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15.13 The ESS_moderator_short McStas Component
A parametrised pulsed source for modelling ESS short pulses.
Identification
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Author: KL, February 2001
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Origin: Risoe
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Date:
Description
Produces a time-of-flight spectrum, from the ESS parameters Chooses evenly in lambda, exponentially decaying in time . Adapted from Moderator by: KN, M.Hagen, August 1998
Units of flux: n/cm^2/s/Å/ster (McStas units are in general neutrons/second)
Example general parameters (general): size=0.12 Lmin=0.1 Lmax=10 dist=2 focus_xw=0.06 yh=0.12 nu=50 branchframe=0.5
Example moderator specific parameters (From F. Mezei, "ESS reference moderator characteristics for ...", 4/12/00: Defining the normalised Maxwelian M(lam,T) = 2 a^2 lam^-5 exp(-a/lam^2); a=949/T; lam in Å; T in K, and the normalised pulse shape function F(t,tau,n) = ( exp(-t/tau) - exp(-nt/tau) ) n/(n-1)/tau, the flux distribution is given as Phi(t,lam) = I0 M(lam,T) F(t,tau,n) + I2/(1+exp(chi2 lam-2.2))/lam*F(t,tau2*lam,n2) )
a1: Ambient H20, short pulse, decoupled poisoned
T=325 tau=22e-6 tau1=0 tau2=7e-6 n=5 n2=5 chi2=2.5 I0=9e10 I2=4.6e10 branch1=0 branch2=0.5
a2: Ambient H20, short pulse, decoupled un-poisoned
T=325 tau=35e-6 tau1=0 tau2=12e-6 n=5 n2=5 chi2=2.5 I0=1.8e11 I2=9.2e10 branch1=0 branch2=0.5
a3: Ambient H20, short pulse, coupled
T=325 tau=80e-6 tau1=400e-6 tau2=12e-6 n=20 n2=5 chi2=2.5 I0=4.5e11 I2=9.2e10 branch1=0.5 branch2=0.5
b1: Liquid H2, short pulse, decoupled poisoned
T=50 tau=49e-6 tau1=0 tau2=7e-6 n=5 n2=5 chi2=0.9 I0=2.7e10 I2=4.6e10 branch1=0 branch2=0.5
b2: Liquid H2, short pulse, decoupled un-poisoned
T=50 tau=78e-6 tau1=0 tau2=12e-6 n=5 n2=5 chi2=0.9 I0=5.4e10 I2=9.2e10 branch1=0 branch2=0.5
b3: Liquid H2, short pulse, coupled
T=50 tau=287e-6 tau1=0 tau2=12e-6 n=20 n2=5 chi2=0.9 I0=2.3e11 I2=9.2e10 branch1=0 branch2=0.5
Input parameters
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
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size |
m |
Edge of cube shaped source |
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Lmin |
Å |
Lower edge of wavelength distribution |
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Lmax |
Å |
Upper edge of wavelength distribution |
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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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nu |
Hz |
Frequency of pulses |
50 |
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T |
K |
Temperature of source |
325 |
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tau |
s |
long time decay constant for pulse, 1a |
22e-6 |
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tau1 |
s |
long time decay constant for pulse, 1b (only for coupled water, else 0) |
0 |
|
tau2 |
s/Å |
long time decay constant for pulse, 2 |
7e-6 |
|
n |
1 |
pulse shape parameter 1 |
5 |
|
n2 |
1 |
pulse shape parameter 2 |
5 |
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chi2 |
1/Å |
lambda-distribution parameter in pulse 2 |
2.5 |
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I0 |
flux |
integrated flux 1 (in flux units, see above) (default 9e10) |
9e10 |
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I2 |
flux*Å |
integrated flux 2 (default 4.6e10) |
4.6e10 |
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branch1 |
1 |
limit for switching between distribution 1 and 2. (has effect only for coupled water, tau1>0) |
0 |
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branch2 |
1 |
limit for switching between distribution 1 and 2. (default value 0.5) |
0.5 |
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branchframe |
1 |
limit for switching between 1st and 2nd pulse (if only one pulse wanted: 1) |
0 |
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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 ESS_moderator_short.comp.
