Module Collimator

# VITESS Modules Space, Slit and Spacewindow

There are 3 modules that can be used for propagation of the neutrons to a certain area. They all take into account gravity (if this option is chosen). Space propagates the neutrons over a certain distance and takes into account attenuation (in air). Slit propagates the neutrons over a certain distance to a rectangular window centered around the beam axis. Spacewindow propagates the neutrons over a certain distance to a rectangular or circular window whose center might be out of the center. It can consider attenuation inside the window and non-perfect absorption outside the window. Their x-coordinates are reset to 0 (as in most others modules) after finishing the propagation treatment.

### VITESS Module Space

The module space simply simulates the propagation over a certain distance. But an attenuation inside the medium can be considered by giving the macrosopic absorption and total scattering cross-section. The macroscopic cross-section can be calculated from the microscopic cross-section by multipyling with the particle density.

### VITESS Module Slit

The module slit simulates the propagation to a rectangular window in a certain distance, which is centered around a beam axis and has its normal vector along the x-axis. Only those neutrons hitting the window are considered further. This common case can be easily handled using this simple module. In all othe cases the module spacewindow has to be used.

### VITESS Module Spacewindow

The module spacewindow simulates free propagation of neutrons from (x=0) to a rectangular or circular window. This window may have a thickness, an inner and/or an outer material. In case of ideal transmission inside and ideal absorption outside, only those hitting the window are considered further on. Otherwise, attenuation is calculated for neutrons passing inside ("inner material") or outside ("outer" material.)
There is also the possibility to simulate a beamstop by defining a negative window, where neutrons pass outside and are absorbed inside. But it is a better choice to use the module 'beamstop' for that. To simulate beamstops and windows between sample and detector, the option 'use previous frame' has been included. It prevents the usual shift of the origin to the center of the window, thus leaving the origin in the center of the sample, which is necessary for a proper working of the detector module.
Two kinds of material can be chosen. Outer material is means the "absorption" material for the collimator. Inner material is meaning the "transmission" material for the collimator. The thickness for the each material must be given. Setting the thickness of the material to zero means ideal absorption for the outer and ideal transmission for the inner material. For the inner material a data can be read from the file, which has the structure, given below. If the file is given, the exp. transmission is activated automatically, otherwise the ideal transmission is used.

    The file format describing the transmission is:
    Wavelength [Å]           attenuation µ [1/cm]
    Note: the wavelength range must be from shortest to longest wavelength in the table

    For the outer materials, the following options exist
    Value 0 - data from file (created by user)
    Value 1 - Gadolinium.
    Value 2 - Cadmium.
    Value 3 - Bor10.
    Value 4 - Europium
    Value 5 - Silicon.
    Value 6 - Ideal absorption
    For materials 1 - 4, the maximal wavelength range is 0.3 ... 28 A
    For silicon, the maximal wavelength range is 1 .. 20 A    (Data from Thomas Krist, HZB, Berlin)

### Parameters for module 'Space'

Parameter Unit | Description | Range or Values | Command Option
distance [m] | the distance from the origin (usually the exit of the previous component) to a plane vertical to the x-axis where they reach the next component | >= 0 | -d
total scattering [1/cm] | macroscopic total scattering cross-section | >= 0 | -M
absorption [1/cm] | macroscopic absorption cross-section for 1.798 Ã The absorption cross-section for the wavelength under consideration is calculated in the module from this value. | >= 0 | -m

### Parameters for module 'Slit'

Parameter Unit | Description | Range or Values | Command Option
distance [m] | the distance from the origin (usually the exit of the previous component) to the slit (with its normal vector along the x-axis). It is the projection of the vector from the origin to the center of the slit on the x-axis | >= 0 | -d
width [cm] | width of the rectangular slit | >= 0 | -W
height [cm] | height of the rectangular slit | >= 0 | -H

### Parameters for module 'Spacewindow'

Parameter Unit | Description | Range or Values | Command Option
distance orig. <-> win [cm] | the distance from the origin (usually the exit of the previous component) to the center of the window. It is the projection of the vector from the origin to center of the window on the x-axis | >= 0 | -l
window shape | hor. direction to the center of the collimator circular : circular window defined by its center (relative to beam axis) and radius (default) rectangular: rectangular window defined by the distances from the 4 edges to the center of the beam axis | circular, rectangular | -R
radius [cm] | radius of the circular window | >= 0 | -r
center y center z [cm] | horizontal (y) and vertical (z) position of the center of the window relative to the center of the beam axis | >= 0 | -y -z
min. y max. y [cm] | right and left edge of the rectangular window relative to the center of the beam axis | >= 0 | -w -W
min. z max. z [cm] | lower and upper vertical edge of the rectangular window relative to the center of the beam axis | >= 0 | -h -H
rot. angle [deg] | orientation of the normal vector of the window 0 deg: direction along the x-axis 90 deg: to the left (= y-axis) | [-180°,180°] | -A
used as beamstop | yes: negative window: absorption inside and transmission outside no : normal window: transmission inside | 'no', 'yes' | -S
use previous frame | yes: origin of the co-ordinate system remains unchanged no : x-ordinate is shifted to the center of the window (default) Prevention of the shift of the origin is necessary for beamstops and windows between sample and detector | 'no', 'yes' | -F
material description file | File with data characterizing the transmission of the outer material of the window. |  | -C
thickness of material [cm] | Thickness of material used for the absorbing part of the window | >= 0 | -t
material | material used to produce the absorbing part of the window |  | -c
material description file | File with data characterizing the transmission of the inner material of the window. |  | -m
thickness of material [cm] | Thickness of material used for the transmitting part of the window | >= 0 | -T

Back to VITESS overview
vitess@helmholtz-berlin.de

Last modified: Tue May 8 17:08:06 MET DST 2001