A new site for 85Kr measurements on groundwater samples

Analysis of stable and radioactive isotopes is essential as a complement to geochemistry and geohydraulic investigations on groundwater regimes and their genesis. This is widely acknowledged also for the determination of the specific activity of 85Kr in groundwater. The geochemical inertness and well-defined input function of 85Kr allow estimates of groundwater age and enhance characterization of groundwater flow and components in many aquifer systems. A new site for measurement of the 85Kr specific activity has been established at the Institute of Applied Physics at the Freiberg University, Saxony. Under normal conditions ca. 80 μl krypton are dissolved in 1 m3 of water in contact with air. Therefore gas extraction has to be most effectively. A modified CO2 extractor of 45 cm x 10 cm was chosen. The water is continuously pumped under pressure (3 - 4 bar) passing a Venturi-type nozzle, which simultaneously operates as a water-jet pump. The extracted gas flows through a CO2 trap (NaOH 10 %), a H2O cold trap, through molecular sieves (5, 3 A) and a charcoal column, cooled by liquid nitrogen, where krypton, nitrogene and other components are adsorbed. Remaining gases re-enter the extractor at the Venturi-type nozzle. A small membrane pump supports the circulation. Due to the special design of the water outlet, contamination of the sample is avoided. Optional a compact stove heats the water to improve the extraction efficiency. If pressure supply is high enough, additional extractors can be run simultaneously. In a test run the recovery for radon was around 65 to 70 %. Further preparation steps of the raw krypton sample is performed in the laboratory. To obtain a good first enrichment a tube furnace filled with chrome powder is used to separate nitrogen and oxygen from the sample at 900 deg C. The following enrichment steps are performed by a preparation setup developed at GSF-Institute for Hydrology, Neuherberg. (author)