783620H / 1013 6. ANALYSIS METHODS
The true coincidence correction (TCC) is the correction necessary to account for all of the pulses removed from the full-energy peak. This correction is a simple divisor of the net peak area, that is the net peak area is increased by the correction factor. The correction factor is detector and sample geometry dependent. The correction factor depends on the full-energy efficiency, that is the ability of the detector to detect the total energy of the gamma ray, and the total efficiency, that is the ability of the detector to detect any part of the gamma ray energy. The fullpeak efficiency is determined in the efficiency calibration (Calibrate/Efficiency) and the total efficiency is determined in the TCC part of Calibrate/Calibration Wizard....
6.17. ISO NORM Implementation in GammaVision
This section details the analysis methods for the GammaVision implementation of ISO NORM.
1
Net peak area has been taken as the measurand defined in ISO NORM (see Eq. 141–143). The background variance is calculated from Eq. 145, and special cases are addressed in Eq. 197–200.
The critical level, in counts, is calculated from Eq. 160; and the MDA, in counts, is calculated from Eq. 162. The conditions to converge to Currie’s MDA method (see GammaVision MDA
Method 12, “Regulatory Guide 4.16,” Section 6.9.2.12) are presented in Eq. 169–171. The best estimated activity, in counts, is calculated from Eq. 183, and the uncertainty in the best estimated activity, in counts, is calculated from Eq. 186. The confidence intervals are calculated from
Eq. 187 and 188, respectively. Negative peak area and confidence interval are discussed in
Section 6.17.7.2.
6.17.1. The ISO NORM Model in GammaVision
The ISO NORM model is discussed in Section 5.2 of ISO NORM, and is described mathematically as:
(141)
The parameters above are generic. As an example in ISO NORM, x
1
is the gross count rate, x
2
is the background count rate, x
3
is a shielding factor, x
4
is an additional background correction term, and w is a conversion factor.
In GammaVision, the model is a simplified version of Eq. 141 above:
(142)
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