Module Grid

# VITESS Module Grid

The module grid simulates propagation of neutrons from the origin (x=0) to a multi-aperture collimator of circular or rectangular outer shape. This grid collimator represents a plate with multiple rectangular windows transmitting the neutrons. The size of the circular or rectangular plate and the distance from the former module to the plate along the x-axis have to be given by the user. (The plate must be perpendicular to the x-direction.)
The size and center position of each hole (= window) is described in a parameter file, which has to be created by the user. The collimator plate can be shifted horizontally and/or vertically. The vertical shifting is useful to simulate a special 'gravitational monochromator'. The vertical shift can be calculated automatically by the module, if the parameters belonging to 'Options gravity monochromator' are set. Deviations of some parameters from ideal values can be simulated (see 'Deviation of parameters'). Non-ideal absorption in the plate can also be treated. The transmission is calculated then as:

Transmission = exp(-μ*distance)
where 'distance' is the length of the path of the neutron in the given material, (which is calculated automatically). The x-coordinates are reset to 0 (as usual) at the end of the treatment in this module.

Figure: propagation to grid plate

### 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 hole
2nd column: (z-coordinate): vertical position of the center of the hole
3rd column: half width = half 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   halfwidth
    #----------------------
      -0.6  -0.6   0.5
       0.6  -0.6   0.5
      -0.6   0.6   0.5
       0.6   0.6   0.5

### Parameters for module 'grid'

Parameter Unit | Description | Range or Values | Command Option
Holes description | File that characterizes the positions and sizes of the holes of a grid | - | -I
distance orig - grid [cm] | distance from origin to grid along the beam direction (x-axis) | >=0 | -D
thickness of material [cm] | Thickness of the material used for the grid | >=0 | -t
outer hor size or radius [cm] | for circular outer shape : radius of the grid for rectangular outer shape: width of the grid | >0 | -a
outer vert size [cm] | for rectangular outer shape: height of the grid | >0 | -b
horizontal/vertical shift [cm] | horizontal/vertical shift of the grid | any | -d -e
shape of the grid | shape of the collimator plate containing the holes | 'circular' 'square' | -N
crosstalk between channels tracking | 'yes': analysis of cross-talk between channels within the grid system activated 'no' : no analysis of cross-talk | 'no' 'yes' | -K
material description file | for 'material from file': File which characterizes the transmission of the (outer) material of the grid plate. | - | -C

material

material of the grid plate:
'from file' needs a 'material description file' as input
in all other cases the necessary data are in the program

'from file'
'gadolinum'
'cadmium'
'bor-10'
'Eu'
'silicon'
'ideal absorber'

-c

Deviation: horizontal/vertical shift [cm] | random deviation of horizontal/vertical shift of the grid a linear distribution within [shift-dev_shift, shift+dev_shift] is assumed | >=0 | -y -q
Deviation: radius of window [cm] | random deviation of the size of the apertures in the grid plate a linear distribution within [radius-dev_radius, radius+dev_radius] is assumed | >=0 | -h -H
Deviation: center of window [cm] | random deviation of the center position of the windows (in the plate) a linear distribution within [pos-dev_pos, po+dev_pos] is assumed | >=0 | -h -H
Deviation: distance orig - grid [cm] | random deviation from the defined distance between origin and grid a linear distribution within [distance-dev_dist, distance+dev_dist] is assumed | >=0 | -X
Grav.Monochr.: calc dist orig - grid [cm] | distance from the end of previous module, usually the previous window used to calculate the ideal shift of the grid plate for a certain wavelength | >0 | -M
Grav.Monochr.: calc total dist [cm] | distance from the beginning of the grid system used to calculate the ideal shift of the grid plate for a certain wavelength | >0 | -m
Grav.Monochr.: Grav.Monoch. wavelength [Å] | wavelength for which the grid system should be optimized | >0 | -n

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