Fission Product Mass Distributions

This observation, along with the observation that the lower edge of the heavy fragment peak is anchored at A = 132 has suggested that the preference for asymmetric fission is due to the special stability of having one fragment with Z = 50, N = 82, a doubly magic spherical nucleus. 

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The observed width of the distribution functions in mass number of the fission products indicates that the spherons that lie in the plane of the fissure are essentially randomly distributed between the two daughter nuclei, as discussed below for asymmetric fission. 

What convinces scientists that sustained fission once occurred at Oklo is the presence of characteristic fission products in the ore. Elements of mass numbers between 75 and 160 occur in the ore in larger amounts than elsewhere. Furthermore, mass analysis of the elements in Oklo ore shows that they are distributed in the characteristic pattern shown in Figure 22-12. This isotopic signature, which is not found in any other naturally occurring materials, is so characteristic that it has convinced most scientists that the ore deposits at Oklo once formed a huge nuclear reactor. 

Figure 1 shows some of the single fallout particles collected after the Chinese nuclear explosion on May 14, 1965 (14). Figure 2 shows the mass-yield distribution of the fission products in some of the single fallout particles (5). The values of H calculated in this manner range from 30-50 sec., as shown in Table 1(4). 

Figure 2. Mass-yield distribution of the fission products in single fallout particles collected at Osaka, Japan and Fayetteville, Ark. after the May 9, 1966 Chinese nuclear explosion (4)...

Just as earlier we were able to observe mass-yield distributions of the fission products from the fissionable nuclide used in the Chinese nuclear device, it is possible to see part of the mass-yield curve from the fission of 244Pu, which was synthesized originally in a supernova. Figure 6 shows the mass-yield distribution of the excess fissiogenic xenon observed in the meteorite Pasamonte (15). 

Zysin, Y. A., Fission Product Yields and Their Mass Distribution, p. 63,... 

A knowledge of the size distribution function of the radioactive debris and the specific activity of individual fission product chains as a function of particle size suffice to define many important radiological properties of the land-surface nuclear explosion. If is the function of a radionuclide or fission mass chain distributed between particle sizes Di and D2, then... 

The population of fission product elements as a function of time is changing rapidly. These may be estimated from a knowledge of the half-lives of the fission product chain members, the mass chain yield, and the independent yield distribution along the mass chains. Although there are some uncertainties in these procedures largely because of lack of data on short-lived species, and a less than perfect understanding of the charge distribution function, reasonable estimates of radioactive atom... 

Figure 11.1 suggests fission proceeds in two steps, the ascent to the saddle point and the passage through the scission point. We shall present our discussion of fission from this point of view. We shall assert that like chemical reactions, the reaction probability is determined by the passage through the transition state. We shall also assert, more controversially, that the distribution of fission product energies, masses, and so forth is determined at or near the scission point. 

Fission products span a very 9.33 Smoothed mass distribution Tor the ... 

In Fig. 8.13 the yield of fission products obtained by thermal fission of is plotted as a function of the mass number A (mass distribution). The maxima of the yields are in the ranges of mass numbers 90-100 and 133-143. In these ranges the fission yields are about 6%, whereas symmetrical fission occurs with a yield of only about 0.01%. The peaks in the mass distribution curve A = 100 and at 4 = 134 are explained by the fact that formation of even-even nuclei is preferred in the fission of the even-even compound nucleus It should be taken into account that the sum of the fission yields is 200%, because each fission gives two fission products. 

The mass distribution obtained by fission of and with thermal neutrons (Fig. 8.14) is similar to that observed for Whereas the maximum for heavy fission products is nearly at the same place in the case of and Pu, the maximum for light fission products is shifted to the right in the case of Pu. This tendency continues with increasing mass of the fissioning nuclei, and in thermal-neutron fission of Fm the two maxima merge into one another. 

The influence of the energy of the neutrons on the mass distribution of the fission products is shown in Fig. 8.15 at higher neutron energies, the probabihty of symmetric fission increases strongly. 

The mass distribution curves in Figs. 8.13 to 8.15 give the total yields of the decay chains of mass numbers A. The independent yields of members of the decay chains, i.e. the yields due to direct formation by the fission process, are more diflicult to determine, because the nuclides must be rapidly separated from their precursors. Only a few so-called shielded nuclides (shielded from production via decay by a stable isobar one unit lower in Z) are unambiguously formed directly as primary... 

As an example, the mass distribution of the products obtained by the bombardment of with " Ar is plotted in Fig. 8.24. The curve is explained by superposition of the processes described above only few nucleons are transferred by quasielastic reactions (a), and many nucleons by deeply inelastic processes (b). Fusion followed by fission of highly excited products leads to a broad distribution of fission products around l/2(y4i + A2), where A and Ai are the mass numbers of and Ar, respectively (c), and asymmetric fission of heavy products of low excitation energy gives two small maxima (d). 

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