McStas logo McStas - A neutron ray-trace simulation package ILL; PSI; ESS Niels Bohr Institute DTU Physics NEXMAP


About McStas
 Conditions of use
 Project funding

 Other Downloads (share)

Mailing list

Search web/mailinglist

 Wiki (GitHub)
 McStas manual


Report bugs (GitHub)

Code-repository (GitHub)

McStas: ESS_moderator_short

[ Identification | Description | Input parameters | Links ]

The ESS_moderator_short Component

A parametrised pulsed source for modelling ESS short pulses.


  • Site:
  • Author: KL, February 2001
  • Origin: Risoe
  • Date:


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/AA/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

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 AA; 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.
sizemEdge of cube shaped source
LminAALower edge of wavelength distribution
LmaxAAUpper edge of wavelength distribution
distmDistance from source to focusing rectangle; at (0,0,dist)
focus_xwmWidth of focusing rectangle
focus_yhmHeight of focusing rectangle
nuHzFrequency of pulses50
TKTemperature of source325
tauslong time decay constant for pulse, 1a22e-6
tau1slong time decay constant for pulse, 1b (only for coupled water, else 0)0
tau2s/AAlong time decay constant for pulse, 27e-6
n1pulse shape parameter 15
n21pulse shape parameter 25
chi21/AAlambda-distribution parameter in pulse 22.5
I0fluxintegrated flux 1 (in flux units, see above) (default 9e10)9e10
I2flux*AAintegrated flux 2 (default 4.6e10)4.6e10
branch11limit for switching between distribution 1 and 2. (has effect only for coupled water, tau1>0)0
branch21limit for switching between distribution 1 and 2. (default value 0.5)0.5
branchframe1limit for switching between 1st and 2nd pulse (if only one pulse wanted: 1)0
target_index1relative index of component to focus at, e.g. next is +1 this is used to compute 'dist' automatically.+1


[ Identification | Description | Input parameters | Links ]

Generated on 2023-09-19 20:01:36

Last Modified: Wednesday, 03-Jan-2024 19:59:10 CET
Search website mailinglist archive GitHub repos