[ Identification | Description | Input parameters | Links ]

The Isotropic_Sqw_legacy Component

Identification

• Site:
• Author: Virginie Hugouvieux, E. Farhi
• Origin: ILL
• Date: August 2003

Description

An isotropic sample handling multiple scattering and including as input the
dynamic structure factor of the chosen sample (e.g. from Molecular
Dynamics). Handles elastic/inelastic, coherent and incoherent scattering -
depending on the input S(q,w) - with multiple scattering and absorption.
Only the norm of q is handled (not the vector), and thus suitable for
liquids, gazes, amorphous and powder samples.

The component at hand provides the legacy implementation from McStas 2.0.

If incoherent/self S(q,w) file is specified as empty (0 or "") then the
scattering is constant isotropic (Vanadium like).
In case you only have one S(q,w) data containing both coherent and
incoherent contributions you should e.g. use 'Sqw_coh' and set 'sigma_coh'
to the total scattering cross section.
The implementation assumes that the S(q,w) data is normalized, i.e. S(q)
goes to 1 for large q. If this is not the case, the component can do that
when 'auto_norm=-1'. Alternatively, the S(q,w) data will be multiplied by
'auto_norm' for positive values.

Focusing on the relevant [q,w] data range corresponding to the instrument
setting may improve computation accuracy and prevent some neutrons
to be removed when energy transfer is higher than actual neutron energy.
An automatic such process may be activated when 'auto_qw=1'.
On the other hand, if the input S(q,w) data is too restricted in q, the
scattering conditions will be limited, leading to incomplete result.
Best conditions are to get the widest S(q,w) data and use 'auto_qw=1'.
Additionally, for single order scattering (order=1), you may restrict the
vertical spreading of the scattering area using d_phi parameter.

An important option to enhance statistics is to set 'p_interact' to, say,
30 percent (0.3) in order to force a fraction of the beam to scatter. This
will result on a larger number of scattered events, retaining intensity.

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

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 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.

Concentric components:
This component has the ability to contain other components when used in
hollow cylinder geometry (namely sample environment, e.g. cryostat and
furnace structure). Such component 'shells' should be split into input and
output side surrounding the 'inside' components. First part must then use
'concentric=1' flag to enter the inside part. The component itself must be
repeated to mark the end of the concentric zone. The number of concentric
shells and number of components inside is not limited.

COMPONENT S_in = Isotropic_Sqw_legacy(Sqw_coh="Al.laz", concentric=1, ...)
AT (0,0,0) RELATIVE sample_position

COMPONENT something_inside ... // e.g. the sample itself or other materials

COMPONENT S_out = COPY(S_in)(concentric=0)
AT (0,0,0) RELATIVE sample_position

Sqw file format:
File format for S(Q,w) (coherent and incoherent) should contain 3 numerical
blocks, defining q axis values (vector), then energy axis values (vector),
then a matrix with one line per q axis value, containing Sqw values for
each energy axis value. Comments (starting with '#') and non numerical lines
are ignored and used to separate blocks. Sampling must be regular.

Example:
# q axis values
# vector of m values in Angstroem-1
0.001000 .... 3.591000
# w axis values
# vector of n values in meV
0.001391 ... 1.681391
# sqw values (one line per q axis value)
# matrix of S(q,w) values (m rows x n values), one line per q value,
9.721422  10.599145 ... 0.000000
10.054191 11.025244 ... 0.000000
...
0.000000            ... 3.860253

See for instance file He4_liq_coh.sqw. Such files may be obtained from e.g. INX,
Nathan, Lamp and IDA softwares, as well as Molecular Dynamics.

Powder file format:
Files for coherent elastic powder scattering may also be used.
Format specification follows the same principle as in the PowderN
component, with parameters:

powder_format=Crystallographica
or  powder_format=Fullprof
or  powder_format=Lazy
or  powder_format={j,d,F2,DW,Delta_d/d,1/2d,q,F,strain} (column indexes 1:n)

or column indexes (starting from 1) given as comments in the file header
(e.g. '#column_j 4'). Refer to the PowderN component for more details.
Delta_d/d and Debye-Waller factor may be specified for all lines with the
'powder_Dd' and 'powder_DW' parameters.

Additionally a special [q,Sq] format is also defined with:
powder_format=qSq
for which column 1 is 'q' and column 2 is 'S(q)'.

Examples:
1- Vanadium-like incoherent elastic scattering
Isotropic_Sqw_legacy(radius=0.005, yheight=0.01, V_rho=1/13.827,
sigma_abs=5.08, sigma_inc=4.935, sigma_coh=0)

2- liq-4He parameters
Isotropic_Sqw_legacy(..., Sqw_coh="He4_liq_coh.sqw", T=10, p_interact=0.3)

3- powder sample
Isotropic_Sqw_legacy(..., Sqw_coh="Al.laz", save_sqw=1)

%BUGS:
When used in concentric mode, multiple bouncing scattering
(traversing the hollow part) is not taken into account.
When adapting the (q,w) range automatically, the guess may be wrong for highly
polychromatic beam.

%VALIDATION
For Vanadium incoherent scattering mode, V_sample, PowderN, Single_crystal
and Isotropic_Sqw_legacy produce equivalent results, eventhough the two later are
more accurate (geometry, multiple scattering). Isotropic_Sqw_legacy gives same
powder patterns as PowderN, with an intensity within 20 %.

Input parameters

Links

• Source code for Isotropic_Sqw_legacy.comp.
• E. Farhi, V. Hugouvieux, M.R. Johnson, and W. Kob, Journal of Computational Physics 228 (2009) 5251-5261 "Virtual experiments: Combining realistic neutron scattering instrument and sample simulations"
• Hugouvieux V, Farhi E, Johnson MR, Physica B, 350 (2004) 151 "Virtual neutron scattering experiments"
• Hugouvieux V, PhD, University of Montpellier II, France (2004).
• Cross sections for single elements
• Web Elements
• Fullprof powder refinement
• Crystallographica software
• Example data file He4_liq_coh.sqw
• The PowderN component.
• The test/example instrument Test_Isotropic_Sqw.instr.
• Geomview and Object File Format (OFF)
• Java version of Geomview (display only) jroff.jar
• qhull
• powercrust

[ Identification | Description | Input parameters | Links ]