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

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СКАЧАТЬ an established technique (Aitken 1998; Bøtter-Jensen et al. 2003) and the method demonstrated that the OSL signal stimulated from defects with large activation energies could be stable for thousands of years in the right environmental circumstances.

      Use of OSL in conventional radiation dosimetry started with the development of oxygen-deficient Al₂O₃, doped with carbon. This material was first suggested as a sensitive TL dosimeter at the Urals Polytechnical Institute in Russia (Akselrod and Kortov 1990), but the TL signal from this material was found to be very sensitive to visible light, such that exposure to daylight after irradiation reduced the subsequent TL signal. The group at Oklahoma State University in the United States then turned this apparent disadvantage into an advantage and showed that the material was a very sensitive OSL material (McKeever et al. 1996; Akselrod and McKeever 1999). The era of OSL dosimetry and the search for new OSL dosimetry materials had begun.

      The term radiophotoluminescence (RPL) appears in the literature in the early 1920s with the work of Przibram and colleagues in Vienna (Przibram and Kara-Michailowa 1922; Przibram 1923). These researchers showed that photoluminescence (PL) can be induced in some materials only after exposure to ionizing radiation. Without irradiation, no PL is observed and, therefore, these authors gave their observation the name radiophotoluminescence. The distinction between RPL and OSL lies in the stability of the radiation-induced luminescence centers during stimulation with visible or infra-red light. In OSL, the luminescence signal decays under continued light stimulation, whereas in RPL, the signal remains constant. OSL is a destructive readout process (involving ionization) whereas RPL is a non-destructive process involving electron excitation, but not ionization. (These concepts will be discussed in more detail in later sections and chapters.)

The association between the RPL signal induced by the radiation and the initial dose of absorbed radiation was not exploited until the work of Schulman et al. (1951) who used RPL from a variety of materials as a means of determining the dose of radiation initially absorbed. (Interestingly, in addition to RPL, Schulman et al. (1951) also studied the other two major phenomena that comprise the subject of this book – namely, TL (termed radiothermoluminescence by Schulman and co-workers) and OSL (termed radiophotostimulation.) Schulman and colleagues examined the RPL properties of alkali halides and the relationship between RPL and the coloration of these materials after irradiation. These authors also examined Ag-doped phosphate glasses as RPL dosimeters, marking the introduction of what was to become the dominant RPL dosimeter material.

      The development of RPL as a dosimetry tool expanded in Germany with the work of Becker (Becker 1968) and Piesch and colleagues (Piesch et al. 1986, 1990), and in Japan with Yokota and colleagues (Yokota et al. 1961; Yokota and Nakajima 1965). Emphasis was on the development of methods for reading the RPL signal as well as a search for improved materials. Although RPL dosimetry was slow to penetrate the dosimetry market because of the competition offered by TL dosimetry (in particular) and later OSL dosimetry, today RPL dosimetry retains an important place within the luminescence dosimetry community and the commercial marketplace.

      Exercise 1.1

      Landmark developments in the use of TL, OSL, and RPL in dosimetry have been scattered over many decades, beginning with the study of radioactivity. To understand where the field resides at present – and to ensure that one does not “re-invent the wheel” – familiarity with this background is very important. Some of the highlights and seminal papers have been referenced here; many other important publications are available.

      Choose one of the three dosimetry methods – TL, OSL, or RPL – and perform a bibliography search to trace the development of the technique from its earliest beginnings to its present-day use.

      1.2 Introductory Concepts for TL, OSL, and RPL

      1.2.1 Equilibrium and Metastable States

Consider a luminescence dosimetry material in its equilibrium state (Figure 1.1) in which all electrons are in their equilibrium energy levels. Perturbation from the equilibrium state by absorption of energy from an external energy source (radiation) raises the system to a metastable state, in which some electrons now occupy higher, non-equilibrium energy levels. The metastable state is characterized by potential energy barrier(s), which need(s) to be overcome before the system can return to equilibrium. The system may be stable in the metastable state from fractions of a second to thousands of years, depending on the size of the potential energy barrier(s). Absorption by the system of energy from an external stimulus while in the metastable state can overcome the potential barrier(s) and cause the system to relax to its stable equilibrium state. The stored energy may then be released, usually in the form of heat, but a portion of it may be released in the form of visible light and, thus, luminescence may be emitted. The intensity of the luminescence is related to the amount of energy initially absorbed during the irradiation phase. If the stimulus provided is in the form of heat energy, the luminescence emission is TL. If the stimulus involves the absorption of optical energy, the luminescence emission is OSL. (An animated version of Figure 1.1 is given on the web site, under Exercises and Notes, Chapter 1.)

      Figure 1.1 Conceptual notion of TL and OSL in which a system in an equilibrium state is perturbed from that state to a metastable state through the absorption of energy from radiation. Once a stimulus is applied, in the form of heat or light, the system is triggered to return to equilibrium, along with the release of a portion of the absorbed energy in the form of light. If the stimulus is heat, the light emission is TL; if the stimulus is light, the emission is OSL. An animated version of this figure is available on the web site, under Exercises and Notes, Chapter 1.

To examine the nature of the equilibrium and metastable states it is necessary to consider the energy band gap of the material and Fermi-Dirac statistics.

      1.2.2 Fermi-Dirac Statistics

Insulators and semiconductors are characterized by an energy gap between the uppermost filled energy band (the valence band) and the next empty band (the conduction band). At absolute zero, the valence band is completely full and the conduction band is completely empty. For a “perfect” crystal, no energy states are allowed in the energy gap between the top of the valence band (at energy Ev) and the bottom of the conduction band (at energy Ec). That is, if Z(E) is the density of available states at any energy E, then Z(E) = 0 for Ev < E < Ec, and the energy gap (Ec – Ev) is known as the “forbidden” gap or zone. However, real crystals contain defects such that Z(E) ≠ 0 in this forbidden zone. Energy states E can exist for which Ev < E < Ec and Z(E) > 0, and electrons can occupy energy states that are above the valence band but below the conduction band. Since such energy levels arise because of defects (e.g., impurities, vacancies, interstitials, and larger defect complexes), these states are localized at specific lattice sites within the crystal whereas the conduction and valence bands are delocalized. As a result, excitation of valence band electrons to one of these higher energy states, through the absorption of energy from a radiation field, requires not just a transition to a higher, excited energy level, but it also requires transport of the electron from one atomic or molecular site to another within the host crystal. That is, movement through the crystal is needed. This can only occur via a “transfer state” – in other words, via the conduction band (Figure 1.2a). Once the excited electrons have been transported to their new positions in the lattice, they relax into lower energy levels E, where Ev < E < Ec.

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