The module sample_environ simulates coherent and incoherent elastic scattering in material around the sample.
This sample environment is supposed to have the form of a hollow cylinder centered around the sample.
As the beam has to pass through it twice, it also has to be added twice to the list of components in the instrument.
For more than one sample environment, all have to appear twice in the order in which the neutrons pass through them.
In each case, a parameter has to be set defining if it is before or after the sample.
The modules before the sample have to be marked as direction 'in', those after the sample as direction 'out'.
So the order of modules for 2 sample environments around a sample would be:
...
outer_environment (in)
inner_environment (in)
sample
inner_environment (out)
outer_environment (out)
...
Coherent as well as incoherent scattering is treated. Therefore, data about the properties of each sample environment have to be given in a parameter file:
size, scattering coefficients, structure factor file and the unit cell volume.
The first sample environment defines the center position of the sample and all sample environments.
The sample and all following sample enviroment modules have to give (0,0,0) as sample (environment) position to express that they are all concentric.
Passing through the sample environment creates new trajectories:
For each incoming trajectory, N trajectories for Bragg scattered neutrons, 1 for a transmitted neutron and, if the incoherent scattering cross-section is greater 0, also 1 for an isotropically scattered neutron.
(N is the number of d-spacings in the structure factor file that fulfill the Bragg condition.)
All these trajectories can then be scattered in the sample and in the same way in all following sample environment modules.
The colour marking the coherently scattered neutrons can be set. The incoherently scattered neutrons are marked by a number that is 1 higher than the coherently
scattered neutrons. The neutrons scattered at the sample can also be marked by a colour.
In this way, scattering from all parts of sample and sample environment can be distinguished and separated later on.
Additional note: The trajecory describing a transmitted neutron keeps its ID, the others get new ones. For direction 'in', the first letter is changed, for direction 'out', the second letter.
In VITESS 3.n, the parameter input is separated between direct input parameters that are given on the main window ('Parameters for module sample_environment') and others that are stored in a file ('File input parameters'). In VITESS 4.n, all parameters can be given as direct input parameters, but using the file is still possible. If a parameter is given in both ways, the direct input parameter will be used.
| Parameter Unit |
Description |
Range or Values |
Command Option |
| parameter file | The parameter file describes the geometry and compositions of the sample environment These parameters are only used if they are not given as direct input parameters. |
- | -F |
| x y z [cm] |
x-, y and z-position of the centre of the sample environment (usually the sample position) in the frame of the previous module typical: (30, 0, 0) for direction 'in' (center of the sample environnment 30 cm after the last device) and : ( 0, 0, 0) for direction 'out' (center identical to sample center) |
any | -x -y -z |
| direction | in : sample environment before sample out: sample environment after sample |
in out |
-r |
| colour | The trajectories will be marked by a so-called 'colour' to show that they are scattering in this sample environment the given colour is used for coherent scattering, the incoherent scattering is marked with (colour+1) |
>=0 | -c |
| Parameter Unit |
Description |
Range or Values |
Command Option |
| structure factor file |
The structure factor file contains information to determine the scattering probability for each reflection.
It can be given as a .dat, .laz and .lau format. The columns (where to find the data) are set by the program. In VITESS format (.str), each line contains: d and M |Fd|² fdw (see FILES folder). |
- | -s |
| thickness [cm] |
Thickness of the hollow cylinder surrounding the sample | >0 | -t |
| diameter [cm] |
Outer diameter of the hollow cylinder | >0 | -d |
| height [cm] |
Outer height of the cylinder | >0 | -h |
| incoherent scattering [1/cm] |
User defined macroscopic incoherent scattering cross-section of the sample environment material If a non-zero value is given, incoherent scattering is applied. |
>=0 | -i |
| total scattering [1/cm] |
User defined macroscopic total scattering cross-section of the sample environment material This parameter only affects the attenuation in the sample environment, not the scattering. It is assumed to be wavelength independent. |
>=0 | -T |
| absorption [1/cm] |
User defined macroscopic absorption cross-section of the sample environment material (with respect to a wavelength of 1.798 A) This parameter only affects the attenuation in the sample environment. Absorption is assumed to increase linearly with the neutron wavelength. |
>=0 | -m |
| unit cell volume [ų] |
Unit cell volume of the sample environment material (this is necessary for normalisation of the scattering probability) |
> 0 | -U |
Last modified: Tue May 8 17:08:06 MET DST 2001