GammaVision
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v7 (A66-BW) 783620H / 1013
5.3.3.6. TCC Polynomial Fit
The TCC polynomial fit is several polynomial fits to different energy parts of the spectrum (up to a six-order polynomial). The different energy regions are below 200 keV and above 200 keV.
The details of the polynomial fit are given in the papers referenced below.
21,22,23
This fit can be used for p- or n-type detectors, and is used in the GammaVision TCC correction method.
5.3.3.7. Performing the Efficiency Calibration
To efficiency calibrate the system, collect an energy-calibrated spectrum of the known standard for a time sufficient to get well-formed peaks with small uncertainty. The certificate supplied with the source will have the energies, gammas/sec, nuclide names, and measurement date needed in the calibration.
The working library is used in the calculations. Load the library for the calibration source as shown in Section 3.2.3 or 5.6.2.
Expand the spectrum horizontally to show the peaks completely. Select Calibrate from the menu, then Efficiency. This will open the Efficiency Calibration Sidebar (Fig. 104). If the
Efficiency item is disabled (gray), the system is not energy calibrated. Because there is yet no efficiency calibration, no graphs or tables are shown. Choose a spectrum peak listed in the source data sheet.
Use the Full Spectrum View (Fig. 105) to approximately locate the peak, or use the
Library List and the Expanded Spectrum
View to put the marker on the center of the peak. This selects the peak. The peak area and count rate are calculated in the same manner as in the analysis program.
Click the Calc... button to open the Efficiency Calculation Worksheet for entering the data about the peak (Fig. 106).
Fig. 104. Efficiency Calibration Sidebar.
21
M. Blaauw, “The use of sources emitting
γ-rays for determination of absolute efficiency curves of highly efficient Ge detectors,” NIM A322, 1993, pp. 483–500.
22
R. Gunnink, “New method for calibrating a Ge detector by using only zero to four efficiency points,” NIM
A299, 1990, pp. 372–376.
23
Gunnink, R., and A.L. Prindle, “Nonconventional methods for accurately calibrating germanium detectors,” J.
Radioanalytical and Nuclear Chemistry, 160(2), 1992, pp. 304–314.
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783620H / 1013 5. MENU COMMANDS
In the Assay (from Certificate) section of the dialog, enter the calibration Date and Time from the source data sheet. Enter the Activity from the source and select the units from the droplist.
Fig. 105. Select Peak in Full Spectrum Window.
Fig. 106. Efficiency Calculation Worksheet.
The source Uncertainty is entered here but can be left at 0.0. This uncertainty is used in the total uncertainty calculation.
When the values are correct, click the Calculate Efficiency = button at the top of the dialog and the calculated efficiency will be entered in the field beside the button. If this value is acceptable, click OK to save the input and efficiency values and leave the worksheet. The graphs and tables will now be displayed.
Select the next peak and repeat the process. The Date and Time will default to the previously entered values, but the Nuclide Half-Life and Activity must be entered for each energy.
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As each peak (above the minimum) is entered, the table and graph will update and new fits will be made. The fitting mode can be changed at any time to see how the various functions model the data. Note that for quadratic fit, a linear fit is made for one or two points and a quadratic fit is not done until three points are entered. For a polynomial fit, no fit is made until five points are entered. Because of the separate energy regions, the TCC polynomial fit requires more points.
Any point in the Efficiency Table can be deleted by selecting the point then clicking on the sidebar’s Delete Entry button. Any point in the table can be modified by selecting it and clicking
Calc.... When the worksheet opens, the previously entered values will be shown. These values can be changed and a new efficiency generated by clicking on Calculate Efficiency=. To retain the changes, click OK; to discard them, click Cancel.
You can change the knee energy by clicking on the Knee... button in the calibration sidebar. This will open the Knee dialog (Fig. 107). It will display the energy value for the knee. To change it, enter a new number and click Apply.
This will move the knee to this energy and update the fit, graph, and table. The value in the Knee dialog field will be
Fig. 107. Set Knee Energy.
set to the marker energy when you move the marker and click in the spectrum or the Efficiency graph window. This is most easily seen in the Efficiency graph window. The knee energy is not changed until you click Apply. To close the Knee dialog, click the Close box. GammaVision will use the knee value shown when Apply was last clicked.
The Merge... button on the Energy Calibration Sidebar allows you to merge two efficiency tables. Clicking on it opens a standard file-recall dialog. Choose the
.EFT
file to be used. and click on Open. GammaVision will use the efficiency table data to calculate the efficiency at each energy from the spectrum and fill in the efficiency table.
The table of worksheet entries, including the gammas/sec, half-life, and certification date, contains all the information needed to do the calibration. It can be saved by clicking the Save... button in the Table area of the sidebar. This opens a standard file-save dialog. Enter a filename and click Save; GammaVision will assign a default extension of
.EFT
.
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