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McXtrace: Bragg_crystal_bent Component

[ Identification | Description | Input parameters | Output parameters | Links ]

The Bragg_crystal_bent Component

Based on: Perfect_crystal.comp written by Anette Vickery, Andrea Prodi, Erik Knudsen Bent, perfect, reflecting crystal with common cubic structures (diamond, fcc, or bcc, and others if symmetry form factor multipliers provided explicitly)

Identification

  • Author: Marcus H Mendenhall, NIST
  • Origin: NIST, Gaithersburg, MD, USA
  • Date: December 1, 2016
  • Version: 2.0-a

Description

Bragg_crystal_bent.comp supercedes Perfect_Crystal_bent.comp with major edits and corrections.

For details see:
The optics of focusing bent-crystal monochromators on X-ray powder diffractometers with application to lattice parameter determination and microstructure analysis, 
Marcus H. Mendenhall,* David Black and James P. Cline, J. Appl. Cryst. (2019). 52, https://doi.org/10.1107/S1600576719010951

Reads atomic formfactors from a data input file.

The crystal code reflects ray in an ideal geometry, does not include surface imperfections or mosaicity
The crystal planes from which the reflection is made must lie in the X-Z plane on the unbent crystal rotated
by an angle alpha about the x axis with respect to the crystal surface.

The external geometry of the crystal follows that of Elliptic_mirror.comp.
I.e. the crystal is positioned such that the a-axis of the ellipsoid is on the
z-axis, the b-axis is along the y-axis and the c is along the x-axis.
The reference point of the crystal is the ellipsoid centre, offset by one half-axis along the y-axis.
(See the component manual for Elliptic_mirror for a drawing).

Notation follows Tadashi Matsushita and Hiro-O Hashizume, X-RAY MONOCHROMATORS. Handbook on Synchrotron Radiation,North-Holland Publishing Company, 1:263–274, 1983.

Non-copyright notice:
Contributed by the National Institute of Standards and Technology; not subject to copyright in the United States. 
This is not an official contribution, in that the results are in no way certified by NIST.

NOTE: elliptical coordinate code and documentation taken from Mirror_elliptic.comp distributed in McXtrace v1.2
However, the coordinates are rotated to be consistent with Bragg_crystal_flat.comp and Perfect_Crystal.comp.
Idealized elliptic mirror with surface ellipse and lattice ellipses independent, to allow construction of
Johansson optics, for example.

Input parameters

Parameters in boldface are required; the others are optional.
Name Unit Description Default
x_a m 1st short half axis (along x). Commonly set to zero, which really implies infinite value, so crystal is an elliptic cylinder. 0
y_b m 2nd short half axis (along y), which is also the presumed near-normal direction, reflection near the y-z plane. 1.0
z_c m long half axis (along z). Commonly a=0. b=c, which creates a circular cylindrical surface. 1.0
lattice_x_a m curvature matrix for underlying lattice, for bent/ground/rebent crystals THERE HAS BEEN NO TESTING for the case in which lattice_x_a != x_a. 0
lattice_y_b m curvature matrix for underlying lattice, for bent/ground/rebent crystals 1.0
lattice_z_c m curvature matrix for underlying lattice, for bent/ground/rebent crystals 1.0
length m z depth (length) of the crystal. 0.05
width m x width of the crystal. 0.02
V AA^3 unit cell volume 160.1826
form_factors "FormFactors.txt"
material maybe also GaAs? [ ] Si, Ge "Si.txt"
alpha rad asymmetry angle (alpha=0 for symmetric reflection, ie the Bragg planes are parallel to the crystal surface) alpha is defined so that positive alpha reduces the Bragg angle to the plane i.e. alpha=Thetain grazes the planes. if alpha!=0, one should restrict to rays which have small kx values, since otherwise the alpha rotation is not around the diffraction axis. 0.0
R0 Reflectivity. Overrides the computed Darwin reflectivity. Probably only useful for debugging. 0
debye_waller_B AA^2 Debye-Waller temperature factor, M=B*(sin(theta)/lambda)^2*(2/3), default=silicon at room temp. 0.4632
crystal_type 1 => Mx_crystal_explicit: provide explicit real and imaginary form factor multipliers structure_factor_scale_r, structure_factor_scale_i, 2 => Mx_crystal_diamond: diamond 3 => Mx_crystal_fcc: fcc 4 => Mx_crystal_fcc: bcc 1
h Miller index of reflection 1
k Miller index of reflection 1
l Miller index of reflection 1
structure_factor_scale_r 0.0
structure_factor_scale_i 0.0

Output parameters

Name Unit Description Default
prms  
a2inv  
b2inv  
c2inv  
l_a2inv  
l_b2inv  
l_c2inv  

Links


[ Identification | Description | Input parameters | Output parameters | Links ]

Generated automatically by McDoc, Peter Willendrup <peter.willendrup@risoe.dk> / Tue Nov 26 12:17:24 2019


Last Modified: Tuesday, 26-Nov-2019 13:17:24 CET
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