On October 28,1663, Sir Robert Boyle reported to the Royal Society in London certain experimental findings ( I ) conducted on a large, superbly cut diamond . . ‘%leventhly, I also brought it to some kind of glimmering light, by taking it into bed with me, and holding it a good while upon a warm part of m y naked body.” The phenomenon this distinguished scientist had observed was thermoluminescence. Most geological minerals such as quartz possess the ability to emit visible light at temperatures below that at which the red-glow appears (Figure 1).Modern solid-state physics has attributed this ability to emit light to the release of trapped electrons from crystal lattice imperfection sites to lower valency levels, the trapped electron populations being first created by ionization and diffusion by radioactive emissions (2). Little did Sir Robert know that his primary fundamental observations would lead eventually to the foundations of solid-state dosimetry and a modern method of dating pottery. Within the clay fabric of almost all low-fired earthenware, there are many inclusions of free quartz and feldspars in a wide assortment of sizes. In 1953 Farrington Daniels ( 3 )suggested that thermoluminescence might be applied to the archaeological age determination of such crystalline extracts, since the original firing in antiquity releases any residual geologic thermoluminescence (Figure l),and any reacquisition of thermoluminescence, although somewhat less intense (Figure 2), is directly related to the radiation dose (Le., time frame) since firing, Le.:
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Age (years) = Accumulated Dose (rads)/Annual Dose Rate (rads) Present address, Food and Drug Administration, 1521 W. Pic0 Blvd., Los Angeles, Calif. 90015. 266A
Seven years later 16 dated sherds of Greek pottery, ranging in age from 1000 B.C. to 4000 B.C., were examined by George Kennedy ( 4 )who confirmed the age of all samples, with a maximum error of 400 years. During the past decade, various workers ( 5 )at the Research Laboratory for Archaeology and the History of Art a t Oxford University in England have contributed major advances in the development and improvement of routine thermoluminescent dating of pottery. Such established techniques are now being actively applied (6) at the National Museum of Antiquities, Scotland; the Research Laboratory of the British Museum; the Department of Physics, University of Birmingham, England; the Danish Atomic Energy Research Establishment, Riso; Kyoto University, Japan; the Museum of Fine Arts, Boston; the Applied Science Center for Archaeology, University of Pennsylvania; the Brookhaven National Laboratory, Long Island, New York; the Smithsonian Institution, Washington, D.C.; the Laboratory for Space Physics, Washington University, St. Louis; and the Conservation Center of the Los Angeles County Museum of Art. Theory Basic Mechanism. Within the clay matrix, trace amounts (ppm level) of uranium-238 and thorium-232 togeth-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
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er with potassium-40 emit mostly alpha and beta particles. Such energetic, charged particles pass through the clay body and its various crystalline inclusions, causing both ionization and displacement effects. Ionization causes electrons from the outer valency orbitals of the parent atoms composing the lattice matrix to be excited to the conduction band (Av cz 10 eV), thereby creating a mobile electron in the conduction band and a mobile hole (missing electron) in the valency band (Figure 3a): i.e., no atom or nuclei movements. Displacement effects can occur whereby a struck atom is ejected from its normal lattice position, creating a vacancy in its original site and a defect in its new interstitial location. Such vacancies and defects are potential energy traps for the mobile electrons and mobile holes produced by the ionization process. However, it has been demonstrated that the growth of new traps created by displacement effects by radiation doses of the magnitude to be experienced, 1000 rads, in the history of pottery making, ca. 7000 years, was negligible (7) compared to the number of already existing naturally occurring traps in the lattice network: vacancies, interstitial impurity atoms, and substitutional impurity atoms. In the study of geologic dating by thermoluminescence, displacement effects must be considered to play an important role. Once in the conduction band, free mobile electrons may elect to stabilize themselves by either recombination with an electron hole or trapping by a defect. The mobile electron hole, also produced by the ionization process, has a very short lifetime and therefore is easily and quickly trapped by defects, the so-called hole trap (Figure 3b). Once such a defect has captured a mobile hole, there then exists a high
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Thomas Cairns' Conservation Center Los Angeles County Museum Of Art 5905 Wilshire Blvd. Los Angeles. Calif. 90036
probability of also capturing a free mobile electron. When this situation results, the two entities neutralize each other, and the site of neutralization is called a recombination or luminescent center. This almost instantaneous recombination process (Figure 3c) results in fluorescence and the release of the defect for reavailahility as a recombination center for-additional neutralizations and fluorescence. A large majority of mobile electrons and mobile holes recombine in this manner in a reasonably short space of time; a small fraction of mobile electrons produced during the ionization process are permanently trapped in negative ion vacancies and by impurity atoms (Figure 3d). These traps can he considered as wells of differing depths depending on their potential energy profiles, usually 0-2 eV. They provide retention of free electrons over longer periods of time, unless such trapped electrons are deliheratelv stimulated by thermal energy trans'king) to traverse hack over this al energy profile (eject from
dergo recornhination at room temperature without any additional thermal stimulation (phosphorescence). Finally, it is the integrated intensity of this light output (at a preselected temperature which demonstrates thermal stability over the archaeological time period, usually in excess of 300 "C) which is proportional to the ahsorbed dose of ionizing radiation (annual dose rate for internal and environmental) the clay and its inclusions have received since the clay was last fired above 500 "C. Kinetics. As already mentioned, electrons in traps which can he ejected at 100 "C immediately after irradiation are not visible in the T L glow curve profile of archaeological samples. Such electrons can he thought of as being in very shallow traps or wells and even at room temperature are thermally unstable, undergoing recombination usually within a few days without any additional thermal stimulation. This rate of escape of electrons from traps governs the observed T L glow curve profile (9).Before applying
the trap well) into the conduction hand, to find a recombination or luminescent center, to neutralize with an electron hole, and to emit part of their energy (
