True coincidence summing correction

The latter restriction is a consequence of lost counts due to true coincidence summing. I will discuss this fully in the next chapter. For a nuclide emitting a single gamma-ray, such a restriction would not necessarily apply. In general, the use of corrections of this type is best avoided. It is much more satisfactory to standardize on a small number of counting positions and create a separate efficiency calibration at each one. If it does become necessary to make mathematical corrections, it should be borne in mind that. 

In deriving Equation (7.16), a number of simplifications have been made and one would not expect to be able to correct for gross changes in geometry in this manner. It should also be noted that changes in true coincidence summing are not accounted for explicitly although the empirical manner in which the correction factor is derived would tend to do this if the nuclide used were the same as that to be measured in practice. 

The test spectra are accompanied by the spectra of calibration sources and background spectra, together with the appropriate documentation. Because measurement of low-activity sources will often be made close to the detector, errors due to true coincidence summing could be expected. Some programs do allow such corrections to be made and to facilitate that point source spectra of a number of nuclides needed to generate a total efficiency calibration are provided. 

In Chapter 6,1 mentioned the moves being made in the direction of providing detector calibrations based upon the dimensions and physical parameters of the detector and its encapsulation. Once this happy state of affairs has been attained, then the extension of that to include true coincidence corrections becomes possible. Again, one can envisage a situation where a new detector is delivered with a CD-ROM holding the theoretical calibration curve and the latest nuclear data for the nuclides of most common interest that would be accessed by spectrum-analysis programs for automatic summing corrections. For that, we must wait, but help does now appear to be on the horizon. 

Summing of events, either a result of coincident emission of gamma rays in the decay chain of the nuclide of interest or of random coincident emissions, can lead to significant losses or potential additions to an otherwise clean peak (De Bruin and Blaauw 1992 Becker et al. 1994). While coincidence losses are not an issue for comparator NAA, calibration, and/or computational correction must be applied (Debertin and Helmer 1988 Blaauw and Celsema 1999) to arrive at true peak areas for other methods of calibration. 

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