[ Identification | Description | Input parameters | Output parameters | Links ]

The ESS_moderator_short Component

A parametrised pulsed source for modelling ESS short pulses.

Identification

• Author: KL, February 2001
• Origin: Risoe
• Date:(Unknown)
• Version: 1.22
• Modification history:
  • KL, 18 November 2001

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/AA/ster  
(McStas units are in general neutrons/second)

Example general parameters (general):
         size=0.12 l_low=0.1 l_high=10 dist=2 xw=0.06 yh=0.12 freq=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 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.

Name	Unit	Description	Default
size	m	Edge of cube shaped source
l_low	AA	Lower edge of lambda distribution
l_high	AA	Upper edge of energy distribution
dist	m	Distance from source to focusing rectangle; at (0,0,dist)
xw	m	Width of focusing rectangle
yh	m	Height of focusing rectangle
freq	Hz	Frequency of pulses
T	K	Temperature of source
tau	s	long time decay constant for pulse, 1a
tau1	s	long time decay constant for pulse, 1b (only for coupled water, else 0)
tau2	s/AA	long time decay constant for pulse, 2
n	1	pulse shape parameter 1
n2	1	pulse shape parameter 2
chi2	1/AA	lambda-distribution parameter in pulse 2
I0	flux	integrated flux 1 (in flux units, see above) (default 9e10)
I2	flux*AA	integrated flux 2 (default 4.6e10)
branch1	1	limit for switching between distribution 1 and 2. (has effect only for coupled water, tau1>0)
branch2	1	limit for switching between distribution 1 and 2. (default value 0.5)
branchframe	1	limit for switching between 1st and 2nd pulse (if only one pulse wanted: 1)

Output parameters

Name	Unit	Description	Default
M
F
l_range
w_mult

Links

• Source code for ESS_moderator_short.comp.

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[ Identification | Description | Input parameters | Output parameters | Links ]

Generated automatically by McDoc, Peter Willendrup <peter.willendrup@risoe.dk> / Fri Feb 10 19:36:33 2012

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Last Modified: Friday, 10-Feb-2012 20:36:33 CET