Tham khảo tài liệu 'modern developments in x-ray and neutron optics episode 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 272 A. Snigirev and I. Snigireva Fig. . Parabolic compound refractive lens CRL . The individual lenses a and stacked behind one another to form a CRL was shown that focusing by X-ray lenses is possible 106 . Since the 1 Ỏ in the index of refraction is smaller than 1 lenses must have a concave shape 106-108 . To obtain a focal length F in the range of 1 m many single lenses have to be stacked behind each other to form a compound refractive lens CRL as shown in Fig. . Fabricating the lenses from low-Z materials like Li Be B C and Al minimizes the problems associated with absorption. The focal length of such CRL with a parabolic profile J 2Ry and N individual biconcave lenses is F R 2NỖ where R is the radius of curvature at the apex of parabola. A lens with thickness 2y0 d has an aperture 2R0 2ỵ 2Ry0. Refractive lenses act as a conventional lens and one can apply the Gauss lens formula which relates the source distance p the image distance q and the focal distance F via q Fp p F . The diffraction-limited resolution of the lens A is defined by an effective aperture A 2NA where the numerical aperture is NA leff 2q. leff is the effective aperture of the lens reduced by photon absorption and scattering compared with the geometrical aperture 2Ro. The first lenses consisted of a row of holes about 1 mm in diameter drilled in a material such as Al or Be 107 . Two of these lenses in crossed geometry are able to focus an X-ray beam to a spot size of a few microns. Soon after this first successful experimental demonstration it was understood that refractive lenses can be used as a condensers or collimators with relatively long focal distances. Be Graphite and Al lenses were installed at the front-ends FE about 24 m from the source at various beamlines 109-112 . The typical FE 17 Hard X-Ray Microoptics 273 Table . Typical parameters of the FE CRL lenses at the ESRF Lens N E for L2 41 m keV E for collimation keV f for E 20 keV m 1 7 42 2 16 .
