Pendellösung suppression in the diffraction pattern of
a set of thin perfect crystals in a Bonse-Hart camera

M. Sánchez del Río, C. Ferrero and A. K. Freund

European Synchrotron Radiation Facility. B.P. 220, 38043 Grenoble Cedex 9, France
(presented on 21July 1994)

Abstract

In a Bonse-Hart camera, a set of two or more grooved (channel cut) perfect crystals
are used to provide a very high angular resolution experiment. The resulting
multi-reflection profiles can be calculated in a first approximation by the dynami-cal
theory of diffraction, where usually only coherent scattering is considered. In
addition, there is experimental evidence of incoherent compton scattering (ICS)
and thermal diffuse scattering (TDS) contributions. Such contributions are espe-cially
important when performing small angle scattering experiments where in-tensities
in an interval of ten orders of magnitude are often recorded. We propose
and analyze here the use of thin (a few micrometers) crystals for a Bonse-Hart cam-era
in order to decrease the TDS and ICS components, thus to increase the perfor-mances
of the device. However, using thin crystals causes the occurrence of
interference fringes (Pendellösung) which degrades the instrument resolution. We
study in this paper the possible elimination of Pendellösung fringes by angularly
offsetting one crystal with respect to the others. Optimizing the offset value, the
Pendellösung oscillations of the crystals interfere destructively, then significantly
reducing their contribution to the total resolution function.