VITESS Modules to simulate Propagation and Apertures

There are 5 modules that can be used to simulate propagation of the neutrons to a certain area, where it may pass an aperture: space, slit, spacewindow, spacewindow_multiple and grid. 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 of the beamline. It can consider attenuation inside the window and non-perfect absorption outside the window and can also be used as a beamstóp. Additionally, it has some filter options.
Spacewindow_multiple works like Spacewindow, but can treat up to 100 rectangular or circular windows in any geometry. It is described in spacewindow_multiple.html
Their x-coordinates are reset to 0 (as in most others modules) after finishing the propagation. gridsimulates a regular set of N x M apertures of equal size. A series of such ensembles of apertures which can be used for collimation in SANS instruments.

VITESS Module 'space' to simulate Propagation over a Certain Distance

The module space simply simulates the propagation over a certain distance. 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.

Parameters for module 'space'

VITESS Module 'slit' to simulate Propagation to a Rectangular Slit

The module slit simulates the propagation to a rectangular window in a certain distance, which is centered around the 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 other cases the module spacewindow has to be used.

Parameters for module 'slit'

VITESS Module 'spacewindow' to simulate Propagation to a Rectangular or Circular Aperture

The module spacewindow simulates free propagation of neutrons (from x=0) to a rectangular or circular window. The (rectangular) window can be rotated about the x-axis. There is the possibility to let only neutrons of a certain range in flight direction and a certain color pass.
The 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 denotes the material of the window frame absorbing neutrons outside the window. Inner material denotes the material of the window pane (if any) through which the neutrons have to pass. For each material used, the thickness has to be given. Setting the thickness of the material to zero means ideal absorption for the outer and ideal transmission for the inner material. For both kinds of materials, a file describing the absorption can be given (see below). If the file is given, the treatment of transmission is activated, otherwise the ideal transmission/absorption is assumed. For the window frame, there is also the option to choose from a number of absorbing materials.
The file format describing the transmission is:

    First column : wavelength  [Å] 
    Second column: attenuation [1/cm] 
  

    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
  

Parameters for module 'Spacewindow'

VITESS Module 'spacewindow_multiple' to simulate Propagation to a Ensemble of Apertures

The module multiplewindows simulates free propagation of neutrons from (x=0) to the multi-aperture circular collimator (circular slits) or just characterising a space region of interest. The influence of gravity can be included in the calculation or omitted(-G option, values 1 or 0). If gravity is included, neutrons travel from plane x=0 to the collimator plane on parabolic trajectories. This is very useful for simulating a SANS (pinhole collimation) instrument with long distances for the long wavelength range. This collimator represents a window with multiple holes (with an ideally absorbing circular plate) transmitting the neutrons. The radius of the circular plate has to be given by the user. The radius and center position of each hole is described in the parameter file, which must be created by the user. 


The format of parameter file: Y, Z give the center position of the hole, R the radius of the hole.
Example (two holes):
-10 0.0 2.0
0.0 10.0 3.0
The number of holes for each collimator must be < 100. The number of lines is equal to the number of holes in the multiple collimator. The neutrons at x=0 has been calculated by the previous module (naturally they may differ in y and z-coordinates, divergence, wavelength and time) and those hitting the holes (or outside of circular plate) are considered further on in case of ideal transmission or transmited via the "inner material of collimator". Other neutrons are absorbing in case of ideal absorption or transmitting via the "outer material of collimator".
Their x-coordinates are reset to 0 as is usually  done by most of the modules after finishing their specific calculations.

Also the module calculates the center of the beam at the collimator plate and the time of flight. The user must give the outer radius of the circular plate and the distance from the former module to the plate along the x-axis. (The plate must be perpendicular to the x-direction). An example of visualization with such a parameter file is given below:
(For visualization use the module monitor or visual (see visual module help file))

with
-4.0 0.0 1.3
3.5 0.0 1.3
0.0 3.5 1.3
0.0 -3.5 1.3
0.0 0.0 1.3
-2.5 2.5 1.3
2.5 -2.5 1.3
2.5 2.5 1.3
-2.5 -2.5 1.3
 
---------------------------------------------
There are two kinds of material can be chosen. Outer material is meaning the "absorbtion" material for the collimator. Inner material is meaning the "transmission" material for the collimator. The thickness for the each material must be given too. If the thickness of material is given zero, so for outer material is meaning the ideal absorbtion. For the inner material is meaning the ideal transmission.

For the inner material data can be red from the file, which has the structure, given below. If the file is given, the exp. transmission is activating automaticly, otherwise the ideal transmission is acivated.


List of outer materials, which are included in the module
Value 0 - Read data from file, which created by user
Value 1 - Gd: Gadolinium.
Value 2 - Cd: Cadmium.
Value 3 - B: Bor10.
Value 4 - Eu
Value 5 - Si: Silicon.
Value 6 - Ideal absorbtion
For values 1-4 wavelength range must be 0.3 ... 28 A in the source module or virtual source
Foe value 5 wavelength range must be 1 .. 20 A in the source module or virtual source
This data was given by Thomas Krist, HMI, Berlin.

The format of file, which is describing the transmission is:
Wavelength, A Value MU, cm^(-1)
...............................................................

The wavelength values in the table must be increasing!!!
Then, the transmission is calculated by formula: Transmission = exp(-MU*Distance);
where Distance is length of flight of neutron in given material in cm, which calculated automaticly. During passing the neutrons via material, the Transmission is multiply on the neutron probability.

Please note.
The wavelength range must be from smallest to longest wavelength in the table. If you used material from list and/or material from file, please choose the minimal and maximum wavelength values in the source module (or virtual source), which do not extend the wavelength range for the absorption (outer) and transmission (inner) materials.

Parameters for module 'Spacewindow_multiple'

VITESS Module 'grid' to simulate Propagation to a Multi-Aperture Collimator

The module grid simulates the propagation of neutrons from the origin (x=0, previous module, e.g. the previous grid item) to a multi-aperture collimator of rectangular or circular holes and outer shape. This grid collimator represents a disk with multiple rectangular or circular apertures transmitting neutrons. The size of the circular or rectangular disk and the distance from the former module to the current disk (along the x-axis) have to be given by the user. (The disk must be perpendicular to the x-direction.)
The size and center position of each aperture is defined in a parameter file, which has to be created by the user (see example below). The collimator disk can be shifted horizontally and/or vertically. The vertical shifting is useful to consider gravitational effects.
Deviations of some parameters from ideal values can be simulated (see table below).

The vertical shift can be calculated automatically by the module, if the parameters belonging to 'Gravity Option' are set. At the same time, the ideal sizes of the aperatures are calculated from the length of the grid system and the distance between the beginning of the grid system and the current grid. In this case, the file describing the first grid item is sufficient for the whole grid system; otherwise a file has to be written for each grid item.

Non-ideal absorption in the plate can also be treated. The transmission is calculated then as:

   Transmission = exp(-μ*pathlen) 

where 'pathlen' is the length of the path of the neutron in the given material, (which is calculated automatically from the thickness of the material). The x-coordinates are reset to 0 (as usual) at the end of the treatment in this module.

Parameter file defining the holes

The user has to create a file describing the windows of the grid system. The number of lines is equal to the number of holes in the multiple collimator grid. The format of each line is like this:
1st column: (y-coordinate): horizontal position of the center of the aperture
2nd column: (z-coordinate): vertical position of the center of the aperture
3rd column: width = height of the hole

Example:
4 quadratic holes of 1 x 1 cm² in a distance of 1.2 cm (horizontally and vertically) would look like that:

#   y     z   width
#----------------------
  -0.6  -0.6   1.0  
   0.6  -0.6   1.0
  -0.6   0.6   1.0 
   0.6   0.6   1.0

Parameters for module 'grid'

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