Refractory chains, specific activity

In this expression A(i) is the specific activity of radionuclide i, A(i) (r = 1) is the specific activity of nuclide i at r = 1, r is the radius of a debris particle, and mis a number between 0 and 1. A value of m = 0 implies that the activity is proportional to the volume of the particles. Such a volume dependence results for nuclides which combine with the matrix material in the cooling fireball and applies to the so-called refractory chains, such as the 95-, 11-, and 147-chains. A value of m = 1 or a surface-... 

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. 

Absolute Magnitude of Specific Activities. In the Tewa and Zuni atoll samples absolute magnitude of specific activities for particles of the same kind (e.g., irregulars) are not too different when a refractory chain such as Chain 144 is compared. The absolute magnitudes on a normalized basis are around 3 X 1015 to 5 X 1015 equivalent fissions per gram. Bravo (15 megatons on coral) and Lacrosse (40 kilotons on coral) also give similar values. 

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. 

We note that fission product activity balance requires that the specific activity of a volatile chain increase more rapidly than a refractory chain with decreasing diameter because of the large depletion factors noted in 89Sr/144Ce in large sizes. 

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