GammaVision
®
v7 (A66-BW) 783620H / 1013
Fig. 253. Random Summing. The Sum Peak Is the Sum of Two
Coincident Low Full-Energy Peaks.
6.11.1. Random Summing Correction
The uncorrected peak area is multiplied by RSF to obtain the corrected peak area. This is done before the peak area is converted to activity.
The slope of the correction curve is dependent on the detector and the source/detector geometry.
It is experimentally determined by the following simple procedure. The procedure requires two radioactive sources: one of low activity and high energy, and one of high activity and a lower energy. The energy of the high-activity source must be chosen so that it will not interfere with the high-energy peak, i.e., it should be less than half the energy. Two such sources are
88
Y and
137
Cs.
Position the high-energy source in front of the detector so that the count rate is low and collect a spectrum with a small counting statistical error (e.g., 100000 counts in the peak area). Measure the net peak area of the high energy gamma ray and the total counts in the spectrum (see
Section 5.4.3, Calculate/Peak Info...). Now position the low-energy source in front of the detector to increase the count rate and collect a second spectrum for the same live time as the first spectrum. Again measure the net peak area and total counts in the spectrum. More spectra can be collected by moving the low-energy source closer to the detector to increase the count rate.
For example, six spectra were collected and the results are shown in Table 14.
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