Rotational bands in neutron-richEr169,171,172

Abstract

The neutron-rich $^{169,171,172}\mathrm{Er}$ nuclei were populated by few-neutron transfer reactions between $^{170}\mathrm{Er}$ and $^{238}\mathrm{U}$ at a near barrier energy. The spectroscopy of these $\mathrm{Er}$ isotopes was studied using prompt $\ensuremath{\gamma}$ rays correlated with delayed transitions or events involving at least three prompt transitions. The ground-state band of $^{172}\mathrm{Er}$ was populated up to spin ${22}^{+}$ at an excitation energy of $5528\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$. Rotational bands built on the $1∕{2}^{\ensuremath{-}}[521]$, $5∕{2}^{\ensuremath{-}}[512]$, and $7∕{2}^{+}[633]$ neutron configurations in $^{169,171}\mathrm{Er}$ were extended to substantially higher spins than previously known. The signature splitting observed in these rotational bands is addressed within the framework of the particle-rotor model in terms of triaxiality and Coriolis attenuation. The signature inversion observed in the $5∕{2}^{\ensuremath{-}}[512]$ band is well reproduced by including the triaxial degree of freedom in the calculation. Attenuating the Coriolis interaction in the calculation is found to be necessary to reproduce the signature splitting observed in the $7∕{2}^{+}[633]$ band. A similar Coriolis attenuation also is needed to account for the signature splitting as well as the $B(M1)∕B(E2)$ ratios in the $7∕{2}^{+}[633]$ ground-state band in the neighboring $N=99$ isotones, $^{167}\mathrm{Er}$ and $^{169}\mathrm{Yb}$.