Cross-section measurement and analysis for the 149Sm(n,α)146Nd reaction at 6.0 MeV

We have measured the 149Sm(n,α)146Nd cross section at 6.0 MeV to be 0.12±0.018 mb. This is the first reported result for this cross section in the MeV region and so should be helpful for constraining nuclear data evaluations (which differ by a factor of 40 for this reaction at this energy) and for testing and improving nuclear models. The experiment was performed at the at the 4.5-MV Van de Graaff accelerator of Peking University. Neutrons were produced via the 2H(d,n)3He reaction using a deuterium gas target, and absolute neutron flux was determined with a small 238U fission chamber. Alpha particles were detected using a two-section gridded ionization chamber in which two large-area 149Sm2O3 samples were placed back-to-back so that the cross section and forward/backward ratio were measured for nearly the entire 4π solid angle. The data were compared to statistical-model predictions using the code talys. Good agreement between the measured and theoretical cross sections could be obtained with a small modification of the α-optical potential parameters from their default values in talys. However, we were not able to reproduce the measured forward/backward ratio. In addition, using the previously reported 147Sm(n,α)144Nd cross section at 6.0 MeV together with our new result, good agreement was found between the measured and predicted cross-section ratio for these two isotopes. Finally, none of the existing evaluated data libraries are in agreement with our new data to within the experimental uncertainties.