Review of photon interaction cross section data in the medical and biological context

The probability of a photon (x-ray, gamma-ray, bremsstrahlung, etc) of a given energyE undergoing absorption or scattering when traversing a layer of materialZ can be expressed quantitatively in terms of a linear attenuation coefficient (cm 1 ). Since is dependent on the material's density, (g cm 3 ), which can be variable, the quantity usually tabulated is the mass attenuation coefficient = (cm 2 g 1 ) in which the dependence on the density has been removed. =, in turn, can be obtained as the sum of the different types of possible interactions of photons with atoms of the material. For photon energies below 1 MeV the major interaction processes to be considered are incoherent (Compton) scattering, coherent (Rayleigh) scattering and atomic photoeffect absorption. Above 1 MeV one must also include nuclear-field pair production and atomic-field (triplet) production, and above 5 MeV one in principle should include photonuclear absorption, although the latter is neglected in data tabulations up to the present time. This review includes a selective history of measurements and theory relating to = from the turn of the century up to the present time, and is intended to provide a basis for further calculations and critical tabulations of photon cross section data, particularly as required by users in radiation medicine and biology. The mass energy-absorption coefficient en= is also briefly discussed.