γ -soft rotor with configuration mixing in the O(6) limit of the interacting boson model

Abstract

To describe obvious intruder states and nonzero quadrupole moments of $\ensuremath{\gamma}$-soft nuclei such as $^{194}\mathrm{Pt}$, a rotor extension plus intruder configuration mixing with $2n$-particle and $2n$-hole configurations from $n=0$ up to $n\ensuremath{\rightarrow}\ensuremath{\infty}$ in the O(6) ($\ensuremath{\gamma}$-unstable) limit of the interacting boson model is proposed. It is shown that the configuration mixing scheme keeps the lower part of the $\ensuremath{\gamma}$-unstable spectrum unchanged and generates the intruder states due to the mixing. It is further shown that almost all low-lying levels below 2.17 MeV in $^{194}\mathrm{Pt}$ can be well described by modifying the O(6) quadrupole-quadrupole interaction into an exponential form. The third-order term needed for a rotor realization in the interacting boson model seems necessary to produce nonzero quadrupole moments with the correct sign.