Measurement of the 235U fission cross-section for 252Cf spontaneous fission neutrons

The absolute 235U fission cross section for 252Cf spontaneous fission neutrons is a normalization datum for establishing the energy dependent cross section for 235U fission. It is also a measurement standard for deriving integral cross sections from ratio determinations in benchmark neutron fields which serve reactor physics and neutron dosimetry. The measurements reported here were carried out with a small volume 252Cf neutron source and two lightweight double fission chambers in compensated beam geometry. The important characteristics of the measurement are: (1) except for neutron scattering corrections, the cross section obtained depends only upon the source strength of a natural neutron source, a distance measurement, and the isotopic mass of four 235U deposits, and (2) for any credible departure of the 252Cf fission neutron spectrum from its experimentally determined shape, the cross section does not vary more than 0.4%. The present measurement represents an extension of a previous measurement performed by Heaton et al. at the NBS a decade ago. Since then progress has been made in the characterization of fissionable deposit masses; in the determination of the source strength of 252Cf sources relative to the internationally compared Ra-Be photoneutron standard neutron source NBS-I; and in the ability to accurately measure the distance between fission chambers in compensated beam geometry and to determine the position of the fissionable deposits inside the chambers. New calculational techniques have been developed to determine the residual scattering corrections for the fission chambers, deposit backings, and neutron source. Finally, new experiments have been performed to measure the effect of neutron return from the environment. (author)