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McStas: Guide_gravity_psd Component

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

The Guide_gravity_psd Component

Neutron straight guide with gravity. Can be channeled and focusing. Waviness may be specified, as well as side chamfers (on substrate).


  • Author: Emmanuel Farhi
  • Origin: ILL (France).
  • Date: Aug 03 2001
  • Version:(Unknown)
  • Modification history:
    • E. Farhi, from Gravity_guide by K. Lefmann (buggy).
    • E. Farhi, focusing channels are now ok (Sept 4th, 2001).
    • E. Farhi, 2D channel array. Correct focus bug (Dec 14th 2002)
    • E. Farhi, Waviness+chamfers+nelements (Aug 19th 2003)
    • P. Link, included side wall psd's for neutron loss registration (Oct 9th 2015)


Models a rectangular straight guide tube centered on the Z axis, with
gravitation handling. The entrance lies in the X-Y plane.
The guide can be channeled (nslit,d,nhslit parameters). The guide coating
specifications may be entered via different ways (global, or for
each wall m-value).

Waviness (random) may be specified either globally or for each mirror type.
Side chamfers (due to substrate processing) may be specified the same way.
In order to model a realistic straight guide assembly, a long guide of
length 'l' may be splitted into 'nelements' between which chamfers/gaps are

The reflectivity may be specified either using an analytical mode (see
Component manual) or as a text file in free format, with 1st
column as q[Angs-1] and 2nd column as the reflectivity R in [0-1].
For details on the geometry calculation see the description in the McStas
component manual.

There is a special rotating mode in order to approximate a Fermi Chopper
behaviour, in the case the neutron trajectory is nearly linear inside the
chopper slits, i.e. the neutrons are fast w/r/ to the chopper speed.
Slits are straight, but may be super-mirror coated. In this case, the
component is NOT centered, but located at its entry window. It should then
be shifted by -l/2.

Example: Guide_gravity_psd(w1=0.1, h1=0.1, w2=0.1, h2=0.1, l=12,
          R0=0.99, Qc=0.0219, alpha=6.07, m=1.0, W=0.003)

May 2005: extensive internal test, all problems solved
Validated by: K. Lieutenant

Example values: m=4 Qc=0.02 W=1/300 alpha=6.49 R0=1

WARNING: This is a contributed Component.

Input parameters

Parameters in boldface are required; the others are optional.
Name Unit Description Default
w1 m Width at the guide entry  
h1 m Height at the guide entry  
w2 m Width at the guide exit. If 0, use w1. 0
h2 m Height at the guide exit. If 0, use h1. 0
l m length of guide  
R0 1 Low-angle reflectivity 0.995
Qc AA-1 Critical scattering vector 0.0218
alpha AA Slope of reflectivity 4.38
m 1 m-value of material. Zero means completely absorbing. m=0.65 glass/SiO2 Si Ni Ni58 supermirror Be Diamond m= 0.65 0.47 1 1.18 2-6 1.01 1.12 for glass/SiO2, m=1 for Ni, 1.2 for Ni58, m=2-6 for supermirror. m=0.47 for Si 1.0
W AA-1 Width of supermirror cut-off 0.003
nslit 1 Number of vertical channels in the guide (>= 1) (nslit-1 vertical dividing walls). 1
d m Thickness of subdividing walls 0.0005
mleft 1 m-value of material for left. vert. mirror -1
mright 1 m-value of material for right. vert. mirror -1
mtop 1 m-value of material for top. horz. mirror -1
mbottom 1 m-value of material for bottom. horz. mirror -1
nhslit 1 Number of horizontal channels in the guide (>= 1). (nhslit-1 horizontal dividing walls). this enables to have nslit*nhslit rectangular channels 1
G m/s2 Gravitation norm. 0 value disables G effects. 0
aleft 1 alpha-value of left vert. mirror -1
aright 1 alpha-value of right vert. mirror -1
atop 1 alpha-value of top horz. mirror -1
abottom 1 alpha-value of left horz. mirror -1
wavy deg Global guide waviness 0
wavy_z deg Partial waviness along propagation axis 0
wavy_tb deg Partial waviness in transverse direction for top/bottom mirrors 0
wavy_lr deg Partial waviness in transverse direction for left/right mirrors 0
chamfers m Global chamfers specifications (in/out/mirror sides). 0
chamfers_z m Input and output chamfers 0
chamfers_lr m Chamfers on left/right mirror sides 0
chamfers_tb m Chamfers on top/bottom mirror sides 0
nelements 1 Number of sections in the guide (length l/nelements). 1
nu Hz Rotation frequency (round/s) for Fermi Chopper approximation 0
phase deg Phase shift for the Fermi Chopper approximation 0
reflect 0-1 [str] Reflectivity file name. Format q(Angs-1) R "NULL"


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

Generated automatically by McDoc, Peter Willendrup <> / Wed Jan 12 15:13:20 2022

Last Modified: Wednesday, 12-Jan-2022 16:13:20 CET
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