Measurement of the 235U(n,f) cross section from 0.3 to 3.0 MeV using the NBS electron linac

Progress is reported on a measurement of the 235U(n,f) cross section from 0.3 to 3.0 MeV using the NBS electron linac as a pulsed neutron source. Fission events were detected using a parallel-plate, ionization fission chamber located 69.5 m from the neutron-producing tungsten target and containing 170.9 +- 2.1 mg of 235U. The absolute neutron flux was measured with the NBS black neutron detector, located at 200.4 m and coaxial with the fission chamber. A Monte Carlo program was used to calculate the neutron detection efficiency of the black detector. The present paper gives preliminary results. The fission cross section shape is shown compared to that of the ENDF/B-V 235U(n,f) cross section. Absolute values await a complete analysis of the experimental data and the measurement of the black detector efficiency at 2.6 MeV using the associated-particle technique at the 0BS Van de Graaff. Accuracies of +-2% are expected in the final analysis of this 235U(n,f) cross section measurement. (Auth.)