Activity standardization of 131I at CENTIS-DMR and PTB within the scope of a bilateral comparison
Introduction
Although 131I is being increasingly replaced by shorter lived radionuclides such as 18F, 99mTc and 123I it is still frequently used in nuclear medicine, e.g. for the diagnosis of thyroid carcinoma. In Cuba, it is utilized in several hospitals which can obtain activity standards from the Radionuclide Metrology Department of the Center of Isotopes of Cuba (CENTIS-DMR). CENTIS-DMR also organizes national intercomparisons to control and improve the quality of activity measurements in the country (Oropesa et al., 2006).
The traceability of the Cuban activity standards to the International Standard (SI) is desirable to ensure a high quality of applications. However, a direct submission of an ampoule to the International Reference System (SIR) at the Bureau International des Poids et Mesures (BIPM) is not possible since Cuba is only an associate member of the Metre Convention.
In this work, a bilateral comparison between CENTIS-DMR and the German National Metrology Institute, the Physikalisch-Technische Bundesanstalt (PTB), is presented. Aliquots of a radioactive solution of 131I were measured in both institutes, using liquid scintillation counting in combination with the CIEMAT/NIST efficiency tracing method. In addition, secondary activity standardization techniques by means of reentrant 4π ionization chambers and gamma-ray spectrometry were applied. The aim of the comparison was to check the results for consistency and to check the measurement capabilities of CENTIS-DMR.
In 2004, the PTB submitted two samples of known activity of 131I to the SIR at the BIPM. Thus, the comparison presented in this work also allows the creation of a virtual link to the SIR for CENTIS-DMR. Another possibility to obtain this link would be given via the recent International Atomic Energy Agency (IAEA) comparison in which CENTIS-DMR participated (Zimerman et al., 2008). The solution used for that comparison was provided by QSA Global (Braunschweig, Germany) and is traceable to the activity standards of PTB. Two participants of the IAEA comparison also submitted ampoules to the SIR recently, which would allow a further link to the key comparison value (KCRV) of the SIR (see Ratel et al., 2008 and references therein).
Section snippets
131I solution
A solution of the radionuclide 131I used for the comparison with a total volume of 34 mL was prepared at CENTIS-DMR. The approximate activity concentration was 3.5 MBq/g on the reference date of March 14, 2006, at 16:00 UTC. The chemical composition of the solution was Na131I in 0.9% NaCl, 0.25% Na2S2O3×5H2O, 0.26% KI, 0.1% NaH2PO4 and 0.44% (NH4)2HPO4.
A weighed portion of about 4 g of the 131I solution was transferred into a P6-type ampoule, which was then sent to PTB. Another part of the
Analysis and results
The uncertainty budgets for the activity concentration determined at CENTIS-DMR and PTB by means of liquid scintillation counting are listed in Table 1. The main uncertainty components at CENTIS-DMR are assigned to the gravimetrically determined dilution factor (0.50%) the input parameters (decay data) and statistical model (0.45%), and other effects (0.30%) such as the asymmetry of the photomultiplier tubes. At PTB, the main uncertainty components are assigned to the dead-time (life-time)
Conclusion and outlook
The activity concentration determined at CENTIS-DMR was found to be in good agreement with the PTB result. The comparison of the uncertainty budgets for liquid scintillation reveals possibilities to reduce the uncertainties at CENTIS-DMR. In particular, the overall uncertainty could be reduced, improving the procedures for dilution and the asymmetry of the counter system. The latter problem could be overcome with another counter at the institute. Moreover, further comparisons are desirable to
Acknowledgments
The first author is indebted to Y. Moreno León, R.A. Serra Águila and A.T. Hernández Rivero for their kind help during the measurements. The second author wishes to thank his colleagues from PTB Working Group 6.11 “Activity Unit” for their valuable assistance during the sample preparation and the measurements.
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