Thermoluminescence

Luminescence is the emission of light from some solids called phosphors.This emission, which does not include black-body radiation, is the release of energy stored within the solid through some type of prior excitation of the solid electronic system, i.e. by ionising radiation. The ability to store the radiation energy is important in luminescence dosimetry and is generally associated with the presence of activators (i.e. impurity atoms and structural defects). Table I lists some types of luminescence phenomena and the methods of excitation. The three last phenomena are linked together for the time scale over which the light emission takes place. Fluorescence is a luminescent process which persist only as long as the excitation is continued. the decay time of fluorescence is independent of temperature: it is determined by the probability of the transition from an excited level Ee to the ground state Eo. This process is shown in fig.1. Phosphoresce is the luminescence observable after removal of the exciting source. The decay time of phosphoresce is dependent on the temperature. referring to fig. 2, one can observe that this situation arises when an electron is excited (e.g. by ionising radiation) from a ground state Eo to a metastable state Em (electron trap), from which it does not return to the ground level with emission of a photon (e.g. the transition from Em to Eo), because it is completely or partially forbidden by the selection rules. If one supposes that a higher excited level, Ee, exists to which the system can be raised by absorption of the energy Ee-Em and that the radiative transition Ee-Em is allowed, then one can provide the energy Ee-Em by thermal means at room temperature. After that a continuing luminescence emission (phosphorescence) can be observed even after the excitation source is removed. This emission will continue with diminishing intensity until there are no longer any charges in the metastable state. If the system is raised to a higher temperature, the transition from Em to Ee will occur at an increased rate; consequently the phosphorescence will be brighter and the decay time will be shorter due to the faster depopulation of the metastable state. The phosphorescence is then called thermoluminescence. The luminescence effects can be used in solid-state dosimetry for measurement of ionising radiation dose; the main luminescence dosimetric techniques are a) radio-thermoluminescence (RTL): it consists in a transient emission of light from an irradiated solid when heated; b) radio-lyoluminescence (RLL): it consists in a transient emission of light from an irradiated solid upon dissolving it in water or some other solvent; c) radio-photoluminescence (RPL): it consists in an emission of light from an irradiated solid by excitation with UV light. Techniques a) and b) are linked by three facts: i) they all show transient effects, superimposed on an intrinsic background, ii) they offer a memory but not a permanent record of irradiation dose, iii) the basic memory mechanism for all techniques is similar for a given inorganic material; only the readout of the radiation-induced signal is different for the two different techniques. This paper is relate to the RTL technique, generally called thermoluminescent dosimetry