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To use more than one stored spectrum to make a single calibration:
1) Calibrate using one spectrum.
2) Exit the calibration function.
3) Save the calibration in a file.
4) Recall the second spectrum.
5) Recall the calibration (because recalling the spectrum has replaced the first calibration with the calibration from the spectrum).
6) Select Calibrate/Energy... and enter the peak energies for the second spectrum.
The process can be repeated for additional spectra.
5.3.3. Efficiency...
5.3.3.1. Introduction
The Efficiency... calibration function calculates the detection efficiency of the HPGe detector system as a function of energy. The efficiency of the detector system is the relation between the number of gamma rays emitted from the source to the number of gamma rays collected in the full-energy peak.
The HPGe detector system efficiency includes effects from the detector itself, the detector/ source geometry, the materials surrounding the detector, and absorption in the source material or matrix (Fig. 101).
In general, it is not good practice to use efficiency calibrations from one detector/source geometry for other geometries. Therefore, different calibration files should be made for all the different detector/source combinations to compensate for the differences between the geometries. It might be useful to assign calibration files names that give some indication of the detector/source geometry to which they apply.
In addition, you should always make sure the library Match Width setting (on the System tab under Analyze/Settings/Sample Type...; see page 152) in effect during efficiency calibration is the same setting you will be using during sample analysis. This is because the net peak area calculation can be affected by the Match Width setting. The default setting is 0.5. If using a different setting, we recommend a value between 0.4 and 0.75.
Since the efficiency is defined as a function of energy, the Energy... calibration must be done first. The Efficiency... command remains disabled (gray) until the spectrum has been energy calibrated.
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Fig. 101. Detector with Extended Source.
The energy recalibration can be redone (to account for gain changes) without the need to redo the efficiency calibration.
P-type germanium detectors, such as the ORTEC GEM Series, have a maximum efficiency at about 150 keV; for n-type detectors, such as the GMX Series, it is about 100 keV. For detectors above about 50% relative efficiency, these values will be somewhat higher (Fig. 102). For both types, these maxima, or knee values, depend on the individual detector. For p-type GEM detectors, the efficiency goes down as the energy goes down from the knee. For n-type GMX detectors, the efficiency is nearly constant at energies below the knee. For both types, the efficiency goes down at energies above the knee.
The efficiency calibration is critically important to the accuracy of the activity results from
GammaVision. It is recommended that only calibrated sources traceable to a known standard be used. The time between the calibration of the radionuclide source by its manufacturer and the time the spectrum is collected is important, as this defines the decay correction needed to calculate source strength for the spectrum.
A source should be selected that contains isolated singlets over the entire energy range of interest. If the energy region near the knee is important to the analysis, several points around the knee should be used for both the two-function and polynomial type of fits. If you wish, you can perform the efficiency calibration using one or more spectra to minimize the difficulty of obtaining the required number of singlets.
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Fig. 102. Detector Efficiency as a Function of Energy.
As of v7, GammaVision calculates and stores the counting uncertainty in the calibration record, similar to the way the fit uncertainty is calculated and stored. There is an above-the-knee counting uncertainty and a below-the-knee counting uncertainty. Above-the-knee counting uncertainty is the averaged counting uncertainty of all the calibration peaks with energy above the knee energy. Below-the-knee counting uncertainty is the averaged counting uncertainty of all the calibration peaks with energy below the knee energy. Both are stored in the calibration data record.
See the discussion in Section 6.12.7.
To perform the calibration, you need an energy-calibrated spectrum of the radionuclides and their source strengths and calibration dates. These data are entered into GammaVision in convenient menu-type forms, and you can review the results of each step. Questionable points can be deleted, additional points added, and the fitting process repeated until the desired result is obtained.
If there are many well-separated peaks, GammaVision can use two energy regions for separate fitting. The energy separating the two regions (called the knee) is specified by you. The best fit to the two regions is often obtained by entering a knee energy that corresponds to a region where the efficiency is slowly varying and not at the maximum point. This is usually about 400 keV to 500 keV. By using the calibration plotting feature, the effect of the knee energy can be seen and the best value can be easily determined.
There are several options for the type of fit used to describe the efficiency/energy relationship
(see Fig. 103). These are:
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