X-ray optics for modulation spectroscopy

This paper reports new techniques for hard x-ray modulation spectroscopy, helicity modulation and energy modulation, which have been developed at SPring-8. The helicity modulation technique has been realized by combination of fast helicity switching at 40 Hz using a transmission diamond x-ray phase retarder and a phase-sensitive (lock-in) detection system. Performance of the helicity modulation technique is demonstrated by measured spectra of x-ray magnetic circular dichroism of extremely high signal-to-noise ratio in the region near the Fe K-edge. A quick and precise method for the polarization tuning is presented, and the helicity modulation technique is shown to be applicable to magnetic EXAFS spectroscopies in wider energy region. For the energy modulation technique, fast energy switching has been made using a Si channel-cut crystal as a second monochromator of high energy resolution. An energy-derivative XAFS spectrum was directly obtained at the Mn K-edge, and illustrates the advantages of this technique in high energy resolution and noise reduction.