X‐Ray Photoeffect Cross Sections in Low‐ and Medium‐Z Absorbers for the Energy Range 852 eV to 40 keV

摘要

Experimental determinations of the mass‐attenuation coefficient (μm) using both a two‐crystal (quartz) and single‐crystal (beryl) monochromator for Be, C, N, O, Ne, Al, Ar, Cu, and Sn at selected photon energies between 852 eV and 40 keV are presented. The experimental precautions necessary to ensure freedom from effects such as small angle scattering, x‐ray fluorescence, and the Borrmann effect guarantee that the measured values of μm are characteristic of atomic absorbers. Values of the photoelectric cross section (σpe) for these elements are determined by subtraction of the contribution of coherent (Thomson) and incoherent (Compton) scattering to enable direct comparison with calculated values of σpe based on modified Coulomb wavefunctions (Stobbe theory). Modification of inner screening constants gives calculated values of σpe in fair agreement with experiment, whereas fully screened calculations or calculations made with no screening underestimate or overesimate, respectively, the cross section. The use of nonrelativistic sum rules and the precaution necessary when using Hartree‐Fock wavefunctions for the determination of σpe are discussed.