Radiative-capture cross sections for the ${}^{139}$La($n,{\hat{I}}^3$) reactionusing thermal neutrons and structural properties of ${}^{140}$La

Abstract

A set of prompt partial ${\hat{I}}^3$-ray production cross sections from thermal neutron-capture were measured for the ${}^{139}$La($n,{\hat{I}}^3$) reaction using a guided beam of subthermal (thermal and cold) neutrons incident upon a ${}^{\mathrm{nat}}$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($n,{\hat{I}}^3$) cross sections from the irradiation of a stoichiometric ${}^{\mathrm{nat}}$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 $\ddot{I}{ƒ}_0^{\mathrm{expt}}=8.58(50)$ 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 $\ddot{I}{ƒ}_0=9.36(74)$ 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 ${\hat{I}}^3$-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.

37 pages, 11 figures, 8 tables