THE WORLD-WIDE DEPOSITION OF LONG-LIVED FISSION PRODUCTS FROM NUCLEARTEST EXPLOSIONS” By N. G. Stewart, R. G. D. Osmond, R. N. Crooks, and Miss E. M. Fisher U.KA.E.A, Research Group, Atomic Energy Research Establishment, Harwell ABSTRACT Measurementsof the gross fission product activity deposited since 1951 at two stations in the U. K. have been described in an earlier report.! A network of six stations in the U. K. and thirteen in other parts of the world has now been set up at which rain water is collected over monthly or three-monthly periods and analyzed for Sr®, sr®, Cs'8", and Ce'“4. The stations have been commissioned at intervals since the first was set up in May 1954 and an account is given of the results obtained so far, particularly on Sr® for which the records are the most complete. It is shown that most of the Sr*° deposited is derived from those nuclear explosions whose clouds enter the stratosphere and return to earth slowly over a number of years. In the suc- cessive yearly periods between May 1954 and May 1957 the Sr® deposition at a representative Station in the U. K. amounted to 2.06, 2.24 and 2.55 mc/km?, respectively, and the cumulative total in May 1957 was 7.5 mc/km?. Cesium-137 levels are about 50% higher than those of Sr®. The experimental data indicate that, within a given region, fallout is proportional to rainfall. Based on some simple assumptions about the frequency of nuclear weapon tests in the future, estimates are made of the possible future levels of Sr” in soil in the U. K. It has been found that the mean Sr™ concentration in rain water in the U. K. shows a marked seasonal variation with peaks in the late spring and troughs in the late autumn of 1955 and 1956, and the concentration in the lower stratosphere appears to vary in a similar manner. The maximum to minimum ratio is about 6:1. A similar but less marked variation of the opposite phase has been observed in New Zealand rain water. It has also been observed that the deposition rate of Sr® has a minimum value near the equator, and there appears to be a pronounced maximum in the middle latitudes of the northern hemisphere. These results are shown to be consistent with a model for the general circulation of the atmosphere proposed by Dobson® and Brewer’ as a result of their observations of ozone and water vapour in the atmosphere. Some proposals for future work on the meteorological problems of long-range fallout are given in the report. 1 INTRODUCTION In a recent paper,! an account was given of the method used in the U. K. for measuring the. rate of deposition of Sr* from nuclear test explosions. The measurements, which were con*Received from the Atomic Energy Research Establishment as report A.E.R.E. HP/R 2354, dated October 1957. 231 aor eisai.