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

Hurst, A. M. and Sweet, A. and Goldblum, B. L. and Firestone, R. B. and Basunia, M. S. and Bernstein, L. A. and Révay, Zs. and Szentmiklósi, L. and Belgya, T. and Escher, J. E. and Harsányi, I. and Krtička, M. and Sleaford, B. W. and Vujic, J. (2019) Radiative-capture cross sections for the La139(n,γ) reaction using thermal neutrons and structural properties of La140. Physical Review C, 99 (2). ISSN 2469-9985

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Abstract

A set of prompt partial γ-ray production cross sections from thermal neutron capture were measured for the 139La (n,γ) reaction using a guided beam of subthermal (thermal and cold) neutrons incident on a nat La2O3 target at the Prompt Gamma Activation Analysis facility of the Budapest Research Reactor. Absolute 140La cross sections were determined relative to the well-known comparator 35Cl(n,γ) cross sections from the irradiation of a stoichiometric nat LaCl3 sample. The total cross section for radiative thermal neutron capture on 139La from the sum of experimentally measured cross sections observed to directly feed the 140 La ground state was determined to be σ0 = 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 quasi continuum to arrive at a total cross section of σ0 = 9.36(74) b. In addition, a neutron-separation energy of Sn = 5161.001(21) keV was determined from a least-squares fit of the measured primary γ-ray energies to the low-lying levels of the 140La 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.

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