An absolute energy characterisation of scandium Kβ to 2 parts per million

We present an absolute energy measurement of the Kβ1,3 (KM2,3) emission spectrum of scandium (Z = 21) accurate to 2.1 parts per million (ppm). The previous experimental uncertainty was estimated as 105 ppm, or 0.47 eV, therefore we improve the accuracy of this measurement by a factor of 50 for use in any x-ray standards. There is a long-standing discrepancy between the most recent experimental and theoretical values. This work reports a Sc Kβ peak energy of 4460.845 eV with estimated standard error uncertainty of 0.0092 eV. The satellite component centroids, line-widths, and relative intensities are determined as a sum of five Voigt functions. The same analysis and experimental method shown here can be applied to advanced experiments in quantum electrodynamics, astrophysics and particle physics on soft x-ray spectra. This value has reconciled some of the previous discrepancy. However, the theoretical value is still discrepant from the new experimental measurement by 1.745 eV with a much tighter constraint on the experimental uncertainty. This strongly strengthens the need for new theoretical calculations and experimental measurements.