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in the spectrum. The spectrum can be uncalibrated, calibrated, or even incorrectly calibrated.
Spectra such as the mixed gamma standard or uranium ore can be used with the appropriate library.
Auto Calibrate works as follows: the spectrum is searched for all the major peaks, then this peak list is compared to the library peak list to find the calibration that gives the best match.
NOTE
If the No FWHM Cal checkbox on the Energy Calibration sidebar is marked, the
FWHM calibration is not changed during calibration. This might be desirable when decay of short-lived isotopes in a calibration source results in peaks that are adequate for an energy calibration but whose shape might not be optimal for the FWHM calibration.
5.3.2.3. Manual Calibration
If there is no energy calibration, the energy of one or two known peaks must be entered. Using the Full Spectrum View, select a peak in the high energy (channel) part of the spectrum. When this part of the spectrum is visible in the expanded display, move the cursor to the known peak.
At this time the centroid of the peak will be calculated and displayed in the upper part of the
Energy Calibration Sidebar as, in this example, Channel: 11342.66. This is the channel number of the peak centroid. Now click in the E= input box and enter the energy of this peak. Click the
Enter button or press <Enter>. A table and graph will appear on the screen (Fig. 93). They can be moved around and sized if they obscure the spectrum.
The table shows one value (the one just entered), and the graph shows a straight line fit from
(energy = 0, channel = 0) to the energy and channel just entered. This is an approximate calibration; it should be fairly accurate if the zero offset is small.
The Energy and FWHM radio buttons at the bottom of the Energy Calibration Sidebar display the table and graph for either energy vs. channel (Fig. 93) or FWHM vs. energy (Fig. 94).
Click FWHM. The table shows the one value entered and the graph shows a horizontal line. For a single point, the FWHM is assumed to be a constant.
Using the Full Spectrum View (or the Library List window), select a peak in the low-energy part of the spectrum and move the marker to the centroid of the peak. Again click in the E= field at the top of the sidebar and enter the energy of this peak. Both the energy function and FWHM function, as well as their corresponding tables, will update with the new entry so progress can be monitored.
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Fig. 93. Energy Calibration Display.
At this time, the cursor can be positioned using the calibration graphs, calibration table, Full
Spectrum View, or Expanded Spectrum View. The cursor will show the energy based on the calibration up to this point.
The calibration can be refined by adding as many points as desired. Any point can be deleted by selecting that point in the table of values (Energy or FWHM) and clicking on the Delete
Energy button on the calibration sidebar. The fit updates when a point is removed.
NOTE
If the No FWHM Cal checkbox on the Energy Calibration sidebar is marked, the
FWHM calibration is not changed during calibration. This might be desirable when decay of short-lived isotopes in a calibration source results in peaks that are adequate for an energy calibration but whose shape might not be optimal for the FWHM calibration.
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Fig. 94. FWHM Calibration Display.
After the desired number of points have been entered, you can save the energy values by clicking the sidebar’s Save... button. This opens a file-save dialog (Fig. 95).
Assign a filename and GammaVision will append the default energy-calibration extension,
.ENT
.
The saved table of values now contains the Energy-Channel and Energy-FWHM pairs used in the current calibration. This table can be used for future calibrations using the same nuclides. It can also be edited within GammaVision. To do this, click on the sidebar’s title bar icon to open the control menu (Fig. 96). Select Edit File... and open the
.ENT
file to be edited. It will be displayed in the Energy Table Editor dialog shown in Fig. 97.
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The Energy Table Editor will list each
Energy and its associated Channel and FWHM as determined during the calibration. The values in each column can be edited. In addition, you can Add
New rows or Delete existing ones. This allows you to fine-tune existing calibrations or generate calibrations using data pairs for non-ORTEC spectra. Note that if all Channel and FWHM values are deleted from the table, the Energy list and spectrum data will be used to generate a calibration when the table is
Merged as described in the next section.
Fig. 95. Save Energy Calibration Table.
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Fig. 96. Calibration
Sidebar Control
Menu.
Fig. 97. Editing an .ENT File in GammaVision.
After modifying the table, click OK to save the table in the current
.ENT file, Save As... to save it under a different filename, or Cancel to close the editor and retain the original values.
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