A detector for circular polarization of soft x rays

To satisfy the increasing need for research using circularly polarized x rays we present a compact prototype of a polarization detector based on magnetic circular dichroism (MCD) in bulk transmission using a multilayer at grazing incidence as bandpass filter. The detector features reliability, heat load stability, and for Fe-MCD a polarization sensitivity from 700 to 740 eV. The technique can be easily extended to the L edges of other 3d transition elements giving rise to other energy ranges by just tuning the incidence angle and changing the material of the transmission foil. The detector calibration was made using off plane bend magnet radiation at the BESSY I storage ring and it turns out that the polarization degree of an unknown source can be well detected within a 3% error rms. The detector dimensions are less than 10 cm and only modest vacuum conditions are required. Once calibrated in white light, it can be used to determine the degree of circular polarization behind monochromators, insertion devices as well as in polychromatic light with up to 100 Hz repetition rate without any rotation of its components. (C) 1996 American Institute of Physics.