GammaVision
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v7 (A66-BW) 783620H / 1013
The shape of the second derivative can be used to reject Compton edges and other non-peak structure in the data (Fig. 242).
Fig. 242. Second Difference for a Compton Edge.
6.3.7.1. Peak Acceptance Tests
The second difference must pass the following tests to be considered a peak.
(53)
where G is related to S, the PEAK SEARCH SENSITIVITY factor, set on the System tab under Analyze/ Settings/Sample Type..., as follows:
(54)
where:
G = a constant proportional to the resolution of the detector
F = 1.35 for the wide-peak filter and 1.0 for the regular-peak filter
In addition, the spectrum data at the channel indicated by the second derivative must pass these tests.
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783620H / 1013 6. ANALYSIS METHODS
(55)
where:
C(n)
= the channel data of the n th
channel
C
O(n) = the second difference at the n th
channel
If the wide-peak filter is used, the second derivatives must also meet the following criterion:
(56)
The peak centroid is calculated using the weighted channel sum method as follows:
(57)
where:
P = peak centroid in channels
C i
= net contents of channel i
i = channel number
l = peak low limit
h = peak high limit
Once a peak is located, it is recorded and the peak search starts again.
After a peak has been located, if it is not in the library, the peak background, net area, and uncertainty are calculated in the same manner as library peaks. If the peak uncertainty is less than the sensitivity threshold level you entered, the peak is added to the list of unknowns. If the peak is within the deconvolution width (approximately 3.3 times the FWHM) of a library peak, then the peak is marked on the output list. The unknown peaks are included in a deconvolution when they are close enough to affect the peak area calculation.
If a peak is located in the spectrum and the library peak is a subsidiary peak, where the major peak has not been found, the peak will be maintained in the unknown list.
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