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McStas: FermiChopper_ILL Component

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

The FermiChopper_ILL Component

Fermi Chopper with rotating frame.


  • Author: M. Poehlmann, C. Carbogno, H. Schober, E. Farhi
  • Origin: ILL Grenoble / TU Muenchen
  • Date: May 2002
  • Version:(Unknown)
  • Modification history:
    • K.Lieutenant, June 2005: added phase parameter. Comp validation.
    • E. Farhi, Jul 2008: uniformize parameter names ()
    • EF, Oct 2008: fix chopper orientation
    • EF, Mar 2009: fixed infinite recursion which may cause SEGV. Cleanup of code.


Models a Fermi chopper with optional supermirror coated blades
supermirror facilities may be disabled by setting m = 0, R0=0
Slit packages are straight. Chopper slits are separated by an infinitely
thin absorbing material. The effective transmission (resulting from fraction
of the transparent material and its transmission) may be specified.
The chopper slit package width may be specified through the total width 'xwidth'
of the full package or the width 'w' of each single slit. The other parameter
is calculated by: xwidth = nslit*w.

FermiChopper_ILL(phase=-50.0, radius=0.04, nu=100,
  yheight=0.08, w=0.00022475, nslit=200.0, R0=0.0,
  Qc=0.02176, alpha=2.33, m=0.0, length=0.012, eff=0.95,

Markus Poehlmann     
Christian Carbogno   
and Helmut Schober   

Apr 2005: extensive external test, most problems solved (cf. 'Bugs')
Validated by: K. Lieutenant

no blade width used

- overestimates peak width for long wavelengths
- does not give the right pulse position, shape and width for slit widths below 0.1 mm
- fails sometimes when using MPI

WARNING: This is a contributed Component.

Input parameters

Parameters in boldface are required; the others are optional.
Name Unit Description Default
phase deg chopper phase at t=0 0
radius m chopper cylinder radius 0.04
nu Hz chopper frequency 100
yheight m Height of chopper 0.08
w m width of one chopper slit 0.00022475
nslit 1 number of chopper slits 200.0
R0 1 low-angle reflectivity 0.0
Qc AA-1 critical scattering vector 0.02176
alpha AA slope of reflectivity 2.33
m 1 m-value of material. Zero means completely absorbing. 0.0
W AA-1 width of supermirror cut-off 2e-3
length m channel length of the Fermi chopper 0.012
eff 1 efficiency = transmission x fraction of transparent material 0.95
zero_time 1 set time to zero: 0: no 1: once per half cycle 0
xwidth m optional total width of slit package 0
verbose 1 optional flag to display component statistics, use 1 or 3 (debuging) 0

Output parameters

Name Unit Description Default
FCVars - structure containing many neutron counters for the data in 'verbose', mean velocities, angular frequency of the chopper etc.  


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

Generated automatically by McDoc, Peter Willendrup <> / Mon Jun 26 12:27:17 2017

Last Modified: Monday, 26-Jun-2017 14:27:17 CEST
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