Particle-coupled octupole collectivity in 91Zr

Excited states in the semi-magic ⁹¹Zr nucleus have been populated by the heavy-ion fusion-evaporation reaction ⁸²Se(¹³C, 4n) at 52 MeV beam energy for investigating the phenomenon of octupole collectivity in odd-A nuclei. The half-life of the $11/2^{-}$ state at 2170.8 keV excitation energy has been measured by the electronic fast-timing technique using a hybrid array of clover HPGe and LaBr₃(Ce) detectors, and is found to be ${\tau }^{\mathrm{expt}}=791.6\pm 44.1$ ps. The reduced E3 transition probability $B(E3;11/2^{-}\to 5/2^{+})=18.5\pm 1.2$ W.u. is significant and confirms its collective nature. The experimentally obtained $B\left(E3\right)$ value along with the excitation energy of particle-coupled octupole state is well reproduced in the theoretical calculations performed using random-phase approximation with three different interactions. In contrast to ⁹⁰Zr where the large $B\left(E3\right)$ value is made mainly by proton particle-hole excitations, the change in the $B\left(E3\right)$ value in ⁹¹Zr is due to the valence neutron density of $2d_{5/2}$ orbital to $1h_{11/2}$ orbital.