Dispersive sandwich-type monochromators based on bent perfect crystals (BPCs) for high-resolution neutron scattering experiments

Direct comparison of plastically deformed Ge mosaic crystal and bent perfect Si crystal for neutron monochromatization

Bent perfect crystals in asymmetric diffraction geometry in neutron scattering experiments

Abstract. In conventional diffractometry with a mosaic monochromator due to a nonnegligi-ble λ−θ correlation of the monochromatic beam impinging the sample, the resolution consid-erably differs in the parallel (+,-) and anti-parallel (+,+) setting. However, a bent perfect crystal (BPC) monochromator at monochromatic focusing condition can provide equal reso-lution properties at both parallel and anti-parallel setting and gives a chance to use both sides of the scattering plane. Three diffraction geometries with the BPC monochromators were tested and the received data of FWHM of the line profiles and the corresponding ∆ d/d prove the possibility of simultaneous measurements in both settings. Introduction Conventional neutron diffractometers employing mosaic monochromators are used only in the parallel (+,-) measurement setting, because the resolution of the anti-parallel (+,+) setting is much worse [1]. However, if we could use bot h settings with the equivalent resolution, the measurement efficiency could be twice of that related to the traditional mode. A strong dif-ference in the resolution properties in the parallel and antiparallel settings is brought about by a nonnegligible λ−θ correlation of the monochromatized beam impinging the sample in the scattering plane. In order to overcome this problem the momentum distribution of the monochromatized neutrons should be symmetric with respect to the axis of the beam imping-ing the sample. Three monochromator arrangements which could provide a rather highly collimated monochromatic neutron beam with a negligible λ−θ correlation and a reasonable ∆λ (keeping the luminosity and the instrument resolution on a feasible level) have been al-ready tested. These are: 1.

Observation of multiple Bragg reflections of neutrons in bent perfect crystals

Cylindrically bent perfect crystals (BPC) as neutron monochromators/analyzers have been proved as an excellent alternative of conventional mosaic crystals providing a way how to increase luminosity and angular/energy resolution of some dedicated scattering devices installed at steady state sources [1][2][3].In our contribution we present the recent results of test experiments with dispersive monochromators based on a double-reflection process realized either in one cylindrically bent perfect crystals (often called as Renninger or Umweganregung effect) or by means of a sandwich using two bent perfect crystal slabs of a different cut.Depending on the bending radius of the crystal slab (or the sandwich of two slabs) the resolution / and the collimation of the monochromatized beam can be continuously adjusted in the range of 5x10 -5 -1x10 -3 .Of course, that the dispersive bent perfect crystal elements can be used also for the high-resolution analysis of the scattered beam as well as for a high precision -calibration of the TOF neutron scattering devices.Neutron diffraction investigations are in this field partially supported by the projects AV0Z10480505, GACR.202/03/0891 and

Multiple reflections (MRs)—a new challenge for high-resolution neutron diffractometry and spectrometry

After preliminary results obtained and published recently [1], in our contribution focusing and reflectivity properties of the dispersive double bent-crystal arrangement are presented in much more detail. It has been found that two different bent perfect crystals in (+n,-m) setting can be good candidates for high efficiency neutron microfocusing as well as high-resolution monochromatisation. Due to the (+n,-m) setting of two different bent perfect crystals, a high resolution is expected in both Δ(2θ (2θ is the scattering angle) as well as Δλ/λ(λ is the neutron wavelength). Experimental tests were carried out with the setting employing the bent Si(111) slab and Si(220)-sandwich, which contained either one, or two or four 1.3 mm thin simply stacked slabs. Thanks to a high reflection probability of both bent elements and an easy manipulation with the curvature of the Si(220)-sandwich, an excellently focused intensive monochromatic beam of the width from one to several millimetres was obtained. The properties of the double bent-crystal setting were studied in Rez at the neutron optics diffractometer for the neutron wavelength of 0.162 nm and for various thicknesses and curvatures of the Si(220)-sandwich. It has been also found that besides an excellent focusing and reflectivity properties of the dispersive double bent-crystal setting the obtained monochromatic neutron current is sufficiently high for standard high-resolution diffraction experiments even at the medium power research reactor.

In this paper studies of neutron diffraction properties of the double crystal (+n,-m) setting containing Si(220) and Si(311) bent perfect crystals (BPC) in symmetric and fully asymmetric diffraction (FAD) geometry with the output beam expansion (OBE), respectively, are presented. Namely, our attention was focused on the properties of the FAD geometry of the BPC Si(311) crystal slab. It has been found that after a beam expansion this FAD geometry can provide a monochromatic beam of a rather large cross-section and of very small divergence with some possible application use.

Monte Carlo simulations of parasitic and multiple reflections in elastically bent perfect single-crystals

Bent perfect crystals as neutron reflectors: Test experiments with Si plates

The focusing properties of cylindrically bent crystals in symmetric Laue geometry are discussed using the formalism of Fresnel diffraction and the analytical solution of the Takagi-Taupin equations for a point source on the entrance surface. The existence of a focal shift in the dynamical focusing effect is pointed out and discussed. The present theoretical framework is applied to experiments performed at the energy-dispersive X-ray absorption spectroscopy beamline of the European Synchrotron Radiation Facility concerning the position and the size of the focal spot obtained from a polychromatic source at a large distance from the bent crystal.

Neutron imaging with bent crystals is considered in the linear approximation of neutron optics. A matrix formalism analogous to that of conventional lens optics is developed. Various imaging conditions are discussed in relation to the crystal deformation type and to possible applications in neutron scattering. All focusing conditions known previously are included as particular cases. Arrangements combining high resolution in imaging with high resolution in scattering are examined. The concept of Bragg mirrors as devices for non-dispersive imaging is introduced. The most powerful application appears to be that of Bragg mirrors combined with the time-of-flight method. Imaging with a thick packet of silicon wafers at the spatial resolution of a single thin wafer is demonstrated.

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.

Focusing 3-axis diffractometer set-up equipped with bent perfect crystal (BPC) monochromator and analyzer offers the sensitivity in determination of strains Dd/d < 10-4 in polycrystalline materials which is about one order of magnitude higher with respect to that of conventional 2-axis neutron scanners. It also offers possibility of line profile analysis for reasonable sample volumes and counting times. In this paper, the feasibility of using the 3-axis set-up even for measurements of rather large bulk polycrystalline samples with an acceptable resolution is presented. As the 3-axis set-up exploits focusing in real and momentum space, by a proper adjustment of the curvature of the analyzer, a high-resolution determination of the lattice changes can also be achieved even on large irradiated gauge volumes, though with a slightly relaxed resolution. It can be successfully exploited namely, in the strain/stress measurements on samples exposed to an external load, e.g. in tension/compression rig, in aging machine etc. In addition to the original performance where the analysis is carried out by rocking the BPC analyzer and the neutron signal registered by a point detector, a new alternative is offered which uses a fixed rocking angle position of the analyzer and the detector signal is registered by a one-dimensional position sensitive detector (PSD). This trick makes possible in some cases the elastic strain/stress measurements considerably faster and thus reduces the drawback of the time consuming step-by-step analysis.

Cylindrically bent perfect crystals as neutron monochromators