X-Ray Wavelengths

TLDR

Recent studies have revealed inconsistencies in accepted x‑ray reference line values, despite their basis on supposedly high‑quality calcite crystals. The authors propose defining a new unit Å* so that the W Kα₁ wavelength is exactly 0.2090100 Å*, making 1 Å* equal to 1 Å ± 5 ppm. The paper critically evaluates factors for selecting the W Kα₁ line as the X‑ray wavelength standard, reviews experimental measurements establishing its absolute wavelength, and incorporates 61 additional x‑ray lines from a comprehensive reevaluation of over 2700 emission and absorption wavelengths. The W Kα₁ wavelength is determined to be 0.2090100 Å with ±5 ppm uncertainty, and secondary standards for Ag Kα₁, Mo Kα₁, Cu Kα₁, and Cr Kα₂ are established with ~1 ppm error, with recommended wavelengths and energies provided in Å* units and numerical order.

Abstract

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 $K{\ensuremath{\alpha}}_{1}$ 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 $\ensuremath{\lambda}$ W $K{\ensuremath{\alpha}}_{1}=(0.2090100\ifmmode\pm\else\textpm\fi{}5 \mathrm{ppm})$ \AA{}. This may be used to define a new unit, denoted by \AA{}*, such that the W $K{\ensuremath{\alpha}}_{1}$ wavelength is exactly 0.2090100 \AA{}*; hence 1\AA{}*=1\AA{}\ifmmode\pm\else\textpm\fi{}5 ppm. The wavelengths of the Ag $K{\ensuremath{\alpha}}_{1}$, Mo $K{\ensuremath{\alpha}}_{1}$, Cu $K{\ensuremath{\alpha}}_{1}$, and the Cr $K{\ensuremath{\alpha}}_{2}$ 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 \AA{}* 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.