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McStas: Single_crystal Component

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

The Single_crystal Component

Mosaic single crystal with multiple scattering vectors, optimised for speed with large crystals and many reflections.

Identification

  • Author: Kristian Nielsen
  • Origin: Risoe
  • Date: December 1999
  • Version: $Revision$
  • Modification history:
    • EF, 22nd Apr 2003 : now uses Read_Table library
    • M. Schulz, March 2012 : allow to curve the crystal planes
    • EF, PW, May 2014: code efficiency improvement when SPLIT is used
    • EF, PW, 2015: powder/PG and texture mode

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.
The crystal lattice can be bent locally, keeping the external geometry unchanged.
Curvature is spherical along vertical and horizontal axes.

In order to dramatically improve the simulation efficiency, we recommend to
use a SPLIT keyword on this component (or prior to it), as well as to disable
the multiple scattering handling by setting order=1. This is especially powerfull
for large reflection lists such as with macromolecular proteins.

Sample shape:
Sample shape may be a cylinder, a sphere, a box or any other shape
  box/plate:       xwidth x yheight x zdepth
  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 PLY, OFF and NOFF file format but not COFF (colored faces). 
  Such files may be generated from XYZ data using:
    qhull < coordinates.xyz Qx Qv Tv o > geomview.off 
  or
    powercrust coordinates.xyz
  and viewed with geomview or java -jar jroff.jar (see below).
  The default size of the object depends on the OFF file data, but its 
  bounding box may be resized using xwidth,yheight and zdepth.

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.
The reflection list should be ordered by decreasing d-spacing values.
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 
   #column_k 
   #column_l 
   #column_F2 
   #column_F  
and for material specification:
   #sigma_abs 
   #sigma_inc 
   #Delta_d/d 
   #lattice_a 
   #lattice_b 
   #lattice_c 
   #lattice_aa 
   #lattice_bb 
   #lattice_cc 

See the Component Manual for more defails.

Example: Single_crystal(xwidth=0.01, yheight=0.01, zdepth=0.01,
          mosaic = 5, reflections="YBaCuO.lau")

A PG graphite crystal plate, cut for (002) reflections
  Single_crystal(xwidth = 0.002, yheight = 0.1, zdepth = 0.1, 
    mosaic = 30, reflections = "C_graphite.lau", 
    ax=0,      ay=2.14,   az=-1.24,
    bx = 0,    by = 0,    bz =  2.47,
    cx = 6.71, cy = 0,    cz =  0)

A leucine protein, without multiple scattering
  Single_crystal(xwidth=0.005, yheight=0.005, zdepth=0.005,
    mosaic = 5, reflections="leucine.lau", order=1)

A Vanadium incoherent elastic scattering with multiple scattering
  Single_crystal(xwidth=0.01, yheight=0.01, zdepth=0.01,
          reflections="", sigma_abs=5.08, sigma_inc=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 has been validated.

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. Puts the crystal 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) or PLY file for complex geometry. The OFF/PLY file may be generated from XYZ coordinates using qhull/powercrust 0
xwidth m Width of crystal 0
yheight m Height of crystal 0
zdepth m Depth 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 arc minutes Horizontal (rotation around lattice vector a) mosaic (anisotropic), gaussian RMS. Put the crystal in the anisotropic crystal vector state. I.e. model mosaicity through rotation around the crystal lattice vectors. Has precedence over in-plane mosaic model. -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 Flag to indicate if |F|^2 from 'reflections' is in barns or fm^2. barns=1 for laz and isotropic constant elastic scattering (reflections=NULL), barns=0 for lau type files 0
ax AA or AA^-1 Coordinates of first (direct/recip) unit cell vector 0
ay - a on y axis 0
az - a on z axis 0
bx AA or AA^-1 Coordinates of second (direct/recip) unit cell vector 0
by - b on y axis 0
bz - b on z axis 0
cx AA or AA^-1 Coordinates of third (direct/recip) unit cell vector 0
cy - c on y axis 0
cz - c on z axis 0
p_transmit 1 Monte Carlo probability for neutrons to be transmitted without any scattering. Used to improve statistics from weak reflections -1
sigma_abs barns Absorption cross-section per unit cell at 2200 m/s 0
sigma_inc barns Incoherent scattering cross-section per unit cell Use -1 to unactivate 0
aa deg Unit cell angles alpha, beta and gamma. Then uses norms of vectors a,b and c as lattice parameters 0
bb deg Beta angle 0
cc - Gamma angle [deg]. 0
order 1 Limit multiple scattering up to given order (0: all, 1: first, 2: second, ...) 0
RX m Radius of horizontal along X lattice curvature. flat for 0 0
RY m Radius of vertical lattice curvature. flat for 0 0
RZ m Radius of horizontal along Z lattice curvature. flat for 0 0
powder 1 Flag to indicate powder mode, for simulation of Debye-Scherrer cones via random crystallite orientation. A powder texture can be approximated with 0 0
PG 1 Flag to indicate "Pyrolytic Graphite" mode, only meaningful with choice of Graphite.lau, models PG crystal. A powder texture can be approximated with 0 0

Output parameters

Name Unit Description Default
hkl_info - .l:  
offdata  

Links


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

Generated automatically by McDoc, Peter Willendrup <peter.willendrup@risoe.dk> / Fri Mar 17 20:40:06 2017


Last Modified: Friday, 17-Mar-2017 21:40:06 CET
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