The Thermoluminescence Mechanism
The inorganic scintillation materials, when exposed to ionizing radiation, emit light in the form of prompt fluorescence. The scintillation photons are given off when the electron-hole pairs that were formed by the incident radiation recombine at an activator site. These materials are purposely kept free of other impurities and defects in order to maximize the yield of prompt scintillation light. A different class of inorganic crystals, known as thermoluminescent dosimeters (TLDs), are based on a somewhat opposite approach. Instead of promoting the quick recombination of electron-hole pairs, materials are used which exhibit high concentrations of trapping centers within the band gap. As illustrated in Fig, the desired process is now one in which electrons are elevated from the valence to the conduction band by the incident radiation but are then captured at various trapping centers. If the distance of the trap energy level below the conduction band is sufficiently large, there is only a small probability per unit time at ordinary room temperatures that the electron will escape the trap by being thermally excited back to the conduction band. Therefore, exposure of the material to a continuous source of radiation, although not resulting in a significant yield of prompt scintillation light, leads to the progressive buildup of trapped electrons. Holes can also be trapped in an analogous process. An original hole created by the incident radiation may migrate through the crystal until reaching a hole trap with energy somewhat above the top of the valence band. If this energy difference is large enough, the hole will not migrate further and is then locked in place unless additional thermal energy is given to the crystal. A sample of TLD material will therefore function as an integrating detector in which the number of trapped electrons and holes is a measure of the number of electron-hole pairs formed by the radiation exposure. After the exposure period, the trapped carriers can be measured through a process also illustrated in Fig. 19.16. The TLD sample is placed in a stream of heated gas or on a heated support, and its temperature is progressively raised. At a temperature that is deter- mined by the energy level of the trap, the trapped electrons can pick up enough thermal energy so that they are re-excited back to the conduction band. Assuming that this temperature is lower than that required to free the trapped holes, the liberated electrons then migrate to near a trapped hole, where they can recombine with the emission of a photon Alternatively, if the holes are released at a lower temperature, they may migrate to a trapped electron and their recombination also results in a radiated photon. In either case, if the magnitude of the energy difference is about 3 or 4 eV, the emitted photons are in the visible region and are the basis of the TLD signal. Ideally, one such photon is emitted per trapped carrier. Therefore, the total number of emitted photons can be used as an indication of the original number of electron-hole pairs created by the radiation. TLD systems thus derive a signal by using a heater in which the sample can be viewed by a photomultiplier tube. The light yield is recorded as a function of sample temperature in a glow curve of the type illustrated in Fig. 19.17. The basic signal related to the radiation exposure is the total number of emitted photons, or the area under the glow curve. If the sample is raised to a relatively high temperature, all the traps are depleted and the expo- sure record of the sample is “erased.” TLD materials therefore have the very practical advantage of recycl ability, and a single sample may be reused many times.