International comparison measurements in the range of the activity unit

In the period under report, Working Group 6.11 has participated in a comparison for activity determination on a ³²P solution and sent activity standards of the nuclides ⁵⁷Co, ¹⁰⁹Cd, ¹¹¹In and ²⁰¹Tl to the international reference system SIR at the BIPM. In addition, an activity standard of the radioactive inert gas ²²²Rn and a solution of the short-lived radionuclide ¹⁸F were forwarded for comparison purposes.

The Mutual Recognition Arrangement (MRA) under the auspices of the Comité International des Poids et Mesures (CIPM) is a worldwide arrangement for the cross-national mutual recognition of calibration certificates of national metrology institutes for all commercially or socially relevant measurands. The Arrangement serves to remove barriers to trade and demands from the participants - in addition to proof of the existence of a quality management system - that they also participate in comparison measurements.

In the range of the activity unit, such comparison measurements are performed by sending aliquots of a radioactive stock solution of a nuclide to the participants in the comparison. In 2005 PTB has, for example, forwarded ampoules of a ³²P solution to metrology institutes all over the world. The participants then determine the specific activity as well as possibly existing radioactive contaminations and send the results together with a complete documentation and an uncertainty analysis to the Bureau International des Poids et Mesures (BIPM) in Paris where the data are evaluated.

In addition, the international reference system SIR for activity measurements was established at BIPM in 1976. In the case of this system, the participants fill ampoules provided by the BIPM with their national activity standards in the form of solutions or gases and send them to Paris where the ampoules are measured in an ionization chamber and compared with a long-lived ²²⁶Ra standard. Traceability to a radium reference source and the excellent long-term stability of the measuring system associated with it allow the results of the participants to be compared even when periods of several years lie between the transmittals of the individual institutes. Meanwhile, more than 872 samples from 62 different radionuclides have been forwarded and measured.

The SIR system offers a great number of advantages. The metrology institutes can handle the schedules of their comparison measurements and select the radionuclides with greater flexibility. Moreover, the number of comparisons can be adapted to their own efficiency. And, above all, comparison measurements can also be carried out on very short-lived radionuclides, whereas the logistical problems are often insuperable when a corresponding solution is distributed in the whole world.

In the period under report, Working Group 6.11 has sent ampoules with solutions of radionuclides ⁵⁷Co, ¹⁰⁹Cd, ¹¹¹In and ²⁰¹Tl to the SIR. In addition, an ampoule with the radioactive inert gas ²²²Rn was submitted. Particular emphasis must be placed on the transmittal of the short-lived radionuclide ¹⁸F which plays an important role in the field of medicine. With a half-live less than 110 minutes, ¹⁸F not only represents a metrological but also a logistical challenge. The specific activity of the solution was determined with both the aid of the 4pß⁺-γ-coincidence method and 4p-liquid scintillation counting. An ampoule was taken to Paris by direct travel from Braunschweig in less than 6 half-lives.