[ Identification | Description | Input parameters | Links ]

# The `PowderN` Component

General powder sample (N lines, single scattering, incoherent scattering)

## 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))

    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           <value of unit cell volume Vc [Angs^3]>
    #sigma_abs    <value of Absorption cross section [barns]>
    #sigma_inc    <value of Incoherent cross section [barns]>
    #Debye_Waller <value of Debye-Waller factor DW>
    #delta_d_d/d    <value of delta_d_d/d width for all lines>
    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, by defining 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}

    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     <index of the multiplicity 'j' column>
    #column_d     <index of the d-spacing 'd' column [Angs]>
    #column_F2    <index of the squared str. factor '|F|^2' column [b]>
    #column_F     <index of the structure factor norm '|F|' column>
    #column_DW    <index of the Debye-Waller factor 'DW' column>
    #column_Dd    <index of the relative line width delta_d_d/d broadening 'Dd' column>
    #column_inv2d
    #column_q     <index of the scattering wavevector 'q' column [Angs-1]>
    #column_strain <index of the strain line shift Delta/d [ppm]>

    Last, CIF, FullProf and ShelX files can be read, and converted to F2(hkl) lists
    if 'cif2hkl' is installed. The CIF2HKL env variable can be used to point to a
    proper executable, else the McCode, then the system installed versions are used.

    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.

    This sample component can advantageously benefit from the SPLIT feature, e.g.
    SPLIT COMPONENT pow = PowderN(...)

## Input parameters

Parameters in boldface are required; the others are optional.

Name | Unit | Description | Default |
reflections | string | Input file for reflections (LAZ LAU CIF, FullProf, ShelX). Use only incoherent scattering if NULL or "" | "NULL" |
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. | "NULL" |
format | no quotes | Name of the format, or list of column indexes (see Description). | {0, 0, 0, 0, 0, 0, 0, 0, 0} |
radius | m | Outer radius of sample in (x,z) plane | 0 |
yheight | m | Height of sample y direction | 0 |
xwidth | m | Horiz. dimension of sample, as a width | 0 |
zdepth | m | Depth of box sample | 0 |
thickness |  | Thickness of hollow sample. Negative value extends the hollow volume outside of the box/cylinder. | 0 |
pack | 1 | Packing factor. | 1 |
Vc | AA^3 | Volume of unit cell=nb atoms per cell/density of atoms. | 0 |
sigma_abs | barns | Absorption cross section per unit cell at 2200 m/s. Use a negative value to unactivate it. | 0 |
sigma_inc | barns | Incoherent cross section per unit cell. Use a negative value to unactivate it. | 0 |
delta_d_d | 0/1 | Global relative delta_d_d/d broadening when the 'w' column is not available. Use 0 if ideal. | 0 |
p_inc | 1 | Fraction of incoherently scattered neutron rays. | 0.1 |
p_transmit | 1 | Fraction of transmitted (only attenuated) neutron rays. | 0.1 |
DW | 1 | Global Debye-Waller factor when the 'DW' column is not available. Use 1 if included in F2 | 0 |
nb_atoms | 1 | Number of sub-unit per unit cell, that is ratio of sigma for chemical formula to sigma per unit cell | 1 |
d_omega | deg | Horizontal focus range (only for incoherent scattering), 0 for no focusing. | 0 |
d_phi | deg | Angle corresponding to the vertical angular range to focus to, e.g. detector height. 0 for no focusing. | 0 |
tth_sign | 1 | Sign of the scattering angle. If 0, the sign is chosen randomly (left and right). ONLY functional in combination with d_phi and ONLY applies to bragg lines. | 0 |
p_interact | 1 | Fraction of events interacting coherently with sample. | 0.8 |
concentric | 1 | Indicate that this component has a hollow geometry and may contain other components. It should then be duplicated after the inside part (only for box, cylinder, sphere). | 0 |
density | g/cm^3 | Density of material. rho=density/weight/1e24*N_A. | 0 |
weight | g/mol | Atomic/molecular weight of material. | 0 |
barns | 1 | Flag to indicate if |F|^2 from 'reflections' is in barns or fm^2 (barns=1 for laz, barns=0 for lau type files). | 1 |
Strain | ppm | Global relative delta_d_d/d shift when the 'Strain' column is not available. Use 0 if ideal. | 0 |
focus_flip | 1 | Controls the sense of d_phi. If 0 d_phi is measured against the xz-plane. If !=0 d_phi is measured against zy-plane. | 0 |

AT ( | , | , |  | ) RELATIVE |  |
ROTATED ( | , | , |  | ) RELATIVE |

## 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 ]