6. FALLOUT MECHANISM There is need for more basic knowledge on the mechanism of fallout. Theoretical treatment of the deposition process is possible, but there is little information on the primary characteristics of the radioactive material that constitutes long-range fallout. Certain general observations have been made and are generally agreed upon: 1. The Sr® in fallout is largely water soluble, although fallout debris consisting of silicate materials from continental tests may have a larger fraction that is water insoluble. 2. Eighty to ninety per cent of the Sr® fallout, and presumably of total radioactive fallout, comes down during periods of precipitation and only 10 to 20 per cent is deposited by dry fallout. 3. Radioactive debris injected into the troposphere is deposited within a few months, whereas material injected in the stratosphere may have a residence time of several years. 4. There is a markedlatitudinal variation in fallout, apparently greater than can be explained on the basis of tropospheric fallout being confined to a narrow zoneof latitude. There are many details of deposition, however, which are not fully understood, and some experimental work is in progress to study fallout mechanisms. The data reported in this section are results of relatively small, short-term experiments intended to point the way toward more extended work if the approach appears promising. Those projects that are being continued will be mentioned in the discussion relating to the individual study. 6.1 PRECIPITATION SAMPLES COLLECTED AT MOUNT WASHINGTON OBSERVATORY A number of preliminary samples consisting of condensed fog and precipitation have been collected at the Mount Washington Observatory. These have been taken to determine whether there is a relation between cloud content and precipitation content for Sr, The results are only preliminary, but a more definite program is under way at the present time. 6.2 NEW HAVEN DUSTFALL The Bureau of Environmental Sanitation of the New Haven Department of Health collects monthly dustfall samples at several stations in and around the city. The collectors are standard 1500-ml beakers, and duplicate samples are analyzed for dust content, one by evaporation of any rainfall and the other by filtration. During 1956, certain samples supplied to HASL were measuredfor total MFP activity and Sr”, and the results are shown in Table 61. Although the data are not as complete as desired, there are several interesting points: (1) filtration or evaporation is equally effective for dustfall by weight, (2) filtration loses both MFPand Sr® by solubility, and (3) the agreementin activity values between stationsis fairly good and is independent of the dustfall. 6.3 FALLOUT COLLECTIONS IN HARTFORD, CONN., AREA A series of monthly pot collections was made in the Hartford, Conn., area to determine the variability in local fallout around an industrial city. The data are presented in Table 62, and the location of the stations are mapped in Fig. 14. 136