[ Identification | Description | Input parameters | Links ]

The PowderN Component

Identification

• Site:
• Author: P. Willendrup, L. Chapon, K. Lefmann, A.B.Abrahamsen, N.B.Christensen, E.M.Lauridsen.
• Origin: McStas release
• Date: 4.2.98

Description

General powder sample with
many scattering vectors
possibility for intrinsic line broadening
incoherent elastic background ratio is specified by user
No multiple scattering. No secondary extinction.

Based on Powder1/Powder2/Single_crystal.
Geometry is a powder filled cylinder, sphere, box or any shape from an OFF file.
Incoherent scattering is only provided here to account for a background.
The efficient is highly improved when restricting the vertical scattering
range on the Debye-Scherrer cone (with 'd_phi' and 'focus_flip').
The unit cell volume Vc may also be computed when giving the density,
the atomic/molecular weight and the number of atoms per unit cell.
A simple strain handling is available by mean of either a global Strain parameter,
or a column with a strain value per Bragg reflection. The strain values are
specified in ppm (1e-6).
The Single_crystal component can also handle a powder mode, as well as an
approximated texture.

Sample shape:
Sample shape may be a cylinder, a sphere, a box or any other shape.
box/plate:       xwidth x yheight x zdepth (thickness=0)
hollow box/plate:xwidth x yheight x zdepth and thickness>0
cylinder:        radius x yheight (thickness=0)
hollow cylinder: radius x yheight and thickness>0
sphere:          radius (yheight=0 thickness=0)
hollow sphere:   radius and thickness>0 (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 of the OFF file data, but its
bounding box may be resized using xwidth,yheight and zdepth.

If you use this component and produce valuable scientific results, please
cite authors with references bellow (in Links).

Example: PowderN(reflections = "c60.lau", d_phi = 15 , radius = 0.01,
yheight = 0.05, Vc = 1076.89, sigma_abs = 0, delta_d_d=0, DW=1,
format=Crystallographica)

Powder definition file format
Powder structure is specified with an ascii data file 'reflections'.
The powder data are free-text column based files.
The reflection list should be ordered by decreasing d-spacing values.
... d ... F2
Lines begining by '#' are read as comments (ignored) but they may contain
the following keywords (in the header):
#Vc           
#sigma_abs    
#sigma_inc    
#Debye_Waller 
#delta_d_d/d    
These values are not read if entered as component parameters (Vc=...)

The signification of the columns in the numerical block may be
set using the 'format' parameter. Built-in formats are:
format=Crystallographica
format=Fullprof
format=Lazy
and these specifications it is important NOT to use quotes, as shown.

An other possibility to define other formats is to set directly
the signification of the columns as a vector of indexes in the order
format={j,d,F2,DW,delta_d_d/d,1/2d,q,F,Strain}
Signification of the symbols is given below. Indices start at 1.
Indices with zero means that the column are not present, so that:
Crystallographica={ 4,5,7,0,0,0,0,0,0 }
Fullprof         ={ 4,0,8,0,0,5,0,0,0 }
Lazy             ={17,6,0,0,0,0,0,13,0}
Here again, NO quotes should be around the 'format' value.

At last, the format may be overridden by direct definition of the
column indexes in the file itself by using the following keywords
in the header (e.g. '#column_j 4'):
#column_j     
#column_d     
#column_F2    
#column_F     
#column_DW    
#column_Dd    
#column_inv2d 
#column_q     
#column_strain 

Concentricity

PowderN assumes 'concentric' shape, i.e. can contain other components inside its
optional inner hollow. Example, Sample in Al cryostat:


COMPONENT Cryo = PowderN(reflections="Al.laz", radius = 0.01, thickness = 0.001,
concentric = 1, p_interact=0.1)
AT (0,0,0) RELATIVE Somewhere

COMPONENT Sample = some_other_component(with geometry FULLY enclosed in the hollow)
AT (0,0,0) RELATIVE Somewhere

COMPONENT Cryo2 = COPY(Cryo)(concentric = 0)
AT (0,0,0) RELATIVE Somewhere


(The second instance of the cryostat component can also be written out completely
using PowderN(...). In both cases, this second instance needs concentric = 0.)
The concentric arrangment can not be used with OFF geometry specification.

Input parameters

Links

• Source code for PowderN.comp.
• "Validation of a realistic powder sample using data from DMC at PSI" Willendrup P, Filges U, Keller L, Farhi E, Lefmann K, Physica B-Cond Matt 385 (2006) 1032.
• See also: Powder1, Single_crystal
• See ICSD Inorganic Crystal Structure Database
• Cross sections for single elements
• Web Elements
• Fullprof powder refinement
• Crystallographica software (free license)
• Geomview and Object File Format (OFF)
• Java version of Geomview (display only) jroff.jar
• qhull
• powercrust

[ Identification | Description | Input parameters | Links ]