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3.5 The Moderator McStas Component
A simple pulsed source for time-of-flight.
Identification
-
Author: KN, M.Hagen
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Origin: Risoe
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Date: August 1998
Description
Produces a simple time-of-flight spectrum, with a flat energy distribution
Example: Moderator(radius = 0.0707, dist = 9.035, focus_xw = 0.021, focus_yh = 0.021, Emin = 10, Emax = 15, Ec = 9.0, t0 = 37.15, gamma = 39.1)
Input parameters
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
|
|
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|
radius |
m |
Radius of source |
0.07 |
|
Emin |
meV |
Lower edge of energy distribution |
|
|
Emax |
meV |
Upper edge of energy distribution |
|
|
dist |
m |
Distance from source to the focusing rectangle |
0 |
|
focus_xw |
m |
Width of focusing rectangle |
0.02 |
|
focus_yh |
m |
Height of focusing rectangle |
0.02 |
|
t0 |
mus |
decay constant for low-energy neutrons |
37.15 |
|
Ec |
meV |
Critical energy, below which the flux decay is constant |
9.0 |
|
gamma |
meV |
energy dependence of decay time |
39.1 |
|
target_index |
1 |
relative index of component to focus at, e.g. next is +1 this is used to compute ’dist’ automatically. |
1 |
|
flux |
1/(s cm 2 st meV) |
flux |
1 |
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Links
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Component source code found in file Moderator.comp.
A time-of-flight source (pulsed)
| Name: | Moderator |
| Author: | (System) Mark Hagen, SNS |
| Input parameters | \(r_s\), \(E_0\), \(E_1\), \(z_f\), \(w\), \(h\), \(\tau _0\), \(E_c\), \(\gamma \) |
| Optional parameters |
|
| Notes |
|
The simple time-of-flight source component Moderator resembles the source component Source_simple described in 3.1. Moderator is circular with radius \(r_s\) and focuses on a rectangular target of area \(w \times h\) in a distance \(z_f\). The initial velocity is chosen with a linear distribution within an interval, defined by the minimum and maximum energies, \(E_0\) and \(E_1\), respectively.
The initial time of the neutron is determined on basis of a simple heuristical model for the time dependence of the neutron intensity from a time-of-flight source. For all neutron energies, the flux decay is assumed to be exponential, \begin {equation} \Psi (E,t) = \exp (-t/\tau (E)) , \end {equation} where the decay constant is given by \begin {equation} \tau (E) = \left \{ \begin {array}{cc} \tau _0 & ; E<E_c \\ \tau _0 / [ 1 + (E-E_c)^2/\gamma ^2 ] & ; E \geq E_c \end {array} \right . \end {equation}
The decay parameters are \(\tau _0\) (in \(\mu \)s), \(E_c\), and \(\gamma \) (both in meV).
Other pulsed source models are available from contributed components. See section 3.10.
