Instrumental aspects of x-ray microbeams in the range above 1 keV

Abstract

X-ray microscopy has the capability of looking into normally opaque samples with high resolution. X rays are sensitive to elemental, structural, and chemical content and thus can provide microscopic maps of the composition and structure of a sample. X-ray microscopy has seen great growth in the last two decades in the number and types of operating instruments as well as their capabilities. This growth is due to two developments. The first is the development of high-brightness second- and third-generation synchrotron light sources that can be used with small-aperture optics. The second is a revolution in x-ray optics. In addition to the extension of commonly used visible optics, such as Fresnel zone plates and multilayer mirrors, into the x-ray regime, there has also been a dramatic improvement in grazing-incidence optics fabrication. In the range up to a few keV, Fresnel zone plates offer the highest resolution, which is below 100 nm in several instruments. Recent developments in fabrication may lead to their application at higher energies; for now, however, sub-μm diffractive microfocusing at higher energies is usually achieved by Bragg–Fresnel optics, Fresnel optics operated in reflection using either crystal planes or multilayer coatings. Although these offer very high resolution, they have small collection apertures and limited wavelength range of operation. The Kirkpatrick–Baez mirror combination remains the most popular and versatile microprobe in the x-ray regime. These systems can operate over a very broad energy range and several facilities are now operating with micron-scale resolution. We will discuss these and some newer types of x-ray focusing schemes.