The atomic number of a material exhibits a strong and fundamental relationship with the nature of radiation interactions within that medium.
There are numerous mathematical descriptions of different interaction processes that are dependent on the atomic number, Z.
When dealing with composite media (i.e. a bulk material composed of more than one element), one therefore encounters the difficulty of defining Z.
For bulk interaction properties, it can be useful to define an effective atomic number for a composite medium and, depending on the context, this may be done in different ways.
In many textbooks and scientific publications, the following - simplistic and often dubious - sort of method is employed.
One such proposed formula for the effective atomic number, Zeff, is as follows:[1]
The effective atomic number is important for predicting how photons interact with a substance, as certain types of photon interactions depend on the atomic number.
This 'power law' method, while commonly employed, is of questionable appropriateness in contemporary scientific applications within the context of radiation interactions in heterogeneous media.
This approach dates back to the late 1930s when photon sources were restricted to low-energy x-ray units.
[3] As such, for polyenergetic photon sources (in particular, for applications such as radiotherapy), the effective atomic number varies significantly with energy.
[4] It is possible to obtain a much more accurate single-valued Zeff by weighting against the spectrum of the source.
[4] The effective atomic number for electron interactions may be calculated with a similar approach.