Indirect Corrections

Overview

../_images/Corrections_interface.png

Provides correction routines for quasielastic, inelastic and diffraction reductions.

These interfaces do not support GroupWorkspace as input.

Action Buttons

?
Opens this help page.
Py
Exports a Python script which will replicate the processing done by the current tab.
Run
Runs the processing configured on the current tab.
Manage Directories
Opens the Manage Directories dialog allowing you to change your search directories and default save directory and enable/disable data archive search.

Calculate Paalman Pings

../_images/Corrections_tabCalculatePaalmanPings_widget.png

Calculates absorption corrections in the Paalman & Pings absorption factors that could be applied to the data when given information about the sample (and optionally can) geometry.

Options

Input
Either a reduced file (_red.nxs) or workspace (_red) or an S(Q,
\omega) file (_sqw.nxs) or workspace (_sqw).
Use Can
If checked allows you to select a workspace for the container in the format of either a reduced file (_red.nxs) or workspace (_red) or an S(Q,
\omega) file (_sqw.nxs) or workspace (_sqw).
Sample Shape
Sets the shape of the sample, this affects the options for the shape details (see below).
Sample/Can Number Density
Density of the sample or container.
Sample/Can Chemical Formula
Chemical formula of the sample or can material. This must be provided in the format expected by the SetSampleMaterial algorithm.
Plot Output
Plots the A_{s,s}, A_{s,sc}, A_{c,sc} and A_{c,c} workspaces as spectra plots.
Save Result
If enabled the result will be saved as a NeXus file in the default save directory.

Correction Details

These options will be automatically preset to the default values read from the sample workspace, whenever possible. They can be overridden manually.

Emode
The energy transfer mode. All the options except Efixed require the input workspaces to be in wavelength. In Efixed mode, correction will be computed only for a single wavelength point defined by ` Efixed` value. All the options except Elastic require the Efixed value to be set correctly. For flat plate, all the options except Efixed, are equivalent. In brief, use Indirect for QENS, Efixed for FWS and diffraction. Efixed can be used for QENS also, if the energy transfer can be neglected compared to the incident energy. See CylinderPaalmanPingsCorrections for the details.
Efixed
The value of the incident (indirect) or final (direct) energy in mev. Specified in the instrument parameter file.
NumberWavelength
Number of wavelength points to compute the corrections for. Ignored for Efixed.
Interpolate
Whether or not to interpolate the corrections as a function of wavelength. Ignored for Efixed.

Shape Details

Depending on the shape of the sample different parameters for the sample dimension are required and are detailed below.

Flat Plate

../_images/Corrections_pgFlatPlate_widget.png

The calculation for a flat plate geometry is performed by the FlatPlatePaalmanPingsCorrection algorithm.

Sample Thickness
Thickness of sample in cm.
Sample Angle
Sample angle in degrees.
Can Front Thickness
Thickness of front container in cm.
Can Back Thickness
Thickness of back container in cm.

Cylinder

../_images/Corrections_pgCylinder_widget.png

The calculation for a cylindrical geometry is performed by the CylinderPaalmanPingsCorrection algorithm, this algorithm is currently only available on Windows as it uses FORTRAN code dependant of F2Py.

Sample Inner Radius
Radius of the inner wall of the sample in cm.
Sample Outer Radius
Radius of the outer wall of the sample in cm.
Container Outer Radius
Radius of outer wall of the container in cm.
Beam Height
Height of incident beam cm.
Beam Width
Width of incident beam in cm.
Step Size
Step size used in calculation.

Annulus

../_images/Corrections_pgAnnulus_widget.png

The calculation for an annular geometry is performed by the CylinderPaalmanPingsCorrection algorithm, this algorithm is currently only available on Windows as it uses FORTRAN code dependant of F2Py.

The options here are the same as for Cylinder.

Background

The main correction to be applied to neutron scattering data is that for absorption both in the sample and its container, when present. For flat plate geometry, the corrections can be analytical and have been discussed for example by Carlile [1]. The situation for cylindrical geometry is more complex and requires numerical integration. These techniques are well known and used in liquid and amorphous diffraction, and are described in the ATLAS manual [2].

The absorption corrections use the formulism of Paalman and Pings [3] and involve the attenuation factors A_{i,j} where i refers to scattering and j attenuation. For example, A_{s,sc} is the attenuation factor for scattering in the sample and attenuation in the sample plus container. If the scattering cross sections for sample and container are \Sigma_{s} and \Sigma_{c} respectively, then the measured scattering from the empty container is I_{c} = \Sigma_{c}A_{c,c} and that from the sample plus container is I_{sc} = \Sigma_{s}A_{s,sc} +
\Sigma_{c}A_{c,sc}, thus \Sigma_{s} = (I_{sc} - I_{c}A_{c,sc}/A_{c,c}) /
A_{s,sc}.

References:

  1. C J Carlile, Rutherford Laboratory report, RL-74-103 (1974)
  2. A K Soper, W S Howells & A C Hannon, RAL Report RAL-89-046 (1989)
  3. H H Paalman & C J Pings, J Appl Phys 33 2635 (1962)

Apply Paalman Pings

../_images/Corrections_tabApplyPaalmanPings_widget.png

The Apply Corrections tab applies the corrections calculated in the Calculate Corrections tab of the Indirect Data Analysis interface.

This uses the ApplyPaalmanPingsCorrection algorithm to apply absorption corrections in the form of the Paalman & Pings correction factors. When Use Can is disabled only the A_{s,s} factor must be provided, when using a container the additional factors must be provided: A_{c,sc}, A_{s,sc} and A_{c,c}.

Once run the corrected output and can correction is shown in the preview plot, the Spectrum spin box can be used to scroll through each spectrum. Note that when this plot shows the result of a calculation the X axis is always in wavelength, however when data is initially selected the X axis unit matches that of the sample workspace.

The input and container workspaces will be converted to wavelength (using ConvertUnits) if they do not already have wavelength as their X unit.

The binning of the sample, container and corrections factor workspace must all match, if the sample and container do not match you will be given the option to rebin (using RebinToWorkspace) the sample to match the container, if the correction factors do not match you will be given the option to interpolate (SplineInterpolation) the correction factor to match the sample.

Options

Input
Either a reduced file (_red.nxs) or workspace (_red) or an S(Q,
\omega) file (_sqw.nxs) or workspace (_sqw).
Geometry
Sets the sample geometry (this must match the sample shape used when running Calculate Corrections).
Use Can
If checked allows you to select a workspace for the container in the format of either a reduced file (_red.nxs) or workspace (_red) or an S(Q,
\omega) file (_sqw.nxs) or workspace (_sqw).
Scale Can by factor
Allows the container intensity to be scaled by a given scale factor before being used in the corrections calculation.
Use Corrections
The Paalman & Pings correction factors to use in the calculation, note that the file or workspace name must end in either _flt_abs or _cyl_abs for the flat plate and cylinder geometries respectively.
Plot Output
Gives the option to create either a spectra or contour plot (or both) of the corrected workspace.
Save Result
If enabled the result will be saved as a NeXus file in the default save directory.

Absorption Corrections

../_images/Corrections_tabAbsorptionCorrections_widget.png

The Absorption Corrections tab provides a cross platform alternative to the previous Calculate and Apply Corrections tabs.

Common Options

Sample Input
Used to select the sample from either a file or a workspace already loaded into Mantid.
Use Container
Used to enable or disable use of a container and selects one from either a file or loaded workspace.
Shape
Select the shape of the sample (see specific geometry options below).
Number Wavelengths
Number of wavelengths for calculation
Events
Number of neutron events
Mass Density/Number Density
Mass density or Number Density for either the sample or container.
Chemical Formula
Chemical formula for either the sample or container in the format expected by SetSampleMaterial.
Use Container Corrections
Enables full container corrections, if disabled only a can subtraction will be performed.
Scale
Scale factor to scale container input by.
Keep Correction Factors
If checked a Workspace Group containing the correction factors will also be created, this will have the suffix _Factors.
Plot Result
If clicked the corrected workspace and correction factors will be plotted.
Save Result
If Clicked the corrected workspace and (if Keep Correction Factors is checked) the correction factor workspace will be saved as a NeXus file in the default save directory.

Flat Plate

../_images/Corrections_pgAbsCorFlatPlate_widget.png

Flat plate calculations are provided by the IndirectFlatPlateAbsorption algorithm.

Sample Width
Width of the sample in cm.
Sample Height
Height of the sample in cm.
Sample Thickness
Thickness of the sample in cm.
Sample Angle
Angle of the sample to the beam in radians.
Container Front Thickness
Thickness of the front of the container in cm.
Container Back Thickness
Thickness of the back of the container in cm.

Annulus

../_images/Corrections_pgAbsCorAnnulus_widget.png

Annulus calculations are provided by the IndirectAnnulusAbsorption algorithm.

Sample Inner Radius
Radius of the inner wall of the sample in cm.
Sample Outer Radius
Radius of the outer wall of the sample in cm.
Container Inner Radius
Radius of the inner wall of the container in cm.
Container Outer Radius
Radius of the outer wall of the container in cm.
Sample Height
Height of the sample in cm.

Cylinder

../_images/Corrections_pgAbsCorCylinder_widget.png

Cylinder calculations are provided by the IndirectCylinderAbsorption algorithm.

Sample Radius
Radius of the outer wall of the sample in cm.
Container Radius
Radius of the outer wall of the container in cm.
Sample Height
Height of the sample in cm.

Container Subtraction

../_images/Corrections_tabContainerSubtraction_widget.png

The Container Subtraction Tab is used to remove the containers contribution to a run.

Once run the corrected output and can correction is shown in the preview plot, the Spectrum spin box can be used to scroll through each spectrum. Note that when this plot shows the result of a calculation the X axis is always in wavelength, however when data is initially selected the X axis unit matches that of the sample workspace.

The input and container workspaces will be converted to wavelength (using ConvertUnits) if they do not already have wavelength as their X unit.

Options

Input Sample
Either a reduced file (_red.nxs) or workspace (_red) or an S(Q,omega) file (_sqw.nxs) or workspace (_sqw) that represents the sample.
Input Container
Either a reduced file (_red.nxs) or workspace (_red) or an S(Q,omega) file (_sqw.nxs) or workspace (_sqw) that represents the container.
Scale Can by Factor
Allows the container intensity to be scaled by a given scale factor before being used in the corrections calculation.
Plot Output
Gives the option to create either a spectra or contour plot (or both) of the corrected workspace.
Save Result
If enabled the result will be saved as a NeXus file in the default save directory.

Categories: Interfaces | Indirect