NIST Tables of X-Ray Mass Energy-Absorption Coefficients and Mass Attenuation Coefficients - SRD 126
Tables of X-Ray Mass Attenuation Coefficients and Mass Energy - Absorption Coefficients from 1 keV to 20 MeV for Elements Z = 1 to 92 and 48 Additional Substances of Dosimetric Interest include tables covering energies of the photon (x-ray, gamma ray, bremsstrahlung) from 1 keV to 20 MeV.  Version 1.4