Lichens radionuclide deposition

The deposition of Tc from weapon fallout has been estimated to be 540 + 50mBqm at 62°N. Frequently, carpets of mosses and lichens are used as bioindicators for the estimation of deposited radionuclides. The concentration of Tc in lichens (mBq per kg dry weight), the fission yields (Mt) for nuclear weapons testing, and the observed and theoretical Tc/ Cs radionuclide ratios are given in Figure 1. The present Tc/ Cs ratio is expected to be 3.2 X 10 " due to the physical decay of Cs. In general, the observed Tc/ Cs ratio is lower than expected from fission probably due to a shorter residence time for Tc in the lichen carpet. The concentration of Tc and the Tc/ Cs ratio in lichens has increased since 1975. Unusual mixtures of shortlived radioisotopes, e.g., Np and Mo, in fallout... 

Accurate values of deposition of radionuclides near the sampling locations are required to determine the mean residence time or effective half-life of radionuclides in lichens. Liden and Gustafsson (1967) obtained in Cladonia lichens the value of 17 4 years for T y and 11 years for Teff. They made Cs determinations in Swedish Lapland in 1962-1965. Near the sampling location there was an observation station where Cs has been... 

Hanson et al. (1967) performed similar experiments on the effect of seasonal snow cover on the distribution of fallout radionuclides in Cladonia alpestris and the movement of artificially applied radionuclides within the thalli. Strontium-90 concentrations increased during summer months when the lichens were exposed to atmospheric fallout deposition, stabilized at... 

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