Beryllium and lithium x-ray lenses at the APS

Abstract

Compound refractive lenses (CRLs) are arrays of concave lenslets used to focus X-rays. For a given incident X-ray beam energy, the focal length of a CRL depends on the material and shape of the individual lenslets, and in particular is inversely related to the number of lenslets in the array. The throughput of a lens array is heavily affected by absorption of the X-rays in the lens. For this reason, it is necessary to employ low-atomic-number materials and fabricate the lenses as thin as possible, especially for low to moderate X-ray energy range (~ 5 - 20 keV) photons. Lithium and beryllium are two of the best candidate materials for X-ray lenses due to their relatively high (real decrement) index of refraction and low X-ray absorption. Lithium is very malleable, however, and reacts strongly with moisture in the air, requiring a special fabrication environment and housing. Beryllium, on the other hand, is a solid metal and is easy to machine and handle. This paper summarizes the recent work at the Advanced Photon Source (APS) on parabolic lithium and cylindrical beryllium lenses. These lenses have been tested on APS X-ray beamlines. Their performance in terms of the focal size and gain is described and further improvements including tighter manufacturing tolerances and thinner lens walls are discussed.