McStas: ESS_moderator_long Component

[ 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: 1.25
- Modification history:
  - E Klinkby, October 2012 - updated geometry, bispectral

## 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

    2012-updates:

    Geometry is now MCNPX-like IMPORTANT: Origin of the component is inside the cylindrical
    moderator, i.e. take care when positioning the next components! (E Klinkby)
    Component implements both the cold moderator and the thermal (pre-)moderator, fraction of statistics for
    the cold moderator is the new cold_frac parameter. New set of input parameters with subscript _t defines the
    thermal flux.(E Klinkby)
    IMPORTANT: The thermal flux corresponds to the 2001 thermal ESS moderator as no update has currently
    been released from the ESS neutronics group.
    By default the component applies a wavelength-dependent correction term to the cold flux, derived from
    2012 MCNPX calculations by ESS neutronics group. Corrections calculated by K Lieutenant (Vitess) and
    implemented here by E Klinkby. In case this is not wanted, the src_2012 parameter can be set to 0.
    Default cold moderator intensity parameters correspond to the "ESS 2012" parameter set. The original
    2001 ESS "Mezei moderator" can be described by setting T=50, tau=287e-6, tau1=0, tau2=20e-6, chi2=0.9, I0=6.9e11,
    I2=27.6e10, branch1=0, branch2=0.5, src_2012=0
    The component can use target_index for focusing to a given beam port. Use an Arm() and ROTATED to position
    relatively to the moderator.
    Time focusing option: Adjusts neutron departure time to match a 'first chopper' defined by parameters tfocus_dist, tfocus_time, tfocus_width (K Lefmann).

    Units of flux: n/cm^2/s/AA/ster
    (McStas units are in general neutrons/second)

    Example general parameters (general):
             size=0.12 Lmin=0.1 Lmax=10 dist=1.6 focus_xw=0.19 focus_yh=0.15 nu=16.67

    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 ESS 2001 thermal
             T_t=325 tau_t=80e-6 tau1_t=400e-6 tau2_t=12e-6 n=20 n2=5 d=2e-3 chi2_t=2.5
             I0_t=13.5e11 I2_t=27.6e10    branch1_t=0.5 branch2_t=0.5

      c2: Liquid H2, long pulse, coupled ESS 2012 cold
             T=50 tau=287e-6 tau1=0 tau2=20e-6 n=20 n2=5 d=2e-3 chi2=0.9
             I0=8.21e11, I2=3.29e11    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
--- | --- | --- | ---
width_c |  |  | 0
yheight |  |  | 0.12
Lmin | AA | Lower edge of wavelength distribution |
Lmax | AA | Upper edge of wavelength distribution |
dist | m | Distance from source to focusing rectangle; at (0,0,dist) | 0
focus_xw | m | Width of focusing rectangle |
focus_yh | m | Height of focusing rectangle |
nu | Hz | Frequency of pulses | 14
T | K | Temperature of cold source | 50
tau | s | long time decay constant for cold pulse tail 1a | 287e-6
tau1 | s | long time decay constant for cold pulse tail 1b | 0
tau2 | s | long time decay constant for cold pulse, 2 | 20e-6
d | s | pulse length | 2.857e-3
n | 1 | pulse shape parameter, 1 | 20
cold_frac | 1 | Fraction of neutron statistics from cold source. It is implicitely assumed that supermirror allows each beamline to choose the desired fraction of cold and thermal neutrons (i.e. extreme idealization). | 1.0
n2 | 1 | pulse shape parameter, 2 | 5
chi2 | 1/AA | lambda-distribution parameter in cold pulse 2 | 0.9
I0 | flux | integrated cold flux, 1 (in flux units, see above) | 8.21e11
I2 | flux | Cold flux, 2 (in flux units, see above) | 3.29e11
target_index | 1 | relative index of component to focus at, e.g. next is +1 this is used to compute 'dist' automatically. | 0
cyl_radius | m | Radius of the cylindershaped cold source | 0.085
branch1 | 1 | limit for switching between two time structures in cold distribution 1 (only for coupled water, else = 1) | 1
branch2 | 1 | limit for switching between cold 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
n_pulses | 1 | Number of pulses simulated. 0 and 1 creates one pulse. The integrated intensity is constant | 1
width_t | m | Edge of cube shaped thermal source | 0.12
T_t | K | Temperature of thermal source | 325
tau_t | s | long time decay constant for thermal pulse tail 1a | 80e-6
tau1_t | s | long time decay constant for thermal pulse tail 1b | 400e-6
tau2_t | s | long time decay constant for thermal pulse, 2 | 12e-6
chi2_t | 1/AA | lambda-distribution parameter in thermal pulse 2 | 2.5
I0_t | flux | integrated thermal flux, 1 (in flux units, see above) | 13.5e11
I2_t | flux | Thermal flux, 2 (in flux units, see above) | 27.6e10
branch1_t | 1 | limit for switching between two time structures in thermal distribution 1 (only for coupled water, else = 1) | 0.5
branch2_t | 1 | limit for switching between thermal distribution 1 and 2. (default value 0.5) | 0.5
src_2012 | bool | Flag to apply 2012 MCNPX-derived, wavelenght-dependent correction to intensity from the cold moderator. | 1
tfocus_dist | m | Position of time window | 0.1
tfocus_time | s | Time position of window | 0.0
tfocus_width | s | Time width of window | 0.0
beamport_angle | deg | Direction within the beamport sector (0 < angle < 60) to direct neutrons | 30

## Output parameters

Name | Unit | Description | Default
--- | --- | --- | ---
M |  |  |
F |  |  |
l_range |  |  |
w_mult |  |  |
w_geom |  |  |
w_geom |  |  |
w_geom_t |  |  |

## Links

- Source code for `ESS_moderator_long.comp`.

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