Indirect Corrections¶

Table of Contents

Overview ¶

Provides correction routines for quasielastic, inelastic and diffraction reductions.

These interfaces do not support GroupWorkspace’s as input.

Action Buttons ¶

?: Opens this help page.
Py: Exports a Python script which will replicate the processing done by 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.

Container Subtraction ¶

The Container Subtraction Tab is used to remove the container’s contribution to a run.

Once run the corrected output and can correction is shown in the preview plot. 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.

../_images/Corrections_tabContainerSubtraction_widget.png

Options ¶

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.
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 Container by Factor: Allows the container’s intensity to be scaled by a given scale factor before being used in the corrections calculation.
Shift X-values by Adding: Allows the X-values to be shifted by a specified amount.
Spectrum: Changes the spectrum displayed in the preview plot.
Plot Current Preview: Plots the currently selected preview plot in a separate external window
Run: Runs the processing configured on the current tab.
Plot Spectrum: Gives the option to create a plot of the spectrum selected in the spin box for the corrected workspace.
Plot Contour: Gives the option to create a contour plot of the corrected workspace.
Save Result: If enabled the result will be saved as a NeXus file in the default save directory.

Calculate Paalman Pings ¶

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 ¶

Sample: 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).
Corrections Details: These options will be automatically preset to the default values read from the sample workspace, whenever possible. They can be overridden manually.(see below)
Sample Shape: Sets the shape of the sample, this affects the options for the shape details (see below).
Sample/Can Mass/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.
Run: Runs the processing configured on the current tab.
Plot: Plots the parameter selected in the neighbouring combobox.
Save Result: Saves the result in the default save directory.

Correction Details ¶

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.
Number Wavelengths: 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 ¶

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 ¶

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 dependent 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 ¶

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 dependent 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:

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

Calculate Monte Carlo Absorption ¶

The Calculate Monte Carlo Absorption tab provides a cross platform alternative to the Calculate Paalman Pings tab. In this tab a Monte Carlo implementation is used to calculate the absorption corrections.

../_images/Corrections_tabAbsorptionCorrections_widget.png

Options ¶

Sample Input: Either a reduced file (_red.nxs) or workspace (_red) or an $S(Q, \omega)$ file (_sqw.nxs) or workspace (_sqw).
Use Container: 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).
Number Wavelengths: The number of wavelength points for which a simulation is attempted.
Events: The number of neutron events to generate per simulated point.
Interpolation: Method of interpolation used to compute unsimulated values.
Maximum Scatter Point Attempts: Maximum number of tries made to generate a scattering point within the sample (+ optional container etc). Objects with holes in them, e.g. a thin annulus can cause problems if this number is too low. If a scattering point cannot be generated by increasing this value then there is most likely a problem with the sample geometry.
Beam Height: The height of the beam in $cm$ .
Beam Width: The width of the beam in $cm$ .
Shape Details: Select the shape of the sample (see specific geometry options below).
Sample/Can Mass/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.
Run: Runs the processing configured on the current tab.
Plot: Plots the parameter selected in the neighbouring combobox.
Save Result: Saves the result in the default save directory.

Shape Details ¶

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

Flat Plate ¶

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 degrees.
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 ¶

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 ¶

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 $cm$ .

Apply Absorption Corrections ¶

The Apply Corrections tab applies the corrections calculated in the Calculate Paalman Pings or Calculate Monte Carlo Absorption tabs of the Indirect Data Corrections 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 $A_{c,c}$ .

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.

../_images/Corrections_tabApplyAbsorptionCorrections_widget.png

Options ¶

Sample: Either a reduced file (_red.nxs) or workspace (_red) or an $S(Q, \omega)$ file (_sqw.nxs) or workspace (_sqw).
Corrections: The calculated corrections workspace produced from one of the preview two tabs.
Geometry: Sets the sample geometry (this must match the sample shape used when calculating the corrections).
Use Container: 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 Container by factor: Allows the container intensity to be scaled by a given scale factor before being used in the corrections calculation.
Shift X-values by Adding: Allows the X-values of the container to be shifted by a specified amount.
Rebin Container to Sample: Rebins the container to the sample.
Spectrum: Changes the spectrum displayed in the preview plot.
Plot Current Preview: Plots the currently selected preview plot in a separate external window
Run: Runs the processing configured on the current tab.
Plot Spectrum: Gives the option to create a plot of the spectrum selected in the spin box for the corrected workspace.
Plot Contour: Gives the option to create a contour plot 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