Measurements of Neutron Capture Cross Section of 237Np for Fast Neutrons

The neutron capture cross section of Np-237 has been measured for fast neutrons supplied at the center of the core in the Yayoi reactor. The activation method was used for the measurement, in which the amount of the product Np-238 was determined by gamma-ray spectroscopy using a Ge detector. The neutron flux at the center of the core calculated by the Monte Carlo simulation code MCNP was renormalized by using the activity of a gold activation foil irradiated simultaneously. The new convention is proposed in this paper to make possible a definite comparison of the integral measurement by the activation method using fast reactor neutrons with differential measurements using accelerator-based neutrons. "Representative neutron energy" is defined in the convention at which the cross section deduced by the activation measurement has a high sensitivity. The capture cross section of Np-237 corresponding to the representative neutron energy was deduced as 0.80 +/- 0.04 b at 214 +/- 9 keV from the measured reaction rate and the energy dependence of the cross section in the nuclear data library ENDF/B-VII.0. The deduced cross section of Np-237 at the representative neutron energy agrees with the evaluated data of ENDF/B-VII.0, but is 15% higher than that of JENDL-3.3 and 13% higher than that of JENDL/AC-2008.