Inelastic scattering of low energy gamma rays

The total atomic cross sections are determined in carbon, aluminium and copper at photon energies of 145, 129, 100 and 84 kev by the transmission method in good geometry. The values of bound-electron inelastic scattering cross sections σ b are extracted by subtracting the theoretical sum of photoelectric and coherent scattering cross sections from the total cross section. The values of σ b are also calculated theoretically using the S ( q, Z ) values based on the Thomas-Fermi atomic charge distribution. The free-electron incoherent scattering cross sections σ f are computed from the Klein-Nishina formula. Good agreement is obtained between theoretical and experimental ratios of σ b /σ f for carbon and aluminium at 145 kev and carbon at 129 kev. Deviations from experiment, increasing with atomic number and decreasing energy, are observed in all other cases and are ascribed to the systematic errors inherent in the Thomas-Fermi atomic charge distribution.