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 Kα₁ 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 λ W Kα₁=(0.2090100±5 ppm) {}. This may be used to define a new unit, denoted by {}*, such that the W Kα₁ wavelength is exactly 0.2090100 {}*; hence 1{}*=1{}±{}5 ppm. The wavelengths of the Ag Kα₁, Mo Kα₁, Cu Kα₁, and the Cr Kα₂ 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 {}* 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.