NIST: National Institute of Standards and Technology http://www.cstl.nist.gov/nist839/839.01/index.html
NIST: Scientific and Technical Databases http://www.nist.gov/srd/
NIST: X-ray Attenuation Coefficients http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html
X-Ray Periodic Tables http://csrri.iit.edu/periodic-table.html