244Pu fission

Terrestrial isotope ratios are mainly unaffected by these processes and therefore provide valid information about standard abundances of individual nuclear species except in special cases where they have been modified by fractionation (e.g. D/H), differential escape from the atmosphere or radioactive decay, e.g. 40Ar is enhanced in the atmosphere relative to 36Ar by 40K decay and there is extra 136Xe thought to result from fission of 244Pu. 

Plutonium-244 decays by -emission and spontaneous fission with a half-life of 82 Ma. The branching ratio, fission/a-emission, is 0.00125. The former existence of 244Pu in the solar system is indicated by the presence of fission tracks in meteorite samples and... 

The preservation of distinct mantle reservoirs over time in Mars and the limited degassing experienced by the Martian mantle after the initial period, as indicated by the almost quantitative retention of 244Pu-produced fission xenon, show that Mars has been a static planet with no mantle convection since very early in its history. 

The half-life of 244Pu (8.2 X 107 years) is short compared with the age of the earth (4.5 X 109 years), and hence this nuclide is now extinct. However, the time interval (a) between the element synthesis in stars and formation of the solar system may have been comparable with the half-life of 244Pu. It has been found recently in this laboratory that various meteorites contain excess amounts of heavy xenon isotopes, which appear to be the spontaneous fission decay products of 244Pu. The value of H calculated from the experimental data range between 1 to 3 X 108 years. The process of formation of the solar system from the debris of supernova is somewhat analogous to the formation of fallout particles from a nuclear explosion. 

Pu was discovered in the debris of the Bikini test in 1952, and its decay constants have been redetermined by Fields et al. to be as follows a-decay (8.18 0.26) X 107t/, spontaneous fission decay (6.55 0.32) X 1010t/ (5). 

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). 

The decay of extinct 244Pu is deduced from excess abundances of the nuclides 136Xe, 134Xe, and 132Xe, produced by the spontaneous fission of 244Pu. Uncertainties arise because there is no stable isotope of Pu that can be used in the way that 127I is used in Equation (3.60) and the use of other heavy nuclides U or Th as substitutes leads to difficulties due to differences in primordial production and chemistry. 

Figure 6.15 Xe isotopic data are plotted in a 136Xe/132Xe-134Xe/132Xe diagram. The linear trend indicates that the data represent mixing between the atmospheric Xe and fission-derived Xe, either from 244Pu or 238U or from both.

Figure 7.7 Isotopic compositions of atmospheric (present) Xe, SW-Xe, and calculated primordial Earth Xe (see text) are shown relative to the calculated primordial (solar system) Xe (Igarashi, 1995). Data are given in Table 7.1. Note that SW-Xe is indistinguishable from the calculated primordial (solar system) Xe. Slight excesses in the atmospheric 131136Xe relative to the primordial Earth Xe are due to the addition of fission Xe either from 244Pu or from 238U or from both, which were produced in the Earth.

Lewis, R. S. (1975) Rare gases in separated whitlockite from the St. Severin chondrite Xenon and krypton from fission extinct 244Pu. Geochim. Cosmochim. Acta, 39, 417-32. 

The 48Ca + 244Pu experiment was repeated in June-October, 1999. During a period of 3.5 months, two more a-decay sequences, terminating in spontaneous fission, were observed [48]. The two chains were identical within the statistical fluctuations and detector-energy resolution, but differed from the first chain measured in 1998. The two new events were assigned to the decay of 288114, the 4n evaporation channel. The cross section was 0.5 pb. 

Further information about this event has been obtained by studying tracks which nuclear decay processes leave in certain minerals ( 8.1.2). Fission tracks can only persist in minerals that have not been heated because heating above 600°C erases the tracks. The fact that Pu fission tracks have been found in iron meteorites and in lunar samples shows that 244pu existed when the planetary system formed. Because of the short half-life of Pu (8 X10 y) it can be concluded that such mineral samples must have formed within a few hundred million years after the nuclide Pu itself was formed. This is probably also the time for planetary formation. The existence of primordial plutonium indicates that an r-process preceeded the formation of the planets. 

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