P. Mikula

Effect of wavelength-dependent attenuation on neutron diffraction stress measurements at depth in steels

The wavelength dependence of the maximum feasible penetration depth was studied for neutron diffraction stress measurements in ferritic and austenitic steels. This property was examined with wavelengths from the close vicinity of the Bragg edges, where the neutron total cross section has its local minimum and for which the scattering angles are convenient for stress measurements. These wavelengths (e.g. 2.39 and 2.19 A) are longer than those commonly used in stress measurements (∼1.6 A). By using such wavelengths, configured by a focusing bent perfect crystal Si(111) monochromator, it was observed that the available total beam path length is about 85 mm in both ferritic and austenitic steels. This study provides specific information for choosing the instrument configuration suitable for most strain-scanning experimental tasks.

A new version of a medium‐resolution double‐crystal diffractometer for the study of small‐angle neutron scattering (SANS)

A new version of a SANS instrument based on two bent perfect Si crystals in the nondispersive (1, -1) setting is presented. Unlike an earlier proposal which employed bent crystals in symmetric diffraction geometry I-Kulda & Mikula (1983). J. Appl. Cryst. 16, 498-504], use of the crystal analyser in the fully asymmetric geometry enables the angular dependence of the SANS intensity to be transformed into the positional dependence along its longest edge. This makes it possible to employ a one-dimensional position-sensitive detector and thus significantly increase the speed of collection of experimental data. Momentum-transfer resolution may be easily controlled by an adjustment of the bending radii of the crystals. Use was made of diffraction by the (111) crystal planes at a neutron wavelength of 0.2 nm to measure powder samples of Si3N4, poly(vinyl chloride)/poly(methyl methacrylate) and CrO2 in order to demonstrate the practical efficiency of the proposed device.

Bent perfect crystals in asymmetric diffraction geometry in neutron scattering experiments

Abstract Asymmetric cuts of elastically deformed perfect crystals, although less frequently used in practice, may also be attractive for applications. They offer an additional degree of freedom for optimization of instruments setups in terms of real- and reciprocal-space focusing. Some aspects of an useful employment of cylindrically bent perfect Si crystals in a strongly asymmetric geometry in neutron scattering experiments are presented. Special attention is paid to the limiting case of the fully asymmetric diffraction (FAD) geometry of the bent crystals. Here the absence of real-space focusing (the focus is directly at the crystal) is compensated by the effective mosaicity controlable in a large range, permitting to design instrument configurations offering considerably higher resolution for a given luminosity than the mosaic crystals.

An elastically bent silicon crystal as a monochromator for thermal neutrons

Some experimental results are presented for an elastically bent perfect silicon crystal in a strongly asymmetric diffraction geometry as a neutron monochromator. The use of this unconventional geometry of the monochromator appears to be suitable for a wide (several centimetres) incident polychromatic beam, when, thanks to the spatial condensation of the diffracted neutrons (Fankuchen effect), a high monochromatic beam density may be obtained. Furthermore, when using focusing in real and in momentum space by adjusting an optimum bending radius, the intensity diffracted by a sample may be comparable even with the best mosaic monochromators such as highly oriented pyrolytic graphite (PG). A comparison is demonstrated on the rocking curves of a strongly mosaic Ni–Al(020) crystal obtained with the monochromatic beam from bent Si(111), Si(400) and from PG(002), Cu(220) mosaic monochromators.

Investigation of short-range atomic order in glasses from the MoO3-TeO2 system

Short-range atomic order in the MoO3-TeO2 glassy system has been studied by the neutron diffraction method. With increase of the amount of MoO3 from 20 to 50 mol %, a sequential transition 4 → (3 + 1) → 3 of the coordination state of tellurium with respect to oxygen was observed. Throughout this process, a formation of MoO5 and MoO4 groups was detected. The interpretation of the experimental radial distribution functions was performed by comparison with quasi-crystalline model curves (structure diffusion model).

Double-Bent-Crystal Small-Angle Neutron Scattering Setting and its Applications

Theoretical relations for optimum adjustments of two arrangements of a double-bent-crystal (DBC) small-angle neutron scattering (SANS) diffractometer are discussed. Results of several experiments demonstrate the applicability of DBC SANS in the investigation of some technologically important materials in the range of scattering-vector sizes 0.0002-0.02 A -1. Formulas derived for data analysis, which describe the transformation of the SANS cross section to the measured intensity, indicate that water calibration is not required and that the indirect method of data evaluation is the most appropriate.

Interpretation of bent-crystal rocking curves

The optics of successive Bragg reflection by two bent crystals is considered in the lamellar approximation. The optimal curvatures ensuring minimal rocking-curve widths and good reflection efficiencies are determined. The conditions under which a rocking curve reproduces the reflectivity curve of a bent crystal are indicated. Analytical formulae for the rocking-curve width and peak intensity are derived for three simple limiting cases. The computations are supported by experimental results obtained with bent perfect silicon crystals.

Comparative tests of neutron monochromators using elastically bent silicon and mosaic crystals

Abstract The performance of elastically bent perfect (Si) and mosaic (Cu, Ge) monochromator crystals is compared in the range of neutron wavelengths between 0.8 and 1.4 A in terms of neutron beam flux delivered at sample position as well as in terms of resolution in powder diffraction spectra recorded with an Al2O3 standard. The reflectivities of the mosaic crystals are found to be a factor 2–3 less than those predicted by mosaic model calculations neglecting primary extinction. The elastically bent Si crystals provide fluxes comparable or better than those of the germanium crystals of the same thickness. At the same time their effective mosaicity remains about three times lower, a fact that leads to improved resolution in the high-angle part of the powder spectra.

An experimental test of an elastically bent silicon crystal as a thermal-neutron monochromator

The effectiveness of employing an elastically bent perfect Si (111) crystal as a monochromator for neutron-diffraction experiments is demonstrated on rocking curves presented for a flat perfect Si (220) crystal and a mosaic Fe (110) crystal, for two different collimations of the incident beam. For the sake of comparison equivalent rocking curves with a conventional mosaic Zn monochromator are also introduced. The experimental results obtained indicate that curved perfect-crystal monochromators may become a useful alternative to mosaic-crystal ones.

Use of multiple Bragg reflections in a bent perfect single crystal for high resolution monochromatization of neutrons

The effectiveness of the method of obtaining a highly monochromatic and collimated beam based on the well-known “Umweganregung” effect may be considerably improved when an elastically deformed single crystal is employed. In such a way a strongly excited multiple reflection effect produces a beam the intensity of which seems to be of practical use. For this purpose a very intensive umweg-effect simulating the 222 forbidden reflection of an elastically bent silicon single crystal at λ=0.156 nm was treated. The variable curvature of such a monochromator enables one to obtain a beam having a precisely fixed wavelength λ with a bandwidth Δλλ in the range from 10−4 to 10−3 and collimation of the order of minutes of arc. Experimental and theoretical treatment has proved that in an elastically deformed crystal the double diffraction simulating a forbidden reflection (Umweganregung) can be assumed as a diffraction on a bicrystal system realized in one crystal.