A Course in Luminescence Measurements and Analyses for Radiation Dosimetry. Stephen W. S. McKeever

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 Personal Dosimetry

      Following the work by Farrington Daniels and colleagues, the application of TL to the world of dosimetry developed at a rapid pace. TL dosimetry (TLD), using a variety of TLD materials, became the foremost method to be used in the measurement of dose to people. The use of RPL in dosimetry developed slowly in parallel with the dominant application of TLD, while OSL did not become a popular dosimetry tool until the 1990s. Nevertheless, OSL dosimetry (OSLD) has since grown to become perhaps the dominant personal dosimetry method throughout the world, even though the availability of OSLD materials is rather limited. Today, in the world of luminescence personal dosimetry, TLD, OSLD, and radiophotoluminescence dosimetry (RPLD) each have their niches and one or other is the preferred dosimetry method in many institutions around the world.

TLD personal dosimeter badges have been designed by a wide range of companies and institutions, and a myriad of badge designs are available. Figure 1.5 displays several of them. (OSLD and RPLD badges are also included in this figure.) Each TLD badge contains a suitable TL material (the most popular of which is the first TLD material to be developed, namely LiF:Mg,Ti), along with an array of radiation filters (different metals or plastics of different thicknesses) to enable the analysis of the TLD badges to reveal not only dose, but also to provide some information about the radiation type and quality (i.e., energy). This is important to allow the dosimetry service provider to present information concerning how penetrative was the radiation to which the badge wearer was exposed.

      Figure 1.5 Examples of personal dosimeters, including TLD, OSLD and RPLD badges. (Also included are some examples of electronic dosimeters.) Source: Dr. Hannes Stadtmann, © Hannes Stadtmann, European Radiation Dosimetry Group (EURADOS).

TLD badges are used everywhere that requires the monitoring of radiation workers at radiation facilities (nuclear power stations, hospitals, industrial complexes, and many others), including guest visitors to those facilities. Although TLD badges are slowly being phased out and replaced by other technologies (such as electronic dosimeters, some of which are also shown in Figure 1.5), they remain a powerful presence in the world of personal radiation dosimetry.

      Also shown in Figure 1.5 are some of the OSLD and RPLD badges that are commercially available. While OSLD has become an extremely popular luminescence method for personal dosimetry, RPLD occupies a smaller portion of the luminescence dosimetry market share.

      Commercial TLD materials include those based on LiF (e.g. LiF:Mg,Ti and LiF:Mg,Cu,P), CaF₂ (e.g. CaF₂:Mn, CaF₂:Dy, and CaF₂:Tm), Li₂B₄O₇ (e.g. Li₂B₄O₇:Cu),