Measurement and normalization of the relative 235U fission cross-section in the low resonance region

Abstract

Measurements were performed with the Linac of the C.B.N.M. to below thermal energy for a direct normalization of fission cross-sections to the 2200 m/sec reference cross-section. The measurements were performed at a well-collimated short flight-path with low repetition-rate and long burst-width of the Linac. The fission reaction rate and the neutron spectrum were determined simultaneously with solid state detectors on each side of a back-to-back 235U and 10B layer. From the counting-rate ratios σ f -curves were calculated assuming a 1 ν behaviour of the 10B( n, α 7Li cross-section. Several fission integrals are calculated from the normalized σ f curve and compared with other published results. It is shown that the large discrepancies (up to 35 per cent) which exist between the different results can be considerably reduced by a common normalization procedure e.g. using the fission integral 11eV 7·8eVσf(E) dE=240.2±2.1 barn eV obtained from this measurement.