Table t. Activity levels measured in New York City during 1959 (sotopic activines are reported as of the end of the sampling month. Totai beta activity estimates are reported as of the counting dale (in parentheses). Activity leveis (me /mi2) Sampling month Seve Cs Jan. Feb. 1,00 1.81 1.60 3.36 Mar. Apr. May June - 4.45 4.16 ~ 1.52 2.43 6.58 6.93 190 3.46 July 0.52 0.88 Sept. Oct. Nov. Dec. ~ Olt 0.39 0.54 - 0.§2 0.20 0.6! 0.51 0.43 Aug Total deposition = 0.65 0.88 Ruiee 48 56 13 Cette Zr*s 30.9 49.0 -8L.7 - 88.6 19.4 -thg 36.0 83.1 31.3 5.69 1,24 19 2.70 (64 2.53 17.9 27.3 320 36.1 48.5 275 790.0 “79.4 15.7 Sr 36.3 31.3 6. 3197 8.03 8.01 256 <0.78 [of $0.33 $0.24 $0.1? $0.2 $0.36 $0.17 S0.16 419 221 223 $0.49 From estimated radiotungsten yields Wiss 22.4 16.9 18.4 9.84 4.69 3.05 0.94 <0.50 $0.20 $0.4 <0.42 <0,068 83.8 for the Hardtack series (6, 8) and Gross activity 24t 229 356 413 (6 Feb. 59) (23 Mar. 59) (29 Apr. 59) (5 May $9) 78.7 (9 June 59) 97B (t3 July 59) 210 (10 Aug. 59) 23.3 (8 Sept. 59) 406 8 30 5.96 7.54 (12 (24 (15 (13 Oct. Nov Bec. Jan. 59) 59) 59} 60) 1480 Activity level on J) Dee. 1959 it possible to identify coneributions to total fallout from these sources with some degree of accuracy. measured fission-product yields (atoms per fission) for thermonuclear weapons in general (/9), the Sr*/W™ and Sr*/ W" ratios at an assumed mean production date of 1 June 1958 are estimated to be 0.00380 and 0.436, respectively. Monthly depositions of Sr in New York City from Hardtack aré calculated directly from measured W™ levels by extrapolation of the Sr*/W™ produc- tion ratio to the reporting dates for the fallout measurements. Levels of Sr® from 28.1 5.63 9.8 542 nuclides. Emission energies, decay constants, and other pertinent decay-scheme characteristics were taken from the listings of Strominger, Hollander, and Seaborg (/6). The radiation sources are assumed to be evenly distributed over an infinite plane, and no corrections are made for weathering, shielding, Compton scattering, or the effects of the ground-to-air interface. The beta-radiation counters used were standard end-window GeigerMiiller tubes surrounded by anti-coincidence rings and shielded with mercury. The gama-radiation instrument was a 3- by 2-inch sodium iodide scintillator equipped with a transistorized single-channel analyzer (/7). Monthly fallout activity levels measured during 1959 are listed in Table 1. Each value is the mean of at least two determinations. The Zr”, Sr*, and W™ concentrations were considered undetectable after August, June, and July, respectively, when the counting error, expressed as one standard deviation, exceeded the apparent counting rate. In other cases, the relative percentage of uncertainty due to counting factors averaged 3 percent and ranged from 2.1 to 12 percent of the activities reported. The total depositions were ob- and 1.5 curies, respectively, of beta activity per square mile were deposited in New York City during 1958 and 1989 The predominance of shorterlived nuclides noted in 1958 continued through the first half of 1959, with the result that the effects of radioactive decay reduced the two years’ total deposition by a factor of 10 by the end of 1959, and correcting for decay during the sampling period (9). The activity levels shown to exist at the end of 1959 Kingdom, and United States test series tained by summing the monthly levels include totals previously reported for the end of 1958, corrected for decay through the end of 1959. The cumulative gross activity estimate for 1958 was obtained by assuming 2 mean production date of 30 June 1958 and correcting for decay by the T'? law (/8). The decay corrections for the 1959 monthly increments are the results of actual measurements described in the ruthenium analyses. Since the counters used were not sensitive enough to detect the less energetic beta emitters, the gross activity estimates are probably low, and more representative of activities with energies in excess of 0.3 Mev. However, these levels do show that at least 5.1 (9) other sources It is generally conceded that delayed fallout is primarily of stratospheric origin and that the mean atmospheric residence time of weapon-test debris is from one to three years, depending on testing conditions (5, 6). It follows veowucrion gare (s2°*/sa" crag re ’ J 0. ‘ ‘A % s x e < 3 t ‘.3 L ® z '; = z \ \ +e Yo wssseree |e coreetgree New York City fallout reported in Table 1! are limited to Soviet, United 54 by before June 1958. The increases in the Tatio observed during the summer months of 1958 are attributed to the arriva] in New York City of mixed debris. The compositions of these mixtures are determined by extrapolating the ratio curve for the earlier measurements through the middle of 1959 and that the possible major sources of the conducted during 1957 and 1958. Although the sensitivity of debris dating methods is lessened by consideration of monthly collections rather than individual rainfalls, the production of W'S in the United States Hardtack series and the cessation of testing after the Soviet series of October 1958 make obtained éarly 1958 testing in samples taken \ Age and Origin of Debris are subtracting the Hardtack Sr™ and similarly determined Sr” fractions from the total isotopic concentrations and analyzing the Sr*/Sr™ ratios calculated for the non-Hardtack debris. The interpretation of these ratios is illustrated in Fig. 1. The clearest indication of debris age occurs in the 1959 ratios, which uniformly show the Soviet series of October 1958 to be the predominant source. A second trend indicates contributions from Jate 1957 and ' eae t +9o8 TamPlLin® Fig. 1. Fallout’ 2039 ‘e oare from non-Hardtack sources in New York City during 1958 and 1959; Sr®/Sr* ratios, corrected for calculated Hardtack concentrations, are used. Most of the 1958 data are taken from the report of Welford and Collins (9). Strontium-89 values for the first three months of 1958 are taken from the datu summuries of Hurdy ef a/. (2).