Tuning of X-ray phase retarder for magnetic EXAFS spectroscopy in helicity modulation mode

A technique for polarization tuning over a wide energy range has been developed for magnetic EXAFS spectroscopies using a dia mond x-ray phase retarder in helicity modulation mode. A quick rocking scan using a piezo stage enabled us to tune the angle of retarder crystal with the precision of +/-2 arcsecond within 1 s. Available energy range of circular dichroism measurements has been extended to 6-16 keV by adopting not only undulator gap scan but also the present tuning technique. This technique has been applied to magnetic EXAFS measurements in pure iron at the Fe K-edge. Higher signal-to-noise ratio was obtained in reduced data acquisition time.