FitPeak v1¶

Default function parameters’ names¶

Input function parameters’ names should be exactly the same as those defined in Mantid. This brings some inconvenience to users through MantidPlot GUI. Thus user can input function parameters values in the default order, which is instructed in the list of functions. For example, one background function type is defined as ‘Linear (A0, A1)’. As user chooses this function, he does not need to input background function parameter names, but writes values of A0 and A1, respectively.

Output of function parameters¶

The output [[TableWorkspace]] contains the following columns… * parameter name * fitted parameter value * error

Simple fit¶

In the ‘simple fit’ mode, the algorithm will make a composite function including peak and background function and fit it against the observed data.

It works well with good starting values of the peak and background function, especially when the peak is significant with low background.

High background fit¶

In the ‘high background fit’ mode, the background will be removed first; then the fitting is focussed on the ‘pure’ peak function; and a composite function is fit against the original data as the last step.

This approach is developed due to the failure of ‘simple fit’ mode on the cases that background level is much higher than the peak height. Without the background being removed, the optimizer intends to favor the background rather than the peak function.

Starting values of the peak function¶

• Peak height is estimated by the maximum value, with background removed, inside the peak range;
• Peak position can be set up either to the X value of the maximum Y value in the peak range, or to the value specified by user accordin to user’s selection. For example, in the case of calibrating the offsets of detectors of powder diffractometers, the peak positions are unknown. Then it is better to use the X value with the maximum Y value as the starting peak centre. While in the case of striping vanadium peaks, all peaks’ centres should be exactly same as the theortical values.

Criteria To Validate Peaks Found¶

FindPeaks finds peaks by fitting a Gaussian with background to a certain range in the input histogram. Fit v1 may not give a correct result even if chi^2 is used as criteria alone. Thus some other criteria are provided as options to validate the result

1. Peak position. If peak positions are given, and trustful, then the fitted peak position must be within a short distance to the give one.

2. Peak height. In the certain number of trial, peak height can be used to select the best fit among various starting sigma values.

3. Peak width: Peak width cannot be equal or wider than the given fit window.

Fit Window and Peak Range¶

If FitWindows is defined, then a peak’s range to fit (i.e., x-min and x-max) is confined by this window.

If PeakRange is defined and starting peak centre given by user is not within this range, then the situation is considered illegal. In future, FitPeak might be able to estimate the peak centre in this situation by locating the X-value whose corresponding Y-value is largest within user-defined peak range.