GammaVision
®
v7 (A66-BW) 783620H / 1013
where:
A i
= the activity of isotope i
D i
= the allowed value for isotope i
DAC = the fractional allowed value in percent
The value is not calculated for MDA values.
6.16. True Coincidence Correction
In the case where a nuclide emits multiple cascade gamma rays when it decays, these multiple gamma rays can be detected individually or together as one gamma ray. An example decay is
60
Co, as shown in Fig. 255.
The two gamma rays (1173.2 and 1332.5 keV) are emitted in cascade or one after another. The lifetime of the intermediate state is very short so that it appears
Fig. 255.
60
Co Decay to
60
Ni.
that the two gamma rays are emitted in coincidence
(i.e., at the same time). Since the two gamma rays can interact in the detector in a time short compared to the response time of the detector and the resolving time of the electronics, the two gamma rays are recorded as a single gamma ray.
When the two gamma rays are detected as one, it effects the spectrum in two ways. One way is the creation of a “sum” peak that is the sum of the amplitudes of the two individual full-energy peaks. The second way is to remove counts from the full-energy peak. The first creates extra peaks in the spectrum. The second reduces the peak area of these gamma rays and thus the reported activity of the nuclide.
The reduction of the peak area due to summing is more important than the creation of the “sum peak.” The sum peak requires the addition of two full-energy pulses to get the summed energy.
The reduction of the peak area only requires that there be a full-energy interaction at the same time as another interaction from the other members of the cascade. Recall that most of the interactions in the detector do not produce full-energy peaks, but produce partial-energy interactions
(the Compton background). A summing with any of the partial-energy signals will result in the full-energy pulse being removed from the full-energy peak. Further details can be found in many references.
39,40,41
39
Glenn F. Knoll, Radiation Detection and Measurement, 3rd edition, p. 323, Wiley and Sons, 2000.
40
Gilmore, G., and J.D. Hemingway, Practical Gamma-Ray Spectrometry, p. 156, Wiley and Sons, 1995.
41
“Calibration and use of Germanium Spectrometers for the measurement of gamma-ray emission rates of radionuclides,” ANSI N42.14-1991.
310
