True Coincidence Summing Correction in Gamma Spectroscopy

close to the detector, often in Marinelli beakers. The extent to which TCS is ignored is
best illustrated in Figure 2.7, which shows the results of an intercomparison arranged
by the National Physics Laboratory in UK (NPL) in 1989 for the measurements of
various radionuclides, including Cs-134, at environmental levels. Out of 58 Cs-134
results reported, only four were within the range expected by the NPL (i.e. within the
shaded band in the Figure-2.7.) and in only 11 cases, the true results lied within the
68% confidence limit reported by the laboratory. It is obvious that the majority of the
results were low; 64% of results reported were more than 5% below the expected
value. The most likely reason for this is that TCS had not been taken into account by
the laboratories reporting these results.
Figure 2.7 Summarized Cs-134 results from the 1989 NPL environmental radioactivity
intercomparison exercise.[8]
To achieve accurate results, the true coincidence summing must be taken into
account. In fact, further NPL intercomparison exercises in 1990 and 1992 showed
similar biases in the Cs-134 results, although slightly smaller on average. Inspite of
the growing awareness of TCS, many labroratories do not take it into account.
2.8 Achieving Valid Close Geometry Efficiency
Calibrations
A valid efficiency calibration curve can only be measured for close geometry
conditions by using radionuclides which do not suffer from TCS. Table-2.2 lists
several nuclides which emit only a single gamma-ray and therefore cannot
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