The measurement of photofission contribution to fission reaction rate.

Abstract

The knowledge of the energy distribution of neutrons in an irradiation field is of fundamental interest to the neutron dosimetry community. Fission reactions such as $\sp{237}$Np(n,f) and $\sp{238}$U(n,f) are attractive choices in neutron dosimetry because of their unique advantages over other threshold reactions. Since the irradiations are usually performed in a mixed field of photons and neutrons, the measured fission reaction rates include contributions from photofission. A method has been developed to quantify an upper (and a lower) bound on photofission contribution to fission reaction rate measurements. This method does not require knowledge of the photon spectrum in the field of measurement, nor does it require knowing the photofission cross sections of the interfering reactions. This method involves the measurement and calculation of spectral indices, which are defined as the ratios of integral detector responses in an irradiation field. The method was tested in the Materials Dosimetry Reference Facility (MDRF), which is a NIST reference neutron field operating at the University of Michigan. Spectral indices of $\sp{237}$Np and $\sp{238}$U fission reactions were measured and calculated relative to a reaction which had no photon contribution, the $\rm\sp{58}Ni(n,p)\sp{58}Co$ reaction. The differential changes in the spectral indices were measured and calculated with and without a bismuth gamma attenuator present around the detectors. The measurement included contributions from both neutrons and photons whereas the calculations included contribution from neutrons only. The monte-carlo code MCNP was used to calculate the neutron spectra in the MDRF with and without bismuth. These neutron spectra were then used to calculate the spectral indices. An expression was derived for the ratio of photon fission rate to neutron fission rate using the definitions of the spectral indices. From this expression, an algebraic upper bound for the photofission contribution was determined by using a minimum value for the mass attenuation coefficient of bismuth at all energies. The established algebraic upper bounds for the photofission contributions to the $\sp{237}$Np and $\sp{238}$U fission reactions in the MDRF were found to overlap zero. Conservative statistical upper bounds were established at the 1$\sigma$ and 2$\sigma$ levels of confidence. The established statistical upper bounds for the photofission contributions in the MDRF are 0.87% for the $\sp{237}$Np fission reaction and 0.55% for the $\sp{238}$U fission reaction at the 1$\sigma$ level of confidence.