Use of the (γ,γ′) Reaction for Studying the Energy Levels ofLa139

Elastic and inelastic scattering of monochromatic photons were used for studying nuclear energy levels in ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$; the photons were produced by thermal-neutron capture in titanium and iron. Two independent resonance lines at 6418 and 6760 keV were excited with the titanium $\gamma$ source, and three independent resonances at 6018, 7279, and 7632 keV were excited with the iron $\gamma$ source. Some 18 known low-lying levels were populated by deexcitation of these resonance levels. The angular distribution of some intense elastic and inelastic lines were measured and the following spin and parity determinations (keV, $J^{\pi }$) were made: 166, $\frac{5}{2}$; 1220, $\frac{5}{2}$, ($\frac{9}{2}$); 1384, $\frac{7}{2}$; 1419, $\frac{7}{2}$, ($\frac{11}{2}$); 1536, $\frac{7}{2}$; 1580, $\frac{9}{2}$; 2232, ($\frac{7}{2},\frac{11}{2}$); 6018, ${\frac{7}{2}}^{-}$; 6418, ${\frac{9}{2}}^{-}$; and 6760, $\frac{7}{2}$, where parentheses denote uncertainties. The parities of the 6018- and 6418-keV levels were directly determined using a Compton polarimeter. The total radiative widths of the 6018- and 6418-keV levels were measured and found to be ${\Gamma }_{\gamma }=0.051\mathrm{} \mathrm{and} 0.081$ eV, respectively. The ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$ levels are compared with recent theoretical calculations.