X-ray microscopy

Abstract

Ever since W. C. Röntgen discovered his x-rays there was a desire to create a useful x-ray microscopy. The fact that the index of refraction deviates so little from that of vacuum causes, however, great difficulties in developing suitable x-ray optical elements. Quite generally, the low efficiency even of the now existing optical elements and the low brilliance of classical x-ray sources causes stringent limitations. The availability of synchrotron radiation from undulators therefore is the basis of a renewed interest in x-ray microscopy. The realization of such a microscope is achieved either by producing a magnified image of scattered or transmitted radiation or to produce a demagnified image of the source as a probe on the sample. For this purpose Fresnel zone plates, multilayer covered mirrors, and mirrors at grazing incidence are available. The resolution or the probe sizes respectively lie presently between 30 nm and 1 μm. The transmission microscope is mainly used for biological samples in aqueous solution. The scanning microscopes allow to use many different signals and this way have a strong microanalytical capacity. In the scanning microscope at HASYLAB/DESY in Hamburg a grazing incidence ellipsoid mirror is used to focus radiation in the energy range 15 eV to 1200 eV to a spot of 1 μm. The signals which are available for microscopy and microspectroscopy are transmission, photoelectrons, luminescence in the visible, UV and soft x-ray regions, scattered light and desorbed ions. Spectrometers for photoelectrons and visible/UV-luminescence are installed for secondary spectroscopies. Several of the signals are not only element specific but are also specific to the chemical environment and binding character. About 25 projects of different types are presently pursued with synchrotron radiation world-wide.