FaeteEe ame PRE AEAS o er RD oei ies, souwrh end of EngenireairbeY&xplained fairly POERE ee ee * Th PO eMary - . sewprenbout WS mmles- an a. Gen hiv, the vertical easily. It seems to be due to the upper size velocity ci fali ana the os .1€ Garticles for Liat of particles at a given altitude. the ditierta: isiands cu. of 2. uated. The . Surveys were made daily to determine the develocity can be est MMNatec in Pwo ways. Particay of these areas of high intensity. Figure 2.3 cles landing ai half the distane+ would have to includes the surveys of four islands west of fall at twice the speed of 75-.micron particles. Engebi. Figure 2.4 includes six islands east From the graph of island activity versus time, and southeast of Engebi. As can be seen from some estimates can be made of the time of the curves, the results for neighboring islands arrival of one half of the particles, from which agree well even though the readings were made an estimate of the speed of descent from 40,000 hurriedly with a standard gammaionization ft can be calculated. For the three islands chamber. All readings were made approximately nearer Engebi, most of the fall-out had occurred 1 meter off the surface of the ground. In no before the first observations. However, with case can it be said that repeated readings were particles falling so fast, the 50 per cent point taken at exactly the samepoint. must have occurred only a little earlier. The On examination of Fig. 2.3 there are two deffall-out times for the farther three islands are subject to considerable error, both because of inite slopes apparent. There is a break between 24 and 48 hr after burst. This break can the scarcity of points on the built-up portion of probably be best explained by fall-out occurring the curves and because of the shifting of winds during this time. This is reasonable in that during the intervening period. The latter factor continued background determinations made by also introduces uncertainty in the estimate of TU 3.1.5 at the radiological safety laboratory on velocity from distance, because the predicted Parry indicate that fall-out occurred at this distance of 75-micron fall-out changed with time. By examining the slopes of these various time. lines several interesting findings arise. After deciding on compromise values for the It is found that, if I = 1,t™*, where I is intenrate of fall, the particle size can be estimated from Stokes’ law (assuming the specific gravity sity in mr/hr and t is time in hours after burst as determined graphically, the decay up to of the particles to be 2). The relative amounts 24 hr after burst is proportional to t7!*. This of different sized active particles originally in agrees with fission product decay as originally the cloud at 40,000 ft can then be estimated from determined by Way and Wigner. Howeverafter the relative intensities on the islands at some 48 hr the decay is proportional to t™'«4 or t7*+5, time, for instance, H+100 hr, when fall-out was This is significantly different from what is certainly complete. This estimate is probably usually expected for fission products. Why this much too low for the small particles on account should occur was unknown at the time of writing. of the shift in winds with time. The results are Whether it is due to particle size, fission chemgiven in Table 2.5. When plotted they indicate a istry, etc., cannot be determined without more range from 60 microns to nearly 300 microns, data. It is of interest that the change in slope with an average size of about 160 microns. did not become apparentuntil after the fall-out Stokes’ law, together with data on rate of rise occurring between 24 and 48 hr after burst. of the fireball, gives an upper limit of about From Fig. 2.4 it is noted that fall-out was 170 microns for the size of particles at 40,000 continuous from 1 hr to 10 to 20 hr after detonaft. In view of many uncertainties in both calculations, the disagreement is not at all serious, tion. After these times recognizable decay occurred on the slopes shown in Fig. 2.2 which It is emphasized that, although this distribuindicates that the decay is proportional to t~!4, tion of particle sizes may be erroneous with regard to the initial conditions in the cloud, the The six islands, Muzin, Kirinian, Bokon, errors are not of immediate practical signifiBiijiri, Piiraai, and Runit, lie on about the same cance. It is the size distribution of the partibearings (120 to 135 degrees) from ground zero cles that reach the ground which is of imporand at distances from 1 mile to about 10 miles. tance to the people working there. There is no Taking, from the wind soundings at shot time, reason to suspect that the calculated distributhe prediction that 75-micron particles, from tion is seriously in error in this respect. about 40,000 ft above ground zero, would land SERRE Gee sncona aon on wee aoe + mene —_————— een Mee n arn pe ee ae a tea Serre nn seean Le at il NE TM “enereee oe