Detection of 133Xe from the Fukushima nuclear power plant in the upper troposphere above Germany

• Unique measurement of 133Xe from the Fukushima accident in the upper troposphere and tropopause.
• Highly sensitive ultralow-background gas counting technique.
• Detection limit in the range of 100 133Xe atoms.
• Liter-scale air sample sufficient → facilitates approval process for airborne measurements.
• Fukushima plume arrival in high altitude earlier than on ground.

After the accident in the Japanese Fukushima Dai-ichi nuclear power plant in March 2011 large amounts of radioactivity were released and distributed in the atmosphere. Among them were also radioactive noble gas isotopes which can be used as tracers to test global atmospheric circulation models. This work presents unique measurements of the radionuclide 133Xe from Fukushima in the upper troposphere above Germany. The measurements involve air sampling in a research jet aircraft followed by chromatographic xenon extraction and ultra-low background gas counting with miniaturized proportional counters. With this technique a detection limit of the order of 100 133Xe atoms in liter-scale air samples (corresponding to about 100 mBq/m3) is achievable. Our results provide proof that the 133Xe-rich ground level air layer from Fukushima was lifted up to the tropopause and distributed hemispherically. Moreover, comparisons with ground level air measurements indicate that the arrival of the radioactive plume at high altitude over Germany occurred several days before the ground level plume.