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3.1 The Source_simple McStas Component
A circular neutron source with flat energy spectrum and arbitrary flux
Identification
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Author: Kim Lefmann
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Origin: Risoe
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Date: October 30, 1997
Description
The routine is a circular neutron source, which aims at a square target centered at the beam (in order to improve MC-acceptance rate). The angular divergence is then given by the dimensions of the target. The neutron energy is uniformly 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).
This component replaces Source_flat, Source_flat_lambda, Source_flux and Source_flux_lambda.
Example: Source_simple(radius=0.1, dist=2, focus_xw=.1, focus_yh=.1, E0=14, dE=2)
Input parameters
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
|
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|
radius |
m |
Radius of circle in (x,y,0) plane where neutrons are generated. |
0.1 |
|
yheight |
m |
Height of rectangle in (x,y,0) plane where neutrons are generated. |
0 |
|
xwidth |
m |
Width of rectangle in (x,y,0) plane where neutrons are generated. |
0 |
|
dist |
m |
Distance to target along z axis. |
0 |
|
focus_xw |
m |
Width of target |
.045 |
|
focus_yh |
m |
Height of target |
.12 |
|
E0 |
meV |
Mean energy of neutrons. |
0 |
|
dE |
meV |
Energy half spread of neutrons (flat or gaussian sigma). |
0 |
|
lambda0 |
Å |
Mean wavelength of neutrons. |
0 |
|
dlambda |
Å |
Wavelength half spread of neutrons. |
0 |
|
flux |
1/(s*cm**2*st*energy unit) |
flux per energy unit, Å or meV if flux=0, the source emits 1 in 4*PI whole space. |
1 |
|
gauss |
1 |
Gaussian (1) or Flat (0) energy/wavelength distribution |
0 |
|
target_index |
1 |
relative index of component to focus at, e.g. next is +1 this is used to compute ’dist’ automatically. |
1 |
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Links
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Component source code found in file Source_simple.comp.
A simple continuous source with a flat energy/wavelength spectrum
This component is a simple source with an energy distribution which is uniform in the range \(E_0 \pm dE\) (alternatively: a wavelength distribution in the range \(\lambda _0 \pm d\lambda \)). This component is not used for detailed time-of-flight simulations, so we put \(t=0\) for all neutron rays.
The initial neutron ray position is chosen randomly from within a circle of radius \(r_\textrm {s}\) in the \(z=0\) plane. This geometry is a fair approximation of a cylindrical cold/thermal source with the beam going out along the cylinder axis.
The initial neutron ray direction is focused onto a rectangular target of width \(w\), height \(h\), parallel to the \(xy\) plane placed at \((0,0,z_\textrm {foc})\).
The initial weight of the created neutron ray, \(p_0\), is set to the energy-integrated flux, \(\Psi \), times the source area, \(\pi r_\textrm {s}^2\) times a solid-angle factor, which is basically the solid angle of the focusing rectangle. See also the section 3.0.1 on source flux.
This component replaces the obsolete components Source_flux_lambda, Source_flat, Source_flat_lambda, and Source_flux.
