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Studying the contents of very old ice could help scientists understand how the relationship between greenhouse gases, which speed up global warming, and climate has evolved over time, he added.Radiometric krypton dating uses krypton-81, a radioactive isotope of krypton—one of the rarest particles on Earth—that is produced by cosmic rays blasting into the Earth, which are stored in air bubbles within Antarctic ice.The gas traveled from Antarctica to Switzerland, where experts separated the krypton from the rest of the air, and then to the Argonne lab, where scientists measured the krypton isotopes, according to Brook.They determined that the ice they gathered was 120,000 years old.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.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.

“The ultimate thing we would like to know is what the climate was like in the past…because that helps us understand what the climate might be like in the future,” said Edward Brook, co-author of the report.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.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.Unlike other isotopic dating methods, the "daughter" in fission track dating is an effect in the crystal rather than a daughter isotope.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 U ratio is well known and assumed constant in nature.

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