VITESS Guide Modules

There are 3 VITESS to simulate guides: guide, which is called 'guide_parallel' in version 3, allows simulating many a variety of guides consisting of linear sections approching parabolic, elliptic and curved guide sections. guide_ideal simulates a really elliptically shaped guide (or a guide of constant or linearly changing cross-section). So far, the elliptic guide has to be symmetric about the position of maximal width. The module bender simulates a curved guide, which can consist of many channels.
Subsequent modules always refer to the new origin, which is defined as the middle of the exit window; it is generated automatically by the program. This applies also for the orientation of the co-ordinate system, which follows the curvature. Both, guide and guide are curved to the left for a positive radius. guide is curved to the right for a negative radius.

Reflectivity

There is one reflectivity curve for the left side, one for the right side and one for the top and bottom of the guide. For special cases, there is the option to choose a separate file for the bottom plane, otherwise the one for the top plane is used also for the bottom plane. The reflectivity of the coating can be given in 3 different ways, by the m-value or two kinds of reflectivity files describing the reflection properties of the coating.

Vitess reflectivity files

This reflectivity file contains the probability of reflection in dependence of the incident angle for neutrons with a wavelength of 1 Å. Each row contains 10 data points covering 0.01 deg, i.e. each point gives the probability average over an angular interval of 0.001 deg. The first row contains values for 0.000 to 0.009 deg, the second for 0.010-0.019 deg a.s.o.). The number of data points may vary between 2 and 1000. If end of file is reached before finding the corresponding value, the probability of reflection is set to 0. This kind of files can be generated using the tool GenerateMirrorFiles.

Some examples can be found in FILES/reflectivity_files, e.g.:

  mirr0.dat : absorbing coating (reflectivity = 0.0) 
  mirr1a.dat: Ni coating    (theta_Ni    = 0.099138 deg/Å*lambda) 
  mirr1b.dat: Ni-58 coating (theta_Ni-58 = 0.11456  deg/Å*lambda)
  

Calculation from m-values

A second possibility is to calculate the reflectivity as a function of the m-value. From version 3.5 on, VITESS uses the 'Linear 2020 Approach' described in mirror_coating.html. Up to version 3.4, the 'Quadratic SwissNeutronics Equation' was used. This change reflects in progress in the reflectivity of high m-coatings, which drops linearly nowadays and does not differ much from low m-coatings for the same Q-values.
If a m-value and file are given for the same wall, the file is used. If a file is given for the 'top' plane, it is also overwrites the m-value for the bottom plane.

R(Q) files

A third possibility is to give the reflectivity of the coating in a 2 column file with the momentum transfer Q in 1/Å in the first and the reflectivity in the second column. So far, this is only realized for the module guide.

VITESS Module 'guide' for Guides consisting of Linear Sections

The module guide simulates the neutron flight path through a mirrored guide calculating the intensity loss for each reflection (in dependence of the neutron wavelength and the incident angle). It can be straight, diverging or converging, or consist of several straight parts that form a polygon section with a certain curvature or an elliptic or parabolic shape. It can also consist of several vertical channels.

Simulation parameters

The effect of gravity is considered in this module, if this option is switched on for the instrument. Trajectories with a probability/count rate less than the 'minimal weight' are taken out of the simulation.

Guide shapes

The guide consists of one or several planar pieces. The number of pieces and the length of a piece are input parameters. Each piece may be converging or diverging. Elliptical and parabolic guide shapes can be chosen; these shapes are approached by several planar pieces. Linearly converging and diverging as well as guides of constant cross-section can be simulated as one piece. Convergence or divergence is then defined by input and output width and height. This is identical to using several pieces if there is no abutment loss (see Table below) included and runs faster. Special guide shapes can be realized using the option 'shape from file' that allows any symmetric shape and an individual coating for each wall of each piece.

A curved guide is described by a polygon. In this case, a radius of curvature (unequal zero) must be given. The co-ordinate system is rotated by an angle

        beta = 2.0*arcsin(length/(2*radius))

between two succeeding pieces. The first piece is aligned to the preceding module, the last piece is aligned to the following module. The algorithm used here works for guides of constant width and linearly decreasing/increasing width.

Channel option

The guide can be divided into channels (in the x-z-plane). The entrance width of each channel is determined by:

        W_chan = [W_entry - (N_chan - 1) * W_blade] / N_chan

MCPL output

A new option allows saving events of gamma and neutron generation caused by absorption, scattering etc. of the neutrons treated in the simulation. The output is in MCPL format and can be used as a source for shielding calculations using MCNP or similar programs.

Mathematical description

Each piece is described by 5 planes: left, right, top , bottom and exit. Each plane is described by an equation of first order:

        ax + by + cz + d = 0 

The coefficients a,b,c,d are calculated from guide length as well as width and height of entrance and exit. The trajectory is calculated until the exit plane is reached, taking into account the intensity loss for each reflection. A warning is given, if trajectories are found out of the exit area.

If the real guide does not consist of straight pieces, a large number of pieces must be used for a good simulation. Alternatively, the module 'bender' or the module 'guide_ideal' can be used. The module 'bender' uses arcs instead of planes for the surface of the guide. The module 'guide_ideal' uses a real elliptic shape.

Module parameters:

Reflection list parameters:

Reflection plot parameters:

Output of reflection list option

A detailed description of this option was published [1].
The following table is taken from this publication and shows the colums of the output file:

Output of reflection plot option

This option is also published in the paper mentioned above [1].
The output file contains several data blocks introduced in comment lines using the following keywords:

• #==Data== holds a 2D binning
• #==DataX== and #==DataY== are 1D binnings
• #==Data==, #==DataX== and #==DataY==: the same analysis, but done seperately for each guide plane with 1 = top, 2 = bottom, 3 = left, 4 = right, >=5 same order for each plane created by the additional planes option.

Example for a guide shape and from file

  # position     width      height                 reflectivity files 
  #    [m]       [cm]        [cm]        left       right      top       bottom
  #-----------------------------------------------------------------------------------------
      9.815     3.57947     4.74530    m_4.0.dat  m_4.0.dat  m_3.0.dat  m_3.0.dat 
     10.290     3.77560     4.82805    m_4.0.dat  m_4.0.dat  m_3.0.dat  m_3.0.dat 
     10.790     3.96479     4.91063    m_4.0.dat  m_4.0.dat  m_3.0.dat  m_3.0.dat 
     11.290     4.13867     4.98879    m_4.0.dat  m_4.0.dat  m_3.0.dat  m_3.0.dat 
     11.790     4.29909     5.06274    m_3.5.dat  m_3.5.dat  m_3.0.dat  m_3.0.dat 
     12.290     4.44751     5.13267    m_3.5.dat  m_3.5.dat  m_3.0.dat  m_3.0.dat 
     12.790     4.58509     5.19872    m_3.5.dat  m_3.5.dat  m_3.0.dat  m_3.0.dat 
     13.000     4.63988     5.22535    m_3.5.dat  m_3.5.dat  m_3.0.dat  m_3.0.dat
  

VITESS Module 'guide_ideal' for Guides of True Elliptic Shape

The module guide_elliptic simulates the neutron flight path through a mirrored guide calculating the intensity loss for each reflection (in dependence of the neutron wavelength and the incident angle). While the regular Guide module is designed to handle all possible wall shapes, this module is explicitly designed to deal with guides, which are elliptic in one or both planes. No segmentation is used in this guide, thus for all shapes, for which an input file is used, the conventional guide module should be used.

Input parameters

If the elliptic shape is chosen, there are two combinations of parameters that allow a complete definition of the shape: - Length, major and minor axis, in this case the ellipse is symmetric wrt to its center - Length, entrance & exit width and distance from exit to focus
For linear or constant shape, the entrance & exit width (height) and length have to be specified. Note that for a constant shape, the width (height) at entrance and exit must agree.
See below for the full list of parameters.

VITESS Module 'bender' for truly curved guide sections

The module Bender is similar to the module Guide using the bender option. The main difference is that the 'bended guide' consists of several straight parts that form a polygon section. In contrast, the bender surfaces are circles (but straight surfaces also possible). Also the module Bender simulates converging (diverging) bender-polarizator with possibility of enabled or disabled of polarising neutrons. The 2-D visualization of surfaces and trace of neutrons path are included (using PGPLOT and G2 graphic libraries). Only first 10000 trajectories will be visualised. Also you can choose the device for visualisation: display, file or both.
Additionally there is a possibility to have spacing inside the bender.
THE SURFACES ARE SITUATED IN THE XZ PLANE.

Simulation parameters

The effect of gravity is considered in this module, if this option is chosen. Neutrons with a probability/current less than the 'minimal weight' are taken out of the simulation. The possibility take into account the rough of reflected surface of guide is included also. Abutment loss feature is rejecting neutrons, which have got reflection near edges (exit) of bender. If last path of neutron (before exit plane) is smaller than given value, such neutron is rejected. User also must to input this value. The polarizing of neutrons may be enabled or disabled. For each spin direction (-1 or 1) user must be input individual reflectivity file for left, right planes of bender (This is not actually for top and bottom planes, but it is possible too). If polarization is included, neutrons, which has another quantization is rejected.

Bender Geometry characteristics

The geometry of bender is defining by following parameters:
1. Entrance height (along vertical axis 0Z).
2. Exit height (along vertical axis 0Z).
3. Length of the bender.
4. Radius of curvature; Angle beta=(Length of bender)/(Radius of curvature) is defining angle of decline of exit sufrace concerning entrance surface of the bender so the bender axis is part of circle. If radius of curvature is zero, the central axis of bender is a straight line and so angle beta is zero. The parameters which are describing arrangement of vertical surfaces in the horizontal plane (XY - plane) are reading from the parameter file, which have name - surface file.

Please NOTE! Subsequent modules always refer to the new origin, which is defined as the middle of the exit window. It is generated automatically by the program, also in the case of curvature (the orientation of the frame follows the curvature).
 

Surface File

Please NOTE! For positive value of radius of curvature the circle will be concave. If radius of curvature of sufrace is zero, the straigth line will be used instead arc line. This is useful for simualtions of some soller collimators. If negative value of radius of curvature is given, the circle will be convex. Such features are giving the possibility for simulating of many types of benders and collimators!
The number of surfaces must be less than 200. The module defines the number of lines in the parameter files automaticly. Number of lines equally numbers of surfaces in multiple collimator. Please do not to writing incorrect data in the parameter file (For example, description of hole without radius, Line: -10 -5.0 empty). The entrance and exit width of bender is defining automaticly. Also user must be chosen the thickness of surfaces.

Information file

Visualisation

The visualization of the bender, which desciribing above-mentioned file is given at the picture 2. (Length 2m, Radius of curvature 20m, thickness of surfaces is neglecting). During simualtions, the neutron flight paths will be appeared. After first reflection, the color of neutron path is changing, so this is giving the possibility for exploring background of fast neutrons and "straight line" gamma-rays.
For UNIX operation systems (Linux, SunOS, Solaris, OSF1,..) the PGPLOT graphic library is used
For Windows operation system the PGPLOT and G2 graphics libraries are used.

Module parameters:

List of absorption materials, which were included in the module (for the options -c, -z and -w):
Value 0 - Read data from file, which was created by user
Value 1 - Gd: Gadolinium.
Value 2 - Cd: Cadmium.
Value 3 - B: Bor10.
Value 4 - Eu
Value 5 - Si: Silicon.
Value 6 - Vacuum, no attenuation
For values 1-4 wavelength range must be between 0.3 ... 28 A in the source module or virtual source.
Foe value 5 wavelength range must be between 1 .. 20 A in the source module or virtual source too.
This datas were given by Thomas Krist, HMI, Berlin.

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

The wavelength values in the table must be increased!
The number of rows must be less that 500!
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" value is multipling in neutron probability.
The wavelength range must be from smallest to longest wavelength in the table. If you have used material from list and 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 of transmission materials for left and right sides and for bender channel material. Otherwise, the simulation process will be cancelled automaticly.

Literature

[1] A. Houben, W. Schweika. Th. Brückel, R. Dronskowski, New neutron-guide concepts and simulation results for the POWTEX instrument, NIMA 680 (2012) 124-133.