Precision measurements have shown that the x-unit scale of x-ray wavelengths based on Mo $K{\alpha }_1=707.831\mathrm{}$ xu is about 20 ppm smaller than that in general use for Cu $K{\alpha }_1$ and longer wavelengths, and for substantially all crystal parameters. Furthermore, the conversion from the Cu $K{\alpha }_1=1.537400\mathrm{}$ kxu to Å has been increased from 1.00202 to 1.002056±5 ppm. New techniques permit wavelength measurements to be made with relative probable errors of ±1 ppm. New wavelength and crystal-parameter tables are now necessary, and this plus the newly developed precision technique provides the opportunity for introducing an x-ray wavelength standard. In an investigation to select the best wavelength to be designated as the standard, the following studies have been carried out: relative measurements of five wavelengths with five crystals, employing a least-squares adjustment; $\gamma$-ray study of single-crystal diffraction patterns and the use of a $\gamma$-ray wavelength as a standard; crystal imperfections and their effect on wavelength measurements; effect of chemical impurities on grating constants; and redetermination of the absolute wavelength scale. The requirements of a standard are analyzed and the advantages of the W $K{\alpha }_1$ line listed. Its wavelength is 0.209010 0 ű5 ppm on the absolute scale, and without error as the designated standard. With this standard, new wavelength tables (separate publication) have been prepared, which include a large number of new measurements of secondary wavelength standards used as reference lines in many original measurements. Probable errors on both the relative and the absolute scale have been included.

• Received 15 September 1964

DOI:https://doi.org/10.1103/PhysRev.137.B455