Neutron Bragg diffraction optics
Experimental investigations in this field are concentrated on experimental studies of the dispersive monochromators providing high and ultra-high angular and/or energy resolution. We have done several tests of ultra-high resolution dispersive monochromators based on multiple reflections excited in elastically deformed perfect crystals. High-resolution monochromatic neutron beams also opened new possibilities of application for an alternative radiography technique, for the so called phase contrast radiography.
The studies in the field of Bragg diffraction optics has in NPI ASCR, v.v.i. a long tradition starting by investigations of neutron diffraction by ultrasonically vibrating perfect and nearly perfect single crystals and followed by investigations of neutron diffraction by cylindrically bent perfect crystals. In the last decades many papers have been published in which successful employment of curved perfect Si or Ge crystals as neutron monochromators or analyzers in particular diffraction experiments have been reported. It has been found e.g. that due to the spatial condensation of neutrons (because of asymmetric diffraction geometry or focusing in real space), at the sample position one may obtain a high monochromatic beam density comparable or higher than that of the mosaic monochromators. Taking into account reflectivity and the resolution properties of curved crystals Popovici et. al. from Missouri University demonstrated a feasibility of the optimized curved crystal three axis spectrometer for inelastic neutron scattering which had the properties generally better than those of spectrometers employing flat mosaic crystals. New interest in the reflectivity and focusing properties of curved perfect crystals has been evoked also by successful testing of the bent perfect crystals employed in double-monochromator systems, or for medium and high resolution small-angle scattering diffractometers. Favourable properties of the bent Si or Ge crystal for short wavelength monochromatization have also been reported. Furthermore, very efficient original double and triple axis arrangements employing bent perfect crystals have been proposed and then tested for residual stress measurements of polycrystalline materials. In the last decade the neutron diffraction group of NPI ASCR, v.v.i. has been concentrated on the development of new unconventional types of focusing neutron monochromators permitting significantly to increase figure of merit of some dedicated neutron scattering instruments and also on development of the so called dispersive monochromators based on multiple reflections realized either in one bent perfect crystal or by means of sandwich of two curved crystal slabs.
However, in spite of the successful use of the focusing techniques in the field of neutron scattering, their potential has not yet been fully explored. The tools necessary for their implementation have yet to be developed, especially when the whole design of the diffractometer system must be individually considered. A large open area still exists in the field of inelastic neutron scattering, powder diffractometry, neutron reflectometry and particularly in TOF diffractometry and spectrometry. The possibility of using the neutron Bragg diffraction techniques based on the elastically deformed perfect crystals elements for the TOF experiments has appeared highly demanded in relation to new neutron sources and projects as ISIS (England), J-Park (Japan), SNS (USA) and ESS (Sweden).