The γ-ray angular distribution in fast neutron inelastic scattering from iron

The angular distribution of gamma-rays emitted after inelastic scattering of fast neutrons from iron was determined at the nELBE neutron time-of-flight facility. An iron sample of natural isotopic composition was irradiated by a continuous photo-neutron spectrum in the energy range from about 0.1 up to 10 MeV. The de-excitation gamma-rays of the four lowest excited states of 56 Fe and the first excited state of Fe-54 were detected using a setup of five high-purity germanium (HPGe) detectors and five LaBr3 scintillation detectors positioned around the sample at 30 degrees, 55 degrees, 90 degrees, 125 degrees and 150 degrees with respect to the incoming neutron beam. The resulting angular distributions were fitted by Legendre polynomials up to 4th order and the angular distribution coefficients a(2) and a(4) were extracted. The angular distribution coefficients of three transitions in Fe-56 are reported here for the first time. The results are applied to a previous measurement of the inelastic scattering cross section determined using a single HPGe detector positioned at 125 degrees. Using the updated gamma-ray angular distribution, the previous cross section results are in good agreement with reference data.