[ 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: 1.32
- 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_h, mosaic_v, and mosaic_n
    parameters.
    Crystal structure is specified with an ascii data file. Each line
    contains 4 or more numbers, separated by white space. The first three numbers
    are the (h,k,l) indices of the reciprocal lattice point, and the last
    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).

    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")
      Overview of algorithm:

      (1). The neutron intersects the crystal at (x,y,z) with given
           incoming wavevector ki=(kix,kiy,kiz).

      (2). Every reciprocal lattice point tau of magnitude less than 2*ki
           is considered for scattering. The scattering probability is the
           area of the intersection of the Ewald sphere (approximated by
           the tangential plane) with the 3-D Gaussian mosaic of the point
           tau.

      (3). The total coherent scattering cross section is computed as the
           sum over all tau. Together with the absorption and incoherent
           scattering cross section and known potential flight-length
           l_full through the sample, we can compute the probability of
           the four events absorption, coherent scattering, incoherent
           scattering, and transmission.

      (4). Absorption is never simulated explicitly, just incorporated in
           the neutron weight.

      (5). Transmission in the first event is selected with the Monte
           Carlo probability p_transmit, which defaults to the actual
           transmission probability. After the first event, transmission
           is selected with the correct Monte Carlo probability.

      (6). Incoherent scattering is done simply by selecting a random
           direction for the outgoing wave vector kf.

      (7). For coherent scattering, a reciprocal lattice point is selected
           using the relative probabilities computed in (2), and the
           weight is adjusted with the contribution from the structure
           factors (this way all reflections will get equally good
           statistics in the detector).

      (8). The outgoing wave vector direction is picked at random using
           the intersecting 2-D Gauss computed in (2). The vector is
           normalized to the length of ki (elastic scattering) to account
           for the error caused by the planar approximation of the Ewald
           sphere.

      (9). The process is repeated from (2) with kf as new initial wave
           vector ki.

## Input parameters

Parameters in boldface are required; the others are optional.

Name | Unit | Description | Default
--- | --- | --- | ---
reflections | string | File name containing structure factors of reflections |
xwidth | m | Width of crystal |
yheight | m | Height of crystal |
zthick | m | Thichness of crystal (no extinction simulated) |
delta_d_d | 1 | Lattice spacing variance, gaussian RMS | 1e-4
mosaic |  |  | -1
mosaic_h |  |  | -1
mosaic_v |  |  | -1
mosaic_n |  |  | -1
ax |  |  | 3.8186
ay |  |  | 0
az |  |  | 0
bx |  |  | 0
by |  |  | 3.8843
bz |  |  | 0
cx |  |  | 0
cy |  |  | 0
cz |  |  | 11.6777
p_transmit |  |  | -1
absorbtion |  |  | 0
incoherent |  |  | 0
aa |  |  | 0
bb |  |  | 0
cc |  |  | 0

## Output parameters

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

## Links

- Source code for `Single_crystal.comp`.

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