Development of Monte Carlo track structure codes

When ionizing radiation interacts with matter, by its basic definition, it produces free electrons and residual positive ions. Thus, one might think initially that all ionizing radiation should produce the same biologic damage. This, however, is not so. When what we traditionally call x-rays (energies from a few 10s of eV to a few hundred keV) interact with an absorber, whether it is a gas, liquid, or solid, they produce “fast” mono-energetic electrons having energies near that of the incident x-ray, i.e., photoelectrons with energies Epe = Ex-ray– φ, where φ is the initial binding energy of the electron and is generally a few 10s of eV more or less; the binding energy is obviously more, and the difference between photon and electron energy greater, when sufficient energy is available to ionize inner shell electrons. When gamma-rays, traditionally defined as having energies from a few hundred keV to a few MeV, interact with matter, they can also produce photo-electrons, but as the energy increases above a few hundred keV, they produce free electrons with energies from a few eV to a few keV through particle-like collisions with absorber electrons; this is called Compton scattering. Thus gamma-rays of a defined energy produce free electrons with a broad distribution of energies. All fast electrons, regardless of how they are produced will subsequently undergo interactions with the bound electrons of the media and loose energy.