Worldwide Open Proficiency Test: Determination of ... - Nucleus - IAEA

4.4. Recommendations related to the analysis of Pb-210 in moss-soil sample
The majority of participating laboratories measured Pb-210 in moss-soil at 46.52 keV using
gamma spectrometry with high resolution detector of different types and calibration methods.
It can be noticed that the level of analytical performance using gamma spectroscopy has been
improved comparing to the results of the PT conducted in 2006. The percentage of acceptable
results in 2009 PT is 20% higher than in 2006 PT.
It was observed that the main reasons for unacceptable performance in Pb-210 measurements
are the following:
• Extrapolation of the efficiency calibration curve to estimate the efficiency at 46.52
keV which could lead to a substantial error as the efficiency curve at this region has a
significant slope as shown in figure 18,
• Inappropriate estimation of the absorption factor,
• Absence of a rigid quality control and validation of the produced analytical results
such as using duplicate analysis of samples and/or analysis of spiked samples,
• Inappropriate estimation of the spectral background as the background could vary in
time and it has to be verified frequently taking into account different parameters such
as season and ventilation system working time. Many laboratories consider the
background a constant parameter and do not verify its stability. Figure 19 shows the
variation of the background value.
Figures 18: A typical efficiency calibration curve used in gamma spectrometry where Am-241 is the
lower energy used for calibration, extrapolation of Pb-210 could lead to a significant
error
Final Darft, page 31