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

# The `PowderN` Component

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

## Identification

- Author: P. Willendrup, L. Chapon, K. Lefmann, A.B.Abrahamsen, N.B.Christensen, E.M.Lauridsen.
- Origin: McStas release
- Date: 4.2.98
- Version: 1.24
- Modification history:
  - KL, KN 20.03.98 (rewrite)
  - KL, 28.09.01 (two lines)
  - KL, 22.05.03 (background)
  - KL, PW 01.05.05 (N lines)
  - PW, LC 04.10.05 (Merge with Chapon Powder_multi)

## Description

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

    Based on Powder1/Powder2/Single_crystal.
    Geometry is a powder filled cylinder or a box.
    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').
    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.

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

    Powder definition and file format

    Powder structure is specified with an ascii data file 'reflections'.
    The powder data are free-text column based files.
    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>
      #sigma_abs <value of Absorption cross section>
      #sigma_inc <value of Incoherent cross section>
      #Debye_Waller <value of Debye-Waller factor DW>
      #Delta_d/d <value of Detla_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. 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,1/2d,q,F}
    Signification of the symbols is given below. Indexes start at 1.
    Indexes with zero means that the column is not present, so that:
      Crystallographica={ 4,5,7,0,0,0,0,0 }
      Fullprof         ={ 4,0,8,0,0,5,0,0 }
      Lazy             ={17,6,0,0,0,0,0,13}
    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 <index of the multiplicity 'j' column>
      #column_d <index of the d-spacing 'd' column>
      #column_F2 <index of the squared structure factor '|F|^2' column>
      #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 'Dd' column>
      #column_inv2d
      #column_q <index of the scattering wavevector 'q' column>

## Input parameters

Parameters in boldface are required; the others are optional.

Name | Unit | Description | Default
--- | --- | --- | ---
reflections | string | Input file for reflections. Use only incoherent scattering if NULL or "" |
format | no quotes | name of the format, or list of column indexes (see Description). | Undefined
d_phi | deg,0-180 | Angle corresponding to the vertical angular range to focus to, e.g. detector height. 0 for no focusing | 0
radius | m | Radius of sample in (x,z) plane | 0.01
yheight | m | Height of sample y direction | 0.05
pack | 1 | Packing factor | 1
Vc | AA^3 | Volume of unit cell=nb atoms per cell/density of atoms | 0
sigma_abs | fm^2 | Absorption cross section per unit cell at 2200 m/s | 0
sigma_inc | fm^2 | Incoherent cross section per unit cell | 0
Delta_d | Angs | global relative Delta_d/d spreading when the 'w' column is not available. Use 0 if ideal. | 0
frac | 1 | Fraction of incoherently scattered neutron rays | 0
xwidth | m | horiz. dimension of sample, as a width | 0
zthick | m | thickness of sample | 0
h | m | the same as yheight | 0
DW | 1 | global Debey-Waller factor when the 'DW' column is not available. Use 1 if included in F2 | 0
nb_atoms | 1 | number of atoms per unit cell | 1
density | g/cm^3 | density of material. V_rho=density/weight/1e24*N_A. | 0
weight | g/mol | atomic/molecular weight of material | 0

## Output parameters

Name | Unit | Description | Default
--- | --- | --- | ---
line_info |  |  |
Nq |  |  |
my_s_v2 |  |  |
my_s_v2_sum |  |  |
my_a_v |  |  |
my_inc |  |  |
q_v |  |  |
w_v |  |  |
isrect |  |  |
columns |  |  |

## Links

- Source code for `PowderN.comp`.
- See also: Powder1, Powder2 and PowderN
- See ICSD Inorganic Crystal Structure Database
- 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
- Fullprof powder refinement: http://www-llb.cea.fr/fullweb/fp2k/fp2k.htm
- Web Elements http://www.webelements.com/

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