Technetium-99 determination in low-volume samples from the global environment with accelerator mass spectrometry

The long-lived fission product technetium-99 (⁹⁹Tc, t _1/2 = (2.111 ± 0.012) × 10⁵ years) was successfully detected in small-volume (10 L, and 1 g, respectively) environmental samples such as Pacific Ocean and river water, Antarctic snow, and peat. Its presence in the environment is almost exclusively of anthropogenic origin and the determined levels are attributed to nuclear weapons testing in the 1950s and 60s as no local contamination source is known to be present at these sampling sites. A unique capability for Accelerator Mass Spectrometry (AMS) measurements of environmental ⁹⁹Tc at unprecedented sensitivity was established, first using the Gas-filled Analysing Magnet System (GAMS, Germany) and after its shutdown, at the Heavy Ion Accelerator Facility (HIAF, Australia). New chemical extraction and measurement techniques, including improved non-isotopic normalisation, enabled detection limits as low as 0.6 femtograms (fg) per sample for Antarctic snow, and enhancement of precision from 30% (GAMS) to 16% (HIAF). The rather high concentrations of about 280 fg (⁹⁹Tc) per g (dry mass) measured in peat indicate Tc accumulation in this archive, which opens the possibility of studying its migration behaviour even under reducing conditions. Furthermore, it allowed deduction of an improved estimate of the global ⁹⁹Tc inventory to (120-190) TBq. The first direct detection of ⁹⁹Tc at 8 fg L^-1 in river water highlights the need for further studies in urban areas to evaluate potential contributions from nuclear medicine. The achieved sensitivity allows monitoring of on-going releases, improving the risk assessment of releases from nuclear waste repositories and tracer applications in environmental sciences.