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8.4 The Monochromator_pol McStas Component
Flat polarizaing monochromator crystal.
Identification
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Author: Peter Christiansen
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Origin: RISOE
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Date: 2006
Description
Based on Monochromator_flat. Flat, infinitely thin mosaic crystal, useful as a monochromator or analyzer. For an unrotated monochromator component, the crystal surface lies in the Y-Z plane (ie. parallel to the beam). The mosaic and d-spread distributions are both Gaussian. Neutrons are just reflected (billard ball like). No correction is done for mosaicity of reflecting crystal. The crystal is assumed to be a ferromagnet with spin pointing up eta-tilde = (0, 1, 0) (along y-axis), so that the magnetic field is pointing opposite (0, -|B|, 0).
The polarisation is done by defining the reflectivity for spin up (Rup) and spin down (Rdown) (which can be negative, see now!) and based on this the nuclear and magnetic structure factors are calculated: FM = sign(Rup)*sqrt(|Rup|) + sign(Rdown)*sqrt(|Rdown|) FN = sign(Rup)*sqrt(|Rup|) - sign(Rdown)*sqrt(|Rdown|) and the physics is calculated as Pol in = (sx_in, sy_in, sz_in) Reflectivity R0 = FN*FN + 2*FN*FM*sy_in + FM*FM (= |Rup| + |Rdown| (for sy_in=0)) Pol out: sx = (FN*FN - FM*FM)*sx_in/R0; sy = ((FN*FN - FM*FM)*sy_in + 2*FN*FM + FM*FM*sy_in)/R0; sz = (FN*FN - FM*FM)*sz_in/R0;
These equations are taken from: Lovesey: "Theory of neutron scattering from condensed matter, Volume
2", Eq. 10.96 and Eq. 10.110
This component works with gravity (uses PROP_X0).
Example: Monochromator_pol(zwidth=0.2, yheight=0.2, mosaic=30.0, dspread=0.0025, Rup=1.0, Rdown=0.0, Q=1.8734)
Monochromator lattice parameter
PG 002 DM=3.355 AA (Highly Oriented Pyrolythic Graphite) PG 004 DM=1.677 AA
Heusler 111 DM=3.362 Å (Cu2MnAl)
CoFe DM=1.771 AA (Co0.92Fe0.08) Ge 111 DM=3.266 AA Ge 311 DM=1.714 AA Ge 511 DM=1.089 AA Ge 533 DM=0.863 AA Si 111 DM=3.135 AA Cu 111 DM=2.087 AA Cu 002 DM=1.807 AA Cu 220 DM=1.278 AA Cu 111 DM=2.095 AA
Input parameters
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
|
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|
zwidth |
m |
Width of crystal |
0.1 |
|
yheight |
m |
Height of crystal |
0.1 |
|
mosaic |
arcmin |
Mosaicity (FWHM) |
30.0 |
|
dspread |
1 |
Relative d-spread (FWHM) |
0 |
|
Q |
Å\(^{-1}\) |
Magnitude of scattering vector |
1.8734 |
|
DM |
Å |
monochromator d-spacing instead of Q = 2*pi/DM |
0 |
|
pThreshold |
|
if probability>pThreshold then accept and weight else random |
0 |
|
Rup |
1 |
Reflectivity of neutrons with polarization up |
1 |
|
Rdown |
1 |
Reflectivity of neutrons with polarization down |
1 |
|
debug |
|
if debug > 0, print out some info about the calculations |
0 |
|
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Links
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Component source code found in file Monochromator_pol.comp.
