Spontaneous fission decay

Some materials have a spontaneous decay process that emits neutrons. Some shortlived fission products are in this class and are responsible for the delayed neutron emission from fission events. Another material in this class is Cf that has a spontaneous fission decay mode. Cf is probably the most useful material to use as a source of neutrons with a broad energy spectrum. 

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). 

Up to 1970, it was thought that the practical limit of the periodic table would be reached at about element 108. By extrapolating the experimental data on heavy-element half-lives, we concluded that the half-lives of the longest-lived isotopes of the heavy elements beyond about element 108 would be so short (<10-6 s) due to spontaneous fission decay that we could not produce and study them (Fig. 15.10). However, in the late 1960s and early 1970s, nuclear theorists, using techniques developed by Vilen Stmtinsky and Wladyslaw Swiatecki, predicted... 

Fig. 2. Presently known N>152 and Z>102 nuclei and their half-lives (blank box for a decay, light grey for spontaneous fission decay and dark grey for p decay see back book cover for a color version). Decay chains starting from element 110, 111 and 112 were measured at GSI in Darmstadt [15,20,21], that from 114 and 116 at JINR in Dubna [35],...

On-line gas chemical studies of dubnium have been mostly performed with Db. This nuclide can be produced in the reaction Bk( 0,5n) at a beam energy of about 100 MeV. It has a half-life of 34 5 s and decays with 67 % by emission of two sequential a particles via 258Lr (T1/2=4.4 s) to the long-lived 254Md (Ti/2=28m). In addition, 262Db has a spontaneous fission decay branch of 33%. Hence, identification of each separated labeled molecule is based on either detection of two characteristic a-particles and their lifetimes or on the detection of a spontaneous fission decay. 

More advanced applications of neutron counting were based on the expectation that spontaneous fission events of superheavy nuclei should be accompanied by the emission of about ten neutrons [41,42], distinctly more than two to four observed for any other spontaneous fission decay. Such neutron bursts can be recognized by recording neutron multiplicities - events with several neutrons in coincidence - with 3He-filled counting tubes [43,44] or large tanks filled with a liquid scintillator sensitive to neutrons [45],... 

The prior presence of " Pu, the only transura-nic nuclide known to have been present in the early solar system, can be inferred from its spontaneous-fission decay branch, through production of fission tracks and, more diagnostically, by production of fission xenon and krypton. The identification of " Pu as the fissioning nuclide present in meteorites is unambiguous, since the meteoritic fission spectrum is distinct from that of but consistent with that of artificial " Pu (Alexander et al, 1971). The demonstration of the existence of " Pu in the solar system reinforced the requirement (from the presence of I) of a relatively short time between stellar nucleosynthesis and solar-system formation and made it incontrovertible, since while it might be possible to make in some models of early solar system development, the rapid capture of multiple neutrons (the r-process) needed to synthesize Pu could not plausibly be supposed to have happened in the solar system. 

For the separation of elements/complexes in single atom amounts automated micro-scale ion-exchange colunrn are used, like ARCA developed at the GSI, or SISAK in Norway. In ARCA, after adsorption on the colunrn, the adsorbed species are eluted and are determined from the retention times of the species on the column. The collected samples must be dried prior to measurements of a-emission and/or spontaneous fission decay. This limits the detection to nuclides with half-lives longer that 30 sec [11-13]. SISAK is a new on-line technique to be used to perform liquid-liquid extractions on the time scale of a few seconds [18,48]. Elements Rf through Sg were investigated using these techniques. 

Apparently, our solar system was showered by debris from a recent but distant supernova explosion which may have disturbed the "peaceful" gas cloud in our part of the universe and initiated the condensation of the solar system. At this time newly formed elements stopped being added to the solar system from the galaxy. Sufficient amounts of and Pu remained after the formation of solid materials to produce characteristic isotope anomalies and fission track excesses in meteoritic and lunar materials. The spontaneous fission decay of Pu yields which decays as... 

Though the " Ca-produced transactinide elements can be relatively resistant to spontaneous fission decay, the local increase in a-decay Q value with an increase in atomic number results in an overall decrease in half-life [329]. The decays of the " Ca-produced superheavy nuclei lead to chains of sequential a decays to longer lived daughter nuclei lying closer to the line of stability, some of which have surprisingly long half-lives. As an example, the decay of 115, produced in the AmC Ca,3n) reaction, results in a chain of a-emitting superheavy nuclei that culminates in Db (Z = 105), a nuclide with a half-life of one day [285, 286, 330]. This nuclide contains 7 neutrons more than the heaviest dubnium isotope that can be produced by either cold fusion or more asymmetric hot-fusion reactions. 

Viola Jr, V.E., Seaborg, G.T. Nuclear systematics of the heavy elements—II lifetimes for alpha, beta and spontaneous fission decay. J. Inorg. Nucl. Chem. 28, 741-761 (1966)... 

Fig. 6 Large neutron counter with He counting tubes for the recording of neutron multiplicities in the spontaneous fission decay of superheavy nuclei (see text). Reproduced from [43]...

In addition to the decay modes discussed, another decay mechanism, fission decay, should be briefly discussed for a completion of radioactive decay introduction. The products of fission are found in a number of sites as the sources of radioactive contaminants. Fission decay involved two types of process spontaneous fission and neutron-induced fission. Spontaneous fission is a naturally occurring decay process in which a nucleus breaks into two fragments, along with the emission of two to three neutrons. An example of a spontaneous fission decay process is ... 

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