Radiative-capture cross sections for the La139(n,γ) reaction using thermal neutrons and structural properties of La140

A set of prompt partial $\\ensuremath{\\gamma}$-ray production cross sections from thermal neutron capture were measured for the $^{139}\\mathrm{La}(n,\\ensuremath{\\gamma})$ reaction using a guided beam of subthermal (thermal and cold) neutrons incident on a $^{\\mathrm{nat}}\\mathrm{La}_{2}{\\mathrm{O}}_{3}$ target at the Prompt Gamma Activation Analysis facility of the Budapest Research Reactor. Absolute $^{140}\\mathrm{La}$ cross sections were determined relative to the well-known comparator $^{35}\\mathrm{Cl}(n,\\ensuremath{\\gamma})$ cross sections from the irradiation of a stoichiometric $^{\\mathrm{nat}}\\mathrm{LaCl}_{3}$ sample. The total cross section for radiative thermal neutron capture on $^{139}\\mathrm{La}$ from the sum of experimentally measured cross sections observed to directly feed the $^{140}\\mathrm{La}$ ground state was determined to be ${\\ensuremath{\\sigma}}_{0}^{\\mathrm{expt}}=8.58(50)$ 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 ${\\ensuremath{\\sigma}}_{0}=9.36(74)$ b. In addition, a neutron-separation energy of ${S}_{n}=5161.001(21)$ keV was determined from a least-squares fit of the measured primary $\\ensuremath{\\gamma}$-ray energies to the low-lying levels of the $^{140}\\mathrm{La}$ 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.