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

# The `Monochromator_pol` Component

Flat polarizaing monochromator crystal.

## Identification

- Author: Peter Christiansen
- Origin: RISOE
- Date: 2006
- Version: 1.3

## 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(zw=0.2, yh=0.2, mosaic=30.0, dspread=0.0025,
                               Rup=1.0, Rdown=0.0, Q=1.8734)

## Input parameters

Parameters in boldface are required; the others are optional.

Name | Unit | Description | Default
--- | --- | --- | ---
zw | m | Width of crystal |
yh | m | Height of crystal |
mosaic | arc minutes | Mosaicity (FWHM) | 30.0
dspread | 1 | Relative d-spread (FWHM) | 0
Q | AA-1 | Magnitude of scattering vector | 1.8734
DM | Angstrom | 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

## Output parameters

Name | Unit | Description | Default
--- | --- | --- | ---
mos_rms |  |  |
d_rms |  |  |
mono_Q |  |  |
FN |  |  |
FM |  |  |

## Links

- Source code for `Monochromator_pol.comp`.

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