GammaVision
®
v7 (A66-BW) 783620H / 1013
3.2.5.1. Auto Calibration
The Auto Calibrate button is the fastest and easiest way to do the energy calibration. Just click
Auto Calibrate to use the working library to calibrate the displayed spectrum with no other inputs. The current calibration is erased before the new calibration is started.
Select Close from the sidebar’s control menu.
3.2.6. Efficiency Calibration
Select Calibrate from the menu bar, then
Efficiency. This opens the Efficiency Cali-
bration Sidebar, shown in Fig. 26 (note its similarity to the Energy Calibration Sidebar).
We want to start from the beginning of the process, so open the sidebar’s control menu
(click the icon on the left of the title bar) and select Destroy.
The Library List window should be visible, but if not, click Library/Select Peak... to reopen it.
Fig. 26. Efficiency Calibration Sidebar.
From the library, double-click the 1332.5 keV peak of
60
Co. The marker will move to the corresponding peak and GammaVision will enter the energy in the top section of the Efficiency
Calibration Sidebar.
In the upper section of the sidebar, click the Calc... button to open the Efficiency Calculation
Worksheet (Fig. 27).
Enter the date and the time from the calibration certificate (see Table 2), as Oct-01-90 12:00.
Enter the gammas per second (5838) from the calibration certificate for
60
Co at 1332.5 keV.
Select GPS from the units droplist. Note that the from Library box is checked; this shows that the half-life came from the library.
At this point, the Calculate Efficiency= button at the top of the worksheet should be active
(black rather than gray). If it is disabled (gray), one or more of the data inputs is either incorrect or has not been entered. When you have completed all fields and Calculate Efficiency= is activated, click it to obtain the efficiency value at this energy. Click OK to insert the value in the efficiency table. You will see that the value appears in the Efficiency Table window.
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783620H / 1013 3. GETTING STARTED — A GAMMAVISION TUTORIAL
Fig. 27. Efficiency Worksheet.
You could repeat this process for all other peaks in the calibration certificate data, but there is an easier way. All of the information from the calibration certificate can be stored in an efficiency calibration (
.EFT
) file (see
Section 5.3.3.12 for instructions on creating and editing this file).
Using this file to direct the calibration is easy. On the Efficiency Calibration Sidebar, click the Merge... button, then select the file
GVDEMO.EFT
. GammaVision will use the data in the table to calculate the efficiency at each energy from the spectrum and fill in the efficiency table. You can see the marker jump from peak to peak in the spectrum as the peak areas are calculated.
Table 2. List of Peak Energies.
Reference Time: 12:00 GMT 01 Oct 1990
Nuclide Energy (keV) Gammas/second
241
Am 59.54
2078
109
Cd 88.03
2967
57
Co
139
Ce
113
Sn
137
Cs
122.1
165.9
391.7
661.63
1813
2550
4263
4198
88
Y
60
Co
60
Co
88
Y
898.02
1173.2
1332.5
1836.01
10120
5818
5838
10640
When the data points are all calculated, the fit Mode stored in the file is used to make the fit.
You can select another fit Mode from the Calibration Sidebar. In the Above-the-knee list, choose the Polynomial option. This will automatically produce a single-function polynomial fit of the entire energy range, as shown in Fig. 28. The other modes use separate fits above and below the “knee” of the detector efficiency curve.
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GammaVision
®
v7 (A66-BW) 783620H / 1013
Fig. 28. Polynomial Efficiency Fit.
The fit will take place automatically. You can then view it by bringing the Efficiency calibration fit window forward. Click the window’s control menu to choose scaling and grid display options
(see Fig. 29). The marker line in the efficiency graph window will be at the energy of the peak selected in the Library List or the
Efficiency Table window.
We will accept this calibration and close the calibration session by clicking the Calibration Sidebar’s Close button. Now save this calibration to disk by selecting Calibrate and Save Calibration....
Give the calibration file the name
GVDEMO
. GammaVision will attach the default
.CLB
extension to the filename.
Fig. 29. Control Menu.
3.2.7. Energy and Efficiency Calibration Using the Calibration Wizard
32
