  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,
$\langle 3_1^- \| \mathcal{M}(E3) \| 0_1^+ \rangle=0.65(^{+17}_{-23})$
$e$b$^{3/2}$, corresponds to a reduced $B(E3)$ transition probability of
48($^{+25}_{-34}$) 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.
