4.5 Luminescence Dating
Luminescence datingA technique that measures the amount of trapped electrons in minerals like quartz or feldspar to determine when they were last exposed to light or heat. is, even compared to the other intensive dating methods, a particularly complex and involved method. It makes use of ionising radiationRadiation with enough energy to remove electrons from atoms; responsible for the trapped charge in materials dated using luminescence. that accumulates in materials to estimate their age and can only be conducted in extremely precise conditions.
How It Works
Luminescence dating includes both optically stimulated luminescence (OSL)A luminescence dating technique where light is used to release trapped electrons from quartz or feldspar, allowing calculation of the last time the sample was exposed to sunlight. and thermoluminescenceA form of luminescence dating where heat is used to release trapped electrons, often applied to ceramics or burned stone., which work similarly but are used on different materials using slightly different techniques. For this lesson, we will discuss OSL in particular.
We live surrounded by radiation. It is a natural component of our existence and we have adapted to it - if not managing it, then at least not being too impacted by the constant bombardment we receive. Archaeological sediment and materials also constantly experience radiation under the ground. This radiation occurs at a fairly constant rate - the radioactivity of the earth and rock around them is extremely unlikely to increase or decrease remarkably without human intervention or a serious world changing event.
The constant radiation dose that materials undergo impacts them in a number of ways. For OSL, the important impact is trapped chargeElectrons stored in defects within a mineral’s crystal lattice due to ionising radiation; the basis for luminescence dating.. As radioactive materials in an environment decay they produce different kinds of ionising radiation. Some of this ionising radiation gradually becomes trapped in crystal structures of materials - most notably in quartz and feldspar. These trapped electrons build at a constant rate, or functionally constant for our purposes thanks to the enormous half-lives of these radioactive isotopes. When the crystal matrix is excited by light, the trapped electrons are released in a measurable luminescence burst. We can measure this release and calculate the amount of electrons trapped in the matrix.
By dividing the dose released by the environmental dose rate, we get the age at which the sample began to trap the electrons. The dose rate is influenced by internal, external, and cosmic radiation. These come from within the crystal, around the sediment, and penetrate the earth's surface from space respectively. Each of these will vary from site to site and region to region, The dose rate can be determined directly from the environment the sample came from, by using dosimeters or radiation counting devices at the site itself, or by using mass spectrometry to calculate the amount of each radioactive nuclide associated with the crystal. This allows calculation of the ionising radiation they emit and can reveal the dose rate to extreme accuracy. The physical depth of the sample can also reveal the input of cosmic radiation, which is naturally shielded against by geological and archaeological features.