High-accuracy x-ray line standards in the 3-keV region

A set of 14 high-accuracy x-ray transition energies in the 2.4–3.1 keV range is presented, which can be used as x-ray standards. They were measured in two- to four-electron sulfur, chlorine, and argon ions produced in an electron-cyclotron resonance ion source, using a single spherically bent crystal spectrometer. The results include the first measurement of six transitions and improve the accuracy of six other experimental values. These measurements considerably extend the set of high-accuracy x-ray energies reported for highly charged ions. Their relative uncertainties range from 1 to 10 ppm. Theory only reaches such a precision in one- and two-electron ions. Our results thus have two distinct applications. On the one hand, they test predictions in two-electron ions [Artemyev, Shabaev, Yerokhin, Plunien, and Soff, Phys. Rev. A 71, 062104 (2005)], at the precision level of some two-photon QED contributions. We observe an agreement with theory for most of the transitions. On the other hand, the three- and four-electron ion transitions provide new benchmark energies for the calculation of missing theoretical contributions, such as Auger shifts or electronic correlations. Spectra were analyzed with an x-ray tracing simulation that contains all the relevant physics of the spectrometer.

Figure 1

Spectra observed in argon ($Z=18$) in about 1 h (each). Each grid spacing corresponds to approximately 10 eV (100 CCD pixels), the reference line $\left(M1\right)$ being located here approximately at 3104 eV. The spectra for chlorine ($Z=17$) and sulfur ($Z=16$) are similar. The lines studied in this article are $n=2\to 1$ transitions; see Table for more information. All the energies were measured relative to the He-like $M1$ calibration line. The spectral range of 60 eV was obtained on a single image thanks to the large x-ray source and detector.

Figure 2

Experimental arrangement of the ECRIS and crystal spectrometer (adapted from Ref. [17]). The source is connected through a vacuum tube to a crystal chamber, where a bent crystal reflects emitted x rays. The x rays are recorded with a two-dimensional position-sensitive detector made of charge-coupled devices (CCDs).

Figure 3

Measurements of the $1s2s2p{}^2{P}_{3/2}\to 1s^{2}2s{}^2{S}_{1/2}$ line in three-electron sulfur. Internal consistency checks were performed by recording spectra for various distances between the Bragg crystal and the x-ray detector using two different (100) quartz crystals (represented here using different symbols) and four different apertures (represented by different colors). The apertures leave apparent a circle or a band of the crystal, their size being given in the legend.