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Optical dating is a method of determining how long ago minerals were last exposed to daylight. It is useful to geologists and archaeologists who want to know when such an event occurred. It refers to techniques including electron spin resonance (ESR), thermoluminescence (TL) and optically stimulated luminescence (OSL). ==Conditions and accuracy== Ages can be determined typically from 100 to 200,000 years BP, and can be reliable when suitable methods are used and proper checks are done. Ages can be obtained outside this range, but they should be regarded with caution. The optical dating method relies on the assumption that the mineral grains were sufficiently exposed to sunlight ("bleached") before they were buried. This is usually, but not always, the case with aeolian deposits, such as sand dunes and loess, and some water-laid deposits. All sediments and soils contain trace amounts of radioactive isotopes including uranium, thorium, rubidium and potassium. These slowly decay over time and the ionizing radiation they produce is absorbed by other constituents of the soil sediments such as quartz and feldspar. The resulting trapped charge within these minerals remains as structurally unstable ''electron traps'' within the mineral grains. Stimulating samples using either blue, green or infrared light causes a luminescence signal to be emitted as the stored unstable electron energy is released, the intensity of which varies depending on the amount of radiation absorbed during burial and specific properties of the mineral. The trapped charge accumulates at a rate over time determined by the amount of background radiation at the location where the sample was buried. Exposure to sunlight resets the luminescence signal and so the time elapsed since the sample was buried can be calculated. 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「Optical dating」の詳細全文を読む スポンサード リンク
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