The age of neutrons in heterogeneous lattices

The age of neutrons in heterogeneous systems has been considered in terms of the various groups of fast neutrons available for slowing down. Special emphasis is given to the age of those neutrons which have not been inelastically scattered. A general formalism has been developed assuming that only neutrons above the fast-fission threshold of U238 suffer inelastic collisions. A particular formula for natural-uranium, heavy-water systems is presented and a numerical comparison is made between the values as obtained by this formula and another which assumes that neutrons which have not been inelastically scattered have an age corresponding to that of the fission spectrum. The values of the age, as given by the formula in this paper, are found to be significantly lower. It is seen that the inelastic scattering of those neutrons which are below the fast-fission threshold leads to a still further reduction in the values of the age. The theoretical analysis in connection with the age leads incidentally to more precise formulations for the fast-fission factor and the effective delayed neutron fractions. (author). 5 refs, 2 tabs