Multiple Scattering in Neutron Double-Differential Cross-Section Measurements

Calculations have been performed of multiple scattering corrections for several important experimental measurements of double-differential neutron scattering by hydrogenous materials, most notably room- temperature water. The calculations were carried out by the program MUSE, which solves the integral transport equation numerically to obtain the scattered neutron fluxes for single and multiple (up to four) collisions in a slab sample, using a theoretical kernel as input. In many cases these corrections turn out to be large and also quite sensitive to the theoretical model. Such results emphasize the fact that a quantitative treatment of multiple scattering is an absolutely essential step in the interpretation of double-differential data, but is not necessarily a straightforward and unique procedure. To help clarify this situation we have investigated and compared ways of treating multiple scattering in double-differential experiments. Two methods of correcting for multiple scattering are considered. These are: (1) multiplying the data by a factor which is essentially the theoretically computed ratio of single-scattering to total-scattering; and (2) subtracting from the observed scattered flux a theoretically computed multi-collision flux. These correction methods are equivalent and the corrected data are consistent with the theoretical double-differential cross-sections only under stringent conditions relating the theoretical model to the experiment. When these conditions are not satisfied it is still sometimes possible to find circumstances which strongly favour the use of one method over the other. However, there is a possibility that large uncertainties and ambiguities will arise in corrections made by either method, primarily because of model sensitivity. Thus it is not always feasible to present experimental results in the form of corrected data. Instead, the uncorrected data should be used to check theoretical models by direct comparison of the theoretically calculated and observed scattered fluxes. (author)