetime measurements of 0+ states in Er 168 with the Doppler-shift attenuation method

Background: The lowest-lying shape oscillations of deformed nuclei have been described as quadrupole in nature (λ=2), resulting in two types of vibrations or oscillations: β vibrations with oscillations along the symmetry axis (Kπ=0+) and γ vibrations breaking axial symmetry with a projection of Kπ=2+ on the symmetry axis. The γ vibration seems to be well characterized as the first Kπ=21+ (or 2γ+) band in deformed nuclei and exhibits a systematic behavior across the region. The nature of the Kπ=0+ excitations, however, has remained poorly understood and has been open to debate for some decades. Purpose: The goal of this work is to understand the nature of 0+ states observed in Er168 through measurements of the lifetimes of these states and to determine if they are consistent with oscillations built on a deformed ground state, the minima of other coexisting shapes, single-particle states, or a mixture of effects. Method: Lifetimes of excited states in the Er168 nucleus were measured with the Doppler shift attenuation method (DSAM) and the inelastic neutron scattering reaction, (n,n′γ), at the University of Kentucky Accelerator Laboratory. Results: Numerous 0+ states had been observed by the (p,t) reaction [D. Bucurescu, Phys. Rev. C 73, 064309 (2006)0556-281310.1103/PhysRevC.73.064309.]. We confirm the 0+ states at 1217.2, 1421.5, 1833.6, 2364.9, 2392.1, and 2643.0 keV in Er168. We could not, however, support the previous assignments of 0+ levels at 2114.1, 2200.6, 2572.5, and 2617.4 keV. We report measured lifetimes for six confirmed 0+ excitations and additional members of 0+ bands.Conclusions: The results for Er168 show that it is the third excited Kπ=0+ (04+) excitation that carries the collective strength and, therefore, the potential to be an oscillation on the ground state. This result is similar to the case in Er166, where it was also the 04+ state that exhibited greater collectivity than the first excited Kπ=0+ band. The Delaroche et al. [J.-P Delaroche, Phys. Rev. C 81, 014303 (2010)0556-281310.1103/PhysRevC.81.014303.] prediction for a collective Kπ=0+ band is at ET=1.818MeV, which corresponds the third excited Kπ=0+ band.