Physics
NIST:
Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients from 1 keV to 20 MeV for Elements Z = 1 to 92
http://www.nist.gov/physlab/data/xraycoef/index.cfm
http://physics.nist.gov/PhysRefData/XrayMassCoef/tab4.html