Alpha particles are about 90 to 95% less effective in inducing luminescence compared to beta and gamma radiation.Thus, the population of stored electrons in lattice-charge defects increases with prolonged exposure to ionizing radiation and the resolved luminescence emission increases with time.Ensuing research in the 1970s documented that marine and other sediments with a prior sunlight exposure of hours to days were suitable for thermoluminescence dating (Wintle and Huntley, 1980).Discoveries in the 1980s and 1990s that exposure of quartz and feldspar grains to a tunable light source, initially with lasers and later by light emitting diodes, yield luminescence components that are solar reset within seconds to minutes, expanded greatly the utility of the method (Huntley et al., 1985; Hütt et al., 1988; Aitken, 1998).However, some quartz grains yield considerable emissions with infrared excitation and may host feldspathic or other mineral inclusions; such grains should be analyzed as feldspar grains.Sediment grains act as long-term radiation dosimeters when shielded from further light exposure with the luminescence signal a measure of radiation exposure during the burial period.Free electrons are generated within the mineral matrix by exposure to ionizing radiation from the radioactive decay of daughter isotopes in the 235U, 238U and 232Th decay series, and a radioactive isotope of potassium, 40K, with lesser contributions from the decay of 85Rb and cosmic sources.The radioactive decay of 40K releases beta and gamma radiation, whereas the decay in the U and Th series generates mostly alpha particles and some beta and gamma radiation.
Often this luminescence “cycle" occurs repeatedly in many depositional environments with signal acquisition of mineral grains by exposure to ionizing radiation during the burial period and signal resetting (“zeroing") with light exposure concurrent to sediment erosion and transportation. (a) Luminescence is acquired in mineral grains with exposure to ionizing radiation and trapping of electrons.
Often mineral grains that are fresh from a bedrock sources have significantly lower luminescence emissions per radiation dose in comparison to grains that have cycled repeatedly. (b) The luminescence for grains is zeroed by exposure to sunlight with erosion and transport.
OSL dating provides an estimate of the time elapsed with latest period of burial and thus, yields a depositional age (Fig. (c) With burial and exposure to ionizing radiation free electrons are stored in charge defects within grains crystal lattice.
(f) Careful sampling without light exposure and measuring of the natural luminescence, followed by a normalizing test dose (Ln/Tn) compared to the regenerative dose to yield an equivalent dose (De) (from Mellet, 2013).
The exposure of quartz and feldspar grains to sunlight for 60 seconds effectively diminishes the time-stored OSL signal to a low definable level.
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(d) Further light exposure of grains with erosion and transport zeros the luminescence.