FALLOUT MONITORING AND DOCUMENTATION Any prediction of the possible effects of radioactive materials from weapons tests requires a continuing program of monitoring and documentation. Such programs have been in operation in a few countries for several years. Other countries have begun monitoring programs since the formation of the United Nations Scientific Committee on the Effects of Atomic Radiation. This report is limited chiefly to the tabulation of results obtained in the United States by various laboratories. The studies include measurements of deposition, air concentrations, water concentrations, and uptake. Some early data based on mixed fission product determinations have been included, since there were very few Sr* measurements made before 1954. For samples collected since that time, however, an attempt has been made to use only radiochemical data, since the interpretation of mixed fission product analyses is very difficult under present conditions of weapons testing. 1. DEPOSITION The level of fallout deposition on the ground is not a direct measure of hazard to man from radioisotopes such as Sr® or Cs!3’_ For example, Sr® has to pass through the food chain be- fore it can be incorporated into the human body. This passage may consist of several steps, all of them biologically complex. The determination of geographical distribution of fallout, however, is the first step in a scientific study leading to the possibility that unusually high or low concentrations may appear in the food chain or in man himself. The two important features of deposition are the total accumulated fallout and the fallout rate. The Srchain from soil to plants to cattle to milk to humans, for example, is dependent on the accumulated deposit present in the soil. The corresponding chain resulting from retention of fallout on plant surfaces, on the other hand, would be rate dependent. In addition to obtaining data for possible correlation with the uptake of the isotopes by humans, the study of fallout deposition is also important for obtaining a material balance of particular isotopes from the amount produced, the amount deposited, and the amountstill in the atmosphere. The measurement of fallout rate requires collection over relatively short periods, usually on the order of one month, and radiochemical measurement for Sr®. Two types of collectors are in current use —a stainless-steel open vessel or pot and a plastic funnel. These units, when exposed continuously, collect both dry fallout and the material carried down by precipitation. It is also possible to collect the material carried down by individual rainfalls and obtain meteorological information as to the probable atmospheric source of the fallout. Such short term collections may also be analyzed for shorter-lived isotopes to estimate the approximate age of the radioactive debris. The radiochemical analysis of soils allows direct measurementof fallout accumulated since the start of testing. These analyses, however, are extremely time consuming, complex, and subject to considerable sampling error. They are most useful, therefore, for presenting a broad picture of world-wide fallout rather than for detailed studies. Although the gummedfilm technique of deposition measurementallows estimation of Sr only by calculation from amount of mixed fission products obtained, it has the advantage of simplicity and, therefore, possible operation at a large number of sampling stations. 3