Indirect Bayes

Overview

Provides Bayesian analysis routines primarily for use with QENS data.

../_images/Bayes_interface.png

Action Buttons

Settings
Opens the Settings GUI which allows you to customize the settings for the Indirect interfaces.
?
Opens this help page.
Manage Directories
Opens the Manage Directories dialog allowing you to change your search directories and default save directory and enable/disable data archive search.

ResNorm

This tab creates a group ‘normalisation’ file by taking a resolution file and fitting it to all the groups in the resolution (vanadium) data file which has the same grouping as the sample data of interest.

The routine fits the width of the resolution file to give a ‘stretch factor’ and the area provides an intensity normalisation factor.

The fitted parameters are in the group workspace with suffix _ResNorm with additional suffices of _Intensity & _Stretch.

The processing on this tab is provided by the ResNorm algorithm.

../_images/Bayes_ResNorm_widget.png

Options

Vanadium File
Either a reduced file created using the Energy Transfer tab or an S(Q, \omega) file.
Resolution File
A resolution file created using the Calibrtion tab.
EMin & EMax
The energy range to perform fitting within.
Preview 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
Plots the selected parameter stored in the result workspaces.
Save Result
Saves the result in the default save directory.

Quasi

Warning

This interface is only available on Windows

The model that is being fitted is that of a \delta-function (elastic component) of amplitude A(0) and Lorentzians of amplitude A(j) and HWHM W(j) where j=1,2,3. The whole function is then convolved with the resolution function. The -function and Lorentzians are intrinsically normalised to unity so that the amplitudes represent their integrated areas.

For a Lorentzian, the Fourier transform does the conversion: 1/(x^{2}+\delta^{2}) \Leftrightarrow exp[-2\pi(\delta k)]. If x is identified with energy E and 2\pi k with t/\hbar where t is time then: 1/[E^{2}+(\hbar / \tau)^{2}] \Leftrightarrow exp[−t
/\tau] and \sigma is identified with \hbar / \tau. The program estimates the quasielastic components of each of the groups of spectra and requires the resolution file and optionally the normalisation file created by ResNorm.

For a Stretched Exponential, the choice of several Lorentzians is replaced with a single function with the shape : \psi\beta(x) \Leftrightarrow
exp[-2\pi(\sigma k)\beta]. This, in the energy to time FT transformation, is \psi\beta(E) \Leftrightarrow exp[-(t/\tau)\beta]. So \sigma is identified with (2\pi)\beta\hbar/\tau . The model that is fitted is that of an elastic component and the stretched exponential and the program gives the best estimate for the \beta parameter and the width for each group of spectra.

../_images/Bayes_Quasi_widget.png

Options

Sample
Either a reduced file created using the Energy Transfer tab or an S(Q, \omega) file.
Resolution
A resolution file created using the Calibration tab.
Program
The curve fitting program to use.
Background
The background fitting program to use.
Elastic Peak
If an elastic peak should be used.
Sequential Fit
Enables multiple fitting iterations.
Fix Width
Allows selection of a width file.
Use ResNorm
Allows selection of a ResNorm output file or workspace to use with fitting.
EMin & EMax
The energy range to perform fitting within.
Sample Binning
Sample binning to use.
Resolution Binning
Resolution binning to use.
Preview 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
Plots the selected parameter stored in the result workspaces.
Save Result
Saves the result in the default save directory.

Stretch

Warning

This interface is only available on Windows

This is a variation of the stretched exponential option of Quasi. For each spectrum, a fit is performed for a grid of β and σ values. The distribution of goodness of fit values is plotted.

../_images/Bayes_Stretch_widget.png

Options

Sample
Either a reduced file created using the Energy Transfer tab or an S(Q, \omega) file.
Resolution
A resolution file created using the Calibration tab.
Background
The background fitting program to use.
Elastic Peak
If an elastic peak should be used.
Sequential Fit
Enables multiple fitting iterations.
EMin & EMax
The energy range to perform fitting within.
Sample Binning
Sample binning to use.
Sigma
Value of Sigma to use.
Beta
Value of Beta to use.
Preview 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
Plots the selected parameter stored in the result workspaces.
Plot Contour
Produces a contour plot of the selected workspace.
Save Result
Saves the result in the default save directory.

Categories: Interfaces | Indirect