New lifetime measurements for the lowest quadrupole states in Ne20,22 and possible explanations of the high collectivity of the depopulating E2 transitions

Abstract

Shell model and multiparticle-multihole configuration-mixing calculations indicate a missing quadrupole collectivity needed to explain the transition strengths in the even-even Ne isotopes. Even with a significant scaling of the effective charges all details of the data cannot be reproduced. The effect is very pronounced in $^{20,22}\mathrm{Ne}$. Therefore new lifetime measurements in these nuclei were undertaken by us with the Doppler-shift attenuation method to recheck experimentally earlier findings. They confirmed the previous result in the case of $^{20}\mathrm{Ne}$ while in $^{22}\mathrm{Ne}$ the ${2}_{1}^{+}$ level lifetime was found to be shorter by 43%. It turned out that in the latter nucleus the lifetime of the ${4}_{1}^{+}$ level also has to be corrected. The $B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ reduced transition strengths derived from the newly determined lifetimes as well as those for the heavier Ne isotopes are reasonably described by involving a mixing of $\ensuremath{\alpha}$-cluster states of, e.g., the type $\ensuremath{\alpha}\ensuremath{\bigotimes}16\mathrm{O}$ with normal, nearly spherical states. An extended version of the coupled-cluster effective interaction method published earlier describes well the spin dependence of the $B(E2)$'s in $^{20}\mathrm{Ne}$.