Concentrations of ~(238)Pu, ~(239+240)Pu and ~(241)Pu in the surface air in Finnish Lapland in 1963

摘要

Activity concentrations of ~(238)Pu, ~(239+240)Pu and ~(241)Pu (measured both directly by liquid scintillation counting and indirectly by buildup of ~(241)Am) were determined with the aid of archived air filters sampled at Sodankyla (67°22'N, 26°39'E) in 1963. Atmospheric concentrations of ~(238)Pu, ~(239+240)Pu and ~(241)Pu were < 0.1-3.6(± 0.5) μBq m~(-3), 0.4(± 0.1)-95(± 3) μBq m~(-3) and < 16-1730(± 70) μBq m~(-3), respectively. The plutonium concentrations varied seasonally, being highest in spring and summer due to the springtime-enhanced transportation of radioactive aerosols from the stratosphere to the troposphere. Contrary to some earlier assumptions, the ~(239+240)Pu activity concentration in the air was practically at the same level in northern and southern Finland. The ~(238)Pu/~(239+240)Pu activity ratio in the air was 0.014(+ 0.003)-0.32(± 0.11), typically close to that of global fallout from the nuclear weapons testing. The ~(241)Pu/~(239+240)Pu activity ratio varied between 7.7(± 1.2) and 41(± 8). According to these isotope ratios, plutonium in Sodankyla originated mainly from global nuclear test fallout, even though the effect of the Soviet tests at the end of 1962 were observed at the beginning of 1963. However, direct atmospheric transport from Novaya Zemlya to Finnish Lapland could not be detected.