ob i Ali a » Bey it yt ¢ ‘ 4.2.4 Decay of Activity Figure 4.7 shows the gross decay of a particle taken from a lagoon station during the interval D+15 to D+90 day. The slope of this curve varies from —1.9 at D+15 day to -1.0 at p+70 day. An examination of the gamma decay from H+2 hr, as obtained by Project 5.3, in- dicated a slope of approximately —1.2. PHYSICAL DISTRIBUTION OF FALL-OUT 4.3 The mass measurements for determining the physical distribution of particulate were made from the material in the total collectors, It was assumed that the amount of activity in the rain water from leaching of the particulate was proportional to the amountof solid dis- ¥ yd 4, 7b ype id solved. This assumption allows for an error of at least 10 per cent, Also, an unknown amount of particulate did not get into the collecting bottles because of its tendency to adhere to any moist surface such as the collecting funnel. Therefore, as a check, the material collected in the rain-gage buckets was also used to establish a mass distribution. The values recorded in Fig. 4.8 represent the greatest mass, corrected to grams per square foot, that was collected in either the total collector or the rain gage. These values are not absolute but represent the minimum amount of fall-out occurring at any one station. Variation of Mass with Cross-wind Distance, The quantity of fall-out, cross wind, varied from some value over 20 g/sq ft at 4 miles to zero at approximately 15 miles. There is no evidence of an exponential mass distribution between 4 and 15 miles (Fig. 4.9); however, previ- ous test data’? show evidence of an exponential distribution. 4.4 PARTICLE-SIZE DISTRIBUTION, RADIOACTIVE An examination of the particle-size distribution was undertaken to investigate and further document the existing theories of the fall-out mechanism. It was not the purpose of this project to obtain detailed data on this subject but simply to get a gross picture of the existence or nonexistence of particles within various size ranges, This information, together with a knowledge of the time of arrival of the particulate, permitted further work (see Chap. 6) on the determination of f ; fall-out mechanism. 4.4.1 Particle-size Distribution as a Function of Time There was someindication of fra-‘ionation of particle size with respect to time of arrival. Figure 4.10 shows the time distribut. n of particulate at two cross-wind stations, one 8 miles distant and the other 15 miles. In both cases the fall-out arriving at later times did not contain particles as large as were found at early times. The frequency of particles in the range 0 to 25 » is not known; however, in all cases parti cles within this range were identified. In all cases particles from less than 25 yu to at least 300 u and in some cases as large as 5000 u were found to have arrived at the same time. 4.4.2 Particle-size Distribution as a Function of Distance There was no indication of fractionation of particle size with cross-wind distance. The distribution covered approximately the same rangeatall stations from 5 to 15 miles (Table 4.3). 4.5 TIME OF ARRIVAL OF PARTICULATE Four cross-wind stations within the fall-out area were selected as having flawless records of the arrival of fall-out with time. These stations were located at 8, 10, and 15 miles, as 41