Source with up to three Maxwellian distributions
Author: Kim Lefmann
Origin: Risoe
Date: March 2001
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
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
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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) |
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|
focus_xw |
m |
Width of focusing rectangle |
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|
focus_yh |
m |
Height of focusing rectangle |
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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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Component source code found in file Source_Maxwell_3.comp.
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.
Maxwellian parameters for some continuous sources are given in Table ??. As nobody knows exactly the characteristics of the sources (it is not easy to measure spectrum there), these figures should be used with caution.