Distribution of Np and Pu in Swedish lichen samples (Cladonia stellaris) contaminated by atmospheric fallout

The activity concentrations of 237Np and the two Pu isotopes, 239Pu and 240Pu, were determined in lichen samples (Cladonia stellaris) contaminated by fallout from atmospheric nuclear test explosions and the Chernobyl accident. The samples were collected at 18 locations in Sweden, from north to south, between 1986 and 1988 and analysed with high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) and alpha spectrometry. Data on the activity ratios 238Pu/239+240Pu and 134Cs/137Cs measured previously were also included in this study for comparison. The 237Np activity concentration ranged from 0.08±0.01 to 2.08±0.17 mBq kg−1, depending on the location of the sampling site and time of collection. The 239+240Pu activity concentration ranged from 0.09±0.01 to 4.09±0.15 Bq kg−1, with the 240Pu/239Pu atomic ratio ranging between 0.16±0.01 and 0.44±0.03, the higher ratios indicating a combination of weapons test fallout and Chernobyl fallout. The 237Np/239Pu atomic ratios ranged between 0.06±0.01 and 0.42±0.04, the lower ratios indicating combination of weapons test fallout and Chernobyl fallout. At a well-defined sampling site at Lake Rogen (62.32° N, 12.38° E), additional lichen samples were collected between 1987 and 1998 to study the distribution of Np and Pu in different layers. The concentrations of the two elements follow each other quite well in the profile.