On the trail of Cs-137 and Sr-90 – Trace analysis to monitor radioactivity in air

The German Federal Office for Radiation Protection (Bundesamt für Strahlenschutz, BfS) is legally obliged to continuously monitor radioactivity in the environment in Germany. The Atmospheric Radioactivity and Trace Analysis Section of the BfS is operating four different measurement facilities/laboratories in Southern Germany: gamma spectrometry, radiochemistry and noble gas laboratories in Freiburg i.Br. and the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO) monitoring station RN33 on Mt. Schauinsland. The three laboratories provide data of radioactive aerosol bound particulates such as Cs, U, Pu or Sr and noble gases (Kr and Xe) for the German Integrated Measurement and Information System (IMIS). The radionuclide station RN33 is monitoring radioactive particulates and Xe for the International Monitoring System (IMS) of the CTBTO. The variation of radioactivity on aerosol bound particles over time and the analysis of their origin, distribution and transport in the environment will be presented.

To collect aerosol bound particles, ground-level air sampling is carried out by high volume air samplers located in Freiburg i.Br. and on Mt. Schauinsland. The high volume air sampler is operated with two filter layers: an upper polypropylene layer with a collection efficiency of about 85% to 95% and a bottom fibre glass layer with a collection efficiency of almost 100%. The fibre glass filter is used as a control for collection efficiency of the polypropylene filter. As required by the German IMIS monitoring programmes, the routine sampling period is seven days, which can be reduced to daily cycles in case of (un)expected enhanced activity concentrations. The filters are separately pressed to pellets and analysed by high resolution γ-spectrometry. In addition, the polypropylene-filters are processed radioanalytically and analysed with a low level α/β-counting system and α-spectrometry.

The applied methods together with atmospheric transport modelling allow to detect smallest amounts of radioactive substances as well as to investigate their origin, distribution and transport in the environment. In addition, the Cs-137/Sr-90 ratios can be used as a geochemical fingerprint for source identification. Current (2022) median activity concentrations in ground-level air for Cs-137 and Sr-90 (2021) are 0.42 µBq m^-³ and 0.06 µBq m^-³, respectively. Detection limit (LOD) for Cs-137 is on average 0.14 µBq m^-³ in weekly samples and 0.01 µBq m^-³ in monthly samples for Sr-90. In March 2022 dust blown in from the Sahara towards Southern Germany resulted in slightly higher airborne Cs-137 activity concentrations while Sr-90 did not markedly change. Other sources for increased Cs-137 activity concentrations include past above ground nuclear weapons tests or resuspension of fallout from the Chernobyl accident. Thus, trace analysis is used to track short- and long-term changes in radioactivity in the environment at lowest activity levels.