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

The ESS_moderator_long Component

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

Output parameters

Links

• Source code for ESS_moderator_long.comp.

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