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Fission track dating is a radiometric dating technique based on analyses of the damage trails, or tracks, left by fission fragments in certain uranium-bearing minerals and glasses. Fission-track dating is a relatively simple, but robust method of radiometric dating that has made a significant impact on understanding the thermal history of continental crust, the timing of volcanic events, and the source and age of different archeological artifacts. The method involves using the number of fission events produced from the spontaneous decay of uranium-238 in common accessory minerals to date the time of rock cooling below closure temperature. Fission tracks are sensitive to heat, and therefore the technique is useful at unraveling the thermal evolution of rocks and minerals. Most current research using fission tracks is aimed at: a) understanding the evolution of mountain belts; b) determining the source or provenance of sediments; c) studying the thermal evolution of basins; d) determining the age of poorly dated strata; and e) dating and provenance determination of archeological artifacts. ==Method== Unlike other isotopic dating methods, the "daughter" in fission track dating is an effect in the crystal rather than a daughter isotope. Uranium-238 undergoes spontaneous fission decay at a known rate, and it is the only isotope with a decay rate that is relevant to the significant production of natural fission tracks; other isotopes have fission decay rates too slow to be of consequence. The fragments emitted by this fission process leave trails of damage (fossil tracks or ion tracks) in the crystal structure of the mineral that contains the uranium. The process of track production is essentially the same by which swift heavy ions produce ion tracks. Chemical etching of polished internal surfaces of these minerals reveals spontaneous fission tracks, and the track density can be determined. Because etched tracks are relatively large (in the range 1 to 15 micrometres), counting can be done by optical microscopy, although other imaging techniques are used. The density of fossil tracks correlates with the cooling age of the sample and with uranium content, which needs to be determined independently. To determine the uranium content, several methods have been used. One method is by neutron irradiation, where the sample is irradiated with thermal neutrons in a nuclear reactor, with an external detector, such as mica, affixed to the grain surface. The neutron irradiation induces fission of uranium-235 in the sample, and the resulting induced tracks are used to determine the uranium content of the sample because the 235U:238U ratio is well known and assumed constant in nature. To determine the number of induced fission events that occurred during neutron irradiation an external detector is attached to the sample and both sample and detector are simultaneously irradiated by thermal neutrons. The external detector is typically a low-uranium mica flake, but plastics such as CR-39 have also been used. The resulting induced fission of the uranium-235 in the sample creates induced tracks in the overlying external detector, which are later revealed by chemical etching. The ratio of spontaneous to induced tracks is proportional to the age. Another method of determining uranium concentration is through LA-ICPMS, a technique where the crystal is hit with a laser beam and ablated, and then the material is passed through a mass spectrometer. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fission track dating」の詳細全文を読む スポンサード リンク
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