Spin Resonance (ESR);
Photoluminescence (PL).
All of the above phenomena, except PL, require the initial absorption of energy from an external radiation field before the signal (TL, OSL, RPL, EPR, DLTS, etc.) can be observed. The radiation energy must be sufficiently high to cause ionization within the material – i.e. the creation of free electrons and holes. Photoluminescence is an intrinsic property of the material; that is, it may be observed before irradiation and is not created by it. Ionization is not required. (Photoluminescence can be an interference signal in both RPL and OSL.)
The differences between the phenomena are illustrated schematically in Figure 1.4. Here is represented the energy band diagram previously discussed for an insulator or semiconductor. Ionization by the radiation creates free electrons and holes that may be subsequently trapped at electron and hole localized states. Stimulation (heat or light) induces recombination of the electrons and holes. (Note: We can describe these phenomena in terms of releasing trapped electrons to recombine with holes, or vice-versa. In this description, for ease of explanation, the discussion is limited to releasing electrons to recombine with holes.) Production of TL or OSL only occurs when a portion of the energy released during the electron-hole recombination is radiative. Thus, the TL and OSL signals include information about both the trap and the recombination center. Both are required for TL and OSL to be observed.
Figure 1.4 Schematic diagram illustrating the differences between several thermally and optically stimulated phenomena. Only TL and OSL contain information about both the trap and the recombination center.
Phosphorescence is in fact an unstable form of TL in which the electrons are trapped in the localized energy levels for short periods of time. If the traps are characterized by a small energy barrier then trapped charge can be thermally stimulated even at room temperature, leading to recombination and therefore luminescence emission. Similarly, radioluminescence (RL) results when electrons excited to the conduction band recombine with holes localized in hole traps, without the step of becoming trapped themselves. Emission of RL occurs during the irradiation and decays quickly after the cessation of the radiation, on a time frame governed by the recombination lifetime of the free electrons in the conduction band. In practice, both RL and phosphorescence can be observed during irradiation and the timescale for the decay of the luminescence after the radiation ends is then governed by several different lifetimes, including the recombination lifetime and the trapping lifetime of the electrons in the electron traps. The radioluminescence signal contains information about the recombination centers, whereas the phosphorescence signal includes information about both the traps and recombination centers.
In contrast to TL and OSL, TSC or PC require only the stimulated release of the trapped electrons to the conduction band. The free electrons now have the opportunity to participate in conductivity if an external electric field is applied. Thus, TSC and PC signals hold information about the traps only, not the recombination centers.
Similarly, DLTS and TSCap also monitor the release of electrons from traps, but in these cases the change in the electrical capacitance of the system is measured. Again, the signals contain no information about the recombination centers. Likewise, for TSEE and OSEE, which detect the thermally or optically stimulated emission of energetic exo-electrons from the material.
Electron paramagnetic resonance detects the trapped electrons (or holes) when the charges are localized at the defects, giving rise to an unpaired electron spin. Release of the charge from the traps is not required.
Photoluminescence occurs when electrons in a defect are raised to an excited state, but are not ionized. If relaxation to the ground state is radiative, luminescence (PL) results. If photoluminescence is observed before irradiation of the material, it is simply called PL. However, if irradiation is needed before PL is observed, the term used is RPL (radiophotoluminescence) – that is, PL from a defect that is created by the irradiation. An example helps to understand the distinction. In alkali halides, halide ion vacancies, or F-centers, are produced during irradiation. F-centers consist of trapped electrons localized by halide ion vacancies. At high enough doses, the concentration of F-centers is such that they cluster together to form F2-, F3-centers, etc. These higher-order clusters of F-centers produce photoluminescence when stimulated at the appropriate wavelength. Without radiation, however, these centers do not exist and so the PL signal from them is correctly termed RPL. In contrast, luminescent materials may be doped with tri-valent rare-earth (RE) ions, RE3+. The 4f-electrons in such ions may be raised to excited states when stimulated with the right wavelength, and relaxation produces luminescence at wavelengths characteristic of the ion. Such signals are intrinsic to the phosphor, that is, they are not an effect of the absorption of radiation energy. Such signals are correctly called PL.
1.3 Brief Overview of Modern Applications in Radiation Dosimetry
There are thousands of published articles in the modern scientific literature demonstrating the application of TL, OSL, and RPL in the field of radiation dosimetry. A review of such applications is not the intent here. Instead, the purpose of this section is to give the reader a flavor of the types of use to which these techniques have been put in the general field of the radiation dosimetry. In this sense, we may broadly consider the following areas where radiation dosimetry using luminescence has been shown to be an essential and/or highly useful tool. The areas include:
Personal dosimetry (detection and measurement of dose absorbed by people);
Medical dosimetry (measurement of doses delivered to patients during medical treatments – diagnosis and therapy – to check and confirm the doses delivered);
Space dosimetry (measurement of doses to astronauts and to space vehicles while in orbit or during interplanetary travel);
Retrospective dosimetry (estimation of doses to people in the aftermath of radiation accidents, whether they are small accidents involving a handful of people, or large-scale events involving hundreds or thousands of people; also luminescence dating of geomorphological structures or archaeological artefacts);
Environmental dosimetry (measurement of doses delivered to the environment – air, soil, built structures, and others).
There are other applications (e.g., detection of fake art objects) but those listed above are the main areas in which luminescence dosimetry has found important, not to say vital, application. One of the remarkable aspects of using luminescence in dosimetry is the fact that the phenomenon can be used to detect radiation levels as low as the naturally occurring environmental background levels on Earth, or as high as the radiation levels used in food irradiation or industrial processing, and everywhere in between. When expressed in units of Gray (Gy, where 1 Gy = 1 Joule of energy absorbed by one kilogram of a substance), doses ranging from μ Gy, for environmental radiation, to MGy, for industrial processes can be measured. This represents an amazing 12-orders of magnitude spread. While humans cannot survive in high radiation environments, homo sapiens evolved within a background of environmental radiation here on Earth. Humans can also survive for lengthy periods in more harsh radiation environments such as Space, where doses as high as several tens of mGy might be absorbed, depending on the mission. Much higher doses may be experienced by patients who may be treated with localized radiation doses of several kGy during medical radiotherapy. It is remarkable that luminescence dosimetry has found application in all of these areas.
In what follows, dosimetry applications are further discussed, albeit briefly, in order to give the reader a taste of some important examples.
1.3.1
