For simplicity, let us assume that the incident radiation is monoenergetic at energy , and it is monodirectional, traveling down the z-axis. Let us assume that the surface that is being irradiated is in the x--y plane, ranging over and .
In this case, one typically knows the number of particles incident on the boundary per unit area of boundary, as a function of time, position on the boundary, energy, and angle. For example, consider a beam of monoenergetic particles incident normally and uniformly on the negative ``y'' surface of a ``brick'' of edges a, b, and c, corresponding to the three coordinate axes x, y, and z.
In this case, there are particles incident per unit area per unit time on the slab, and they are all travelling perpendicular to the surface of the slab. To start a particle in a Monte Carlo simulation, the source module would sample a position on the incoming surface of the brick.
Nuclear engineers working in reactor physics and radiation shielding areas generally employ the concept of neutron ``flux'' to describe the amount of radiation, while other disciplines employ a ``density'' or ``intensity'' to describe what is in essence a very similar quantity. However, since the Monte Carlo simulation is a direct analog of the physical application, how these terms relate to a real application will be apparent after a few examples.