SINGLE PARTICLES FROM HIGH-YIELD AIR BURSTS 101 Table 1-—THE RADIONUCLIDE CONTENT OF AN UNFRACTIONATED 2357 FISSION-PRODUCT MIXTURE AT 25 DAYS Radionuclide Activity, % Radionuclide Activity, o B9cy 5.4 NSp_ Hy 0.05 133Ne 4.75 aly 387 r 95Nb 6.4 6.5 2.8 WICs 140Ba 14074 0.05 8.1 9,2 8Mo—%Te 103Ru 106Ru 129mTe 0.3 3.8 0.08 1.4 141C@ 144C@ 1d3py 147Nq 182-Te — 182] 0.5 1317 147 Dry 1.7 9,1 1.5 25.6 12.5 0.2 particle volume. Figure 3 shows the activity per unit particle volume as a function of particle diameter for two shots. As is to be expected, the activities per unit particle volume tend to fall at a common average for each of the three shots studied, depending on the fission yield of the nuclear device. For each case, however, there are particles that vary considerably from the norm. This variation appears not to be related to particle color, and on a fractional basis it appears to be as large in large particles as it is in small particles. The beta-energy spectrum, of course, varies as a function of time as the composition of the fission-product mixture changes. Figure 4 shows aluminum absorption measurements on a particle sample at different times. Close to zero time the absorption curve approaches a straight line on semilog plots, but as time passes the absorption curve shows a shape of alow-energy componentplus a high-energy component. When the sample has decayed 200 days, the major high-energy com- ponentis due to '44Pr, the daughter of !44Ce. In the discussions so far, the similarities between particles have been noted. Striking dissimilarities become apparent, however, after de- cay. Figure 5 shows the beta decay of ?*U and 79*Pu fission-product mixtures as measured with a methane-end-window proportional counter. Although it is. readily apparent that the rate of decay of these fission-product mixtures varies with time, a relation of A= kt’ isa close approximation of the decay from 3 to 4000 days. The decay of isolated particles was followed and the data during the period 3 to 200 days were chosen to determine the decay rate by a least-squares curvefit. Figure 6 shows plots of the number of particles vs. the decay slope of those particles. As analytical aids the standard deviation of the decay slope, the standard error of the estimate, the variance in each variable, and the difference between each experimental value and the unfractionated fission-product mixtures, derived