True Coincidence Summing Correction in Gamma Spectroscopy

some sources with short half-lives which would not be appropriate for routine
calibration. Nevertheless, with an appropriate selection of nuclides it is possible to
create a valid close geometry efficiency calibration. The TCS-free single gamma-ray
efficiency curve will still be irrelevant to the estimation of the nuclides with complex
decay cheme. It needs to be corrected for the summing in the sample measurements
In practice spectrum analysis is constrained by the type of the software used. So
it is pertinent to consider how the spectrum analysis program would treat calibration
data such as that in Figure-2.2. Most likely it will construct a 'best' but completely
invalid fit, perhaps as in Figure-2.8. It is clear that all the points are likely to
be lower than they should be and that the true calibration curve should lie above
them all.
For comparative analysis, standardized sources are required. The spectrum
analysis program calculates results by reference to a calibration curve so, a set of
correction factors for each gamma-ray needed to be measured which can be
applied to the output from the program. Taking as an example the data in Figure-
2.8, the procedure would be;
• Construct an efficiency calibration
• Measure calibrated sources of each the nuclides to be determined,
prepared in the standard geometry and of the same density as the samples
to be measured.
• Use the computer program to calculate the activity of these sources
based upon the estimated calibration. The ratio between measured and
actual activities is the correction factor to be applied to the sample
measurements.
A fundamental problem arises when standardized reference sources are not available
for a particular nuclide; in this case it would be necessary to perform
measurements at a large source-to-detector distance using an appropriate TCSfree
calibration curve and compare these with the close geometry measurements.
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