We prove that a beam of non-quantum particles going through a double-slit may concentrate in some locations while
avoiding others. Their detection shows maxima and minima which do not appear from the interference of some waves,
but are a consequence of the interaction between particles and the screen. The model is an application of cellular
automata. This behaviour is the result of two assumptions: we allow small diffusions on both sides of the main stream of
particles; particles are reflected by the screen as if the obstacle would be a particle supplier. We attempt an explanation
of these results.