Reprinted from Science, August 10, 1956, Vol. 124, No. 3215, pages 251-255. same time and are mailed to the U.S. Atomic Energy Commission Health and Safety Laboratory in New York, where their radioactivity is assayed. Studies have continued of the collection characteristics of gummed film in Radioactive Fallout pots. Analyses of the data have been completed for an 84-week test period, indicating that the gummed film is 63 percent efficient if we make the assumption that the high-walled pots collect total fallout. We consider that the pots provide the through September 1955 Merril Eisenbud and John H. Harley best practical estimate of total fallout, and the data obtained from the use of gummed films have therefore been corrected by a factor of 1.6, Soon after the fallout-monitoring network of the U.S, Atomic Energy Commission was established in the United States, it became apparentthat daily fall- out observations could be madeby simple procedures at great distances from a nuclear detonation, and the network was expanded in October 1952 to include a numberof locations beyond the continental United States. Additional stations were again added in February 1954, and since then the sampling program has been conducted at 88 stations, including 26 in the United States. These operate continu- ously. Additional stations in the United States are added when nuclear detona- tions are being conducted in Nevada. The overseasstations are listed in Table 1, the continental stations in Table 2. The data collected within continental United States through August 1954 have been reported in previous publications (1, 2). In this report (3) are summarized the data obtained in the United States and abroad through September 1955. The principal objective of the monitoring program is to enable one to estimate the levels of human exposure produced by radioactive fallout at great distances from nuclear detonations. Such exposure may result from external irradi- ation by radioactive dust deposited on the surface of the earth or from internal irradiation by specific radionuclides that enter the body by ingestion. With respect to ingestion, strontium-90 is the nuclide selected for continuing at- tention because, from the point of view of the long-range behavior of the isotopes The authors are on the staff of the Health and Safety Laboratory, U.S. Atomic Energy Commiscsinn New Vark NY involved in biological systems,it is poten- tially the most hazardous. This is due in partto its relatively long half-life (about 28 years) but, more particularly, because of the chemicalsimilarity of strontium to calcium and the possibility that strontium-90 can therefore be assimilated into biological processes involving calcium and ultimately be deposited in human Methods of Estimation Mixed fission product activity. When the sample is received in the laboratory, it is ashed at 550° to 600°C, and its beta The data from the monitoring network provide the following estimates for each activity is counted. The potassium-40 activity of a known weight of potassium carbonate is used as the basis for converting counts per minute to millicuries. This activity is then calculated as of (i) the cumulative surface deposits of mixed fission products and strontium-90. These estimates are reported in millicuries per date, usually the first day of the following calendar year. These extrapolations depend on knowledge of the age of the bone. place wherea station is located. (i) The square mile. (ii) The cumulative gamma dose (in millirads) from external radiation. Tt will be noted that the earlier publica- tions did not include estimates of the gamma dose. However, in recognition of the increased interest in such estimates, these data have been computed for the full sampling period and will hereafter be available on a continuing basis. Method of Sampling Adhesive-coated films, as described previously, are used for collecting samples. An adhesive coated acetate film is supported horizontally on a frame about 3 feet above the ground. The coating re- sampling day and (ii) an arbitrary future sample, its decay being assumed to be proportional to ¢-1-?. At any given time, the sum of the extrapolated daily values provides an estimate of the cumulative radioactivity deposited at the sampling station as of the arbitrary date. Strontium-90. The strontium-90 content of the samples can be estimated from Hunter and Ballou curvesof relative isotopic abundance (4), or they can be measured directly by radiochemistry’ When the latter procedure is used, it is desirable to collect the samples for a period of at least 1! month to facilitate the counting procedures, and a suitable vessel such as a high-walled pot can be exposed for this purpose. The Hunter and Ballou curves were used during this reporting period, Gammadose. It is not possible to meas- tains its adhesive properties when it is wet, and the radioactive dust particles that are entrapped in raindrops are collected, possibly by impaction against the detonation. The normal gamma-radiation duplicate sampling units, thus providing origin is much larger in magnitude than adhesive surface. Most stations maintain some assurance against loss of samples. ure directly the cumulative gamma dose from fallout at a distance from a nuclear background of cosmic and terrestrial the gamma radiation from fallout, and usually masks the latter completely. It is Ses comparison with those of high-walled