The penetration of radiation intópíane semi-infinite slabs of material has been calculated numerically by means of the time-dependent transport theory (Boltzmann equation) and the approximate "diffusion theory." Quantitative results are given for the case of a non-scattering material with constant absorption cross section, and for the general case of a scattering material (boron) with absorption co¬efficient a function of energy. Results show that the diffusion theory calculation gives an energy penetration which is too large, but that in a time during which the diffusion wave penetrates to a depth corres¬ponding to a few mean free paths, the rate of energy penetration calculated using the diffusion approximation approaches the value cal¬culated using the exact theory.
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