X-ray microscope

X-ray microscopy is a microscopy method, which uses instead of visible light, X-rays, so radiation in the wavelength range of 10 nm to 1 pm.

X-rays will initially offer the advantage of shorter wavelength, which allows potentially higher resolution. The resolution of a microscope is limited by the half wavelength. In addition, the interaction of X -rays with matter of the visible light (for example, penetration, an inherent element of contrast, refractive index ), whereby additional information about the sample can be obtained is different. In particular, the information depth increases. It can be obtained with optical microscopes as well as information from deeper layers. The problem has been that the construction of a microscope condenser lenses are necessary. In order to build a converging lens, to a material for the lens must, however, use of which refractive index is greater than 1. However, the refractive indices for available materials are smaller for the frequency range of X-rays 1 Use is using the so-called Fresnelschemas zone plates to focus the X-ray beam. These act as lenses analogous to the classical microscope; However, they do not use the refraction, but the diffraction of X-ray waves. Usable lenses for X-rays, according to the principle of refraction (ie, the " refraction " ) function are only for wavelengths below 1 nm wavelength produced (see Refractive X-ray lens ), and for the important spectral range of the " water window " between 2.4 nm and 4.4 nm in the aqueous samples show good absorption and phase contrast, therefore, not feasible. The modern, high-resolution X-ray microscopy reaches 20 - 30 nm resolution and uses in this spectral exclusively Fresnel zone plates, see also X-ray optics.

We distinguish between imaging and raster scanning microscopes.

Imaging microscopes typically operate in transmission. In this case, the tested sample piece is of "forward" evenly illuminated and imaged, the radiation penetrating the sample by an optical system on a position- resolving detector (for example, CCD sensor).

The raster scanning microscopes, the X-rays using mirrors under grazing incidence mirrors with multilayer systems, Fresnel zone plates or refractive X-ray lenses is focused. The sample is moved by the focus and the measured total light coming from the sample at each sample position and taken as the brightness value for the image. In addition to the reflected light coming from the other sample of particles or radiation can be used for imaging.

These are for example:

• Scattered radiation ( diffraction analysis)
• Reflected radiation
• Transmitted radiation
• Luminescence
• The total electron yield
• Photoelectron
• Photon -stimulated ion desorption

To be able to capture high- resolution images with an imaging X-ray microscope and in a few seconds with a raster scanning X-ray microscope in a few minutes, very intense radiation ( brilliance ) is required. For this purpose, suitable as X-ray sources mainly directed synchrotron radiation and, more recently, plasma sources.

Against electron microscopes for X-ray microscopes advantage that considerably thicker samples - up to typically 10 microns - can be examined, that while the deposited into the sample dose of up to a factor of 10 000 is less, and that is assuming no electrical conductivity for the samples. Biological samples may remain " natural "; ie they do not - as necessary for the examination in the electron microscope - stained with heavy metal, dried, embedded in a support material and are cut after its hardening in typically 100 nm thin layers. Equally high are the expectations to get to the X-ray microscopy artifact-free images, which is partly located already confirmed.