Reprinted from Science. October 6, 1961, Vol. 134. No. 3484, pages 980-984 Copyright * 196i by the American Association for the Advancement of Science 410974 Fallout from 1957 and 1958 Nuclear Test Series New York City data show contributions from short-lived nuclides for as long as 14 months after testing. William R. Collins, Jr., George A. Welford, Robert S. Morse The unique production of radiotungsten in Operation Hardtack and the moratorium on testing after the autumn of 1958 have made fallout measurements during the past few years very useful in assessing some of the more perplexing aspects of offsite fallout distribution. Several comprehensive summaries and interpretive articles on ground-level contamination during this period (/-5) and new observations on the atmospheric behavior of nuclear debris have been published (6-8). As a result, many questions pertaining to fallout are now answerable. However, the emphasis historically placed on strontium-90 and cesium-137 and the practical difficulties involved in large-scale surveillance of large numbers of nuclides have resulted in a serious lack of detailed information on many of the shorter-lived contaminants that are dispersed by nuclear testing. This article deals with some of the causes and effects of high concentrations of shorter-lived fission products in fallout in New York City during 1958 and 1959. Data previously reported (9) are supplemented with data on conceitrations of strontium-90, cesium-137, ruthenium-106, cerium144, strontium-89, zirconium-95, and wolfram-185, measured in monthly fall- out collections during 1959. Casual examination shows that the shorter-lived fission products predominated over Sc* and“Cs™ from the beginning of 1958 through the middle of 1959. This was due primarily to the heavy rate of testing that prevailed during 1957 and 1958, but further interpretation of the measurements indicates that the conditions under which individual test series were conducted during this pe- riod also had an effect. Through anal- ysis of isotope ratios, W concentra- tions, and monthly rainfall volumes, it has been established (i) that more fallout arrived in New York City from the Soviet series in October 1958 than from earlier series, and (ii) that the Soviet debris was richer in short-lived nuclides because it was deposited sooner after its production. The New York City measurements also provide a means of investigating external doses delivered to the popula- tion from photon-emitting fission products, Theoretical gamma-radiation dose rates and integral doses are computed from the reported amounts of Zr*, Ce™, Ru™, and Cs” that accumulated on the ground during 1958 and 1959. These calculations show that fallout made substantial contributions to open-field dose rates, and that the shorter-lived nuclides, particularly Zr”, produced doses comparable to doses of Cs". Since the New York City observations probably are applicable to other sites in the Northern Hemisphere, a more thorough evaluation of the world-wide effects of the shorter-lived nuclides is indicated. Methods Throughout 1959, replicate monthly fallout samples were taken on the roof of the Atomic Energy Commission's Health and Safety Laboratory in New York City with funnel-shaped-ion-exchange collectors (9, /0). At the end of each exposure the paper pulp and resin were removed, ashed at 450°C, and separated into aliquots for determination of gross beta activity. Tungsten, cesium, strontium, cerium, and zirconium fractions were then sequentially separated, purified, and counted ’ 53 for beta radiation. Disintegration rates were calculated by correcting the observed counts for counter efficiency and background, recovery, self-absorption, build-up, and decay (//)}. The count- ing factors for the separated activities were obtained by counting the beta radiation of known quantities of the individua) nuclides under the counting conditions for the .sample. Self-absorption and efficiency factors for the mixed beta activities were approximated by using potassium chloride as a secondary standard. When sufficiently high levels of activity were indicated by the initial gross beta-radiation assay, at least two samples from the month’s coilections were analyzed for Ru’ by gamma spectrom- etry. The determination was based on the intensity of the Rh“ emission peak at 0.51 Mev. The detection efficiency of the scintillator was calculated from the combined Zr“-Nb” peak at 0.76 Mev, with the radiochemically deter- mined Zr* concentration as a standard. Niobium-95 was assumed to be in transient equilibrium with Zr“, at a daughter-to-parent activity ratio of 2.4. The correction factor was then related to the rhodium measurement through the data of Heath (/2) on peak- to total-emission ratios and total absolute crystal efficiencies. All final determinations were made after the samples had been stored for at least 120 days to minimize interference from Ru™ emissions at 0.49 Mev. The ruthenium estimates were confirmed by beta-absorption analyses in which the 3.53-Mev Rh™ beta com- ponent was resolved from the total counting rate of the sample and corrected for counter efficiency (/3). In addition, periodic beta-decay measurements were made, from immediately after the sampling period through the end of 1959. Mixed longer-lived nuclides were identified by the decay slope apparent from the later counts and extrapolated back to the original counting date (74). Approximate Ru'™ levels were then obtained by subtracting the measured concentrations of Ce, Ce", and Sr*. Gamma-tadiation doses delivered to a point in air 3 feet above the ground were derived by adapting the method of Hallden and Harley for mixed fission products (/5)} to specific Mr. Collins and Mr. Morse dre affiliated with the Health and Safety Laboratory, U.S. Atomic Energy Commission, New York, N.Y. Mr. Wel- ford, formerly with the Health and Safety Laboratory, is now aMliated with the U.S. Nuclear Corporation, Burbank, Callf.