X-Ray Optics for Scanning Fluorescence Microscopy and Other Applications

Abstract

AbstractScanning x-ray fluorescence microscopy is analogous to scanning electron microscopy. Maps of the distribution of chemical elements are produced by scanning the specimen with a very small x-ray beam while collecting the XRF spectrum. Our goal is to perform such scanning microscopy with resolution in the range of <1 to 10 μm, using standard laboratory x-ray tubes. In order to increase the radiation flux on the specimen, we are investigating mirror optics in the Kirkpatrick-Baez (K-B) configuration, K-B optics uses two curved mirrors mounted orthogonally along the optical axis. The first mirror provides vertical focus, the second mirror provides horizontal focus. We have used two types of mirrors: synthetic multilayers and crystals. Multilayer mirrors are used with lower energy radiation such as Cu Kμ. At higher energies such as Ag Kct, silicon wafers are used in order to increase the incidence angles and thereby the photon collection efficiency. In order to increase the surface area of multilayers which reflects x-rays at the Bragg angle, we have designed mirrors with the spacing between layers graded along the optic axis in order to compensate for the changing angle of incidence. Likewise, to achieve a large reflecting surface with silicon, the wafers are placed on a specially designed lever arm which is bent into a log spiral by applying force at one end. In this way, the same diffracting angle is maintained over the entire surface of the wafer, providing a large solid angle for photon collection.