Effective atomic numbers of composite materials for total and partial interaction processes for photons, electrons,...

Effective atomic numbers for total and dominant partial interaction processes of photons (1-50 MeV), electrons (1-50 MeV), and protons (1-200 MeV) for the composite materials bone (cortical), muscle (striated), water, polystyrene, Perspex, and Nylon-6 are derived. For photons, the effective atomic number from pair production in the nuclear field is greater than it is from the incoherent scattering. For electrons the effective atomic number from the radiative losses is greater than it is from the collision losses. In both of these cases however, the effective atomic numbers from partial interaction processes remain more or less the same, whereas the number from the total interaction increases with increasing energy. But in the energy regions from 1 to 5 MeV for photons and from 1 to 10 MeV for electrons, the number from the total interaction remains approximately the same for each of these composite materials. For all these materials, in these energy regions the interaction is predominantly with atomic electrons and the contributions from the pair production for photons and radiative losses for electrons are small. In the case of protons the number from total interaction remains more or less the same in the energy region considered. In this energy region collisions with atomic electrons dominate, and the contribution to the total stopping power is mainly from this process only. Hence the derived effective atomic number is basically from the partial process involving the interactions with atomic electrons. Thus, for photons from 1 to 5 MeV for electrons from 1 to 10 MeV and for protons from 1 to 200 MeV, the dosimetric data collected with composite tissue equivalent phantoms, designed on the basis of interaction with atomic electrons for treatment planning, will have less uncertainty.