Diffractometer

An X-ray diffractometer (of diffraction, Latin for diffraction ) is a device, the diffraction phenomena of X-radiation measures. A distinction is made between Röntgeneinkristalldiffraktometern ( diffraction on single crystals ) and Röntgenpulverdiffraktometern ( diffraction on powdered samples).

This device is used in crystallography for the elucidation of structures (crystal structure analysis) and the identity of each sample. It utilizes the fact that the wavelength of X- radiation in the range of interatomic distance is ( 0.1-0.3 nm).

According to the Bragg equation, an X-ray beam incident on a crystal, so bent that reflections occur at discrete locations in the space. The diffractometer is now detected location and intensity of the reflections occurring.

• 3.1 point detectors
• 3.2 area detectors

X-ray source

Laboratory usually molybdenum or copper anodes are used in X-ray diffractometers. The X-ray radiation is passed through a monochromator and a collimator, so that a primary beam with characteristic Ka wavelength ( Mo 0.71 Å, Cu 1.54 Å) is formed. The present there doublet can be corrected with the help of Rachinger correction. Synchrotron at other wavelengths are used.

Laboratory x-ray tubes used for generating X- rays:

• Fixed anode ( water cooled, about 2 kW)
• Rotating anode ( water-cooled, from about 3 to 6 kW )
• Micro Source ( water-or air-cooled, about 30 W)

In the primary beam, a single crystal is moved. This is attached to an amorphous glass or nylon thread or in a glass capillary, sitting on a so-called goniometer, which allows fine adjustment of the crystal in the primary beam. Usually the crystal is additionally cooled by a stream of nitrogen.

Goniostat

Since the reflexes are dependent on the arrangement of the lattice planes of the crystal to the primary beam and occur in all directions of space, it is necessary to rotate the crystal. Therefore, all diffractometer have a Goniostaten that allows these rotations.

Four-circle diffractometer

When Vierkreisdiffraktrometer the goniostat has 4 circuits. The best results are achieved when the axis of rotation of the crystal is perpendicular to the movement of the primary beam.

In Euler's geometry (see Euler angle) to rotate the ω - circle in the horizontal plane, χ, of the circle in the vertical plane standing on it. In the χ - circle sits the goniometer head, which can continue to rotate about its axis, the φ - circle. In the plane of the ω - circle now rotates the fourth Κreis, the θ - circle on which the detector is mounted. It is thus possible to measure the position and intensity of each reflection. The Cartesian coordinate system of the diffractometer is defined in the Euler geometry such that the axis is the crystal for X-ray source, the axis and the horizontal axis is vertical, so that a right-hand system is formed.

In the kappa geometry χ movement of the Euler geometry is replaced by a rotation. The axis makes an angle of 50 ° with respect to. Kappa geometry has the advantage that, in principle, only the lower half of the ball is needed, so that the upper half is free to be affixed, for example, a nitrogen cooling.

Three-circle diffractometer

Dreikreisdiffraktrometer diffractometer are in Kappa geometry, which is not rotatable but is fixed. To obtain a complete data set, it may be necessary to perform crystal rotations, where the rotation axis is not perpendicular to the primary beam.

Einkreisdiffraktometer

Einkreisdiffraktrometer are diffractometer, in which only the φ circuit is mounted. At low crystal symmetry hereby leave no complete records measured in high resolution.

Detector

As detectors in X-ray diffraction are used:

• Point detectors, such as a Geiger counter
• Area detectors such as CCD detectors, X-ray sensitive image plates ( engl. imaging plate ), etc.

Point detectors

If the detector consists of a counter tube ( scintillation counter ) each reflex must be measured individually. However, since most produced a crystal a five-digit number of reflections, this method is relatively time-consuming. Therefore point detectors come almost exclusively with Vierkreisdiffraktometern apply.

Area detectors

In the area detector multiple reflections are recorded simultaneously. The crystal is rotated through a small angle ( 0.3-2.0 ° C) and exposed to light during the rotation. Then the signal is read out and the crystal is rotated back through a small angle. Area detectors are installed in four-, three-and Einkreisdiffraktometern.

The CCD detector operates as a digital camera, where the X-ray radiation is converted into light by a fluorescent layer, which is the CCD can detect the x-ray spectrum or a sensitive CCD detector directly exposed. The CCD detector allows fast measurements with measurement times of a few seconds to minutes. However, their detector surface is relatively small, therefore all reflections can not be measured.

An x-ray sensitive image plate ( engl. imaging plate) is a round image plate made ​​of organic material, the doped with europium ( Eu 2 ) Bariumbromofluorid ( BaBrF ) is coated. Does an X-ray quantum to the plate, there will be a photo-induced oxidation to Eu3 at this point. The result is a color center having a half-life of about 10 hours. After exposure, the plate is read by a laser light, and then quenched with halogen light. Such image plates need depending on the exposure time up to one hour per ( sub-) image, but there are plate diameter of up to 35 cm.

In the field of surface detectors takes place a lot of development in order to measure more quickly and / or to obtain a better signal -to-noise ratio. As may be used in detectors today CMOS chips.