Changeset 2381

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Timestamp:
Jul 28, 2016 2:02:20 PM (7 years ago)
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Tutorials/BkgFit
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• Tutorials/BkgFit/FitBkgTut.htm

Fitting the Starting Background using Fixed Points

Fitting the Starting Background using Fixed Points

Introduction:

For this example, we will fit data from 11-BM that has a fairly large broad background peak.

Step 1. Load the diffraction data

For this example, we will fit data from 11-BM that has a fairly large broad background peak.

Step 1. Load the diffraction data

Import the data using Import/Powder Data/from GSAS powder data file. Change the directory to the location of this exercise (exercises/FitBkg Change the directory to the location of this exercise (FitBkg/data  yours might be different). Switch the input file type to use zip archives (.zip). Select file

src="FitBkgTut_files/image027.png" v:shapes="Picture_x0020_2">

A prompt is presented where the header of the file is shown.

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At low angles (below 3° the background is relatively flat. There may be some minor ripple in this, but this is better explored late in the refinement by looking at the difference curve.

Step 2. Define fixed points

in this, but this is better explored late in the refinement by looking at the difference curve.

Step 2. Define fixed points

To fit this, we will initially define points where we think the background should be using the mouse (be sure that the plot zoom & reposition buttons are not selected). Select the

src="FitBkgTut_files/image030.png" v:shapes="_x0000_i1034">

Step 3. Fit the background

Step 3. Fit the background

At this point is becomes possible to fit different background models to the points to see how they do.  Change the function, the number of terms, background models to the points to see how they do.  Change the function, the number of terms, refinement flag(s) and then press

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Adding more terms does improve the fit.

Step 4. Add a background peak

A better choice is to reduce the number of terms and add a background peak to match the feature centered around 5.3 degrees. Initially set the intensity and peak width (but not the position) to be refined.

src="FitBkgTut_files/image033.png" v:shapes="Picture_x0020_7">

The initial fit is not that good,

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but increasing the number of terms to 8 and refining the peak position as well does give a good fit:

Step 5. Try other models

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Step 5. Try other models

One of the nice features of this approach is that we can try very good job at higher angle, even if more terms are added:

src="FitBkgTut_files/image037.png" v:shapes="Picture_x0020_11">

with only 6 background terms:

src="FitBkgTut_files/image039.png" v:shapes="Picture_x0020_13">

Step 6. What to do next

Step 6. What to do next

The background points that have been defined are saved in background plot. To do this, use the Fixed Points/Clear menu item. Youll need to Save/Save as the project file if you want to retain this new background model for further use.

mso-hansi-theme-font:minor-latin'>Fixed Points/Clear
menu item. Youll need to Save/Save as the project file if you want to retain this new background model for further use.

• Tutorials/BkgFit/FitBkgTut_files/filelist.xml

 r1904
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