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.