Use of the (γ,γ′) reaction for studying the energy levels of ¹³⁹La

Elastic and inelastic scattering of monochromatic photons were used for studying nuclear energy levels in ¹³⁹La; 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 γ source, and three independent resonances at 6018, 7279, and 7632 keV were excited with the iron γ 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, Jπ) were made: 166, 5/2; 1220, 5/2, (9/2); 1384, 7/2; 1419, 7/2, (11/2); 1536, 72; 1580, 9/2; 2232, (72,112); 6018, 7/2^-; 6418, 9/2^-; and 6760, 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 Γγ=0.051 and 0.081 eV, respectively. The La139 levels are compared with recent theoretical calculations.