Measurement of Neutron Flux Spectra by a Multiple Foil Activation Iterative Method and Comparison with Reactor Physics Calculations and Spectrometer Measurements

A multiple foil activation iterative method has been used to experimentally determine neutron flux spectra in various types of neutron environments. The method involves irradiation of a set of different foil detectors, measurement of resultant activities, and adjustment of a spectrum selected as an initial approximation to obtain a good-fit solution for a set of simultaneous activation integral equations. A computer code, SAND-II, is used to obtain this solution. Spectra from thermal and fast reactors and from beam sources have been measured. In each experiment, a set of more than ten foil detectors, encompassing low- and high-energy neutron-induced reactions, was irradiated and used as input to SAND-II. Solutions obtained are compared with diffusion, transport, or Monte Carlo calculations or with spectrometer measurements. It is concluded that the multiple foil activation iterative method is an important adjunct to calculational and neutron spectrometer techniques used to determine neutron flux spectra.