On the transport of charged particles through spongy materials

Monte Carlo calculations have been made of the transport of charged particles in spongy material. These calculations have shown the anticipated decreased probability of the paths being found in the material for conditions of grazing incidence. However, at slightly larger angles of incidence, the calculation has shown an increased probability of the trajectory being within the material. When averaged over randomly-oriented incident angles, these two effects exactly cancel. Thus, charged particles traversing a medium with randomly distributed voids 'see' an average density identical to the bulk average density of the spongy medium. This result can be shown to be a consequence of Fano's theorem (1954) when it is applied with appropriate boundary conditions. However, if the spongy medium were regularly structured, significant deviations in the apparent density would be expected. (author)