Theory Of Neutron Diffraction Research Articles

Nonstationary neutron diffraction by surface acoustic waves

Theory of neutron diffraction on traveling and standing surface acoustic waves (SAWs) is considered. Results of experiment on observation of neutron diffraction by a SAW traveling both along and against the direction of a neutron wave, as well as a standing SAW, are presented. The experimental results are mostly consistent with theoretical predictions.

Nonstationary Diffraction of Ultracold Neutrons from a Moving Grating and Efficiency of Energy Transfer to a Neutron

The discrete spectrum of ultracold neutrons (UCNs) diffracting from a moving grating has been measured using a time-of-flight Fourier spectrometer. It is shown that, choosing an appropriate depth of grating profile, one can change significantly the ratio of wave intensities for different diffraction orders. The experimental results are found to be in good agreement with the theoretical predictions based on the multiwave dynamical theory of neutron diffraction from a volume periodic grating.

New test of the dynamic theory of neutron diffraction by a moving grating

Recently, multiwave dynamical theory of neutron diffraction by a moving grating was developed. The theory predicts that at a certain height of the grating profile a significant suppression of the zero-order diffraction may occur. The experiment to confirm predictions of this theory was performed. The resulting diffracted UCNs spectra were measured using time-of-flight Fourier diffractometer. The experimental data were compared with the results of numerical simulation and were found in a good agreement with theoretical predictions.

Theory of Neutron Diffraction from the Vortex Lattice in UPt3

Neutron scattering experiments have recently been performed in the superconducting state of UPt3 to determine the structure of the vortex lattice. The data show anomalous field dependence of the aspect ratio of the unit cell in the B phase. There is apparently also a change in the effective coherence length on the transition from the B to the C phases. Such observations are not consistent with conventional superconductvity. A theory of these results is constructed based on a picture of two-component superconductivity for UPt3. In this way, these unusual observations can be understood. There is a possible discrepancy between theory and experiment in the detailed field dependence of the aspect ratio.

Modulated magnetic structures—statics

The theory of neutron diffraction on modulated magnetic structures is developed on the basis of a phenomenological approach with the Ginzburg-Landau functional. The diffraction on a soliton lattice is discussed in detail. In the light of the theoretical results the experimental neutron diffraction data are discussed.

Neutron diffraction by laser-generated volume phase gratings

Cold neutrons are diffracted by thick phase gratings which have been produced by light-optical holography in poly-(methyl methacrylate) (PMMA). The neutron diffraction efficiency is investigated in the Laue case as a function of the angle of incidence by rocking the sample. The dynamical theory of neutron diffraction allows to relate the measured diffraction efficiency to the modulation amplitude of the neutron-optical refractive index. The absence of higher order diffraction peaks proves that this modulation is sinusoidal. Further experimental parameters are shown to influence the diffraction in accordance with the dynamical theory: the wavelength of neutrons, the sample thickness, and the grating period. Laser diffraction experiments allow us to give upper limits for the uniformity of the grating constant throughout the sample volume. The gratings show properties which are promising for the fabrication of high-precision control samples for cold and very cold neutron spectrometers, for the construction of neutron-optical instruments, and the possibility to measure photoinduced changes of neutron-optical potentials.

Neutron diffraction study of the plastic phases of polycrystalline SF6and CBr4

An investigation of the plastic phases of polycrystalline specimens of sulphur hexafluoride (SF6) and carbon tetrabromide (CBr4) by neutron elastic scattering and neutron diffraction experiments is reported. A theory of neutron diffraction in plastic crystals, which treats the Bragg scattering and the diffuse scattering from a unified point of view, is developed and applied in the interpretation of the neutron results. The Debye-Scherrer peaks are analysed, both by a cumulant expansion technique and a cubic harmonic analysis, to determine the crystal structures of the plastic phase which are found to be body-centred cubic (space group Im3m) for SF6 and face-centred cubic (space group Fm3m) for CBr4. The bond-orientation distribution function, f(Ω), has maxima in the directions for SF6 and in the directions for CBr4. Since, in both cases, f(Ω) is appreciably different from zero for all orientations, it is apparent that significant thermal reorientation takes place in both these crystals. The tr...

Dynamical theory of neutron diffraction

We present a review of the dynamical theory of neutron diffraction by macroscopic bodies which provides the theoretical basis for the study of neutron optics. We consider both the theory of dispersion, in which it is shown that the coherent wave in the medium satisfies a macroscopic one-body Schrödinger equation, and the theory of reflection, refraction, and diffraction in which the above equation is solved for a number of special cases of interest. The theory is illustrated with the help of experimental results obtained over the past 10 years by a number of new techniques such as neutron gravity refractometry, Pendellösung interference, and neutron interferometry.

A spin-related contrast effect Visibility of 180° ferromagnetic domain walls in unpolarized neutron diffraction topography

Abstract 180° ferromagnetic domain walls have been observed in unpolarized neutron diffraction topographs of good Fe-3% Si single crystals, although they are not visible on X-ray topographs. The contrast is interpreted in terms of the dynamical theory of neutron diffraction in almost perfect magnetic crystals, by invoking the reflection of wave fields at the boundary.

Theory of neutron diffraction in mosaic crystals

It is shown that the Darwin equations, which describe the effect of multiple Bragg reflexion in mosaic crystals and have previously been obtained by heuristic arguments, arise naturally in a systematic solution of the neutron transport equation. With a method similar to that developed for calculating multiple-scattering corrections for neutron inelastic scattering in liquids, an approximate expression for the secondary extinction factor is obtained, which is applicable to a crystal of arbitrary size and shape. The results are compared with exact results for a plane slab, with Hamilton's [Acta Cryst. (1963), 16, 609-611] results for a cylinder and with Becker & Coppens's [Acta Cryst. (1974), A30, 129-147] results for a sphere.