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

cps
Draft, page 32
1.10E-02
1.05E-02
1.00E-02
9.50E-03
9.00E-03
8.50E-03
8.00E-03
Mean: 9.59E-3 cps
Uc: 3.3E-4 cps
%Unc: 3.4%
2007-04-28 2007-08-06 2007-11-14 2008-02-22 2008-06-01 2008-09-09 2008-12-18 2009-03-28 2009-07-06
Figure 19: Background variations
Some laboratories stated that they used gamma detectors with energy range from 200 keV
which might be not suitable for Pb-210 measurement; also some laboratories extrapolated the
efficiency calibration to cover the Pb-210 energy which resulted in not acceptable results. The
most frequent reasons for failure in Pb-210 are discussed in more details in [5, 6].
4.5. Recommendations related to the analysis of Ra-226 in moss-soil sample
The majority of participating laboratories measured Ra-226 in moss-soil using gamma
spectrometry with high resolution detector of different type.
In general, there are two approaches used for Ra-226 determination by gamma spectrometry.
These are the use of the 186 keV line from Ra-226, and use of gamma lines from the shortlived
progeny Pb-214 and Bi-214. In the former case, the contribution of U-235 to the peak
area needs to be allowed for, and if an inappropriate correction is applied, this may lead to
shortcomings in performance.
As shown in Figure 20, most of the unacceptable results were due to overestimation of the
Ra-226 which could be due to the unresolved U-235 peak or due to an incorrect calibration.
The shortcomings observed in the performance of the participating laboratories in gamma
measurements of Ra-226 could be attributed to several factors. These factors are related to the
gamma spectrometry procedure utilised by each laboratory. Some of these factors are the use
of inappropriate calibration standards or procedure, sample geometry, interference from other
gamma lines, and properties of the sample container used.
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