Strong resonant scattering of bremsstrahlung by a level in ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}$ at 3.225-MeV excitation energy has been observed. Angular distribution and linear polarization measurements have led to a $1^{-}$ assignment for this level. From a self-absorption study, a value ${\Gamma }_0=220\mathrm{}\pm 30$ meV has been obtained for the partial radiative width of the ground-state transition. For the known $1^{-}$ states in ${}_{}{}^{150}{}_{}{}^{}\mathrm{Sm}$ (1.166 MeV) and ${}_{}{}^{154}{}_{}{}^{}\mathrm{Sm}$ (0.921 MeV), partial widths ${\Gamma }_0\mathrm{}\left(1.166\right)=(5.4\mathrm{}\pm 0.4)$ meV, and ${\Gamma }_0\mathrm{}\left(0.921\right)=(7.4\mathrm{}\pm 1.0)$ meV were measured. Combining the results of bremsstrahlung measurements with those of previous resonance fluorescence experiments utilizing high-speed rotation of radioisotopes, a width ${\Gamma }_0=(3.1\mathrm{}\pm 0.4)$ meV was arrived at for the ground-state transition from the 1.465-MeV $1^{-}$ state in ${}_{}{}^{148}{}_{}{}^{}\mathrm{Sm}$. The excitation energies of the $1^{-}$ states studied in the even Sm isotopes below $N=89\mathrm{}$ are fairly close to the sums [$E\left(2_1^{+}\right)+E\left(3_1^{-}\right)$]. The $B\left(E1\right)\text{'}\mathrm{s}$ initially decrease as one moves away from the deformed region. While this is not unexpected, the reversal of this trend near and at the magic neutron number $N=82\mathrm{}$, as evidenced by the large $B\left(E1\right)$ observed in ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}$ and the behavior of the $B\left(E1\right)\text{'}\mathrm{s}$ in ${}_{}{}^{142,144}{}_{}{}^{}\mathrm{N}\mathrm{d},$ is somewhat surprising.

NUCLEAR REACTIONS ${}_{}{}^{144,148,150,154}{}_{}{}^{}\mathrm{Sm}(\gamma ,\gamma )$; bremsstrahlung $E=1.2-3.8\mathrm{}$ MeV; measured $\sigma \left(96\mathrm{}°\right)$ and $\sigma \left(126\mathrm{}°\right)$, LP; deduced $J$, $\pi$, $\Gamma$. Enriched ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}$ and ${}_{}{}^{150}{}_{}{}^{}\mathrm{Sm}$ targets.

• Received 15 April 1976

DOI:https://doi.org/10.1103/PhysRevC.14.543