X-ray optics

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The X-ray optics deals with the spread of X-ray radiation and its interaction with matter. It is used not only in the wavelength range of the actual X-ray radiation (0.01 to 10  nm ), but also at wavelengths up to 100 nm ( VUV radiation ).

In X-ray optics, a distinction is made between soft and hard X-rays. The range in which the wavelength of the radiation is longer than the distance between the atoms in the solid (0.1 nm to 0.5 nm) is called soft X-ray radiation . Here the solid is viewed as a homogeneous medium . Hard X-ray radiation, on the other hand, is the range in which the wavelength is shorter than the distance between the atoms in the solid, i.e. in the range 0.01 to approx. 0.5 nm. This is where the atomic structure of the solid comes into its own.

Differences to optics with visible light

For X-rays, the refractive index of matter is a little smaller than one (deviation in the range 10 −8 to 10 −6 depending on the wavelength). This results in a phase velocity of the wave that is greater than the speed of light . The reason for the deviation below one is that the oscillation frequency of electromagnetic radiation in the X-ray range is greater than the oscillation frequency of the outer electrons of the illuminated atoms, which perform forced oscillations in the electric field of the X-rays above their resonance frequency . This property can be used to define X-rays.

The direction of propagation of electromagnetic radiation can in principle be changed, for example by focusing, by utilizing refraction , reflection and diffraction .

When using the refraction of the X-rays on a surface, the refractive index ratio between the environment and the lens material must be taken into account. In a vacuum environment (refractive index is one) and, for example, a focusing lens (refractive index less than one), in contrast to the visible spectral range, the lens has a concave shape - instead of a convex shape, as is necessary with visible converging lenses.

When using the reflection, it should be noted that this effect is based on the so-called "external total reflection" (cf. internal total reflection in the visible spectral range) for small angles to the surface of the optics. At larger angles, the effect of multiple reflection is used through layered material arrangements in the optics to enable effective reflection.

When the diffraction is used, a path difference is specifically created between the individual wave ranges. For example, interference can create a focus behind the optics.

For X-ray wavelengths - in particular for wavelengths that are shorter than 100 nm - there are no completely radiolucent (“transparent”) media. As a result, X-ray lenses have to be as thin as possible. The simpler ways to focus X-ray light are mirrors and Fresnel zone plates . X-ray mirrors must have a much more planar surface than mirrors for visible light. Diffuse scattering on a surface is caused by imperfections called surface roughness. If the mean distance or the size of the bumps is much smaller than the wavelength, the surface roughness plays only a minor role. However, if this distance is similar to the wavelength of the light, an incident beam is mainly diffusely scattered and hardly reflected as a beam. For X-rays, which have very small wavelengths, surfaces that look absolutely flat in visible light are often very rough.

X-ray mirror

To compensate for the low reflectivity in the X-ray range, essentially three different methods are used:

Grazing idea

The reflectivity of surfaces increases as the angle of incidence becomes flatter . With a refractive index that is less than 1, total reflection can even occur at very flat angles of incidence . This is why mirrors are often used in X-ray optics with grazing incidence. An example of an optical device that works with grazing incidence is the Wolter telescope .

Multilayer systems

If you need mirrors that provide high reflectivity at steep angles of incidence and only have to work at one wavelength, mirrors made of multilayer systems are often used. They consist of two different materials that lie on top of each other in alternating layers. These multilayer systems are always built for a specific wavelength and a specific angle of incidence. As a rule, one uses an optically dense and an optically thin medium at the associated wavelength. The layer thicknesses are coordinated so that the period always corresponds to the wavelength for the intended angle of incidence. A constructive interference then occurs during the reflection on the optically denser layers . A popular multilayer system is, for example, the combination of silicon and molybdenum for wavelengths around 13.5 nm. Here, silicon is the optically thin medium and molybdenum is the optically denser one.

Bragg reflection

With hard X-rays, the structural interference of the waves on the crystal lattice described by the Bragg equation can be used. A diffraction reflex is generated on a crystal at a certain angle at a certain wavelength. However, the reflected beam intensity is very low.

X-ray optical devices

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