Mass attenuation coefficients μ/ρ for H, C, N, O, and Ar, developed at the National Bureau of Standards by the X-Ray and Ionizing Radiation Data Center from the latest theoretical and experimental cross-section data, are tabulated for photon energies from 0.1 keV to 20 MeV, including the cesium-137 and cobalt-60 energies explicitly. Cross sections are listed, for the above elements, for the principal photon-atom interactions: coherent and incoherent scattering, atomic photoeffect, and positron-electron pair and triplet production; the energy-absorption cross sections for these interactions are also listed. From this cross-section data base, mass energy-absorption coefficients <tex-math>$\mu _{{\rm en}}/\rho $</tex-math>, taking into account bremsstrahlung losses and positron annihilation in flight, Compton-scattered and fluorescence photons, and mass-attenuation coefficients μ/ρ, are tabulated for the above elements and for the mixtures: air, water, polystyrene, methyl methacrylate (Lucite, Perspex, Plexiglass, etc.), polyethylene, bakelite, and amber over the range from 0.1 keV to 20 MeV. For application to cavity ionization detector metrology the ratio <tex-math>$\mu _{{\rm en}}/\rho $</tex-math> in air to that in carbon and to that in the above six remaining mixtures is tabulated over the same energy range. The air and carbon <tex-math>$\mu _{{\rm en}}/\rho $</tex-math> values and air/carbon ratios are compared with those in ICRU Report 17 and other earlier compilations.

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