BackToBackExponential1D

From MantidProject

Contents 1 Deprecation notice 2 Summary 3 Properties 4 Source Code

Deprecation notice

Instead of using this algorithm to fit a BackToBackExponential please use the Fit algorithm where the Function parameter of this algorithm is used to specified the fitting function, including selecting a BackToBackExponential.

Summary

Takes a histogram in a 2D workspace and fit it to a back-to-back exponential peakshape function, i.e. to the function:

This peakshape function represent the convolution of back-to-back exponentials and a gaussian function and is designed to be used for the data analysis of time-of-flight neutron powder diffraction data, see Ref. 1.

The parameters A and B represent the absolute value of the exponential rise and decay constants (modelling the neutron pulse coming from the moderator) and S represent the standard deviation of the gaussian. The parameter X0 is the location of the peak; more specifically it represent the point where the exponentially modelled neutron pulse goes from being exponentially rising to exponentially decaying.

References

1. R.B. Von Dreele, J.D. Jorgensen & C.G. Windsor, J. Appl. Cryst., 15, 581-589, 1982

The figure below illustrate this peakshape function fitted to a TOF peak:

Properties

Note the initial default guesses for in particular A and B are only based on fitting a couple of peaks in a dataset collected on the ISIS's HRPD instrument.

Order	Name	Direction	Type	Default	Description
1	InputWorkspace	Input	Workspace	Mandatory	The name of the input Workspace
2	SpectrumIndex	Input	integer	0	The spectrum to fit, using the workspace numbering of the spectra (default 0)
3	StartX	Input	double	Lowest value of x data array	X value to start fitting from
4	EndX	Input	double	Highest value of x data array	last X value to include in fitting range
5	I	InOut	double	0.0	height of peak
6	A	InOut	double	1.0	exponential constant of rising part of neutron pulse
7	B	InOut	double	0.05	exponential constant of decaying part of neutron pulse
8	X0	InOut	double	0.0	peak position
8	S	InOut	double	1.0	standard deviation of gaussian part of peakshape function
8	BK	InOut	double	0.0	constant background
9	MaxIterations	Input	Integer	500	Max iterations
10	Output Status	Output	String	""	whether the fit was successful
11	Output Chi^2/DoF	Output	double	0.0	returns how good the fit was

Source Code

Header BackToBackExponential1D.h

Source BackToBackExponential1D.cpp