Optically Stimulated Luminescence Dosimetry

L. Boetter-Jensen, S.W.S. McKeever, A.G. Wintle

• Publisher: Elsevier Science
• Year: 2003
• ISBN:
  9780080538075
• Topic: Physical Sciences
• Subtopic: Nuclear Physics

Optically Stimulated Luminescence (OSL) has become the technique of choice for many areas of radiation dosimetry. The technique is finding widespread application in a variety of radiation dosimetry fields, including personal monitoring, environmental monitoring, retrospective dosimetry (including geological dating and accident dosimetry), space dosimetry, and many more. In this book we have attempted to synthesize the major advances in the field, covering both fundamental understanding and the many applications. The latter serve to demonstrate the success and popularity of OSL as a dosimetry method.

The book is designed for researchers and radiation dosimetry practitioners alike. It delves into the detailed theory of the process from the point of view of stimulated relaxation phenomena, describing the energy storage and release processes phenomenologically and developing detailed mathematical descriptions to enable a quantitative understanding of the observed phenomena. The various stimulation modes (continuous wave, pulsed, or linear modulation) are introduced and compared. The properties of the most important synthetic OSL materials beginning with the dominant carbon-doped Al2O3, and moving through discussions of other, less-well studied but nevertheless important, or potentially important, materials. The OSL properties of the two most important natural OSL dosimetry material types, namely quartz and feldspars are discussed in depth. The applications chapters deal with the use of OSL in personal, environmental, medical and UV dosimetry, geological dating and retrospective dosimetry (accident dosimetry and dating). Finally the developments in instrumentation that have occurred over the past decade or more are described.

The book will find use in those laboratories within academia, national institutes and the private sector where research and applications in radiation dosimetry using luminescence are being conducted. Potential readers include personnel involved in radiation protection practice and research, hospitals, nuclear power stations, radiation clean-up and remediation, food irradiation and materials processing, security monitoring, geological and archaeological dating, luminescence studies of minerals, etc.

Chapter 1

Introduction

1.1 Optically stimulated luminescence

Optically stimulated luminescence (OSL) is the luminescence emitted from an irradiated insulator or semiconductor during exposure to light. The OSL intensity is a function of the dose of radiation absorbed by the sample and thus can be used as the basis of a radiation dosimetry method. The process begins with irradiation causing ionisation of valence electrons and the creation of electron/hole pairs. Pre-existing defects within the material then localise the free electrons and holes through non-radiative trapping transitions. Subsequent illumination of the irradiated sample with light leads to absorption of energy by the trapped electrons and transitions from the localised trap into the delocalised conduction band. Recombination of the freed electrons with the localised holes results in radiative emission and luminescence. This is the OSL signal, the intensity of which is proportional to the dose of absorbed radiation. OSL signals are often accompanied by photoconductivity phenomena.

OSL is not to be confused with the related phenomenon of photoluminescence (PL) that can be stimulated from similar materials, but which is generally not dependent upon irradiation of the sample. PL is the excitation, via the absorption of light, of an electron in a crystal defect within the material, resulting in excitation of the electron from the defect’s ground state to an excited state. Relaxation back to the ground state results in the emission of luminescence, the intensity of which is proportional to the concentration of excited defects. Ionisation of the electron from the defect (i.e., a transition from a localised to a de-localised state) does not generally occur, however, and there is no associated photoconductivity. As a consequence of the above mechanism, the wavelength of the emitted luminescence is longer than that of the excitation light (Stokes’ shift). Exceptions to that latter rule (the so-called “anti-Stokes” phosphors) may be found in which energy transfer mechanisms dominate. If the defect being excited is itself created by irradiation of the sample, a PL signal that is dependent on absorbed dose may be obtained. This is termed radiophotoluminescence (RPL) and the RPL signal may be utilised in dosimetry, but the mechanism is PL, not OSL.

OSL is one of a class of measurements known as stimulated phenomena. Such phenomena may be stimulated thermally (thermally stimulated phenomena or TSP) or optically (optically stimulated phenomena or OSP). TSP include thermoluminescence (TL), thermally stimulated conductivity (TSC), thermally stimulated exo-electron emission (TSEE), thermally stimulated capacitance (TSCap), deep level transient spectroscopy (DLTS), thermogravimetry (TG), differential thermal analysis (DTA) and others. Likewise, OSP include OSL, photoconductivity (PC) and optically stimulated exo-electron emission (OSEE). The relationship between these different phenomena is illustrated in Fig. 1.1 using a schematic energy band diagram. The reader is referred to works by Bräunlich (1979), Chen and Kirsh (1981) and Chen and McKeever (1997) for general texts on TSPs and related phenomena. See McKeever (1998, 2001) for reviews of OSL and its use in dosimetry.

Fig. 1.1 Schematic representation of several popular thermally and optically stimulated phenomena. Capacitance techniques (DLTS and TSCap) measure signals proportional to the concentration of charges when they reside in the traps. Conductivity techniques (TSC and PC) monitor the charges after release from the traps as they transit through the conduction band. Luminescence techniques (TL and OSL) monitor the charges as they undergo radiative recombination with charge of the opposite sign. Exo-electron processes (TSEE and OSEE) monitor the charges if they are emitted from the surface of the material. Although not the same type of stimulated phenomenon, PL is also indicated.

1.2 Historical development of OSL dosimetry

In recent years, OSL has become a popular procedure for the determination of environmental radiation doses absorbed by archaeological and geological materials in an attempt to date those materials. In this procedure, the target samples (usually natural grains of quartz and/or feldspar) are exposed in the laboratory to a steady source of light of appropriate wavelength and intensity, and the luminescence stimulated from the mineral during this procedure is monitored as a function of the stimulation time. The integral of the luminescence emitted during the stimulation period is a measure of the dose of radiation absorbed by the mineral since it was last exposed to light. Through calibration of the signals against known doses of radiation, the absorbed dose can be obtained and through a separate determination of the environmental dose rate, the age of the sample can be determined. Huntley et al. (1985) first used the method, now known as “continuous-wave-OSL” (CW-OSL), for this purpose and the latest developments in this field have been described in the triennial conferences on luminescence and ESR dating (Faïn et al., 1991; Bailiff et al., 1994; McKeever, 1997, 2000).

The first OSL measurements on quartz and feldspar were made using an argon ion laser (Huntley et al., 1985). However, the development of cheaper stimulation systems based, first on filtered lamps, and then on light emitting diodes (LEDs), have led to a massive expansion in dating applications. Feldspars, particularly sand-sized potassium-rich feldspars that could be isolated using heavy liquids, were the first to be investigated. Hütt et al. (1988) showed that luminescence signals could be stimulated from feldspars using near infra-red wavelengths around 880 nm, where a resonance in the stimulation spectrum had been observed. This led to the measurement of infra-red stimulated luminescence (IRSL) using clusters of inexpensive diodes (Spooner et al., 1990). Green light from filtered halogen lamps was used for quartz (Bøtter-Jensen and Duller, 1992) until sufficiently powerful blue (470 nm) LEDs became available (Bøtter-Jensen et al., 1999b).

Since diodes can be used to give short stimulation pulses, and have far longer working lives than the lamps, it was possible to construct laboratory procedures to determine the equivalent dose (D_e) for single aliquots of sample. Duller (1991) developed an additive dose method for feldspars and this has been widely adopted. A similar procedure was developed for quartz using the filtered lamp system (Murray et al., 1997). More recently, following a five-year study of the OSL properties of quartz, Murray and Wintle (2000) developed the single aliquot regenerative dose (SAR) protocol that has been used in both dating and accident dosimetry. In this method, the sensitivity of all OSL measurements used to obtain D_e is monitored by the OSL response to a test dose. For sedimentary quartz, the method has been shown to be reliable by the accurate dating of 50 samples, for which there is independent age information (Murray and Olley, 2002). The SAR protocol has now been used for single quartz grains (Duller et al., 2000) when stimulated using a focussed solid-state laser as the stimulation source (Duller et al., 1999). This has opened up a whole new level of investigation for sedimentary deposits (Duller and Murray, 2000).

The use of OSL as a personal dosimetry technique, however, is not yet so widespread, despite the fact that its use in this field has a much longer genesis. It was first suggested for this application several decades ago by Antonov-Romanovskii et al. (1956) and was later used by Bräunlich et al. (1967) and Sanborn and Beard (1967). Since these early developments, however, the use of OSL in radiation dosimetry has not been extensively reported, perhaps due to the lack of a good luminescent material, which was both highly sensitive to radiation, and had a high optical stimulation efficiency, a low effective atomic number and good fading characteristics (i.e., a stable luminescence signal at room temperature). MgS, CaS, SrS and SrSe doped with different rare earth elements such as Ce, Sm and Eu were among the first phosphors suggested for OSL dosimetry applications (Bräunlich et al., 1967; Sanborn and Beard, 1967; Rao et al., 1984). They possess a high sensitivity to radiation and a high efficiency under infra-red stimulation at a wavelength around 1 μm, but they suffer from significant fading of the luminescence at room temperature. These phosphors also have a very high effective atomic number and, as a result, exhibit strong photon energy dependenc...