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

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 multireflection profiles can be calculated in a first approximation by the dynamical 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 especially important when performing small angle scattering experiments where intensities 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 camera in order to decrease the TDS and ICS components, thus to increase the performances of the device. However, using thin crystals causes the occurrence of interference (Pendellösung) fringes which degrades the instrument resolution. We study in this paper the possible elimination of Pendellösung fringes by angularly offsetting one or more crystals 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.