Absolute cross section measurements of ²³⁸U(n,f) and ²³⁷Np(n,f) in the neutron energy range 1 - 2.4 MeV

Two experimental campaigns have been performed at the Van de Graaff accelerator of the National Physical Laboratory (NPL, UK) under the European CHANDA project for measuring in an absolute manner the (n,f) cross sections of ²³⁵U, ²³⁸U and ²³⁷Np in the fast neutron energy region. The experimental setup consisted of a long counter (LC) and a twin Frisch-grid ionization chamber (TFGIC) for detecting fission fragments. The LC consists of a BF₃ tube surrounded by a moderating layer and an outer shield. It has a high sensitivity, a negligible gamma response and a nearly flat fluence response from 1 keV up to 7 MeV. During both campaigns five different samples were used. The results show a very good agreement with the most up-to-date evaluations for ²³⁸U(n,f) and ²³⁷Np(n,f). In the case of ²³⁵U(n,f) our data agree with evaluation when using one of the samples but we obtain a higher cross section when using the other sample. We think that further investigations will be able to explain this mismatch. Our uncertainties are dominated by the statistical uncertainty due to the beam current limitation of the facility as well as the fluence uncertainty