The slowing‐down distances and times of fast neutrons are calculated by extending the early work of Fermi to the case in which the neutron mean free path is energy dependent. This calculation is essentially a one‐particle Monte Carlo treatment in which each path length is a mean free path and the neutron is always scattered through a scattering angle ϑ, such that cosϑ=〈cosϑ〉, the average of cosϑ in the laboratory system. The average results derived by this analytic method are shown to be in reasonable agreement with a detailed Monte Carlo computer calculation of the moderation of a 1‐MeV neutron down to 40 eV in pure hydrogen. These calculations illustrate the Monte Carlo method and the broad spread in time and energy of the neutron flux resulting from an instantaneous point source in hydrogen. The results also graphically illustrate the greater penetrating distances of 14‐MeV neutrons compared to 1‐MeV neutrons in hydrogenous materials. Pedagogic, computer‐oriented uses of this material are suggested.

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