6. Photon Interaction Coefficients

As indicated in Section 2.2, the mass attenuation coefficient is given in terms of the sum of cross sections for photon interactions in the material, i.e., ðm=rÞ 1⁄4 ðNA=MAÞ P J sJ. However, for this report, the quantity of key importance for the dosimetry of ionizing radiation, particularly for primary standards, is the mass energy-transfer coefficient, ðmtr=rÞ 1⁄4 ðNA=MAÞ P J fJsJ, where fJ is the fraction of energy transferred to charged particles in the interaction of type J, and the closely related mass energy-absorption coefficient, ðmen=rÞ 1⁄4 ðmtr=rÞð1 gÞ, where g is the fraction of energy transferred to charged particles that is subsequently lost on average in radiative processes. Traditionally, the component cross sections, sJ, considered have been those for the interaction of the photon with atomic electrons and the Coulomb fields of the nucleus and atomic electrons, namely photoelectric absorption, coherent (Rayleigh) scattering, incoherent (mostly Compton) scattering, and pair production in the fields of the screened nucleus (pair) and atomic electrons (triplet). Interactions with nucleons (photonuclear cross sections) are usually not included in critically evaluated compilations. It should be noted that fJ is considered to be zero for coherent scattering, which removes consideration of coherent scattering in the calculation of mtr and men. A recent review by Pratt (2014) highlights our understanding of atomic photoeffect, Comptonscattering, and Rayleigh-scattering cross sections. He points out that, although much progress has been made in the theory and measurement of these photon-interaction cross sections, both theory and measurement still lack the accuracy to resolve discrepancies at levels of from 1 % to about 10 %. Compilations of such cross-section data are now indispensable in applications of radiation science, and figure importantly in the realization of measurement standards for ionizing radiation (see Section 3). A useful review of the history of the information used in such compilations can be found in the work of Hubbell (1969; 1999; 2006). Over more than six decades of such development, the results of theory and numerical computation have shown reasonable agreement with measured data and have largely replaced reliance on measurement as the basis for the standard reference data on photon-interaction cross sections. The work of Hubbell et al. (1975; 1979), culminating in the online database XCOM (Berger and Hubbell, 1987; Berger et al., 2010), has become a standard source of atomic cross sections for photon interactions. This Report is concerned with photon energies above about 1 keV and only with air, graphite, and liquid water; however, virtually all materials are involved in application areas of photon transport and dosimetry. Because our knowledge of the cross sections is still not complete, it appears difficult to draw unambiguous conclusions on which data to recommend. Rather, some relevant comparisons are made, and the focus will be on the uncertainties of the quantities pertinent to this Report.