Radiative-capture cross sections for the 139 La(n, γ ) reaction using thermal neutrons and structural properties of 140 La

A set of prompt partial γ -ray production cross sections from thermal neutron-capture were measured for the 139 La( n, γ ) reaction using a guided beam of subthermal (thermal and cold) neutrons incident upon a nat La 2 O 3 target at the Prompt Gamma Activation Analysis facility of the Budapest Research Reactor. Absolute 140 La cross sections were determined relative to the well-known comparator 35 Cl( n, γ ) cross sections from the irradiation of a stoichiometric nat LaCl 3 sample. The total cross section for radiative thermal neutron-capture on 139 La from the sum of experimentally measured cross sections observed to directly feed the 140 La ground state was determined to be σ expt0 = 8 . 58(50) b. To assess completeness of the decay scheme and as a consistency check, the measured cross sections for transitions feeding the ground state from levels below a critical energy of E c = 285 keV were combined with a modeled contribution accounting for ground-state feeding from the quasicontinuum to arrive at a total cross section of σ 0 = 9 . 36(74) b. In addition, a neutron-separation energy of S n = 5161 . 005(21) keV was determined from a least-squares fit of the measured primary γ -ray energies to the low-lying levels of the 140 La decay scheme. Furthermore, several nuclear structure improvements are proposed for the decay scheme. The measured cross-section and separation-energy results are comparable to earlier measurements of these quantities.