Atmospheric Transport of Artificial Radioactivity

Abstract

The principal shortcoming common to previous attempts to interpret the atmospheric behavior of fall-out is the failure to distinguish the iufluence of latitude, altitude, and time of injection on the storage time and fall-out pattern for stratospheric sources. On the basis of evidence provided by fission- product concentration ratios in precipitation, Mantell pointed out that the short- lived radioisotopes in world-wide fall-out were principally of stratospheric origin and that stratospheric storage times decrease markedly with latitude and increase with altitude. Use was made of 12.8-d Ba140, 53-d Sr89, 28- y Sr90, and 74-d W185 concentrations in individual precipitation samples in interpreting some major features of transpont and deposition patterns for stratospheric fall-out. The usefulness of fission-product ratio data for estimating the relative con tribution of various nuclear-test sources to total fall-out has been questioned on the basis of possibly serious physical and chemical fractionation effects and uncertainties in fission-product production- yield ratios. The chemical similarity of barium and strontium limits chemical fractionation effects on Ba140/Sr90 ratio data; Sr89/Sr s0 ratio data obviously are unaffected by chemical factors. Uncertainties due to the use of production-ratio values, or due to the assumption that there are no fractionation effects, do not seriously affectmore » the usefulness of isotope- ratio data for distinguishing major differences in atmospheric behavior of nuclear-cloud sources of different altitude and latitude for well-spaced nuclear tests. The U. S. Hardtack nuclearteat series provided a useful tracer in W185 for studying the behavior of fall-out debris from equatorial tests. Data are given for Ba140/Sr90 and Sr89/Sr90 ratio s and for Sr90 and W185 concentrations in rainfall for various time periods.« less