Many know or have heard about sediment and rock dating methods, such as carbon-14 or lead uranium (U-Pb). These and other geochronological methods are based on the decay of radioactive isotopes and widely used in different areas of geosciences, from the study of igneous and metamorphic rocks crystallization to research on the flowering and fall of human civilizations, and at different time scales, from hundreds to billions of years. Most of these methods depend on specific minerals or organic compounds, which are not always present or are rare in sediments or rocks.
Since the 1950s, the collaboration between researchers in the fields of radiation physics, geosciences and archeology has enabled the development of dating techniques based on the luminescence of quartz and feldspar grains, ubiquitous minerals on the earth’s surface. These dating methods are based on the amount of charges trapped (electrons and gaps) in defects (traps) of the crystalline structure of quartz and feldspar exposed to environmental ionizing radiation, derived mainly from uranium, thorium and potassium disseminated in minerals and cosmic radiation. These charges record the amount of ionizing radiation absorbed (dose) since the crystallization, heating or sun exposure of quartz and feldspar.Thus, the radiation dose records the time since the crystallization of quartz or feldspar or since the event of heating or sun exposure. By stimulating quartz or feldspar grains by light or heat in the laboratory, the loads recombine and emit light (luminescence). This phenomenon is known as optically stimulated luminescence (LOE or optically stimulated luminescence, OSL), if the stimulus is light, and thermoluminescence (TL), if the stimulus is heat. The speed at which a quartz grain or feldspar absorbs the energy of ionizing radiation in a certain environment is called the dose rate and its calculation is necessary to determine the “age of luminescence” (age = dose/dose rate).For this, it is necessary to measure the contents of uranium, thorium and potassium, whose radioactive decay generates the ionizing radiation of geological materials. This can be done by the gamma ray spectrometry technique emitted by these elements. From the combination of the methods we used to measure the dose rate (gamma spectrometry) and the dose (luminescence), the name “Gamma Spectrometry and Luminescence Laboratory” “LEGaL” emerged.
LEGaL was created in 2012 by professors Paulo César Fonseca Giannini and André Oliveira Sawakuchi and has since applied luminescence dating techniques to solve geosciences problems. LEGaL also seeks to develop new applications of luminescence, with emphasis on the analysis of the provenance of quartz grains. Since 2012, the research developed by students of scientific initiation, master’s and doctorate, under the guidance of LEGaL researchers, has resulted in more than 100 articles published in prestigious scientific journals and in collaboration with researchers from Brazilian and foreign institutions.
In this blog, we will present the scientific papers recently published by LEGaL, with the aim of disseminating and popularizing the particularities of the methods of dating and analysis of provenance by luminescence.