  A set of prompt partial $\gamma$-ray production cross sections from thermal
neutron-capture were measured for the $^{139}$La($n,\gamma$) reaction using a
guided beam of subthermal (thermal and cold) neutrons incident upon a $^{\rm
nat}$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($n,\gamma$) cross
sections from the irradiation of a stoichiometric $^{\rm nat}$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 $\sigma_{0}^{\rm
expt} = 8.58(50)$ 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 $\sigma_{0} =
9.36(74)$ 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
$\gamma$-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.
