The `ESS_moderator_long` Component

A parametrised pulsed source for modelling ESS long pulses.

Identification

Author: KL, February 2001
Origin: Risoe
Date:(Unknown)
Version: 1.23
Modification history:
- KL, 18 November 2001

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

The ESS_moderator_long Component

Identification

Description

Input parameters

Output parameters

Links

The `ESS_moderator_long` Component