Determination of high-purity polarization state of X-rays

We report on the measurement of the highest purity of polarization of X-rays to date The measurements are performed by combining a brilliant undulator source with an X ray polarimeter The polarimeter is composed of a polarizer and an analyzer each based on four reflections at channel-cut crystals with a Bragg angle very close to 45 Experiments were performed at three different X-ray energies using different Bragg reflections Si(400) at 6457 0 eV Si(444) at 11 183 8 eV and Si(800) at 12 914 0 eV At 6 key a polarization purity of 1 5 x 10(-9) is achieved This is an improvement by more than two orders of magnitude as compared to previously reported values The polarization purity decreases slightly for shorter X-ray wavelengths The sensitivity of the polarimeter is discussed with respect to a proposed experiment that aims at the detection of the birefringence of vacuum induced by super strong laser fields (c) 2010 Elsevier BV All rights reserved