[ 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: 1.21
- 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`.

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