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Fig. 119. Select the File that Contains the Desired Energy
Calibration.
5.3.8.2. Efficiency and Efficiency-plus-TCC Calibrations — Setting Up a New Calibration or Recalling from File
Create New
If you chose to create a new efficiency or efficiency-plus-TCC calibration, the dialog shown in
Fig. 120 opens. Use this dialog to enter the source Certificate File, Library file, Source Label, and TCC Calibration Method settings.
When performing this calibration on the MCB, you must enter the live-time preset (Count
Time) in seconds. In addition, you might also wish to mark the Clear Data Before Start checkbox. The counting time must be long enough to accumulate well-formed peaks with low counting uncertainty. This is especially important for TCC calibration, which uses the summed peaks in the calculations.
Also note that your library and TCC table should include only peaks that exist in the spectrum.
To use the spectrum currently in the MCB, unmark the Clear Data Before Start box. This is useful if the same source is used for both energy and efficiency calibration.
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Fig. 120. Create a New Efficiency Calibration.
The Source Label entered here is used in a later step that tells you which source to put on the detector.
Certificate File
Use the Browse button to find the correct certificate. To make changes to the certificate, click
Edit. This opens the wizard’s Certificate File Editor dialog, shown in Fig. 121.
The certificate file is the same as the efficiency standard file, except that the contents of the efficiency field are not used here and all fields must have valid contents. The certificate file also has the
EFT
extension. Tables with all fields entered can be used for both the calibration wizard and the efficiency calibration (Calibrate/ Efficiency...). Any energy in the
.EFT
file that is not completely filled in will be ignored by the wizard. The file contains all the data needed to perform an efficiency or efficiency-plus-TCC calibration using this standard source. This file can be created here, with the Efficiency calibration sidebar (see Section 5.3.3.12) or with an ASCII text editor.
The table contains the following columns:
1) Isotope name (same as library).
2) Gamma-ray energy (keV).
3) Activity in Bq or
μCi at the date and time specified in column 6.
4) Gammas/sec for this energy at the specified date and time.
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Fig. 121. Edit Wizard Certificate File.
5) Uncertainty for this nuclide.
6) Calibration date and time for the gammas/sec calibration. The gammas/sec are automatically decay corrected from the date/time in column 4 to the date/time of the spectrum acquisition.
7) Half-life of this nuclide in days.
8) Branching ratio (yield) as gammas/100 disintegrations.
To add an energy to the table, enter the values directly or click Select from Lib, then click Add
New.
To delete an energy, select the energy and click Delete.
To change the values for an energy, select the energy, enter the new values directly or click
Select from Lib, then click Update to record the changes (you must click Update or the changes will not be made).
When finished, click Save As... to rewrite the new file to disk. To discard the changes you have just made, click Cancel; a dialog will verify that you want to discard.
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Library
Click Browse to find the correct library file. For TCC calibrations, the library (
.LIB
or
.MDB
) file and TCC table should include only the nuclides present in the spectrum.
To view or change the contents of the library, click Edit. This opens the GammaVision library editor, which is discussed in Section 5.6.3, page 208.
When finished, click Next.
TCC Calibration Method
The Single Point Source Method uses a point source with all the nuclides needed in one source.
The point source is normally a small area (1 mm–2 mm diameter). Larger sources can be used if they are more than a few centimeters from the detector. An example of the nuclide mixture is given below.
The Single Extended Source Method uses bulk or large-volume sources such as Marinelli beakers or bottles with all the nuclides needed in one source.
The source must be a mixture of nuclides with gamma rays that do not have true coincidence summing and nuclides with gamma rays that do have summing. The energies of the gamma rays must extend over the range of interest for the unknown samples. A mixture of
109
Cd,
113
Sn,
139
Ce,
203
Hg,
134
Cs,
137
Cs,
88
Y, and
54
Mn will be sufficient for most situations.
241
Am can be added for lower energies.
NOTE
60
Co should not be added because of the interference with gamma rays from
134
Cs.
The selection of point or extended geometry changes the fitting process in the calibration calculation which gives different calibration coefficients for the different geometries. The TCC results depend on the source/detector geometry, so selecting the correct geometry is important.
Absorber
The absorber present/not present selection determines the low-energy fitting function. The efficiency for low-energy gamma rays depends on the absorbing material between the detector and the source. The absorbing material and thickness are not important. GammaVision will automatically account for the loss in the fitting process if the absorber is present. The low-energy coefficients are listed as the last 2 (of 6) of the TCC polynomial coefficients and are zero for absorber Not Present.
When finished, click Next.
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