Luminescence phenomena and techniques

Stimulated luminescence is observed during optical or thermal stimulation of materials previously exposed to ionizing radiation. These materials are mostly solid insulators which can keep track of the ionizing radiation interaction through processes of charge trapping in defects of the crystal lattice (Figure 1).

Figure 1: The phenomenon of stimulated luminescence in a quartz crystal: in the crystal structure there are defects (oxygen vacancies, aluminum impurities) where the charges released by the ionizing radiation are trapped. The optical or thermal stimulation provides the energy needed to free the charges and restore the initial conditions. The recombination processes of trapped charges can produce light emission.

The radio-induced transformations in the structure of the material are quite stable and, generally, reversible. The solid can return to its initial state provided it receives the energy needed to free the trapped charges. The activation energy of the traps can be provided by optical stimulation or by heating; the recombination processes of the charges are accompanied by light emission (stimulated luminescence). If the activation energy is provided by the optical radiation we observe Optically Stimulated Luminescence (OSL) o Photo-Stimulated Luminescence (PSL), whereas when the solid is heated we have Thermically-Stimulated Luminescence (TL).

During the measurement the sample is heated up to the desired temperature, generally lower than 500°C, depending on the material as well as on the purpose of the measurement. The intensity of the light emitted is recorded as a function of time or temperature which normally increases linearly with time during the measurement. In figure 1 the thermoluminescence curve or glow curve of some materials are shown; the shape of the curves, the number and the position of the peaks depend on the type and number of defects in the solid structure as well as on their activation energy.

Figure 1: Examples of glow curves of different materials: a) cooking sea salt, b) lithium fluoride dosimeter (TLD100).

The integral of the curve depends on the energy released by ionizing radiation in the material and is used to evaluate/reconstruct the absorbed dose (energy released per unit of mass). The technique can be used in different fields: dosimetry of ionizing radiation, control of irradiated foods (method CEN EN 1788) and study of defects induced in solids by ionizing radiation.

This technique measures the intensity of the light emitted by the sample under stimulation with pulsed infrared radiation. One of the apparatus used for these measurements is, for example, the instrument made by SUERC (Scotland). The apparatus has been designed for the identification of irradiated foods (method CEN EN 13751) but can be used also with other materials for different scopes (dosimetry of ionizing radiation). The output used for the classification of the sample is the total number of counts recorded in 60 seconds compared with two threshold values, T₁ (lower threshold) and T₂ (upper threshold), which allows to classify the sample as negative (non-irradiated), positive or intermediate (probably or possibly irradiated, respectively with outcome to be confirmed by other analysis procedures). The analysis does not completely destroy the PSL signal which can then be measured again; however, it must take into account that the intensity of the signal decreases if the same sample is measured several times.

This technique measures the intensity of the light emitted by the sample under stimulation with visible radiation (blue or green). During the measurement the luminescence emission is recorded as a function of time. The integral of the decay curve is proportional to the energy released by ionizing radiation in the material, and allows to reconstruct the absorbed dose (energy released per unit of mass). The technique can be used in the dosimetry of ionizing radiations, for analysis on irradiated foods and for studies of defects induced in solids by ionizing radiation.

Figure 2: Example of OSL decay curve.