Databases

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