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

Background: The lowest-lying shape oscillations of deformed nuclei have been described as quadrupole in nature (lambda = 2), resulting in two types of vibrations or oscillations: beta vibrations with oscillations along the symmetry axis (K pi = 0+) and gamma vibrations breaking axial symmetry with a projection of K pi = 2+ on the symmetry axis. The gamma vibration seems to be well characterized as the first K pi = 2+1 (or 2+gamma) band in deformed nuclei and exhibits a systematic behavior across the region. The nature of the K pi = 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 168Er 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 168Er nucleus were measured with the Doppler shift attenuation method (DSAM) and the inelastic neutron scattering reaction, (n, n'gamma), at the University of Kentucky Accelerator Laboratory.Results: Numerous 0+ states had been observed by the (p, t) reaction [D. Bucurescu et al., Phys. Rev. C 73, 064309 (2006).]. We confirm the 0+ states at 1217.2, 1421.5, 1833.6, 2364.9, 2392.1, and 2643.0 keV in 168Er. 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 168Er show that it is the third excited K pi = 0+ (0+4 ) 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 166Er, where it was also the 0+4 state that exhibited greater collectivity than the first excited K pi = 0+ band. The Delaroche et al. [J.-P Delaroche et al., Phys. Rev. C 81, 014303 (2010).] prediction for a collective K pi = 0+ band is at ET = 1.818 MeV, which corresponds the third excited K pi = 0+ band.