Quantities, like photons, alpha/beta particles, can travel with a billiard ball like behavior. There are two quantities we might be interested in, that describe the behavior of these patricles. The quantities are the flux, and the particle density. If we define the divergence of the flux to represent the point where the particle was last scattered or emitted from then it would seem to me that the particles should obey an equation which is like a continuity equation in the sense it conserves particles. I think it should look something like this: [tex]\nabla \cdot F = k D [/tex] where F is the net particle flux and D is the particle density. My reasoning is rather informal. The equation basically says that the scattering should be proportional to the quantity of particles.