Radiative-capture cross sections for the La139(n,γ) reaction using thermal neutrons and structural properties of La

Abstract

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 La2O3 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 LaCl3 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 expt 0 = 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 Ec = 285 keV were combined with a modeled contribution accounting for ground-state feeding from the quasicontinuum to arrive at a total cross section of σ0 = 9.36(74) b. In addition, a neutronseparation energy of Sn = 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.