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

# The `ESS_moderator_long` Component

A parametrised pulsed source for modelling ESS long pulses.

## Identification

- Author: KL, February 2001
- Origin: Risoe
- Date:(Unknown)
- Version: 1.19
- 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=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 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))            ; 0<t<d
                        tau (exp(d/tau)-1) exp(-t/tau) ; t>d ,
      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

## 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 tail 1a |
tau1 | s | long time decay constant for pulse tail 1b |
tau2 | s | long time decay constant for pulse, 2 |
d | s | pulse length |
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) |
I2 | flux | Flux, 2 (in flux units, see above) |
branch1 | 1 | limit for switching between two time structures in distribution 1 (only for coupled water, else = 1) |
branch2 | 1 | limit for switching between distribution 1 and 2. (default value 0.5) |
branch_tail | 1 | limit for switching between pulse and tail (suggested value: tau/d) |
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_long.comp`.

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