Radiative-capture cross sections for the La-139(n, gamma) reaction using thermal neutrons and structural properties of La-140
Abstract
A set of prompt partial gamma -ray production cross sections from thermal neutron capture were measured for the La-139(n, gamma) reaction using a guided beam of subthermal (thermal and cold) neutrons incident on a (La2O3)-La-nat target at the Prompt Gamma Activation Analysis facility of the Budapest Research Reactor. Absolute La-140 cross sections were determined relative to the well-known comparator Cl-35(n, gamma) cross sections from the irradiation of a stoichiometric (LaCl3)-La-nat sample.
The total cross section for radiative thermal neutron capture on La-139 from the sum of experimentally measured cross sections observed to directly feed the La-140 ground state was determined to be sigma(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 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 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 gamma-ray energies to the low-lying levels of the La-140 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.