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3.2 The Source_div McStas Component
Neutron source with Gaussian or uniform divergence
Identification
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Author: KL
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Origin: Risoe
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Date: November 20, 1998
Description
The routine is a rectangular neutron source, which has a gaussian or uniform divergent output in the forward direction. The neutron energy is distributed between lambda0-dlambda and lambda0+dlambda or between E0-dE and E0+dE. The flux unit is specified in n/cm2/s/st/energy unit (meV or Å). In the case of uniform distribution (gauss=0), angles are uniformly distributed between -focus_aw and +focus_aw as well as -focus_ah and +focus_ah. For Gaussian distribution (gauss=1), ’focus_aw’ and ’focus_ah’ define the FWHM of a Gaussian distribution. Energy/wavelength distribution is also Gaussian.
Example: Source_div(xwidth=0.1, yheight=0.1, focus_aw=2, focus_ah=2, E0=14, dE=2, gauss=0)
%VALIDATION
Feb 2005: tested by Kim Lefmann (o.k.)
Apr 2005: energy distribution used in external tests of Fermi choppers (o.k.) Jun 2005: wavelength distribution used in external tests of velocity selectors (o.k.) Validated by: K. Lieutenant
%BUGS distribution is uniform in (hor. and vert.) angle (relative to moderator normal), therefore not suited for large angles
Input parameters
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
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xwidth |
m |
Width of source |
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|
yheight |
m |
Height of source |
|
|
focus_aw |
deg |
FWHM (Gaussian) or maximal (uniform) horz. width divergence |
|
|
focus_ah |
deg |
FWHM (Gaussian) or maximal (uniform) vert. height divergence |
|
|
E0 |
meV |
Mean energy of neutrons. |
0.0 |
|
dE |
meV |
Energy half spread of neutrons. |
0.0 |
|
lambda0 |
Å |
Mean wavelength of neutrons (only relevant for E0=0) |
0.0 |
|
dlambda |
Å |
Wavelength half spread of neutrons. |
0.0 |
|
gauss |
0|1 |
Criterion: 0: uniform, 1: Gaussian distributions |
0 |
|
flux |
1/(s cm 2 st energy_unit) |
flux per energy unit, Å or meV |
1 |
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Links
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Component source code found in file Source_div.comp.
A continuous source with specified divergence
Source_div is a rectangular source, \(w \times h\), which emits a beam of a specified divergence around the direction of the \(z\) axis. The beam intensity is uniform over the whole of the source, and the energy (or wavelength) distribution of the beam is uniform over the specified energy range \(E_0 \pm \Delta E\) (in meV), or alternatively the wavelength range \(\lambda _0 \pm \delta \lambda \) (in Å).
The source divergences are \(\delta _h\) and \(\delta _v\) (FWHM in degrees). If the gauss flag is set to 0 (default), the divergence distribution is uniform, otherwise it is Gaussian.
This component may be used as a simple model of the beam profile at the end of a guide or at the sample position.
