bp C/LITER 4 — oL LILLE L I Lit itt teete ete et EE EEE Ee JFMAMUJUJASONDIVFMAMJJASONDIUFMAMJJASONDI DF MAMUJJASOND 4954 1955 1956 Fig. 3—Sr*® content of rain water at Milford Haven. 1957 (3) The Sr®° deposited is primarily derived from dust which has been stored in the strato- eggebaity sphere and which, for meteorological reasons, is being fed down slowly into the lower atmosSphere in a periodic manner. It is comparatively simple to exclude possibilities (1) and (2) but the detailed arguments are lengthy and they are given in Appendices 1 and 2. It is shown in Appendix 1 that the dry deposition and rainfall factors have little effect on the shape of Fig. 3 and that the latter reproduces the shape of the air concentration curve to within 15%. It would nevertheless be useful to demonstrate by direct measurementthat the concentration of long-lived fission products in the troposphere has varied in the same cyclic manner as Fig. 3. A programme along these lines has been started at Harwell in which the Cs" on tropospheric filters collected over the past few years will be measured on a gammaray spectrometer, The main arguments against hypothesis (2)—that the peaks are due to tropospheric fall- out—-are given in Appendix 2 and are based on the known rapid deposition of tropospheric dust, the amount of 54-day Sr*® present in the samples, the decay properties of the gross deposited radioactivity and knowledge of the very small amounts of Sr*® deposited in the U. K. from the testing of nominal-size nuclear weapons prior to January 1955. The continuing deposition of Sr®® from the 1954 thermonuclear tests was anticipated in our previous report! in which the deposition rate of the dust was calculated to be 12% per year, which is in good agreement with Libby’s estimate of a mean stratospheric storage time of ten years.‘ The possibility of a periodic variation in the rate of deposition was not anticipated, but in a later section this result will be shown to Support an atmospheric circulation model deduced from the measured distribution of ozone in the atmosphere. The exact contributions of explosions subsequent to 1954 to the aeposition of Sr® are not easy to determine, particularly in the absence of information about the amount of material in the stratosphere. Using the sr? gata given in Table 2, it is possible to compute the associated amount of Sr’’ when the date of origin of the Sr® is known, using published fission product yields (Sr*9/Sr*" yield ratio = 0.81). By this method, it is deduced in Appendix 2 that tests staged in 1955 were responsible for less than 5% of the Sr*’ deposited at Milford Haven in that year. The 237 “