<title>Simple x-ray focusing mirror using float glass</title>

Abstract

In our recent x-ray photon correlation spectroscopy (speckle) experiments at NSLS, one of the challenges is to increase the coherent photon flux through a pinhole, whose size is chosen to match the beam's horizontal transverse coherence length l<SUB>h</SUB>. We adopted an approach to vertically focus the x-ray beam so as to match its vertical transverse coherence length l<SUB>v</SUB> (at NSLS X13, l<SUB>v</SUB> approximately 50 l<SUB>h</SUB>, l<SUB>h</SUB> approximately 12 micrometers at 3 KeV) with l<SUB>h</SUB>. By demagnifying the vertical size by a factor of l<SUB>v</SUB>/l<SUB>h</SUB>, we expect to increase the intensity of the x-rays through the pin hole by the same factor while keeping the beam coherent. A piece of commercial 3/8' thick float glass, by virtue of its low surface roughness (approximately 3 angstroms rms), good reflectivity in the low photon energy range of interest and low cost, was chosen as the mirror material. A computer controlled motorized bender with a four point bending mechanism was designed and built to bend the float glass to a continuously variable radius of curvature from -700 m (intrinsic curvature of the glass surface) to &lt; 300 m, measured with the Long Trace Profiler at the BNL Metrology Lab. This mirror bender assembly allows us to continuously change the focal length of the x-ray mirror down to 0.5 m under our experimental conditions. At the NSLS X13 Prototype Small Gap Undulator beamline, we were able to focus the x-ray beam from a vertical size of 0.5 mm to approximately 25 micrometers at the focal point 54 cm from the mirror center, thus increasing the photon flux density by a factor of 20. Results also show that, as expected, at an incident angle of 9 mrad, the mirror cuts off the harmonics of the undulator spectrum, leaving a clean 3 KeV fundamental for our experiments.