783620H / 1013 6. ANALYSIS METHODS
on other gamma rays from that isotope. That activity is then used to calculate the contribution to the overlapped peak of this isotope. That contribution is subtracted from the total peak area to obtain the peak area due to the other isotope.
Table 7. Gamma Peak Overlap Examples.
Isotope
99m
Tc
224
Ra
226
Ra
241
Am
241
Am
241
Am
Energy
(keV)
140.99
241.00
185.99
26.35
33.20
59.54
Probability Isotope with
89.3
3.90
3.28
2.5
0.11
36.3
close energy
99
Mo
92
Sr
235
U
237
U
144
Ce
237
U
In the example above, the activity of
235
U is calculated from the area of the peak at the next most probable energy, i.e., 143 keV. Then the area of the 185.72 keV
235
U peak is calculated using the branching ratio of that gamma ray, the efficiency, and the activity. The
235
U area is subtracted from the area of the peak at 185 keV to give the area due to
226
Ra. From this, the activity of
226
Ra in the sample is calculated.
6.5.5.2. Manual Peak Stripping
The manual method of analysis uses three libraries. The first is the working analysis library with the severely overlapped peak multiplets removed. Multiplets that can be deconvoluted by fitting should remain in the first library. The Second Library contains the peaks that are to be stripped from the overlapped doublet. The amount to be stripped is based on the analysis from Library 1.
The nuclide name for the stripped peak must be the same in both libraries, as this is how the program determines which peak areas to use. The Third Library contains the nuclides with the other peak in the overlapped doublets. The computations for the third library are the same as the first library except that deconvolutions are not performed and MDAs are not calculated for the third-library nuclides. You have complete control over the three libraries and can choose which peaks to use in each step. Nuclides that appear in both Libraries 1 and 3 are reported twice on the report.
For example, consider the case in which a spectrum contains nuclides A, B, C, D, E, and F.
Nuclide D has energies of 200 keV and 500 keV, and nuclide F has only one peak at 500 keV.
Let Library 1 contain five nuclides labeled A, B, C, D, and E. Let Library 2 contain nuclide D, and Library 3 contain nuclide F. In Library 1, nuclide D is listed with only the energy at 200 keV. In Library 2, nuclide D is listed with only the energy at 500 keV.
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