Bragg-Brentano geometry

The Bragg-Brentano geometry , also known as the Bragg-Brentano arrangement , is an arrangement for measuring powder diffractograms . It was first proposed by Brentano in 1925. Since no film recordings are possible in this geometry, it only gained practical importance with the development of X-ray detectors and is now the most common arrangement in powder crystallography. This method achieves good resolution at high intensity. The analysis of these diffractograms can be done by the Rietveld method or by comparison with a database.

construction

The principle of parafocusing. The detector rotates around the sample, which rotates around itself at half speed. The radius and center of the focusing circle change continuously as a result.

Flat, flat samples are used in this geometry. The sample is fixed in the center of a turning circle. The detector aperture is adjusted so that the distance between the tube focus and the sample is the same as the distance between the sample and the detector aperture. The measurement is carried out by rotating the sample around the common center point at half the angular speed of the detector (ω = θ).

Due to this special arrangement, the sample is always tangential to the circle defined by the tube focus, the sample center and the detector diaphragm, the focusing circle. Because of the set of circumferential angles , as in the Guinier method , all rays emanating from the tube focus that are bent at this circular line at the same angle meet again in the detector diaphragm. In order to adhere to this geometry exactly, the sample would not only have to be curved, but the radius of curvature would also have to change continuously during the measurement, since the radius of the focusing circle also changes during the measurement. In practice, therefore, flat specimens are used. The resulting error can generally be neglected. A beam geometry that works according to this principle is called parafocusing.

application

Bragg-Brentano diffractometer

In practice, this beam geometry is implemented in two different ways:

The tube remains fixed, the sample is mostly upright and rotates at half the angular speed of the detector.
The sample remains firm (usually horizontal). The tube and detector rotate in opposite directions at the same angle. This geometry is also called (θ-θ) geometry. It is particularly suitable for measuring liquid samples.

Web links

Geometry of the powder diffractometer. The University of Georgia, accessed June 20, 2016 (representation of Bragg geometry).

Individual evidence

↑ JCM Brentano: Focussing method of crystal powder analysis by X-rays . In: Proc. Phys. Soc. [London] . tape 37 , no. 1 , 1924, p. 184-193 , doi : 10.1088 / 1478-7814 / 37/1/326 .
^ HS Peiser, etc: X-ray Diffraction by Polycrystalline Materials . Chapman and Hall, 1960, ISBN 0-412-06050-7 .
↑ Harold Philip Klug, LeRoy E. Alexander: X-Ray Diffraction Procedures: For Polycrystalline and Amorphous Materials . 2nd Edition. John Wiley & Sons, 1974, ISBN 0-471-49369-4 .

[1] JCM Brentano: Focussing method of crystal powder analysis by X-rays . In: Proc. Phys. Soc. [London] . tape 37 , no. 1 , 1924, p. 184-193 , doi : 10.1088 / 1478-7814 / 37/1/326 .

[2] HS Peiser, etc: X-ray Diffraction by Polycrystalline Materials . Chapman and Hall, 1960, ISBN 0-412-06050-7 .

[3] Harold Philip Klug, LeRoy E. Alexander: X-Ray Diffraction Procedures: For Polycrystalline and Amorphous Materials . 2nd Edition. John Wiley & Sons, 1974, ISBN 0-471-49369-4 .