[ Identification | Description | Input parameters | Output parameters | Links ]

# The `Single_crystal` Component

Mosaic single crystal with multiple scattering vectors.

## Identification

- Author: Kristian Nielsen
- Origin: Risoe
- Date: December 1999
- Version: 1.51
- Modification history:
  - EF, 22nd Apr 2003 : now uses Read_Table library

## Description

    Single crystal with mosaic. Delta-D/D option for finite-size effects.
    Rectangular geometry. Multiple scattering and secondary extinction included.
    The mosaic may EITHER be specified isotropic by setting the mosaic input
    parameter, OR anisotropic by setting the mosaic_a, mosaic_b, and mosaic_c
    parameters.

    Sample shape:
    Sample shape may be a cylinder, a sphere, a box or any other shape
      box/plate:       xwidth x yheight x zthick
      cylinder:        radius x yheight
      sphere:          radius (yheight=0)
      any shape:       geometry=OFF file

      The complex geometry option handles any closed non-convex polyhedra.
      It computes the intersection points of the neutron ray with the object
      transparently, so that it can be used like a regular sample object.
      It supports the OFF and NOFF file format but not COFF (colored faces).
      Such files may be generated from XYZ data using qhull/powercrust, and
      viewed with geomview
      The default size of the object depends of the OFF file data, but its
      bounding box may be resized using xwidth,yheight and zthick.

    Crystal definition file format
    Crystal structure is specified with an ascii data file. Each line contains
    4 or more numbers, separated by white spaces:

          h k l ... F2

    The first three numbers are the (h,k,l) indices of the reciprocal lattice
    point, and the 7-th number is the value of the structure factor |F|**2, in
    barns. The rest of the numbers are not used; the file is in the format
    output by the Crystallographica program.
    Lines begining by '#' are read as comments (ignored). Most sample parameters
    may be defined from the data file header, following the same mechanism as
    PowderN.

    Current data file header keywords include, for data format specification:
       #column_h <index of the Bragg Qh column>
       #column_k <index of the Bragg Qk column>
       #column_l <index of the Bragg Ql column>
       #column_F2 <index of the squared str. factor '|F|^2' column [b]>
       #column_F  <index of the structure factor norm '|F|' column>
    and for material specification:
       #sigma_abs <value of Absorption cross section [barns]>
       #sigma_inc <value of Incoherent cross section [barns]>
       #Delta_d/d <value of Detla_d/d width for all lines>
       #lattice_a <value of the a lattice parameter [Angs]>
       #lattice_b <value of the b lattice parameter [Angs]>
       #lattice_c <value of the c lattice parameter [Angs]>
       #lattice_aa <value of the alpha lattice angle [deg]>
       #lattice_bb <value of the beta  lattice angle [deg]>
       #lattice_cc <value of the gamma lattice angle [deg]>

    See the Component Manual for more defails.

    Example: Single_crystal(xwidth=0.01, yheight=0.01, zthick=0.01,
              delta_d_d=1e-4, mosaic = 5,
              ax = 3.8186, ay = 0, az = 0,
              bx = 0, by = 3.8843, bz = 0,
              cx = 0, cy = 0, cz = 11.6777,
              reflections="YBaCuO.lau")

    Vanadium incoherent elastic scattering with multiple scattering
    Single_crystal(xwidth=0.01, yheight=0.01, zthick=0.01,
              reflections="", absorption=5.08, incoherent=4.935,
              ax=3.0282, by=3.0282, cz=3.0282/2)

    Also, always use a non-zero value of delta_d_d.

    %VALIDATION:
    This component is undergoing validation.

## Input parameters

Parameters in boldface are required; the others are optional.

Name | Unit | Description | Default
--- | --- | --- | ---

mosaic_AB

arc_minutes, arc_minutes,1, 1, 1, 1, 1, 1

In Plane mosaic rotation and plane vectors (anisotropic), mosaic_A, mosaic_B, A_h,A_k,A_l, B_h,B_k,B_l, input as a {}-delimited, 8-element vector. Puts the component in the in-plane mosaic state. Vectors A and B define plane in which the crystal roation is defined, and mosaic_A, mosaic_B, denotes the resp. mosaicities (gaussian RMS) with respect to the the two reflections chosen by A and B (Miller indices).

Mosaic_AB_Undefined

reflections | string | File name containing structure factors of reflections. Use empty ("") or NULL for incoherent scattering only | 0
geometry | str | Name of an Object File Format (OFF) file for complex geometry. The OFF file may be generated from XYZ coordinates using qhull/powercrust | 0
xwidth | m | Width of crystal | 0
yheight | m | Height of crystal | 0
zthick | m | Thickness of crystal (no extinction simulated) | 0
radius | m | Outer radius of sample in (x,z) plane | 0
delta_d_d | 1 | Lattice spacing variance, gaussian RMS | 1e-4
mosaic | arc minutes | Crystal mosaic (isotropic), gaussian RMS. Puts the crystal in the isotropic mosaic model state, thus disregarding other mosaicity parameters. | -1
mosaic_a | - | Horizontal (rotation around lattice vector a) mosaic (anisotropic), gaussian RMS. | -1
mosaic_b | arc minutes | Vertical (rotation around lattice vector b) mosaic (anisotropic), gaussian RMS. | -1
mosaic_c | arc minutes | Out-of-plane (Rotation around lattice vector c) mosaic (anisotropic), gaussian RMS | -1
recip_cell | 1 | Choice of direct/reciprocal (0/1) unit cell definition | 0
barns |  |  | 1
ax | - |  | 0
ay | AA or AA^-1 | Coordinates of first (direct/recip) unit cell vector | 0
az | - |  | 0
bx | - |  | 0
by | AA or AA^-1 | Coordinates of second (direct/recip) unit cell vector | 0
bz | - |  | 0
cx | - |  | 0
cy | AA or AA^-1 | Coordinates of third (direct/recip) unit cell vector | 0
cz | - |  | 0
p_transmit | 1 | Monte Carlo probability for neutrons to be transmitted without any scattering. Used to improve statistics from weak reflections | -1
absorption | barns | Absorption cross-section per unit cell at 2200 m/s | 0
incoherent | barns | Incoherent scattering cross-section per unit cell | 0
aa | deg | . | 0
bb | deg | unit cell angles alpha, beta and gamma. Then uses norms of vectors a,b and c as lattice parameters | 0
cc | deg | . | 0
order | 1 | limit multiple scattering up to given order (0: all, 1: first, 2: second, ...) | 0
powder |  |  | 0

## Output parameters

Name | Unit | Description | Default
--- | --- | --- | ---
hkl_info | structure | Internal |
offdata |  |  |

## Links

- Source code for `Single_crystal.comp`.
- See ICSD powder diffraction data base
- Cross sections for single elements: http://www.ncnr.nist.gov/resources/n-lengths/
- Cross sections for compounds: http://www.ncnr.nist.gov/resources/sldcalc.html
- Geomview and Object File Format (OFF) <http|://www.geomview.org>
- Powercrust/qhull <http://www.cs.utexas.edu/users/amenta/powercrust>

[ Identification | Description | Input parameters | Output parameters | Links ]