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

The angular distribution of γ-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 γ-rays of the four lowest excited states of ⁵⁶Fe and the first excited state of ⁵⁴Fe were detected using a setup of five high-purity germanium (HPGe) detectors and five LaBr₃ scintillation detectors positioned around the sample at 30°, 55°, 90°, 125° and 150° 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 a₂ and a₄ were extracted. The angular distribution coefficients of three transitions in ⁵⁶Fe 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°. Using the updated γ-ray angular distribution, the previous cross section results are in good agreement with reference data.