Deep traps in quartz and their use for optical dating

Abstract

We describe a number of optical properties of a natural quartz that are related to the presence of electron traps having an optical depth of about 3 eV below the conduction band. Such traps are populated as a consequence of a radiation dose, and can be depopulated by optical excitation. An understanding of these processes is necessary in order to make use of these phenomena to determine the ages of sediments by optical dating. The recombination emission spectrum is found to consist mainly of a 0.65 eV wide band at 3.35 eV (370 nm). This and other information are used to deduce that the traps are electron traps. The excitation efficiency is found to increase with photon energy, and with temperature. The data are roughly in accordance with Urbach's rule, and an optical trap depth of about 2.9 eV is deduced. Above 100 °C thermal quenching causes a reduction in the observed luminescence. Isothermal decay studies show that there are actually several traps, possibly including a distribution of traps, with most of the luminescence arising from traps with thermal depths of about 1.6 eV and lifetimes deduced from extrapolation to be >2 × 106 years at 20 °C. The ratio of optical to thermal trap depths is in accordance with the Mott and Gurney prediction. A laboratory dose, besides filling traps, also has an effect that we attribute to population of competing recombination centres; this can be reversed with appropriate heating. Our understanding of the dose response and kinetics was tested on two samples of known age, and correct ages of ~ 120 000 years were obtained.