High-Precision X-ray Polarimetry
The goal of high-precision X-ray polarimetry is to optimize the polarization purity of X-ray radiation, in order to measure effects previously not, or but in an insufficient way, available for investigation. As opposed to the visible range, there exist no filters to polarize X-ray radiation. However, it is possible to suppress one direction of oscillation by reflecting the radiation from perfect single crystals, at angles close to 45°. Successive reflections may increase polarization purity even further.
For this, so-called channel cut crystals are used, single crystals with a groove cut into them so that the radiation is reflected four to eight times. In this way, polarization purities of more than nine orders of magnitude may be achieved, that is in one billion X-ray photons, less than one photon is of the wrong polarization. This permits the measurement of small polarization-changing effects, which in turn allow for conclusions regarding the structure of atoms and molecules, showing how electromagnetic fields may influence nuclear and molecular levels, and even vacuum. Measurement of these effects, such as vacuum birefringence which is predicted by quantum electrodynamics and should occur in the intense light of a petawatt laser, is especially feasible at novel X-ray sources, like the XFEL free electron laser.