GammaVision
®
v7 (A66-BW) 783620H / 1013
The analysis of the spectrum using Library 1 will give an activity for nuclide D based on the peak at 200 keV (as well as activities for A, B, C, and E). Using this activity, the peak count rate for the nuclide D peak at 500 keV will be calculated using Library 2 and the calibration and corrections, if any. This peak count rate will be converted to a peak shape function (a Gaussian) based on the calibration peak shape and the counting time. This peak will be subtracted channel by channel from the net spectrum. This new net spectrum will then be analyzed using Library 3.
The results of all the calculations (that is, the equivalent peak areas and gross areas) will be shown on the full report. If Library 1 or 3 does not contain any peaks in the energy range being analyzed, this option is turned off. If Library 2 does not contain any peaks in the energy range or any peaks in Library 1, the subtraction of peak areas does not take place, but the analysis using
Library 3 is still performed. This can be an easy way to obtain a report with MDA for some nuclides (those in Library 1) and without MDA for other nuclides (those in Library 3).
6.6. Fraction Limit
To verify the identification of a particular nuclide in a spectrum, the number of identified peaks is compared to the number of possible peaks, where “identified” peaks are those with a 1-sigma counting uncertainty less than the Peak Cutoff value, and the possible peaks include only those within the low-/high-energy analysis range. Peaks with the “Not in Average” flag set as NOT
TRUE in the library are excluded. It is expressed as follows:
(58)
where Br is the branching ratio for the peak for the given nuclide, adjusted for TCC where applicable; l is the sum over the located peaks; and p is the sum over the possible peaks. This fraction is between 100 for all peaks located and 0 for no peaks located.
This value is compared to a limit value, entered on the System tab under Analyze/Settings/
Sample Type... (page 152), to determine whether this nuclide’s peaks are present in sufficient measure to say the nuclide is present. The fraction limit test is passed if the fraction is above the selected value.
To disable the fraction limit test, enter a limit of zero on the System tab.
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