Photons of energies primarily above 2m(e)c(2) (1.022 MeV) can interact with the Coulomb field of an atomic nucleus to be transformed into an electron-positron pair, the probability increasing with increasing photon energy, up to a plateau at high energies, and increasing with increasing atomic number approximately as the square of the nuclear charge (proton number). This interaction can also take place in the field of an atomic electron, for photons of energy in excess of 4m(e)c(2) (2.044 MeV), in which case the process is called triplet production due to the track of the recoiling electron adding to the tracks of the created electron-positron pair. The last systematic computations and tabulations of pair and triplet cross sections, which are the predominant contributions to the photon mass attenuation coefficient for photon energies 10 MeV and higher, were those of Hubbell, Gimm and Overbo in 1980, from threshold (1.022 MeV) up to 100 GeV, for all elements Z = 1-100. These computations required some ad hoc bridging functions between the available low-energy and high-energy theoretical models. Recently, Sud and others have developed some new approaches including using DWBA (distorted wave Born approximation) theory to compute pair production cross sections in the intermediate energy region (5.0-10.0 MeV) on a firmer theoretical basis. Also, Bergstrom et al. [Phys. Rev. A 53 (1996) 2865], Agger and Sorensen [Phys. Rev. A 55 (1997) 402] and others discuss bound-free pair production below the above mentioned 1.022 MeV threshold. These and other recent developments are discussed with an eye toward new computations, if warranted, to update and improve the accuracy of the 1980 Hubbell et al. tabulations. (C) 2003 Elsevier B.V. All rights reserved.
