GammaVision
®
v7 (A66-BW) 783620H / 1013
6.3.7. Peak Search
After the library peaks are located, the spectrum is searched for any other peaks. This is needed, even if the list of unknown peak values is not requested, for correct calculation of the peak background near peak multiplets and for determination of the peak centroids for deconvolution of multiplets not in the library. The stepped background test compares the background above the peak area to the background of the peak area (see “Background for Multiplets,” Section 6.5.2).
The peak search method is based on the method proposed by Mariscotti. In this method it is assumed that the spectrum, C(n), is continuous and the background is a linear function of the channel number in the vicinity of a peak. This implies that the second derivative is zero for background regions and non-zero in the peak regions. In order to reduce the effect of statistical fluctuations, the smoothed second difference is used (see Fig. 240).
Fig. 240. Second Difference.
The second difference can be represented as: where:
k i
= the smoothed second difference weighting functions
2j +1 = the smoothing width
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783620H / 1013 6. ANALYSIS METHODS
For regular peaks, j = 4; for wide peaks, j = 9. There are nine coefficients for regular peaks and
19 for wide peaks. The criterion for using the wide-peak filter is that the spectrum resolution in keV per channel at the center of the spectrum is <0.15 keV/channel.
The peaks are located where the second derivative varies significantly from zero.
A typical gamma-ray spectrum is shown in Fig. 241. This gamma-ray spectrum is far from the ideal spectrum of a well-formed peak on a smooth background. Shown are seven features that can be distinguished and accounted for in the peak-detection algorithm.
Fig. 241. A Typical Gamma-Ray Spectrum.
1) The full-energy photopeak that results from the complete capture of all the photon energy in the detector and is the most well-defined feature
2) The Compton edge for the full-energy peak
3) The Compton plateau
4) The backscatter peak
5) The pulse pileup or sum peaks from the addition of the peak energies in the detector or electronic processing
6) The single-escape peak
7) The double-escape peak
Not all of these will appear in a given spectrum. For example, escape peaks cannot occur for photons less than about 1 MeV.
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