Nuclear fission product mass distribution

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). 

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... 

Only brief mention is made here of the large number of radiochemical and photographic emulsion studies of the capture of jr -mesons. An excellent review of this work up to 1953 has been given by Turkevitch and mofe recent work can be found in the bibliography of Winsberg s article . The main conclusion is that the 7r -meson when it is captured gives up its entire rest mass to the nucleus and a cascade and nuclear evaporation ensue (see Sects. 11 and 12). The end products have distribution similar to those described for nucleon reactions (Sect. 36) and photon reactions (Sect. 47). Meson induced fission has been studied by Al-Salam , Sugarman , and John and Fry . 

Consequently, in a nuclear reactor, fission products as well as isotopes of heavier elements are produced. These heavy elements are formed by the neutron capture followed by beta decay and are often referred to as the transuranic elements. Because the fission process is asymmetric, the products of the fission reaction tend to distribute themselves around mass 83-105 (light fragment) and mass 129-149 (heavy fragment). Some of the more common light fragments are Kr, Sr, Zr, Tc, and the corresponding heavy fragments are Cs, Ce, and Nd. 

FIGURE 1.7 The mass distribution of the fission products from MOX fuel. (From World Nuclear Association, http //www.world-nuclear.org/uploadedImages/org/info/Nuclear Fuel Cycle/distribution of fission products.png, accessed July 26, 2014. With permission.)... 

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