This component simulates a Fermi chopper with absorbing walls. The rotation axis is
vertical (yaxis), i.e. the path length through the channels is given by the length
'depth' along the zaxis.
The shape of the channels can be straight, curved with circular, or curved with ideal
(i.e. close to a parabolic) shape. This is determined by the parameter 'GeomOption'.
Geometry for straight and circular channels:
The geometry of the chopper consists of a rectangular shaped object with a channel
system. In transmission position, there are 'Nchannels' slits along the xaxis,
separated by absorbing walls of thickness 'wallwidth' giving a total width 'width'.
The rectangular channel system is surrounded by a socalled shadowing cylinder
(cf component manual).
Geometry for parabolic channels:
In this case, the Fermi chopper is supposed to be a full cylinder, i.e. the central
channels are longer than those on the edges (cf. figure in the component manual).
The other features are the same as for the other options.
Apart from the frequency of rotation, the phase of the chopper at t=0 has to be given;
phase = 0 means transmission orientation.
The option 'zerotime' may be used to reset the time at the chopper position. The
consequence is that only 1 pulse is generated instead of several.
NOTE: This component must NOT be located at the same position as the previous one.
This also stands for monitors and Arms. A non zero distance must be defined.
Examples:
straight Fermi chopper, 18000 rpm, 20 channels a 0.9 mm separated by 0.1 mm walls,
16 mm channel length, minimal shadowing cylinder, phased to be open at 1 ms,
generation of only 1 pulse, normal precision (for short wavelength neutrons)
Vitess_ChopperFermi(GeomOption=0, zerotime=1, Nchannels=20, Ngates=4,
freq=300.0, height=0.06, width=0.0201,
depth=0.016, r_curv=0.0, diameter=0.025691, Phase=108.0,
wallwidth=0.0001, sGeomFileName="FC_geom_str.dat")
Fermi chopper with circular channels, 12000 rpm, optimized for 6 Ang, several pulses,
highest accuracy (because of long wavelength neutrons used), rest as above
Vitess_ChopperFermi(GeomOption=2, zerotime=0, Nchannels=20, Ngates=8,
freq=200.0, height=0.06, width=0.0201,
depth=0.016, r_curv=0.2623, diameter=0.025691, Phase=72.0,
wallwidth=0.0001, sGeomFileName="FC_geom_circ.dat")
%VALIDATION
Apr 2005: extensive external test, most problems solved (cf. 'Bugs' and source header)
Validated by: K. Lieutenant
limitations: slow (10 times slower than FermiChopper), especially for a high number of channels
%BUGS
reduction of transmission by a large shadowing cylinder underestimated
Input parameters
Parameters in boldface are required;
the others are optional.
Name
Unit
Description
Default
sGeomFileName
str
name of output file for geometry information
0
GeomOption
1
option: 0:straight 1:parabolic 2:circular
0
zerotime
1
option: 1:'set time to zero' 0: 'do not'
0
Nchannels
1
number of channels of the Fermi chopper
20
Ngates
1
number of gates defining the channel: 4=default, 6 or 8 for long wavelengths
4
freq
Hz
number of rotations per second
300.0
height
m
height of the Fermi chopper
0.05
width
m
total width of the Fermi chopper
0.04
depth
m
channel length of the Fermi chopper
0.03
r_curv
m
radius of curvature of the curved Fermi chopper
0.5
diameter
m
diameter of the shadowing cylinder
0.071
Phase
deg
dephasing angle at zero time
0.0
wallwidth
m
thickness of walls separating the channels
0.0002
Output parameters
Name
Unit
Description
Default
Option

1: straight FC, 2: curved FC
CurvGeomOption

1: ideal shape (nearly parabolic) 2: circular
TOF
ms
TOF of neutron under consideration
WL
Ang
wavelength of neutron
radius_of_curv
curved FC
[cm] radius of curvature
main_depth
cm
max. channel length due to diameter and total_width
shift_y
cm
shift to channel actually written to geometry file
angle_channel
rad
half of the curvature of a curved Fermi chopper
phase0
rad
chopper phase at TOF of neutron to chopper centre
y_ch
cm
position of gates perpendicular to flight direction