Cloud samples specific activity

Integration of the specific activities over the particle size yields a much lower amount of 90Sr than of 147Pm produced, even if estimates from analyses of particles greater than 100/x are included (7). Such a result suggests that the sample was not representative of the true radioactive cloud distribution. 

The specific activities of radionuclides in samples from surface burst debris are not necessarily unique functions of the particle size. The uniqueness is determined by the degree of mixing in the cloud, and is, therefore, expected to be a function of the time elapsed between the shot time and the sampling time. 

Figure 6 shows the specific activity of 140Ba in the fallout and 90Sr in the Zuni cloud (7). The similarity in the shapes of the curves for the various samples is quite evident. Again the Zuni 85-km. fallout agrees quantitatively with the cloud. For the semivolatile mass chain 140, the distinction between the Tewa spheres and irregulars is less marked than it was for the refractory 99 and 144. 

Magnitude of Specific Activity in Cloud Samples. The specific activities of all nuclides—refractory and volatile—show higher values in the cloud samples and in the 85-km. downwind fallout samples from Zuni. It is to be emphasized that these are undifferentiated samples—i.e., no attempt is made to distinguish between radioactive and nonradioactive particles—and that both the cloud and the downwind fallout samples are not representative of the core of the nuclear cloud but only of its lower fringes. Accordingly, specific activities within the core of the cloud should be even higher than those observed. 

The specific activity for a refractory chain in cloud and fallout samples of lower yield surface bursts may not be too different. Data exist for Johnie Boy (10) and for Lacrosse (7) which bear this out. Apparently mixing of the fission products with the debris is much more efficient in low yield explosions where the toroidal circulation is weaker and less persistent and where cloud rise is heavily damped by the atmosphere. This is the basis for drawing the specific activity of the refractory chain at low particle diameters more flatly for lower yield cases, as illustrated schematically in Figure 1. 

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