Asymptotic solutions of neutron transport equation and the limits of correct use of diffusion approximation for rocks

The diffusion approximation solution for neutron transport has been used in well-logging geophysics for calculating tool responses in boreholes, somet imes with success. The problem of the dimension of different materials to w hich it can be applied with success is important for the borehole environme nt. The results obtained show that the diffusion approximation can be used for distances greater than a few millimetre in some rock types. For iron, b arium, and other highly absorbing media the use of the diffusion approximat ion is inappropriate even for large distances. (C) 2001 Elsevier Science Lt d. All rights reserved.