Spontaneous fission tracks

Relatively little work has been carried out on fission track dating of phosphate minerals other than apatite, and merrillite, P-Ca3(P04)2, is the only other example to have received any significant attention. Two early studies examined the potential of monazite, (Ce,La,Y,Th)P04, and pyromorphite Pb5(P04)3Cl, but these minerals have not been studied further. Pyromorphite was shown by Haack (1973) to contain numerous spontaneous fission tracks, but these were very unevenly distributed, making its use in dating very difficult. 

The application of fission track length studies to the interpretation of fission track ages depends on three properties of spontaneous fission tracks. 

Green PF (1980) On the cause of shortening of spontaneous fission tracks in certain minerals. Nucl Tracks 4 91-100... 

In 1964, workers at the Joint Nuclear Research Institute at Dubna (U.S.S.R.) bombarded plutonium with accelerated 113 to 115 MeV neon ions. By measuring fission tracks in a special glass with a microscope, they detected an isotope that decays by spontaneous fission. They suggested that this isotope, which had a half-life of 0.3 +/- 0.1 s might be 260-104, produced by the following reaction 242Pu + 22Ne —> 104 +4n. 

At Dubna, 280-MeV ions of 54Gr from the 310-cm cyclotron were used to strike targets of 206Pb, 207Pb, and 208Pb, in separate runs. Foils exposed to a rotating target disc were used to detect spontaneous fission activities. The foils were etched and examined microscopically to detect the number of fission tracks and the half-life of the fission activity. 

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... 

Fig. 14. Measured distribution of spontaneous fission events attributed to the decay of an isotope of element 106 (Sg). The dotted histogram shows the data corrected for the relative detection efficiency due to annealing of fission tracks. The thick solid curves show the smoothed corrected thermochromatograms for Sg, denoted "[106] , and for l76W. Figure reproduced from [28] with the permission of Oldenbourg Verlag.

Examinations of the same and of other lead-bearing samples for spontaneous fission events with large proportional counters in Dubna seemed to confirm these findings, but further measurements [37] of thin samples sandwiched between two plastic fission-track detectors showed that the events were background caused by cosmic-ray induced reactions of lead. Other groups [38] found no evidence for spontaneous fission activities in lead and other samples at a lower detection limit of 10" 3 g/g achieved with the sandwich technique. Even lower limits down to 10"17 g/g can be reached by etching... 

The largest collector surface for elements impinging on the Globe is the sea, of course. Heavy elements deposited in seawater are enriched in certain sediments such as manganese nodules, iron-manganese hydroxides. Fission tracks were found [83] in feldspar inclusions in such nodules, but no evidence was obtained [40,45] for spontaneous fission activities by counting nodules with neutron detectors. 

The detection techniques applied in those early attempts were often surprisingly simple searches for spontaneous fission activities. The whole product mixture was collected on a catcher foil and exposed to mica, glass or polymer sheets to produce tracks of spontaneous fission events. By quickly rotating the catcher between detector foils during bombardment, this technique allows the detection of short-lived nuclides down to millisecond half-lives [88],... 

Direct searches for superheavy elements in the U+ U reaction were undertaken at the unilac by several groups. All these efforts remained without positive evidence. The data are summarized in Figure 13. The curve labeled chem [106] was obtained with off-line chemical separations [107] and an assay for a-and spontaneous fission activities here, the 10 picobam level was reached for half-lives between several days and years. Attempts to detect short-lived nuclides were less sensitive. The curve labeled gas holds for an on-line search [108] for components volatile at room temperature. wheel [106] refers to fission track detection in the unseparated product mixture deposited on a rotating catcher, rec [109] to implantation of recoil atoms in a surface barrier detector, and JET to on-line transport from target to detector with a gas jet [91,110],... 

Solid-track detectors have found application in the investigation of spontaneous fission of transuraiuum nuclides, of cosmic radiation at high altitudes of the order of 20 km and in dating of minerals by counting the number of tracks. 

Fission tracks are observed in solids due to spontaneous or neutron-induced fission of heavy nuclei. The primary tracks can be made visible under an optical microscope by treatment with chemicals, by which track diameters of the order of 0.1 to 0.5 pm are obtained. The method is the same as that used with track detectors (section 7.12). The length of the fission tracks depends on the nature of the minerals and varies between about 10 and 20 pm. With respect to dating, the only important source of fission tracks is spontaneous fission of Spontaneous fission of other naturally occurring heavy nuclides gives no measurable density of fission tracks neutron-induced fission is, in general, negligible and the tracks of recoiling atoms due to a decay are very short (of the order of 0.01 pm). 

The track density (number of fission tracks per cm ) in a mineral is a function of the concentration of U and the age of the mineral. For the purpose of dating, a sufficient number of tracks must be counted, which means that the concentration of U or the age (or both) should be relatively high. Usually, first the fission track density due to spontaneous fission of is counted, then the sample is irradiated at a thermal neutron flux density in order to determine the concentration of U in the sample by counting the fission track density due to neutron-induced fission of The age t of the mineral is calculated by the formula... 

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. 

Two Other factors unique to these extraterrestrial samples must also be accounted for. The first is that tracks from heavy cosmic ray particles (mainly Fe-group nuclei) and other nuclear interactions with cosmic rays may also be present, in addition to fission tracks. The simplest method for correcting for the cosmic ray background is to measure the track density in adjacent silicate mineral grains, such as feldspar and pyroxene, which do not contain uranium and are therefore free of fission tracks. The second factor is that the samples are so old as to contain tracks from the spontaneous fission of now-extinct transuranic elements, particularly with a half-life of 82 Myr. Such Pu tracks will only be present in samples older than about 3.9 Gyr (Crozaz and Tasker 1981). 

The occurrence of extinct Pu tracks first became apparent when a large excess of fission tracks over that which would be expected from spontaneous fission alone was discovered (e g., Burnett et al. 1971). If calculated as a fission track age in the usual... 

Determination of a fission track age requires several further experimental steps to measure the uranium concentration. The uranium concentration is not measured directly, but a second set of fission tracks is created artificially in the sample by a thermal neutron irradiation. This irradiation induces fission in a tiny fraction of the atoms, which are present in a constant ratio to U in natural uranium. Knowing the total neutron fluence received during irradiation, the number of induced tracks provides a measure of the uranium concentration of the grain. Because the induced tracks are derived from a different isotope of uranium than the spontaneous tracks an important consideration in fission track dating is the assumption that the isotopic ratio of the two major isotopes of uranium, and is constant in nature. With the notable exception of the unique natural nuclear reactors of Oklo in Gabon (Bros et al. 1998), where this isotopic ratio is disturbed, this is a very safe assumption. Numerous measurements have shown that and are always present in their natural abundances of 0.73% and 99.27%, respectively. 

The fission track age, t, is then calculated from the ratio of spontaneous (ps) to induced (pi) track densities according to the standard fission track age equation (Fleischer and Price 1964, Naeser 1967) ... 

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