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Uncertainty propagation formulae are presented for age dating in support of nuclear forensics. The age of radioactive material in this context refers to the time elapsed since a particular radionuclide was chemically separated from its decay product(s). The decay of the parent radionuclide and ingrowth of the daughter nuclide are governed by statistical decay laws. Mathematical equations allow calculation of the age of specific nuclear material through the atom ratio between parent and daughter nuclides, or through the activity ratio provided that the daughter nuclide is also unstable. The derivation of the uncertainty formulae of the age may present some difficulty to the user community and so the exact solutions, some approximations, a graphical representation and their interpretation are presented in this work. Typical nuclides of interest are actinides in the context of non-proliferation commitments. The uncertainty analysis is applied to a set of important parent-daughter pairs and the need for more precise half-life data is examined.

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The international reference system for radioactivity relies on stable instrumentation for comparing primary standards of radioactivity; these instruments enable national metrology institutes to demonstrate the equivalence of their primary standards. The international bureau of weights and measures (BIPM) holds two such instruments for gamma-ray emitting radionuclides, and is working with the POLATOM, LNE/LNHB, NIM, NPL and PTB to develop a new system called the 'extended international reference system' (ESIR). The ESIR will address pure beta-particle emitting radionuclides as well as other radionuclides (such as 55 Fe) which cannot be measured in the established international systems. The ESIR will be a liquid scintillation system based on three photon-counting channels operating in coincidence. This article reports the results from validation studies carried out using solutions of 3 H, 55 Fe, 63 Ni and 14 C to assess the reproducibility of the results. Several key comparison indicators have been developed and tested to find the best way to obtain a robust and reproducible international reference value. An important conclusion is that the new ESIR can deliver accurate comparison values, immune from changes in detection efficiency or asymmetry of the counting channels. A relative uncertainty better than 0.2% can be expected for radionuclides emitting beta particles with an end-point energy above 150 keV while using commercial liquid scintillation cocktails.

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