– the intergovernmental organization through which Member States act together
     on matters related to measurement science and measurement standards.
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Radionuclide metrology

In the field of radionuclides, the international reference system (SIR) for activity measurements of gamma-ray-emitting radionuclides, established in 1976, makes it possible for national laboratories to check the reliability of their activity measurements by comparison with other laboratories world-wide. It is the basis of the BIPM key comparison database of the MRA for radionuclide measurements. The gamma-ray emitters, in liquid or gaseous form, are sent to the BIPM where they are compared with standard sources of very long life times using pressurized ionization chambers. The SIR was recently extended to include pure beta-ray emitters using liquid-scintillation counting methods.

For radionuclides with a short half-life the BIPM has developed a travelling standard that can be taken to the NMI for a measurement on-site. This SIR Transfer Instrument (TI) links the NMI result through reference measurements to the SIR itself. This enables distant NMIs to have degrees of equivalence for medical diagnostic radionuclides that are short-lived. Links for 99Tcm are ready and are in development for 18F.

Since the formation of the Ionizing Radiation section, the BIPM has, in close cooperation with the CCRI, organized some thirty international comparisons of radionuclides, of which seventeen are now CCRI(II) key comparisons. These have usually been preceded by a trial comparison designed to identify and eliminate potential problems. The radionuclides under study are generally chosen so as to address a specific problem, such as the measurement of low-energy photons or the presence of a metastable state. The number of participating laboratories is typically about twenty-five. Detailed analysis of the results of these studies has often contributed to a better understanding of the measurement techniques.

[ Picture of a hospital radiopharmacy ]

Taking part in BIPM and CCRI(II) comparisons is a requirement of the MRA and supports the calibration and measurement capabilities (CMCs) declared by the NMIs. This in turn gives confidence to the hospitals and other users of national calibration facilities regarding the traceability of their activity measurements.

Primary activity measurements are performed at the BIPM using proportional counters in coincidence with NaI(Tl) scintillators. Calibrated Ge spectrometers are available for measuring gamma-emitting impurities which may influence both SIR and primary measurements.