This investigation is designed to determine with high accuracy the wavelength ratios of 5 standard x-ray reference lines: Cr Ka2, Cu Kai, Mo Kai, Ag Kai, and W Kai. Most measurements were obtained with a double-crystal spectrometer in reflection. However, a few were taken with the instrument converted to a high-precision single-crystal spectrometer, and some tungsten determinations were made in transmission. Since estimated errors (± 1 ppm) are the lowest ever claimed for this type of work, the apparatus, including adjustment, calibration, and corrections, are described in considerable detail. Novel features include: use of a high-intensity, narrow-focus rotating-target tube for most measurements, calibration of the spectrometer circle to 0.1 sec with an angular interferometer, and a careful selection of 5 crystals (1 calcite, 2 quartz, and 2 silicon) from more than 100 samples tested. Bragg angles were determined for 20 different combinations of crystals and wavelengths. With an assumed wavelength value for one selected line, this gave an overdetermined set of 20 equations for 9 unknowns (4 wavelengths and 5 grating constants), which was solved by a least-squares adjustment. If the value for Mo Kai is provisionally taken as 707.831 xu, the resulting values for the other wavelengths are: Cr Ka 2 , 2288.854±0.003 xu; Cu Kai, 1537.370±0.002 xu; Ag Kai, 558.2486 ±0.0006 xu; W Kai, 208.5770±0.0003 xu. However, the primary quantities determined are the wavelength ratios, which are invariant with respect to a choice of the primary defining reference line. The question of adopting a definitive standard is left open for the present.
