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

2.3.4. Homogeneity study and characterization of the IAEA-447
To assess the homogeneity of the IAEA-447 moss-soil reference material, 10 bottles covering
the whole bottling range were randomly selected; three independent test portions at ~45 g and
~0.5 g from each bottle were used to perform gamma spectrometry measurements and
radiochemical analysis respectively. Gamma spectrometer equipped with broad energy
detector was used to analyze K-40, Cs-137, Tl-208, Pb-210, Pb-212, Pb-214, Bi-214, Ra-226,
Ac-228, Th-234 and Am-241. Po-210, Ra-226, U-234, U-238, Pu-239+240, Pu-238 and Am-
241 were determined by isotope dilution alpha spectrometry; Pb-210 and Sr-90 by liquid
scintillation spectrometry following sample dissolution by wet mixed acid dissolution or
microwave digestion or molten salt fusion and sequential radiochemical separation
procedures.
The analysis of homogeneity study was performed under repeatability conditions to minimize
variations.
The outcome of the homogeneity study demonstrated that the uncertainties due to between
and within bottles heterogeneity ubb were generally very small and the material could be
considered sufficiently homogeneous for the tested radionuclides at the range of mass used.
2.3.4.1. Characterization and assignment of target values of the measurands of IAEA-447
The analysis related to the homogeneity study and characterization of the reference material
were performed at the IAEA Terrestrial Environment Laboratory in Seibersdorf (Austria).
Then the analytical results of the characterization were confirmed by two external expert
laboratories.
The gamma emitting radionuclides were measured using a metal (radon tight) sample
container. The spectrum was collected after 30 days when the Ra-226 Rn-222 equilibrium
reached. A special low background n-type HPGe detector 30% relative efficiency with
CARBON-EPOXI window was used for the spectrum collection. The shielding was made of
7 cm low background lead and 3 mm of copper. An example for the measurement of gamma
emitting nuclides in the IAEA-447 the moss-soil reference material is shown on the Figure 4.
The property values of all radionuclides of the IAEA-447 were established on the basis of a
robust approach proposed by David L. Duewer [1] and the Mixture Model Median (MMmedian)
of the analytical results reported by the IAEA Terrestrial Environment Laboratory
was calculated. The MM-median is a direct analogue of the median. It is the location which
divides the Mixture Model Probability Density Function (MM-PDF) into two sections of
equal area. The MM-median is closely related to the median. It is robust to outliers and also
accounts for the reported uncertainty of each measurement result.
To estimate the standard uncertainty associated with the property value the MM-median based
Standard Deviation S(MM-median) was calculated from the span of the central 50% of the
MM-PDF density function [1].
2.3.4.2. Determination of Pb-210 and Po-210 in the moss-soil reference material
Preparation of the samples
Half g aliquots of the sample from the different bottles were transferred into microwave
containers, about 30 mg of Pb 2+ carrier; about 0.4 Bq of Po-209 tracer and 15ml 65% HNO3
were added. After digestion the sample solutions were then transferred to plastic centrifuge
tubes and centrifuged for 10 minutes at 3000 rpm. The supernatants were transferred to Teflon
beakers and the residues were transferred back into the microwave containers with 3 ml
HNO3 and 2 ml of 40% HF. The residues were digested again using the same microwave
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