Luminescence dating

Luminescence dating refers to a group of chronological dating methods of determining how long ago mineral grains were last exposed to sunlight or sufficient heating. It is useful to geologists and archaeologists who want to know when such an event occurred. It uses various methods to stimulate and measure luminescence. It includes techniques such as optically stimulated luminescence (OSL), infrared stimulated luminescence (IRSL), radiofluorescence (RF), infrared photoluminescence (IR-PL) and thermoluminescence dating (TL). "Optical dating" typically refers to OSL and IRSL, but not TL. The age range of luminescence dating methods extends from a few years to over one million years for red TL. Since the early applications of luminescence dating in the 1960/1970s, the field has received growing attention in the scientific community, with more than 3500 publications per year and >200 laboratories across the globe in 2020.

Conditions and accuracy Background ionizing radiation around the minerals—for instance quartz or potassium feldspar—buried in the ground causes charge to build up within the minerals. The sources of this natural radiation are cosmic rays and trace amounts of radioactive isotopes of elements such as potassium, uranium, thorium, and rubidium. The trapped charge accumulates over time at a rate determined by the amount of background radiation at that location. Exposure to light causes the charge to quickly decay and produce light; however, if these mineral grains are stimulated using either light (blue or green for OSL; infrared for IRSL) or heat (for TL), a luminescence signal is emitted as the stored electron energy is released, the intensity of which varies depending on the duration of burial (and hence radiation absorbed) and specific properties of the mineral. Most luminescence dating methods rely on the assumption that the mineral grains were sufficiently exposed to light or "bleached" at the time of the event being dated. For example, in quartz a short daylight exposure in the range of 1–100 s before burial is sufficient to effectively "reset" the OSL dating clock. This is usually, but not always, the case with aeolian deposits, such as sand dunes and loess, and some water-laid deposits. Single Quartz OSL ages can be determined typically from 100 to 350,000 years BP, and can be reliable when suitable methods are used and proper checks are done. Feldspar IRSL techniques have the potential to extend the datable range out to a million years as feldspars typically have significantly higher dose saturation levels than quartz, though issues regarding anomalous fading will need to be dealt with first. Ages can be obtained outside these ranges, but they should be regarded with caution. The uncertainty of an OSL date is typically 5-10% of the age of the sample. The most common methods of OSL dating are the so-called multiple-aliquot-dose (MAD) and single-aliquot-regenerative-dose (SAR) technique. In multiple-aliquot testing, a number of grains of sand are stimulated at the same time and the resulting luminescence signature is averaged. The problem with this technique is that the operator does not know the individual figures that are being averaged, and so if there are partially prebleached grains in the sample it can give an exaggerated age. In contrast to the multiple-aliquot method, the SAR method tests the burial ages of individual grains of sand which are then plotted. Mixed deposits can be identified and taken into consideration when determining the age.

History The concept of using luminescence dating in archaeological contexts was first suggested in 1953 by Farrington Daniels, Charles A. Boyd, and Donald F. Saunders, who thought the thermoluminescence response of pottery shards could date the last incidence of heating. Experimental tests on archaeological ceramics followed a few years later in 1960 by Grögler et al. Over the next few decades, thermoluminescence research was focused on heated pottery and ceramics, burnt flints, baked hearth sediments, oven stones from burnt mounds and other heated objects. In 1963, Aitken et al. noted that TL traps in calcite could be bleached by sunlight as well as heat, and in 1965 Shelkoplyas and Morozov were the first to use TL to date unheated sediments. Throughout the 70s and early 80s TL dating of light-sensitive traps in geological sediments of both terrestrial and marine origin became more widespread. Optical dating using optically stimulated luminescence (OSL) was developed in 1984 by David J. Huntley and colleagues. Hütt et al. laid the groundwork for the infrared stimulated luminescence (IRSL) dating of potassium feldspars in 1988. The traditional OSL method relies on optical stimulation and transfer of electrons from one trap, to holes located elsewhere in the lattice – necessarily requiring two defects to be in nearby proximity, and hence it is a destructive technique. Nearby electron/hole trapping centres, in particular in feldspars, may suffer from localised tunnelling, which leads to so-called athermal fading of the signal of interest over time. In 1994, the principles behind optical and thermoluminescence dating were extended to include surfaces made of granite, basalt and sandstone, such as carved rock from ancient monuments and artifacts. Ioannis Liritzis, the initiator of ancient buildings luminescence dating, has shown this in several cases of various monuments.

Physics Luminescence dating is one of several techniques in which an age is calculated as follows:

A = D e D ˙ {\displaystyle A={\frac {D_{e}}{\dot {D}}}}

Where A is the age, typically given in years or thousand years (ka, ky, kyr), D e {\displaystyle D_{e}} the equivalent dose in Gy (Gray) and D ˙ {\displaystyle {\dot {D}}} in Gy ka−1 the environmental dose rate.

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