A simple method for calculating the linear extrapolation distance at the surface of a black body

We adapt a method introduced by Fuchs for calculating evaporation rates from small droplets to the evaluation of the linear extrapolation distance of the neutron density at the surface of a black body immersed in an infinite, absorbing medium. Explicit results are obtained for spheres and cylinders and, by comparison with some very accurate calculations carried out by others, these results are shown to be accurate to within about 9% for a range of parameters. Modifications are introduced which enable the method to deal with absorbing and scattering bodies which are not black. (author)