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3.3 The Source_Maxwell_3 McStas Component
Source with up to three Maxwellian distributions
Identification
-
Author: Kim Lefmann
-
Origin: Risoe
-
Date: March 2001
Description
A parametrised continuous source for modelling a (cubic) source with (up to) 3 Maxwellian distributions. The source produces a continuous spectrum. The sampling of the neutrons is uniform in wavelength.
Units of flux: neutrons/cm^2/second/ster (McStas units are in general neutrons/second)
Example: PSI cold source T1=150.42 K / 2.51 AA I1 = 3.67 E11 T2=38.74 K / 4.95 AA I2 = 3.64 E11 T3=14.84 K / 9.5 AA I3 = 0.95 E11
Input parameters
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
|
|
|||
|
size |
m |
Edge of cube shaped source (for backward compatibility) |
0 |
|
yheight |
m |
Height of rectangular source |
0 |
|
xwidth |
m |
Width of rectangular source |
0 |
|
Lmin |
Å |
Lower edge of lambda distribution |
|
|
Lmax |
Å |
Upper edge of lambda distribution |
|
|
dist |
m |
Distance from source to focusing rectangle; at (0,0,dist) |
|
|
focus_xw |
m |
Width of focusing rectangle |
|
|
focus_yh |
m |
Height of focusing rectangle |
|
|
T1 |
K |
1st temperature of thermal distribution |
|
|
T2 |
K |
2nd temperature of thermal distribution |
300 |
|
T3 |
K |
3nd temperature of - - - |
300 |
|
I1 |
1/(cm**2*st) |
flux, 1 (in flux units, see above) |
|
|
I2 |
1/(cm**2*st) |
flux, 2 (in flux units, see above) |
0 |
|
I3 |
1/(cm**2*st) |
flux, 3 - - - |
0 |
|
target_index |
1 |
relative index of component to focus at, e.g. next is +1 this is used to compute ’dist’ automatically. |
1 |
|
lambda0 |
Å |
Mean wavelength of neutrons. |
0 |
|
dlambda |
Å |
Wavelength spread of neutrons. |
0 |
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Links
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Component source code found in file Source_Maxwell_3.comp.
A continuous source with a Maxwellian spectrum
This component is a source with a Maxwellian energy/wavelength distribution sampled in the range \(\lambda _\textrm {low}\) to \(\lambda _\textrm {high}\). The initial neutron ray position is chosen randomly from within a rectangle of area \(h \times w\) in the \(z=0\) plane. The initial neutron ray direction is focused within a solid angle, defined by a rectangular target of width \(xw\), height \(yh\), parallel to the \(xy\) plane placed at \((0,0,d_\textrm {foc})\). The energy distribution used is a sum of 1, 2, or 3 Maxwellians with temperatures \(T_1\) to \(T_3\) and integrated intensities \(I_1\) to \(I_3\).
For one single Maxwellian, the intensity in a small wavelength interval \([\lambda , \lambda +d\lambda ]\) is \( I_1 M(\lambda ,T_1) d\lambda \) where \(M(\lambda ,T_1) = 2 \alpha ^2 \exp (-\alpha /\lambda ^2) / \lambda ^5 \) is the normalized Maxwell distribution (\(\alpha =949.0\) K Å\(^2/T_1\)). The initial weight of the created neutron ray, \(p_0\), is calculated according to Eq. (3.2), with \(\Psi (\lambda )\) replaced by \(\sum _{j=1}^3 I_j M(\lambda ,T_j)\).
The component Source_gen (see section 3.4) works on the same principle, but provides more options concerning wavelength/energy range specifications, shape, etc.
Some example transmission data files usable alongside sources like this one are given in Table 4.5.
