X-Ray Wavelengths

Inconsistencies in accepted values (in x units) of x-ray reference lines have recently been demonstrated, although all are supposedly based on "good" calcite crystals. Factors supporting the selection of the W $K{\alpha }_1$ line as the X-Ray Wavelength Standard are critically discussed. A review is given of the experimental measurements which are used to establish the wavelength of this line on an absolute angstrom basis. Its value is $\lambda$ W $K{\alpha }_1=(0.2090100\mathrm{}\pm 5 \mathrm{ppm})$ Å. This may be used to define a new unit, denoted by Å*, such that the W $K{\alpha }_1$ wavelength is exactly 0.2090100 Å*; hence 1Å*=1ű5 ppm. The wavelengths of the Ag $K{\alpha }_1$, Mo $K{\alpha }_1$, Cu $K{\alpha }_1$, and the Cr $K{\alpha }_2$ have been established as secondary standards with probable error of approximately one part per million. Sixty-one additional x-ray lines have been used as reference values in a comprehensive review and reevaluation of more than 2700 emission and absorption wavelengths. The recommended wavelength values are listed in Å* units together with probable errors; corresponding energies are given in keV. A second table lists the wavelengths in numerical order, and likewise includes their energies in keV.