Evidence for octupole vibration in the triaxial superdeformed well ofLu164

Abstract

High-spin states in $^{164}\mathrm{Lu}$ were populated in the $^{121}\mathrm{Sb}$($^{48}\mathrm{Ca}$,$5n$) reaction at 215 MeV and \ensuremath{\gamma}-ray coincidences were measured with the Gammasphere spectrometer. Through this experiment the eight known triaxial superdeformed bands in $^{164}\mathrm{Lu}$ could be confirmed. Some of these bands were extended to higher as well as to lower spins. Evidence is reported for the first time for weak $\ensuremath{\Delta}I=1,E1$ transitions linking TSD3 and TSD1. This observation may imply coupling to octupole vibrational degrees of freedom. The decay mechanism is different from the one observed in the neighboring even-$N$ isotopes, which exhibit wobbling excitations built on the $\ensuremath{\pi}{i}_{13/2}$ structure with $E2(M1),\ensuremath{\Delta}I=1$ interband decay. An additional sequence decaying at high spin into TSD1 was observed up to ${I}^{\ensuremath{\pi}}=({50}^{\ensuremath{-}})$. This band has a constant dynamic moment of inertia of \ensuremath{\sim}$70\ensuremath{\hbar}{}^{2}{\mathrm{MeV}}^{\ensuremath{-}1}$ and an alignment that is $\text{\ensuremath{\sim}}2\ensuremath{\hbar}$ larger than that found for TSD1. A revision of the assumed spin-parity-assignment of TSD2 is based on the observed decay-out to normal-deformed structures. The parity and signature quantum numbers of TSD2 are now firmly assigned as $(\ensuremath{\pi},\ensuremath{\alpha})=(+,0)$, in disagreement with the former assignment of $(\ensuremath{\pi},\ensuremath{\alpha})=(\ensuremath{-},1)$, which was based on the assumption that TSD2 is the signature partner of TSD1. TSD1 and TSD2 show an alignment gain at $\ensuremath{\hbar}\ensuremath{\omega}\text{\ensuremath{\sim}}0.67$ and 0.60 MeV, respectively. In TSD1 the involvement of the ${j}_{15/2}$ neutron orbital is suggested to be responsible for the high-frequency crossing.