Open Access
ND 2007
Article Number 063
Number of page(s) 4
Section Nuclear models
Published online 17 June 2008
International Conference on Nuclear Data for Science and Technology 2007
DOI: 10.1051/ndata:07545

Effect of pre-equilibrium spin distribution on neutron induced 150Sm cross sections

D. Dashdorj1, 2, T. Kawano3, G.E. Mitchell1, 4, J.A. Becker2, U. Agvaanluvsan2, M. Chadwick3, J.R. Cooper5, M. Devlin3, N. Fotiades3, P.E. Garrett6, S. Kunieda7, R.O. Nelson3, C.Y. Wu2 and W. Younes2

1  North Carolina State University, Raleigh, NC 27695, USA
2  Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
3  Los Alamos National Laboratory, Los Alamos, NM 87545, USA
4  Triangle Universities Nuclear Laboratory, Durham, NC 27708, USA
5  USEC Inc., Piketon, OH 45661, USA
6  University of Guelph, Guelph, Ontario, N1G 2W1, Canada
7  Japan Atomic Energy Agency, Tokai, Naka, Ibaraki 319-1195, Japan

Published online: 21 May 2008

Prompt γ-ray production cross section measurements were made as a function of incident neutron energy (En = 1 to 35 MeV) on an enriched (95.6%) 150Sm sample. Energetic neutrons were delivered by the Los Alamos National Laboratory spallation neutron source located at the Los Alamos Neutron Science Center (LANSCE) facility. The prompt-reaction γ rays were detected with the large-scale Compton-suppressed Germanium Array for Neutron Induced Excitations (GEANIE). Above En ≈ 8 MeV the pre-equilibrium reaction process dominates the inelastic reaction. The spin distribution transferred in pre-equilibrium neutron-induced reactions was calculated using the quantum mechanical theory of Feshbach, Kerman, and Koonin (FKK). These pre-equilibrium spin distributions were incorporated into the Hauser-Feshbach statistical reaction code GNASH and the γ-ray production cross sections were calculated and compared with experimental data. Neutron inelastic scattering populates 150Sm excited states either by (1) forming the compound nucleus 151Sm* and decaying by neutron emission, or (2) by the incoming neutron transferring energy to create a particle-hole pair, and thus initiating the pre-equilibrium process. These two processes produce rather different spin distributions: the momentum transfer via the pre-equilibrium process tends to be smaller than in the compound reaction. This difference in the spin population has a significant impact on the γ-ray de-excitation cascade and therefore in the partial γ-ray cross sections. The difference in the calculated partial γ-ray cross sections using spin distributions with and without pre-equilibrium effects was significant, e.g., for the 558-keV transition between 8+ and 6+ states the calculated partial γ-ray production cross sections changed by 70% at En = 20 MeV with inclusion of the spin distribution of pre-equilibrium process.

© CEA 2008