Observation of new levels and proposed octupole correlations in neutron-rich150Ce

Abstract

Background: The very neutron-rich ${}^{150}$Ce is located at the edge of the $Z=56$, $N=88$ octupole deformed island. Studying its high spin states and octupole correlations is important for systematically understanding the nuclear structural characteristics in this region. Purpose: To investigate the high spin state levels and to search for octopole correlations in ${}^{150}$Ce. Methods: The high spin states of ${}^{150}$Ce are studied by measuring the prompt $\ensuremath{\gamma}$ rays in the spontaneous fission of ${}^{252}$Cf. The data analysis uses the $\ensuremath{\gamma}$-$\ensuremath{\gamma}$-$\ensuremath{\gamma}$ coincidence methods. The $\ensuremath{\gamma}\ensuremath{\rightarrow}\ensuremath{\gamma}(\ensuremath{\theta})$ angular correlation measurements are used to determine the spin and indirectly determine the parity of particular levels. Results: A new level scheme of ${}^{150}$Ce is established. A total of 47 new transitions and 25 new levels are identified, compared with previous results. Six collective bands have been observed, and five of them are newly established. An octupole band structure with $s=+1$ in ${}^{150}$Ce has been proposed. Systematic analysis of the $B(E1)/B(E2)$ branching ratios, the levels of the octupole bands, the energy differences between negative- and positive-parity bands, and the moments of inertia of the bands is carried out for ${}^{144,146,148,150}$Ce. This analysis gives evidence for our assignment of octupole correlations in ${}^{150}$Ce. The other characteristics of the octupole bands are discussed. Conclusions: An octupole band structure is proposed in ${}^{150}$Ce. The octupole correlations in ${}^{150}$Ce are weaker and show more instability than the neighboring lighter Ce isotopes.