Crow-flight distance

For this calculation we use the problem of neutron diffusion from a point source in an infinite medium. Similar calculations can be made for the plane and line source, but the basic relationship between L and the crow-flight distance will prove to be the same in all cases. Consider then the diffusion of neutrons in an infinite medium from a point source of strength go neutrons per unit time. For convenience, we place the source at the brigin of a suitable coordinate system. As previously noted, this situation has spherical symmetry, and the neutron distribution may be described completely by the radial coordinate r alone. Let us compute now the quantity r for this system. We define as the average square... 

We wish to determine the mean square distance neutrons travel from the source before being absorbed. This distance is related to the crow-flight distance we mentioned in Fig. 24.1 ... 

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