Direct Evidence of Octupole Deformation in Neutron-Rich ${}^{144}$Ba

Abstract

The neutron-rich nucleus ${}^{144}$Ba ($t_{1/2}$=11.5 s) is expected to exhibit some of the strongest octupole correlations among nuclei with mass numbers $A$ less than 200. Until now, indirect evidence for such strong correlations has been inferred from observations such as enhanced $E1$ transitions and interleaving positive- and negative-parity levels in the ground-state band. In this experiment, the octupole strength was measured directly by sub-barrier, multi-step Coulomb excitation of a post-accelerated 650-MeV ${}^{144}$Ba beam on a 1.0-mg/cm${}^2$ ${}^{208}$Pb target. The measured value of the matrix element, $⟨3_1^{-}\Vert \mathcal{M}(E3)\Vert 0_1^{+}⟩=0.65{(}_{-23}^{+17})$ $e$b${}^{3/2}$, corresponds to a reduced $B(E3)$ transition probability of 48(${}_{-34}^{+25}$) W.u. This result represents an unambiguous determination of the octupole collectivity, is larger than any available theoretical prediction, and is consistent with octupole deformation.

5 pages, 2 figures, 1 table, Physical Review Letters 2016