Module Collimator

# VITESS Modules Collimator, Soller Collimator and Radial Collimator

### VITESS Module Collimator

The module collimator simulates a soller collimator of a certain length, width and height consisting of a number of channels separated by blades of a certain thickness. Exit width and height can be different from entrance width and height. For each trajectory, it is checked whether the neutron is outside the exit or entrance area and if a channel wall is hit; in these cases the trajectory is removed. Otherwise it is propagated to the collimator exit (without loss in intensity). This is a realistic description of a Soller collimator and is therefore the recommended choice.

### VITESS Module Soller Collimator

In contrast, the module collimator_soller simulates a collimator by allowing a certain divergence range (in the x-y-plane) which is described by a triangular distribution characterising the probability that a neutron of a certain divergence passes through the collimator. The maximal possible divergence d_phi then corresponds to the width w and length l of a soller collimator channel, i.e. tan d_phi = w/l. The FWHM of the probability distribution (the peak is centered at divergence 0) is also d_phi. The module collimator_soller does neither change the neutron position or direction nor the frame of reference.
This method of simulating collimation is a good approximation. If the module is used, entrance and exit area should be simulated in addition using either the module slit or spacewindow to allow for the limitation in solid angle and take care of the propagation.

This module can also be used to simulate a radial collimator. In this case, the direction of the first channel, the number of channels and the 'additional angular spacing' have to be given:
Each collimation center is defined by an angle corresponding to divergence 0. The first centre is defined by the 'minimum of angle range'. The following centers (always higher angles) are calculated by considering a gap of (2*allowed_div_fwhm + additional_spacing) between two centers. 'allowed_div_fwhm' is the FWHM value of the triangular shape. 'additional_spacing' is the additional angular distance between the collimation centers due to the size of collimator spacers. Therefore, the maximal direction of the last center can be calculated as:

angle_max = angle_min + (num_centers - 1)*(2*allowed_div_fwhm + additional_spacing)

### VITESS Module Radial Collimator

A more appropriate way to simulate radial collimators is to use the module collimator_radial. As the module collimator it calculates the neutron trajectory through the collimator and removes it from the simulation, if it hits the absorbing walls. It considers also the height of the collimator and propagates the neutrons to the end of the radial collimator.
Furthermore, an oscillation of the collimator can be treated. If this is used, the orientation of the collimator (at the time the neutron is passing) is determined by a random choice within a range given. In this way, the shadowing effect of the blades can be simulated and the extent of this effect estimated.

### Parameters for module 'collimator_soller'

Parameter Unit | Description | Range or Values | Command Option
angular collimation | yes: radial collimation with several collimation centers (= directions regarded as zero-divergence directions) no : normal Soller collimator (only 1 zero-divergence direction) | 'yes', 'no' | -k
collimator divergence [deg] | allowed x-y divergence (FWHM of the triangular shape) | >= 0 | -d
peak transmission | maximal probability for passing through the soller collimator (corresponds to an average loss due to blocking neutrons by the finite size of the Soller collimator) | ]0, 1] | -e
minimum of angle range [deg] | The lowest angle defining a zero-divergence direction 0 deg: direction of the beam impinging on the sample (= x-axis) 90 deg: to the left (= y-axis) The following centers (always higher angles) are calculated by considering a gap of (2*allowed_div_fwhm + angle_spacing) between two centers. | [-180°, 180°[ | -m
number of collimation centers | number of orientations corresponding to divergence 0 (or to peak transmission resp.). The difference between two collimation centers is: 2*allowed_div_fwhm + angle_spacing | >= 1 | -n
angle spacing [deg] | additional angular distance between the collimation centers due to the size of collimator spacers | >= 0 | -a

### Parameters for module 'collimator'

Parameter Unit | Description | Range or Values | Command Option
entrance width exit width [cm] | total width of the collimator entrance and exit (between the two walls defining beginning of first and end of last channel) | >= 0 | -w -W
entrance height exit height [cm] | entrance and exit height of the collimator | >= 0 | -h -H
length [cm] | length of the collimator | >= 0 | -l
number of channels | number of collimator channels | >= 1 | -n
blade width [cm] | thickness of the material dividing the collimator into channels | >= 0 | -s

### Parameters for module 'collimator_radial'

Parameter Unit | Description | Range or Values | Command Option
theta [deg] | hor. direction to the center of the collimator 0 deg: direction of the beam impinging on the sample (= x-axis) 90 deg: to the left (= y-axis) | [-180°,180°] | -a
width [deg] | angular range covered by the radial collimator (between the two walls defining beginning of first and end of last channel) | [0°,360°] | -w
oscillation width [deg] | full width amplitude of oscillation of the radial collimator the orientation of the collimator for a certain trajectory is randomly chosen within this range | >= 0 | -a
entrance height exit height [cm] | entrance and exit height of the radial collimator | >= 0 | -h -H
distance [cm] | distance of the collimator entrance from the origin (usually the center of the sample) | >= 0 | -d
length [cm] | length of the collimator channels | >= 0 | -l
number of channels | number of collimator channels | >= 1 | -n
blade width [cm] | thickness of the material dividing the collimator into channels | >= 0 | -s

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