Characterization of the focal quality of micron size beams from x-ray mirrors and zone plates

With the development of high quality x-ray mirrors and x-ray zone plates, the use of micron-size x-ray beams is growing rapidly. The size of the focal spot is frequently characterized by measuring either the transmitted or fluorescence counting rate as a knife edge is scanned through the beam; the full width half maximum (FWHM) of the derivative of this scan is often quoted as the "resolution" of the system. Many systems have been built which are claimed to have a focal spot size of 1 mum(2) or less. Unfortunately, this parameter does not give an accurate presentation of the focal quality of the beam. We have developed a test using a pattern of microfabricated "dots" on a silicon wafer to measure the encircled energy in an x-ray focal spot as a function of radius. Using this test, we have found that focused x-ray beams frequently have a large, low intensity flare that is not represented in the beam profile as measured by the knife edge scan method. This flare can be produced by several factors including diffuse scattering from the mirror surfaces and higher orders of a zone plate. It can cause serious errors in when calculating elemental concentrations in heterogeneous samples. In addition, for micro-XAS experiments where the energy is scanned to identify the compound at a particular point, this flare may vary as a function of energy and therefore change the XAS spectra from a small particle in a complex sample.