Abstract
Exact expressions for the cross power spectral density in a uniform, infinite medium have been calculated. One-speed transport theory is assumed, thereby enabling the deficiencies in diffusion theory for this type of problem to be assessed. It is noted that the exact transport solutions go over to the simple diffusion theory ones when the points under consideration are more than about two mean free paths apart and the absorption is weak. Moreover, the diffusion theory expressions fail at high frequency. We have also examined the nature of the cross correlation function when the points are of the order of a mean free path apart. It is shown that direct free flight correlations exist which could not possibly be predicted with diffusion theory. Specific expressions for the cross and auto power spectral densities in plane, spherical and cyclindrical detectors are given.
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