Prompt ν-values of 235U and 239Pu in some fast reactor spectra

Abstract

The averaged number of prompt neutrons per ³³⁵sU and ²³⁹Pu fission have been measured in fast and thermalized neutron fluxes at the fast zero-power reactor (FRO) at Studsvik, Sweden. The measurements were made with three different reactor cores giving median fission energies for ²³⁵U (core centre) of 50, 170 and 240 keV. The spectrum averaged v data were compared with the corresponding thermal $\text{ν}$ data. The $\text{ν}\text{-}\text{value}$ of the spontaneous fission of ²⁵²Cf was used as an intermediate standard. The fission samples were placed in neutron beams extracted from the reactor cores and a large liquid scintillator was used as the fission neutron detector. The experiment also included relative fission rate measurements, ²³⁸U/²³⁵U, for the three reactor cores.

The results are compared with calculated relative $\text{ν}\text{-}\text{values}$ deduced from different sets of recently evaluated fission cross section and $\text{ν}\text{-}\text{data}$ and from different assumptions concerning the reactor neutron spectra. Within the experimental one standard deviation errors of 1·5 and 2 per cent for ²³⁵U and ²³⁹Pu a good agreement was obtained between the experimental and calculated data assuming $\text{ν}\text{-}\text{values}$ in the fission resonance region equal to $\text{ν}$ at thermal energy.