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

The ESS_moderator_long Component

A parametrised pulsed source for modelling ESS long pulses.

Identification

Description

Produces a time-of-flight spectrum, from the ESS parameters
Chooses evenly in lambda, evenly/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=1.6 xw=0.19 yh=0.15 freq=16.67
         branchframe=0.5

Example moderator specific parameters
(From F. Mezei, "ESS reference moderator characteristics for ...", 4/12/00:
 Defining the normalised Maxwellian
    M(lam,T) = 2 a^2 lam^-5 exp(-a/lam^2); a=949/T; lam in AA; T in K,
  the "pulse integral" function
    iexp(t,tau,d) = 0                              ; t<0
                    tau (1-exp(-t/tau))            ; 0d ,
  and the long pulse shape function
    I(t,tau,n,d) = (iexp(t,tau,d)-iexp(t,tau/n,d)) n/(n-1)/tau/d ,

  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)  )

  c1: Ambient H20, long pulse, coupled
         T=325 tau=80e-6 tau1=400e-6 tau2=12e-6 n=20 n2=5 d=2e-3 chi2=2.5
         I0=13.5e11 I2=27.6e10    branch1=0.5 branch2=0.5

  c2: Liquid H2, long pulse, coupled
         T=50 tau=287e-6 tau1=0 tau2=20e-6 n=20 n2=5 d=2e-3 chi2=0.9
         I0=6.9e11 I2=27.6e10    branch1=0 branch2=0.5

Debugged intensively against Mezei note (4/12 2000) and VitESS @ Rencurel 2006.
The output is now neutrons / second, not as previously neutrons / pulse.

%VALIDATION 
Validated against VitESS and Mezei note (4/12 2000) @ Rencurel 2006

Input parameters

Parameters in boldface are required; the others are optional.
Name Unit Description Default
size m Edge of cube shaped source 0.12
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 50.0/3.0
T K Temperature of source 50
tau s long time decay constant for pulse tail 1a 287e-6
tau1 s long time decay constant for pulse tail 1b 0
tau2 s long time decay constant for pulse, 2 20e-6
d s pulse length 2e-3
n 1 pulse shape parameter, 1 20
n2 1 pulse shape parameter, 2 5
chi2 1/AA lambda-distribution parameter in pulse 2 0.9
I0 flux integrated flux, 1 (in flux units, see above) 6.9e11
I2 flux Flux, 2 (in flux units, see above) 27.6e10
branch1 1 limit for switching between two time structures in distribution 1 (only for coupled water, else = 1) 1
branch2 1 limit for switching between distribution 1 and 2. (default value 0.5) 0.5
branch_tail 1 limit for switching between pulse and tail (suggested value: tau/d - default defined this way) 0.14350
twopulses 1 Flag for selecting one or two pulses. 0 is one pulse. 0

Output parameters

Name Unit Description Default
M  
F  
l_range  
w_mult  

Links

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

Generated automatically by McDoc, Peter Willendrup <peter.willendrup@risoe.dk> / Fri May 11 08:41:08 2012