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Highly Annealed Pyrolytic Graphite (HAPG)

Figure 1: Structure of a graphite-based mosaic crystal. Small crystallites that are slightly tilted towards each other form blocks that form the macroscopic crystal.
Lupe
Figure 2: Mosaic focusing takes place in mosaic crystals as the crystallites that participate in the diffraction are positioned and tilted according to the Johannson geometry. Hence, the radiation from a source placed on the Rowland circle will be refocused on this Rowland circle. Therefore, the integrated reflectivity is strongly enhanced while keeping the spectral resolving power.
Lupe

Highly Annealed Pyrolytic Graphite (HAPG) is a mosaic crystal and a further improvement of the well-known Highly Oriented Pyrolytic Graphite (HOPG) mosaic crystals, by means of a much narrower mosaicity.

Mosaic crystals, in contrast to ideal crystals, consist of many small ideal crystals, called crystallites, see figure 1. One important parameter of these crystals is the so called mosaicity which is the distribution function of the crystallites’ surface normals in comparison to the surface normal of the macroscopic crystal. The full width at half maximum of the mosaicity is the so-called mosaic spread and is usually used to describe these types of crystals.

PG mosaic crystals show the highest integrated reflectivity among all known crystals, which is at least one order of magnitude higher than for example for Silicon 111 reflection. This increased integrated reflectivity is due to the so-called mosaic focusing, which leads to a fulfilment of the Johannson geometry even for flat crystals in the dispersion plane, see figure 2.

HAPG crystals have usually thicknesses in the range of a few tens up to 100 micrometers. The achievable mosaic spread is between <0.1° and 0.4° [1]. With optimized optics spectral resolving powers of up to E/ΔE = 4000 in the first order of reflection are feasible while getting the mentioned high integrated reflectivity. Therefore, they are an ideal candidate for the use in highly efficient spectrometers based on low brilliant, laboratory scale X-ray sources.

As they show a much more complex diffraction behavior as compared to ideal crystals, several tools to model this behavior have been developed [2]. Therefore, it is not only possible to deconvolute measured spectra, but also tailored optic solutions can be designed.

Point of contact:  Wolfgang Malzer, Christopher Schlesiger

Relevant publications:

[1] M. Gerlach, L. Anklamm, A. Antonov, I. Grigorieva, I. Holfelder, B. Kanngießer, H. Legall, W. Malzer, C. Schlesiger, B. Beckhoff, J. Appl. Cryst. 48 (2015), 1381-1390. Link

[2] C. Schlesiger, L. Anklamm, W. Malzer, R. Gnewkow, B. Kanngießer, J. Appl. Cryst. 50 (2017), 1490-1497. Link

 

 

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