An elastically bent silicon crystal as a monochromator for thermal neutrons

1990 ◽  
Vol 23 (2) ◽  
pp. 105-110 ◽  
Author(s):  
P. Mikula ◽  
E. Krüger ◽  
R. Scherm ◽  
V. Wagner
Keyword(s):  
Momentum Space ◽  
Pyrolytic Graphite ◽  
Silicon Crystal ◽  
Thermal Neutrons ◽  
Monochromatic Beam ◽  
Beam Density ◽  
Diffraction Geometry ◽  
Bending Radius ◽  
Neutron Monochromator ◽  
Polychromatic Beam

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.

Dynamical effects in the integrated X-ray scattering intensity from imperfect crystals in Bragg diffraction geometry. I. Semi-dynamical model

2020 ◽  
Vol 76 (1) ◽  
pp. 45-54
Author(s):  
V. B. Molodkin ◽  
S. I. Olikhovskii ◽  
S. V. Dmitriev ◽  
A. I. Nizkova ◽  
V. V. Lizunov
Keyword(s):  
Silicon Crystal ◽  
Azimuthal Angle ◽  
Dynamical Theory ◽  
Bragg Diffraction ◽  
X Ray Diffraction ◽  
X Ray ◽  
X Ray Scattering ◽  
Diffraction Geometry ◽  
Coulomb Type ◽  
Analytical Expressions

The analytical expressions for the coherent and diffuse components of the integrated reflection coefficient are considered in the case of asymmetric Bragg diffraction geometry for a single crystal of arbitrary thickness, which contains randomly distributed Coulomb-type defects. The possibility to choose the combinations of diffraction conditions optimal for characterizing defects of several types by accounting for dynamical effects in the integrated coherent and diffuse scattering intensities, i.e. primary extinction and anomalous absorption, has been analysed based on the statistical dynamical theory of X-ray diffraction by imperfect crystals. The measured integrated reflectivity dependencies of the imperfect silicon crystal on azimuthal angle were fitted to determine the diffraction parameters characterizing defects in the sample using the proposed formulas in semi-dynamical and semi-kinematical approaches.

Real and momentum space focusing of neutrons by a cylindrically bent silicon crystal

1992 ◽  
Vol 180-181 ◽  
pp. 981-983 ◽  
Author(s):  
P. Mikula ◽  
V. Wagner ◽  
P. Lukáš ◽  
R. Scherm
Keyword(s):  
Momentum Space ◽  
Silicon Crystal

Heat transfer analysis study for the design of the New Polychromatic Beam Neutron Reflectometer CANDOR (Chromatic Analysis Neutron Diffractometer Or Reflectometer)

2020 ◽  
Vol 22 (4) ◽  
pp. 417-428
Author(s):  
Lior Uzan ◽  
Juscelino B. Leão ◽  
Christoph Brocker ◽  
Daniel Mattes ◽  
Tanya Dax
Keyword(s):  
Heat Transfer ◽  
Diffuse Scattering ◽  
Pyrolytic Graphite ◽  
Heat Transfer Analysis ◽  
Thermal Diffuse Scattering ◽  
Analysis Study ◽  
Neutron Diffractometer ◽  
Neutron Reflectometer ◽  
Thermal Diffuse ◽  
Polychromatic Beam

A newly developed polychromatic beam neutron reflectometer CANDOR (Chromatic Analysis Neutron Diffractometer Or Reflectometer) on NG-1 at the NIST Center for Neutron research (NCNR) utilizes a wavelength-sensitive neutron detector consisting of 324 analyzing highly-oriented pyrolytic graphite (HOPG) crystals positioned sequentially in rows. Known for having a small thermal diffuse scattering cross section, HOPG crystals can lead to low signal-to-noise ratios in wavelength-sensitive detectors such as CANDOR. Even though it is possible to mathematically separate the desired signal from thermal diffuse scattering; by cooling the detector array of HOPG crystals in order to minimize the Debye Waller effect generates a better solution to this problem. In this heat transfer analysis study we show, within the instrument design constrains and thermodynamic considerations, technical feasibility and test results for the development of the New Polychromatic Beam Neutron Reflectometer CANDOR (Chromatic Analysis Neutron Diffractometer Or Reflectometer) at the NIST Center for Neutron Research.

Neutron diffraction studies of a double-crystal (+n,–m) setting containing a fully asymmetric diffraction geometry of a bent perfect crystal with output beam expansion

2014 ◽  
Vol 47 (2) ◽  
pp. 599-605 ◽  
Author(s):  
Pavol Mikula ◽  
Miroslav Vrána ◽  
Jan Pilch ◽  
Baek Seok Seong ◽  
Wanchuck Woo ◽  
...  
Keyword(s):  
Neutron Diffraction ◽  
Cross Section ◽  
Perfect Crystal ◽  
Large Cross Section ◽  
Output Beam ◽  
Double Crystal ◽  
Monochromatic Beam ◽  
Bent Crystal ◽  
Diffraction Geometry ◽  
Crystal Arrangement

The neutron diffraction properties of a double-crystal (+n,−m) setting, which contains a bent perfect Si(311) crystal in the fully asymmetric diffraction (FAD) geometry with output beam expansion and a bent perfect Si(220) crystal in the symmetric diffraction geometry, are presented. Generally, there are two possibilities for the FAD geometry: either with output beam compression or with output beam expansion. In this case, attention has been focused on the latter. The properties of the (+n,−m) double-bent-crystal arrangement of a bent perfect crystal FAD Si(311) geometry, in combination with a bent Si(220) crystal slab in the symmetric diffraction geometry, were studied. It was found that, after beam expansion, this FAD geometry can provide a monochromatic beam of rather large cross section but very small divergence.

Experimental studies of dispersive double reflections excited in cylindrically bent perfect-crystal slabs at a constant neutron wavelength

2012 ◽  
Vol 45 (1) ◽  
pp. 98-105 ◽  
Author(s):  
Pavol Mikula ◽  
Miroslav Vrána ◽  
Jan Šaroun ◽  
Vadim Davydov ◽  
Vyacheslav Em ◽  
...  
Keyword(s):  
High Resolution ◽  
Crystal Lattice ◽  
Experimental Studies ◽  
Perfect Crystal ◽  
Monochromatic Beam ◽  
Diffraction Geometry ◽  
Neutron Wavelength ◽  
Selected Effects ◽  
Very High

Multiple Bragg reflections (MBRs), which can be realized in a bent perfect crystal (BPC) slab and are mutually in dispersive diffraction geometry, provide a monochromatic beam of excellent resolution. After identifying many MBR effects in a BPC Si crystal by using the method of θ–2θDscanning, we have turned our attention to the study of selected effects using the method of azimuthal rotation of the crystal lattice around the scattering vector of the primary reflection for a fixed chosen wavelength. In this paper, several azimuthal scans with the intention of possible practical exploitation for very high resolution diffractometry are presented.

scholarly journals Characteristics of Pyrolytic Graphite as a Neutron Monochromator

2012 ◽  
Vol 2 (1) ◽  
Author(s):  
M. Adib ◽  
N. Habib ◽  
M. S. El-Mesiry ◽  
M. Fathallah
Keyword(s):  
Pyrolytic Graphite ◽  
Neutron Monochromator

Thin Pyrolytic Graphite Films for Electron Microscope Substrates

1971 ◽  
Vol 29 ◽  
pp. 226-227 ◽  
Author(s):  
George H. N. Riddle ◽  
Benjamin M. Siegel
Keyword(s):  
Vacuum Chamber ◽  
Pyrolytic Graphite ◽  
High Vacuum ◽  
Dark Field ◽  
Ultra High Vacuum ◽  
Graphite Substrate ◽  
Nickel Substrate ◽  
Routine Procedure ◽  
Field Electron Microscopy ◽  
Dark Field Electron

A routine procedure for growing very thin graphite substrate films has been developed. The films are grown pyrolytically in an ultra-high vacuum chamber by exposing (111) epitaxial nickel films to carbon monoxide gas. The nickel serves as a catalyst for the disproportionation of CO through the reaction 2C0 → C + CO2. The nickel catalyst is prepared by evaporation onto artificial mica at 400°C and annealing for 1/2 hour at 600°C in vacuum. Exposure of the annealed nickel to 1 torr CO for 3 hours at 500°C results in the growth of very thin continuous graphite films. The graphite is stripped from its nickel substrate in acid and mounted on holey formvar support films for use as specimen substrates.The graphite films, self-supporting over formvar holes up to five microns in diameter, have been studied by bright and dark field electron microscopy, by electron diffraction, and have been shadowed to reveal their topography and thickness. The films consist of individual crystallites typically a micron across with their basal planes parallel to the surface but oriented in different, apparently random directions about the normal to the basal plane.

An Energy Analyser for Contrast Enhancement in STEM

1974 ◽  
Vol 32 ◽  
pp. 434-435
Author(s):  
H.T. Pearce-Percy
Keyword(s):  
Contrast Enhancement ◽  
Bending Angle ◽  
Magnetic Sector ◽  
Conventional Design ◽  
Bending Radius ◽  
Sector Type

Recently an energy analyser of the uniform magnetic sector type has been installd in a 100KV microscope. This microscope can be used in the STEM mode. The sector is of conventional design (Fig. 1). The bending angle was chosen to be 90° for ease of construction. The bending radius (ρ) is 20 cm. and the object and image distances are 42.5 cm. and 30.0 cm. respectively.

X-ray diffraction properties of curved crystal diffractors

1992 ◽  
Vol 50 (2) ◽  
pp. 1734-1735
Author(s):  
W. Z. Chang ◽  
D. B. Wittry
Keyword(s):  
Diffraction Theory ◽  
X Rays ◽  
Diffraction Image ◽  
X Ray Diffraction ◽  
Rocking Curve ◽  
X Ray ◽  
Bent Crystal ◽  
Diffraction Geometry ◽  
Crystal Parameter ◽  
Quantitative Procedure

Since Du Mond and Kirkpatrick first discussed the principle of a bent crystal spectrograph in 1930, curved single crystals have been widely utilized as spectrometric monochromators as well as diffractors for focusing x rays diverging from a point. Curved crystal diffraction theory predicts that the diffraction parameters - the rocking curve width w, and the peak reflection coefficient r of curved crystals will certainly deviate from those of their flat form. Due to a lack of curved crystal parameter data in current literature and the need for optimizing the choice of diffraction geometry and crystal materials for various applications, we have continued the investigation of our technique presented at the last conference. In the present abstract, we describe a more rigorous and quantitative procedure for measuring the parameters of curved crystals.The diffraction image of a singly bent crystal under study can be obtained by using the Johann geometry with an x-ray point source.

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