Linear polarization of x-ray transitions due to dielectronic recombination in highly charged ions

The linear polarization of x rays produced by dielectronic recombination into highly charged xenon ions was measured at an electron beam ion trap using the Compton polarimetry technique. This opens numerous possibilities for diagnostics of anisotropies of hot plasmas. Moreover, it was observed that the polarization of x rays, following the dielectronic capture populating the ${\left[1s2s^{2}2p_{1/2}\right]}_1$ state, is highly sensitive to the Breit interaction. The experimental results for this transition rule out by $5\sigma$ calculations not taking the Breit interaction into account.

Figure 1

Scheme of the experiment: x rays emitted by the xenon ions, which are produced and trapped in the EBIT, are scattered by a block of boron carbide. The azimuthal distribution of the scattered x rays is measured by an array of SiPIN diodes. A germanium detector registered unscattered x rays.

Figure 2

Intensity of the DR x rays registered by the germanium detector as a function of the electron energy. The dashed line represents the RR background, the thin solid lines correspond to the fits of the marked DR resonances, and the thick solid line represents the fit of the complete spectrum. The fractions $f_{\mathrm{RR}}$ of the x-ray intensity produced by RR at used collision energies are determined from these fits and used in Eq. (2).

Figure 3

A typical x-ray spectrum observed by a SiPIN diode when the electron beam energy was tuned to the DR resonance $\mathrm{Be}{}_2$. The solid line represents the fitted DR peak with the background, shown as the dashed line.

Figure 4

Measured azimuthal distribution of scattered x rays for $\mathrm{Li}{}_1$ (squares) and $\mathrm{Be}{}_2$ (circles) DR resonances. The solid and the dashed lines represent the corresponding fits of Eq. (1) to these distributions.