This website provides the conversion relationship between U.S. customary units (e.g. inch and miles) and SI (International System) units. This website is authored by the Physical Measurement Laboratory of the National Institute of Standards and Technology (NIST).
This database provides access and search capability for NIST critically evaluated data on atomic energy levels, wavelengths, and transition probabilities that are reasonably up-to-date. The Atomic Spectroscopy Data Center has carried out these critical compilations.
A clickable table of elements. This table provides various x-ray properties for each element and it has also a script for calculating x-ray cross sections for a give energy.
A series of online databases hosted by NIST containing physical constants, spectroscopy data, X-ray and gamma-ray data, radiation dosimetry data, and more.