A comparative characterization of Highly Oriented Pyrolytic
Graphite by means of diffraction topography

A. Tuffanelli a), M. Sanchez del Rio b), G.Pareschi c), M. Gambaccini a),
A. Taibi d), A. Fantini a) and M. Ohler b)

a) Dipartimento di Fisica, Universita di Ferrara and INFN sezione di Ferrara
Via Paradiso 12, I-44100 Ferrara, Italy
b) European Synchrotron Radiation Facility
BP 220, 38043 Grenoble-Cedex, France
c) Osservatorio Astronomico di Brera
Via E. Bianchi 46, I-23807 Merate (Lc), Italy
d) Dept. of Medical Physics and Bioengineering, University College London, Shropshire House
11-20 Capper Street, London WC1E 6JA, United Kingdom

ABSTRACT

Highly Oriented Pyrolytic Graphite is a very effcient andwell-known x-ray and neutron monochomator. The crystal
macroscopic properties are determined by its microscopic structure. Our aim is to study the crystal internal structure
and correlate it with the crystal optical behavior. We studied the texture of the crystal, in particular its spatial
homogeneity, for different samples using x-ray diffraction topography. The experiment was performed at the ESRF
beamline BM5 using a laminar 18 keV monochromatic beam.
Several samples supplied by different manufacturers have been studied. Images of (002) re ected beam have been
acquired at the Bragg angle for each sample, using a phosphor coated CCD digital detector. Contrast profiles have
been obtained, and exponential fits has been performed allowing to deduce the secondary extinction coefficient.
It has been found that some samples are quite perfect and the results agree with ideally imperfect crystals model.
Other samples present well defined granular macrostructures (with dimensions of tens of microns) superposed to
the well-known Gaussian-like crystallite distribution. The different behavior between different samples should be
explained in terms of sample internal structure, which is also related to the different graphitization process used by
manufacturers.

Keywords: Mosaic crystals, Highly Oriented Pyrolytic Graphite (HOPG), x-ray topography, secondary extinction,
macrostructures