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McStas: ESS_moderator_long

[ Identification | Description | Input parameters | Links ]

The ESS_moderator_long Component

A parametrised pulsed source for modelling ESS long pulses.


  • Site:
  • Author: KL, February 2001
  • Origin: Risoe
  • Date:


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

  1. 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)
  2. 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)
  3. IMPORTANT: The thermal flux corresponds to the 2001 thermal ESS moderator as no update has currently been released from the ESS neutronics group.
  4. 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.
  5. 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
  6. The component can use target_index for focusing to a given beam port. Use an Arm() and ROTATED to position relatively to the moderator.
  7. 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.
LminAALower edge of wavelength distribution
LmaxAAUpper edge of wavelength distribution
distmDistance from source to focusing rectangle; at (0,0,dist)0
focus_xwmWidth of focusing rectangle
focus_yhmHeight of focusing rectangle
nuHzFrequency of pulses14
TKTemperature of cold source50
tauslong time decay constant for cold pulse tail 1a287e-6
tau1slong time decay constant for cold pulse tail 1b0
tau2slong time decay constant for cold pulse, 220e-6
dspulse length2.857e-3
n1pulse shape parameter, 120
cold_frac1Fraction 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
n21pulse shape parameter, 25
chi21/AAlambda-distribution parameter in cold pulse 20.9
I0fluxintegrated cold flux, 1 (in flux units, see above)8.21e11
I2fluxCold flux, 2 (in flux units, see above)3.29e11
target_index1relative index of component to focus at, e.g. next is +1 this is used to compute 'dist' automatically.0
cyl_radiusmRadius of the cylindershaped cold source0.085
branch11limit for switching between two time structures in cold distribution 1 (only for coupled water, else = 1)1
branch21limit for switching between cold distribution 1 and 2. (default value 0.5)0.5
branch_tail1limit for switching between pulse and tail (suggested value: tau/d - default defined this way)0.14350
n_pulses1Number of pulses simulated. 0 and 1 creates one pulse. The integrated intensity is constant1
width_tmEdge of cube shaped thermal source0.12
T_tKTemperature of thermal source325
tau_tslong time decay constant for thermal pulse tail 1a80e-6
tau1_tslong time decay constant for thermal pulse tail 1b400e-6
tau2_tslong time decay constant for thermal pulse, 212e-6
chi2_t1/AAlambda-distribution parameter in thermal pulse 22.5
I0_tfluxintegrated thermal flux, 1 (in flux units, see above)13.5e11
I2_tfluxThermal flux, 2 (in flux units, see above)27.6e10
branch1_t1limit for switching between two time structures in thermal distribution 1 (only for coupled water, else = 1)0.5
branch2_t1limit for switching between thermal distribution 1 and 2. (default value 0.5)0.5
src_2012boolFlag to apply 2012 MCNPX-derived, wavelenght-dependent correction to intensity from the cold moderator.1
tfocus_distmPosition of time window0.1
tfocus_timesTime position of window0.0
tfocus_widthsTime width of window0.0
beamport_angledegDirection within the beamport sector (0 < angle < 60) to direct neutrons30


[ Identification | Description | Input parameters | Links ]

Generated on 2024-01-03 11:47:43

Last Modified: Tuesday, 16-Apr-2024 20:05:22 CEST
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