137Cs fallout depth distributions in forest versus field sites: implications for external gamma dose rates

The depth profile of 137Cs fallout in soil from atmospheric nuclear weapons tests was measured at neighboring field and forest areas for seven sites in northeastern US. The inferred dose rates in air at 1 m above the ground per unit inventory of 137Cs averaged a factor of 1.8 higher in forest as compared to field areas. Calculations indicate that dose rate in forest areas would be a factor of four higher than that over deeply ploughed land. Based on a limited set of historical measurements made since 1972, it appears that the dose rate per unit inventory in both field and forest areas has more or less stabilized after a sharp decrease following deposition events in the early 1960s. Estimated dose commitments for various land types are compared to the value suggested by UNSCEAR and implications with respect to certain population groups are discussed. The findings may have application in estimating future external doses from deposited 137Cs associated with Chernobyl fallout in Europe. (author)