Bent Si Research Articles

Experiment to observe the spin precession of channeled relativistic Σ+ hyperons

Spin precession of channeled particles in bent crystals has been observed for the first time. Polarized Σ + were channeled using bent Si crystals. These crystals provided an effective magnetic field of 45 T which resulted in measured spin precession of 60° ± 17°. This agrees with the prediction of 62° ± 2° using the world average of Σ + magnetic moment measurements. This new technique gives a Σ + magnetic moment of (2.40 ± 0.46 ± 0.40) μn where the quoted uncertainties are statistical and systematic, respectively. We see no evidence of depolarization in the channeling process.

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 λ 2 beam contamination for the odd reflections such as 111.

One more possibility of using bent crystals for beam extraction from accelerators

Abstract A bent Si crystal placed before the first septum-magnet of the available slow extraction scheme of the IHEP accelerator allowed one to extract, for physical experiments, additional up to 3 × 10 8 protons to be lost otherwise at the septum in nonresonant slow extraction mode. It is about 10–30% of a the beam intensity extracted towards channel no. 22 without a bent crystal. The effect is due to volume channeling. The use of an unbent crystal as an amorphous target results in a decrease of the extracted beam intensity.

Towards ideal focusing monochromators

An analysis of various factors influencing the performance of a focusing monochromator is presented as a complement to our recent experimental comparisons of the reflectivities of mosaic PG, Cu and Ge monochromator crystals with those of elastically bent perfect Si crystals. Attention is drawn to the superior qualities of elastically bent perfect crystals in terms of their sharply bounded reflection curves and to the proper choice of crystal material for maximum output flux at specified focusing conditions.

Bragg optics for strain/stress measurement techniques

Several modifications of the high-resolution strain measurement method developed at NPI Ře — energy dispersive transmission diffraction (EDTD) — using Bragg diffraction optics are described. Bent perfect Si crystals are employed for selection (or analysis) of the quasi-monochromatic beam. This method, which is complementary to Bragg diffractionangle analysis provides sensitivity in lattice-spacing changes Δ d/d down to 10 −5, and may be used even at a mediumpower research reactor for reasonable sample volumes and counting times.

Tunable X-ray quarter-wave plates for X-ray magnetic circular dichroism experiments with the energy dispersive absorption spectrometer

Perfect and mosaic crystals (respectively diamond and beryllium) have been used as X-ray quarter-wave plates to convert linearly polarised X-rays into circularly polarised photons, and to measure X-ray magnetic circular dichroism (XMCD) from ferro-(ferri-) magnetic samples with an energy dispersive absorption spectrometer. Bragg reflections from both the bent Si polychromator and the quarter-wave plate (QWP) have been matched in order to obtain a non-dispersive geometry. This results in a high circular polarisation rate in an energy band pass which is presently QWP size-limited, about 180 eV with the used diamond crystal and 280 eV with the beryllium crystal. The energy tunability of the diamond QWP was demonstrated by recording XMCD at different L-edges from Pr L II (6400 eV) to Tm L III (8648 eV). XMCD spectra were also obtained by switching the helicity of the photons, flipping the QWP between two opposite angular offsets over the diamond Bragg peak. This optical device, delivering polarisation-tunable X-rays sources, is by far better than the classical off-orbit plane source from a bending magnet. It opens up new classes of experiments.

Performance of Beamline 4W1C for x-ray diffuse scattering station at Beijing Synchrotron Radiation Facility

Bent triangular Si crystals with an oblique-cut angle 12.16° are used on the beamline 4W1C at Beijing Synchrotron Radiation Facility, to select monochromatic x-rays and to focus the beam in the horizontal direction. The downstream Pt-coated reflective mirror is bent as a cylindroid focuses the beam vertically. Double-focused monochromatic x rays are available on the diffuse scattering station. Specifications of the beamline with the Si (220) monochromator were measured as follows: size of the focus spot—0.5 mm (horizontal)×0.3 mm (vertical), maximum x-ray density—1.6×109 photons/s mm2, and energy resolution of the monochromatic beam—4.4×10−4.

Prototype grooved and spherically bent Si backscattering crystal analyzer for meV resolution inelastic x-ray scattering

The high-order backscattering reflections from single crystals of silicon have mrad rocking curve widths that can be exploited to produce meV energy-resolution focusing analyzer crystals for use in inelastic x-ray scattering experiments at third-generation synchrotron sources. The first generation of these analyzers has been limited in efficiency principally by slope and/or figure errors. We calculate the effect of slope errors on the theoretical energy resolution and focus spot size of a typical analyzer design using a ray-tracing code to ensure that there are no unforeseen contributions to the energy resolution and efficiency. We also present measurements of the slope errors of the atomic planes for a prototype, spherically bent, strain-relief grooved analyzer as proof that it is in principle possible to obtain the slope and figure error limits required for a high efficiency meV resolution backscattering crystal.

High energy scattering beamlines at European Synchrotron Radiation Facility

The scientific background and scope of research at the High Energy Scattering Beamlines of the European Synchrotron Radiation Facility are reviewed briefly. The beamline has two different insertion devices (IDs), a permanent magnet multipole wiggler and a superconducting wavelength shifter. The IDs can be used alternatingly, and both provide circularly polarized radiation off the orbit plane up to energies of several hundreds of keV. Measurements of the spectral brightness and the polarization components by energy dispersive powder diffraction and Compton scattering from iron in an alternating magnetic field are presented and compared with calculations based on source parameters. The beamline optics are based on use of horizontally reflecting bent Si crystals. These provide various focusing geometries, and the calculated flux of monochromatic radiation is given at energies between 30 and 175 keV. The maximum of 2×1012 photons/s is reached at 80 keV. The main experimental apparatus in the initial operation includes a high-resolution three-axis diffractometer, and a scanning-type Compton spectrometer.

Optical design and performance of the inelastic scattering beamline at the National Synchrotron Light Source

Phase I of the X21 beamline at the National Synchrotron Light Source was commissioned during 1993. The research program at the X21 beamline is focused on the study of electronic excitations in condensed matter with total energy resolution of 0.1–1.0 eV. The source is a 27-pole hybrid wiggler. A water-cooled horizontal focusing Si(220) monochromator and a spherically bent Si(444) analyzer were installed and commissioned. At 8 keV the energy resolution of the monochromator is about 0.7 eV, and the energy resolution of the analyzer is about 0.1 eV. Results from several selected experiments are also discussed.

Fixed-Exit Monochromators for High-Energy Synchrotron Radiation

Current developments in X-ray optics for synchrotron radiation beamlines are briefly reviewed. Reference is made to recent work on adaptive mirrors, cryogenic cooling of monochromators, use of thin diamond crystals, and active correction of the crystal shape for distortions caused by beam heating. The use of bent Si crystals as monochromators is discussed in detail. At high energies Si crystals become transparent to X-rays allowing new monochromator constructions. Bending of the crystal increases the energy bandpass and allows focusing. Different combinations of bent crystals that provide a fixed exit beam are discussed. These include vertically diffracting meridionally focusing Laue-Laue crystals, a Laue crystal combined with a sagittally focusing Bragg crystal, and a Laue-Bragg pair of crystals which provides meridional focusing at two stages.

Energy-dispersive phase plate for magnetic circular dichroism experiments in the X-ray range

A 220 diamond phase plate was combined with an energy-dispersive absorption spectrometer to convert linearly polarized X-rays into circularly polarized photons and to detect circular magnetic X-ray dichroism (CMXD) from ferromagnetic samples. In these experiments, carried out at LURE, the energy-dispersive spectrometer was equipped with a bent Si (311) polychromator and vertically collimating slits accepting essentially a linearly polarized incident beam. The quarter-wave plate was operated in the Bragg geometry but well outside the range of quasi total reflection, with the consequence that the forward-diffracted beam was circularly polarized with a polarization rate approaching 80% over the whole energy bandpass of the polychromator. CMXD spectra of GdFe2 and GdCo2 intermetallic compounds were recorded at ca 7.2 keV near the Gd LIII absorption edge: they are essentially identical to the spectra commonly recorded with elliptically polarized X-ray photons collected out of the orbit plane of the storage ring. It is suggested that the energy-dispersive phase plate will be very useful to detect CMXD spectra with energy-dispersive spectrometers exploiting the well collimated linearly polarized emission of standard undulators installed on the storage rings of the third generation.

Bent perfect crystals in asymmetric diffraction geometry in neutron scattering experiments

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

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.

A doubly bent silicon monochromator

A focusing monochromator consisting of three cylindrically bent Si(311) crystals was constructed and tested. It was designed for a powder diffractometer using neutron wavelengths of about 0.15 nm. The device provides a fixed curvature in the vertical plane and allows a continuously variable bending in the scattering plane. The vertical focusing increased the neutron current at the sample position by a factor of about 2.5. By horizontal focusing the resolution of the powder lines can be adapted to the range of interest.

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

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 Å in terms of neutron beam flux delivered at sample position as well as in terms of resolution in powder diffraction spectra recorded with an Al 2O 3 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.

Investigation of Residual Strains of the Second Kind in Plastically Deformed Metallic Materials

ABSTRACTInvestigation of strongly distorted crystal structures caused by dislocations, stacking-faults etc. in both plastically deformed f.c.c. and b.c.c. metallic materials was performed by the analysis of the neutron diffraction line broadening. Measurements were realized by means of the high resolution triple-axis neutron diffractometer equipped by bent Si perfect crystals as monochromator and analyzer at the NPI Řež. The substructure parameters obtained in this manner are in good agreement with the results of X-ray diffraction analysis.

Theoretical investigations of secondary emission yield and standing waves in curved crystals under dynamical Bragg diffraction of X-rays (Taupin problem)

The first results of the theory of the behaviour of standing waves in curved crystals are presented. The proposed theory is based on the Taupin theory of dynamical X-ray diffraction in curved crystals and the theory of the secondary radiation yield in deformed crystals. This theory is applied for calculation of the angular dependence of the secondary emission yield in a cylindrically bent Si(400) monocrystal, Cu Kalpha radiation. Processes with different depth of secondary emission yield, i.e. photoemission (Lyi=0.1 mu m) and fluorescence (Lyi=70 mu m) are considered. The behaviour of the reflection coefficient and standing waves in the bulk of the curved crystal is investigated. The standing wave intensities in the depth of the curved crystal show much more sensitivity to the phaseshift than in the perfect one. This fact can be used for the investigation of thin layers of impurity atoms embedded in the crystal matrix.

X-Ray Standing Waves in a Bent Crystal. Fluorescence and Photoemission yield

First results of the theory of the standing wave behaviour in curved crystals are presented. The proposed theory is based on the Taupin theory of dynamical X-ray diffraction in curved crystals and the theory of the secondary radiation yield in deformed crystals. This theory is applied for a calculation of the angular dependence of the secondary emission yield in a cylindrically bent Si(400) monocrystal, CuKa radiation. Processes with different depths of the secondary emission yield, i.e. photoemission (Lyi = 0.1 μm) and fluorescence (Lyi = 70 μm), are considered.

First observation of magnetic moment precession of channeled particles in bent crystals.

Spin precession of channeled particles in bent crystals has been observed for the first time. Polarized Σ+ were channeled using bent Si crystals. These crystals provided an effective magnetic field of 45 T which resulted in a measured spin precession of 60±17°. This agrees with the prediction of 62±2° using the world average of Σ+ magnetic moment measurements. This new technique gives a Σ+ magnetic moment of (2.40±0.46±0.40)μN, where the quoted uncertainties are statistical and systematic, respectively. We see no evidence of depolarization in the channeling process.Received 8 September 1992DOI:https://doi.org/10.1103/PhysRevLett.69.3286©1992 American Physical Society