National Institute for Standards and Technology website National Institute for Standards and Technology website http://www.nist.gov/pml/data/xraycoef/index.cfm National Institute for Standards and Technology website Beim NIST Beim NIST http://www.nist.gov/pml/data/xraycoef/index.cfm http://www.nist.gov/pml/data/xraycoef/index.cfm NIST table of x-ray attenuation coefficients NIST Physical Reference Data