Module Disc Chopper

# VITESS module disc chopper

The module chopper simulates the chopping of the neutron beam by a chopper disk.

The orientation, y and z coordinates of the origin defined by the previous VITESS module are maintained, while a shift along the x-axis can be introduced according to the distance between the chopper and the device or window described by the previous VITESS module (x is set to 0 by the chopper module, i.e. a transmitted neutron is written to the output file at the chopper position).

The co-ordinate system, in which the orientation of the chopper disk is given, is a (fixed) system defined by the co-ordinate system of the beamline at the chopper position. The centre is shifted, orientation of y- and z-axes are the same. (Imagine a mark on the edge of the chopper disk pointing into the +z-direction; this defines the phase 0 deg). A positive phase is achieved by a counter-clockwise rotation (seen in the direction of the flying neutrons), cf. Fig. 3.

The following information has to be given as an input:

## Module parameters:

Parameter Unit | Description | range or values | Command option
chopper file | The chopper file describes the geometry and properties of the disk. |  | -C

rounds / min
[cm]

A positive value means that the rotation angle increases with time, and a negative value means that the chopper disk operates in the opposite direction of rotation
Positive rotation is counter-clockwise seen in the direction of the flying neutrons (as indicated in Fig. 3).

no restriction

-s

Offset [deg] | This parameter defines the initial rotation angle of the chopper, i.e.the orientation for time = 0. (The neutron time is initialised in the module source.) Negative values are reasonable for positive rotation (rounds/min > 0) and vice versa | no restriction | -o
distance to previous module [cm] | The distance along the x-axis between chopper and the reference point of the previous module. The influence of gravity can be included or omitted during travels of neutrons from X=0 to the chopper plane. | >= 0 | -l

absorption

ideal: a trajectory that hits the chopper disk (outside the windows) is removed.
Gd: a trajectory that hits the chopper disk (outside the windows) is reduced in intensity
according to a wavelength dependent transmission probability for Gd
coating of 2 x 0.1 mm assumed, cutoff for wavelength above 6 Ang.
Bor: same as Gd, except: transmission coefficients of Bor-10, coating of 2 x 0.25 mm assumed

ideal
Gd
Bor

-g

set zero time

Yes: The time of the trajectory is set to a value between -T/2 and +T/2 (with T being the period of the chopper), depending on the phase of the chopper at the moment of crossing.
t = Phase_red / RotFreq
A time close to zero is only achieved, if the window is open for phase = 0. But in any case the number of pulses is reduced to the number of windows.
This option is useful for time-of-flight instrument on constant wave sources.
No: nothing happens (default, only option until Vitess 2.3)

yes / no

-z

no of equiv. windows | The number of pulses may be further reduced by defining equivalent windows, e.g. 4 windows equally distributed over the chopper yield 2 pulses with 2 'equiv. windows' and 1 pulse with 4 'equiv. windows' | 1...4 | -n

treat neutrons passing by

Yes: If there are trajectories which do not hit the chopper disk area, they are written to the output file and considered further on. In this case, a warning is given including the number of those trajectories (default, only option until Vitess 2.4)
No: Trajectories arriving at the chopper plane outside the disk are removed.

yes / no

-p

set colour | Yes: The number of the window defines the 'colour' of the trajectory. (If the trajectory does not pass a window, the colour is set to zero.) No: nothing happens (default, only option until Vitess 2.4) | yes / no | -c

### Chopper file parameters:

Parameter Unit | Description | range or values
number of windows | Number of apertures of the chopper disk. | 1 - 3
radius [cm] | The radius of the chopper disk. | > 0.0

vertical position
[cm]

z-component of the position of the chopper axle relative to the origin, i.e. the center of the beam.
If the horizontal positon is zero, the chopper is positioned exactly below the beamline for vert. pos < 0 and exactly above the beamline for vert. pos > 0.
If it is below the beamline, the chopper window is open for: rotational angle = 'window position' (see below). If it is above the beamline, the chopper window is open for: rotational angle+180° = 'window position'.

no restriction

horizontal position [cm] | y-component of the position of the chopper axle relative to the origin, i.e. the center of the beam. If the vertical distance is zero, the chopper is positioned exactly left (hor. pos. > 0) or right (hor. pos. < 0) of the beamline (seen in the direction of the flying neutrons). In this case, the chopper window is open for: rotational angle - 90° = 'window position' (left) or rotational angle + 90° = 'window position' (right) resp.. | no restriction

#### For each window

Parameter Unit | Description | range or values
window position [deg] | Position of the window on the disk In case of 1 window, a straightforward choice is to put it to a position that it is open for t=0, i.e. 0 deg for a chopper below the beamline, 180 deg for a chopper above the beamline, 90 deg for a chopper right of the beamline and -90 deg for a chopper left of the beamline (cf. 'vertical position', 'horizontal position'). This value is also given as a note in the output of the run. | -360 - +360
window height [cm] | Height of the window. The window is always open to edge of the disk (see Figure 3). | 0.0 - radius
window width [deg] | Aperture of the window in degree (cf. Figure 3) | >0.0 - 360
left side deviation [deg] | Deviation for left edge of the window in degree. A positive value indicates that the window widens (see Fig. 3). | -90 - 90
right side deviation [deg] | Deviation for right edge of the window in degree. A positive value indicates that the window widens (see Fig. 3). | -90 - 90

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Last modified: Tue Jul 23 17:49:11 MEST 2002