Answer to Question #4920 Submitted to "Ask the Experts"

I am looking for a program that will calculate an x-ray spectrum.

You do not specify the nature of the x-ray source that is responsible for the spectrum. There are various spectral sources including, for example, fluorescence x rays, often used in elemental analysis because of the discrete energies of the characteristic x rays and produced by atomic excitation with higher-energy photons or charged particles; astrophysical sources, whose characteristics are illuminated by study of x rays emitted from certain bodies and gases; and bremsstrahlung x-ray sources that generate a continuous distribution of photons, usually through the slowing down of high-speed electrons as they are deflected in the fields of nuclei. The latter process is common in diagnostic x-ray machines, linear accelerators used as high-energy bremsstrahlung sources in radiation therapy, electron accelerators used in industrial processing, and a variety of other situations in which high-speed electrons interact with materials. There is information available in a mix of materials, depending on the nature of the x-ray source. I am assuming that your interests are probably related to bremsstrahlung spectra produced by electron interactions.

Two of the most popular and powerful computer codes that have been and are used to calculate the energy distributions of bremsstrahlung photons from electron interactions are Monte Carlo probabilistic codes, one being referred to as MCNP (Monte Carlo N-Particle transport code) and the second being EGS4 (Electron-Gamma Shower). They both are available in versions that run on PCs, although you will likely require a suitable FORTRAN compiler. Both of these codes are available for a fee from the Radiation Safety Information Computation Center (RSICC) at Oak Ridge National Laboratory. If you are not experienced with the use of similar codes, you will find the learning curve rather steep. Some national laboratories and private groups offer training courses that instruct participants in the use of these codes; you can usually find course announcements in various technical society publications and on the Internet. The Monte Carlo codes are extremely versatile in that they allow modeling of complex geometries and multiple materials and can track the production of bremsstrahlung photons and follow their penetration through user-specified thicknesses and types of materials.

It is also possible to do some calculations of bremsstrahlung spectral shapes using selected deterministic expressions. Thus, one can calculate the shapes of virgin spectra from monoenergetic electrons incident on thick targets. It is also possible to extend such calculations to polyenergetic electron sources, such as beta particle distributions. This has been discussed somewhat in ATE Question #1540; there are references to some textbooks that provide some discussion pertinent to the topic. While deterministic approaches may be useful to predict the initial photon distributions (i.e., prior to any interactions of these photons), they are often of limited help when it is necessary to ascertain the spectra after passage through or scatter from various materials, especially for anything other than very simple geometries.

George Chabot, PhD, CHP