2.4 MCA Displays

There are three functions under this group.

2.4.1 Preferences

This window allows you to change the display preferences, such as changing the vertical scale, horizontal scale, etc., without affecting the data file.

2.4.2 JCPDS

This allows you to select the JCPDS diffraction file for a given material. Note that although many materials are already in the collection of the JCPDS files, your sample file may not exist. To create a new JCPDS file, following this link.

2.4.3 Fit Peaks

Peak fitting using the new MCA routine is also significantly easier than it once was. First thing you need to do is call up the file you want to fit. Then open up the peak fitting routines by selecting Display from the menu, and then Fit Peaks. A new GUI should open that looks like this. The procedure is relatively straight forward with one little bit of confusion. Let’s look at each section. The top section, Background Parameters, is where you can select the parameters for your background subtraction and actually fit a background if you want.  The choices are pretty self explanatory. I find the default value work best for high count rate peaks, but feel free to play around to get something you find more appealing. The next section, Peak Fit Parameters, are where you'll tell the MCA routine what peaks you want to fit. The Energy Calibration should correspond to the energy calibration at the time you saved the spectra, so there should be no reason to change these values. Likewise, the FWHM of each peak will be individually optimized so typically there's no reason to change the values under Initial FWHM Calibration. 
        In the next section you'll tell the MCA program what peaks to fit. Unlike the ROIs, here you have no limit on the number of peaks you can define. The first peak that will be listed will have no Label and the energy will be meaningless, like the line highlighted in blue in the example above. This is normal, what it's saying is that...OK here's a peak waiting to be defined by you! So let's define it. Highlight the peak (it'll be marked in blue), and in the Label box above enter the name of the peak you wish to fit. Again, follow our naming conventions (i.e. Fe Ka for iron k-alpha, Mn Kb for manganese k-beta, Pb La1 for lead L-alpha 1 etc.) After you've entered the name hit the Enter key and what you'll see is that the name of the line selected in blue will change to reflect this and that the information about this peak (energy, FWHM, etc.) will change to correspond to the values of any peak that the routine found at this position in your spectra. You'll also see that in your spectral display the peak will now be marked with a single green vertical line. If you would rather enter energies instead of the names of the fluorescence lines, just enter the number in the Label field and hit Enter. To add another peak now click on the Insert button. Now select this line (it'll be marked in blue) and enter its information. Repeat this procedure until all your peaks are defined. To change a peak simply select it (it'll be marked in blue) and change its Label. The buttons to the left allow you to manipulate the defined peak positions and should be self-explanatory. Once you've defined all your peaks you have the option of saving this list or recalling a previously saved list of peaks by selecting File and then either Write Peaks... or Read Peaks... from the menu.
        Now we're ready to fit the peaks. Click on that Fit Peaks button at the bottom. In your spectrum display you'll see each peak being fit and then a new window will open showing you the numerical output of this fit listing data about your run conditions, what peaks were fit, their energies, the area calculated for each peak (minus the background), and the area of the background underneath the peak. By default this information is saved in an ascii file named fit_results.txt (and in a delimited file...delimited by the symbol #...named fit_spreadsheet.txt suitable for import into a spreadsheet) that resides in the same directory as the selected spectra. You can change both where the file resides and its name by selecting File and then Results File (or Spreadsheet File) from the menu. Additionally, by default subsequent peak fits are appended to this file. You can change this with the drop down list at the bottom of the display if you'd rather overwrite the file with a new one.