The concentration of Cs 137 : in surface air at Argonne from early 1953 through mid-1960 as determined by these methods is shown in Figure 3. are repeatedly evident. Variations by as much as a factor of 10 over an interval of a few months Such variations are in part seasonal in nature, occurring during the spring months, and in part closely related to the frequency and magnitude of nuclear tests. The cst 8? present in any one monthly sample consists of contributions from a variety of tests conducted prior to the time of collection. The high frequency with which tests of widely varying magnitude and geographic distribution were conducted from 1953 through 1958 coupled with the fact that spectrometric analysis of air filters at Argonne did not commence until mid-1958 have madethe use of activity ratios for the dating of earlier debris quite out of the question. Some very fine work has been done along these lines, however, notably by both Marteu* and Lockhart.” A unique way of following nuclear debris from the Hardtack I Series was provided by the production of - os eae . . . vyas 181 radioisotopes of tungsten by neutron activation in a number of shots in this series. Gamma-emitting W (140-day half-life) has been used in this regard in the work under discussion. In a similar way, Rn? (210-day half-life), also a neutron activation product, formed in the Orange shot at high altitude toward the close of the Hardtack Series may be used to follow high altitude equatorial debris. Figure 4 indicates the concentrations of wi8t cst3?, and rni?? in surface air for the period January 1958 through January 1961. Both wi8t and 02 Rni have been corrected for decay back to shot time; mid-June 1958 for wit and mid-August 1958 for 02 Rn! . Both Cowan and Kalkstein have reported an isomer of Rn Oo? having a half-life of 3.4 + 1.4 years. The 02 Rn) values have been corrected on the premise that 20 percent of the total Rni? activity during the spring of 1960 arose from the 210-day isomer. Method of Determining Sources of Radioactivity In order to avoid using wid production data for the entire series, the validity of which cannot be judged by available literature, we have used an approach which does not involve any estimate of the portion of Hardtack debris tagged with W 181, 185 It was noted during the summer and early fall 1958 that the gross gamma fission activity in surface air at Argonne was predominantly from Hardtack I as indicated by high relative concentrations of Bal 40 1 . » ce am and Ru! ?3, Older debris was also present because of the greater apparent age inferred from 137 140 3 140,141 or Bat? 1038 zr? /Cs » and Ba Cs! ° than from Ba /Ru . In order to remove the older, non3 Hardtack component, the assumption was made that the shape of the cs! ? curve, for example, would have been the same during July through September 1958 as it was in July through September 1959 had it not been for the Hardtack series, since the testing sequence preceding these periods were very similar in both years, The curves for the two years were normalized during the spring months (April and May), and the normalized 1959 values substracted from the corresponding monthly values in 1958 as shown in Figure 5. ant curve was then attributed to cst? from Hardtack only. 0 9 Rul 6 and Zr > The result144 An identical procedure was followed for Ce > . : The activities of these nuclides were in all cases corrected for radioactive decay to the middle of the month of collection prior to normalization and subtraction. The values obtained by subtraction were further corrected for decay back to mid-June 1958, the mid-time of the series. 81 these four fission products relative to wi September 1958. The ratio of Ce The activity ratios of were then determined using the values found during July through 144,181 ¢ py 2B, 1 py l8 95 /Ww s and Ru 106,181 iW used at subsequent times must be