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The prospects for obtaining neutron capture cross sections indirectly from surrogate measurements are discussed. The surrogate reactions approach has been successfully employed to determine (n,f) cross sections from observed fission decays of compound nuclei created with the help of light-ion inelastic scattering or transfer reactions. The challenges encountered in applications of the method to capture reactions are considered. Case studies are presented that shed light on the accuracy to be expected from this approach, and ideas for improving the resulting cross sections are discussed.

Author(s): Sleaford, B.W. | Abstract: The neutron capture reaction is useful in identifying and analyzing the gamma-ray spectrum from an unknown assembly as it gives unambiguous information on its composition. This can be done passively or actively where an external neutron source is used to probe an unknown assembly. There are known capture gamma-ray data gaps in the ENDF libraries used by transport codes for various nuclear applications. The Evaluated Gamma-ray Activation file (EGAF) is a new thermal neutron capture database of discrete line spectra and cross sections for over 260 isotopes that was developed as part of an IAEA Coordinated Research Project. EGAF has been used to improve the capture gamma production in ENDF libraries. For medium to heavy nuclei the quasi continuum contribution to the gamma cascades is not experimentally resolved. The continuum contains up to 90percent of all the decay energy an is modeled here with the statistical nuclear structure code DICEBOX. This code also provides a consistency check of the level scheme nuclear structure evaluation. The calculated continuum is of sufficient accuracy to include in the ENDF libraries. This analysis also determines new total thermal capture cross sections and provides an improved RIPL database. For higher energy neutron capture there is less experimental data available making benchmarking of the modeling codes more difficult. We use CASINO, a version of DICEBOX that is modified for this purpose. This can be used to simulate the neutron capture at incident neutron energies up to 20 MeV to improve the gamma-ray spectrum in neutron data libraries used for transport modelling of unknown assemblies.

J. T. Burke, ∗ J. J. Ressler, J. E. Escher, N. D. Scielzo, I. J. Thompson, R. Henderson, J. Gostic, L. Bernstein, D. Bluel, M. Weideking, V. Meot, O. Roig, L. W. Phair, R. Hatarik, J. Munson, 4 C. Angell, B. Goldblum, C. W. Beausang, T. Ross, R. Hughes, M. Aiche, G. Barreau, N. Cappelan, S. Czajkowski, B. Hass, B. Jurado, L. Mathieu and I. Companis 8 Physics Division, Lawrence Livermore National Laboratory, Livermore, California 94550, USA Service de Physique Nucleaire, Commissariat a l’Energie Atomique, Bruyeres-le-Chatel, Arpajon, France Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Department of Nuclear Engineering, University of California, Berkeley, California 94720, USA Nuclear Physics Department, University of Richmond, Richmond, Virginia 23173, USA Centre d’Etudes Nucleaires Bordeaux Gradignan, CNRS/IN2P3, Univ. Bordeaux 1, Chemin du Solarium, 33175 GRADIGNAN, France Centre d’Etudes Nuclaires Bordeaux Gradignan, CNRS/IN2P3, Univ. Bordeaux 1, Chemin du Solarium, 33175 GRADIGNAN, France Horia Hulubei National Institute for Physics and Nuclear Engineering, 077125 Bucharest-Magurele, Romania