PHI-RHO-Z
In developing models of x-ray generation, analytical scientists decided that the easiest way was to consider "mass depth" or Rho-Z, the product of the sample density and the linear depth Z. The function that describes the relative intensity of x-ray generation with mass depth, is call Phi. The expression is often written as ρφ(z) Phi (Rho-z). At the surface there is a moderate amount of x-rays produced, but it rapidly increases as the electrons scatter laterally, and then quickly reaches a maximum, and trails off to zero. Some matrix corrections use this explicitly, combining the absorption and atomic number corrections. This is an alternative method to the ZAF matrix correction, which can be more easily verified using so called tracer and wedge experiments.