J. Kulda

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.

Bragg diffraction optics in high resolution strain measurements

Abstract Using Bragg diffraction optics focusing conditions for a triple-axis setup equipped with a bent Si single crystal monochromator and analyzer for investigation of stress fields in polycrystalline materials are derived. Results of experimental tests of such a setup are presented. Further it is demonstrated that if certain focusing conditions for a bent monochromator are fulfilled the beam diffracted by a polycrystalline sample becomes quasi-parallel which enables high resolution measurements directly with a PSD without the use of a collimator or a crystal-analyzer. In the three axis setup maximum sensitivity in determination of Δd d ≤ 10 −4 can be achieved permitting profile-broadening analysis for reasonable sample volumes and counting times.

Elastically bent silicon monochromator and analyzer on a TAS instrument

The elastically deformed perfect crystals are ideally suited for focusing of neutron beams thanks to the deterministic nature of their “mosaic” structure. During several decades it was, however, believed that their integrated reflecting power cannot compete with that of mosaic crystals and their practical application remained reserved to special high resolution instruments. In reality supplementary gains in intensity are achieved thanks to more efficient real space focusing and to avoiding the Soller collimators. In the present paper we demonstrate that a properly optimized horizontally and vertically focusing assembly of elastically bent Si 111 crystals can serve as a highly efficient general purpose monochromator and analyzer on a three-axis spectrometer. Typically, a gain factor of 3 to 5 intensity is obtained when replacing a horizontally flat PG 002 analyzer with Soller collimators by the Si 111 in open geometry at the same or better energy and momentum resolution. The excellent performance of the silicon crystals is accompanied by their low cost, standard crystalline quality and by the absence of the λ2 beam contamination for the odd reflections such as 111.

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.

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.

Multiple Bragg reflections of neutrons in an elastically deformed single crystal

The large positive diffraction intensity changes on forbidden (222) planes, which are produced by simultaneous diffraction, were observed when a perfect Si single crystal was excited into vibration. The excited Umweganregung effect is of the same order as the diffraction intensity obtained on (111) planes under the same experimental conditions. The theoretical consideration is an extension of the lamellae model usually used in the two-wave approximation. Both the theory and the experimental results demonstrate that such an effect may cause very large errors especially in diffraction experiments with non-perfect single crystals.

Neutron diffraction by an absorbing vibrating InSb crystal

Following our recent paper [Lukas, Kulda, Mikula, Sedlakova, Alexandrov & Vrana (1991). Acta Cryst. A47, 166–169], a comparison of the dynamical description of diffraction by an absorbing elastically deformed perfect crystal with the experimental data taken at diffraction by a longitudinally vibrating InSb crystal is presented. The comparison is performed on the time-dependent integrated intensity spectra for different values of the vibration amplitudes. The applicability of the analytical formulas derived earlier by two of the authors for an arbitrary homogeneous deformation is in this case fully approved by the good agreement between the theory and the experiment.

Neutron diffraction by perfect crystals excited into mechanical resonance vibrations

Abstract Our contribution presents results of investigations concerning neutron diffraction by vibrating perfect crystals of Si and SiO 2 . They were excited in longitudinal, flexural, thickness and thickness-shear modes of vibration with resonance frequencies ranging from 1 to 1500 kHz. In this case, when the acoustic wavelength is much larger than the extinction distance the following phenomena caused by the deformation and Doppler shift were observed: broadening of the perfect crystal rocking curve, enhancement of the integrated reflectivity by up to two orders of magnitude and time modulation of the diffracted beam. When neutron time-off-flight through the vibrating crystal is comparable to the vibration period, multiple reflections having influence on the reflectivity take place. While investigating the forbidden reflections in the crystal excited into vibrations, a strong multiple diffraction phenomena resulting in “umweganregung” effect was observed. Also the vibrating crystals as neutron choppers with 1 μs pulses and a repetition rate of up to 100 kHz were treated.

Cylindrically bent perfect crystals as neutron monochromators

The integrated reflectivity and flux density of monochromatic neutrons delivered to a sample were examined when testing cylindrically-bent perfect Si and Ge crystals as monochromators. The presented experimental data, and comparison with a conventional mosaic Cu and Ge crystals, demonstrate favourable reflectivity properties of bent perfect crystals approaching or surpassing those of their mosaic counterparts.

ThALES—three axis low energy spectroscopy at the Institut Laue–Langevin

Building on the strength of the present cold neutron three-axis spectrometer IN14, but using state-of-the-art neutron optics, we conceived the next generation three-axis instrument for low energy spectroscopy (ThALES) at the Institut Laue–Langevin (ILL). The main aims of the new instrument are: (i) to increase the overall data collection rate by rebuilding the neutron optics of the primary spectrometer achieving a higher incident neutron flux as well as by multiplexing the analyser–detector system, (ii) to provide an efficient and easy-to-use polarized neutron option, (iii) to extend the incident neutron range towards higher energies bridging the gap with thermal instruments, and (iv) to be able to use high-field magnets—such as the currently available 15 T cryomagnet—under all possible experimental conditions, i.e. in a wider range of incident energies. The expected increase in count rate by at least one order of magnitude allows for new experiments such as high pressure experiments on small sample sizes or investigations of magnetic excitations in thin films. Polarized inelastic neutron measurements should equal count rates of the present IN14 in unpolarized mode. The implementation of various optical elements enhances the flexibility of the instrument and allows trading momentum resolution for high neutron intensity.