Читать книгу A Course in Luminescence Measurements and Analyses for Radiation Dosimetry - Stephen W. S. McKeever - Страница 16

Luminescence, the eerie glow of light emitted by many physical and biological substances, is familiar to us all. The bright speck of a firefly, the luminous glow from seawater in the evening, the glow of a watch dial in the dark – all are examples of luminescence phenomena that are familiar to most of us. Familiarity and understanding are not synonymous, however. Indeed, an understanding of the various luminescence phenomena has a very long genesis and over the centuries there have been several “divers kinds of Degrees of Explication”. Luminescence has had, and continues to have, practical uses in both every-day and in more esoteric applications. Computer screens, electronic indicators, lighting, lasers, and many, many other examples are indications that the field of luminescence is very broad and potentially very useful.

One such field of use is in the detection and measurement of radiation – a field generally known as “dosimetry,” or the act of measuring the dose of radiation absorbed by an object. The amount of radiation absorbed by an object and the subsequent amount of luminescence emitted from it is the basis of the use of luminescence in dosimetry. The connection between radiation and luminescence was made many years ago and, in fact, those of us active in the field of luminescence dosimetry can take pride in the fact that the study of luminescence can be traced to the beginning of the modern scientific method. Although it would be surprising if ancient Islamic or, perhaps, Chinese scholars had not already noted the phenomenon, in one of its many guises, it can nevertheless be argued that the first modern description of luminescence stems from the work of Robert Boyle in mid-seventeenth-century England, published in the Philosophical Transactions of the Royal Society. Boyle – considered to be the “father” of chemistry, as well as being a physicist, an inventor, a philosopher, and a theologian – gives an evocative description of (what we now term) luminescence emitted from a remarkable piece of diamond, loaned to him by a friend, John Clayton (Boyle 1664). The word “luminescence” was not used by Boyle who referred to it as the “glow” from the stone. In a later publication concerning luminescence from a liquid he uses the wonderfully suggestive term “self-shining” to describe the phenomenon (Boyle 1680).

Boyle’s 1664 account of luminescence from diamond is generally accepted as the first scientific description of the phenomenon of thermoluminescence (TL). Boyle described various ways of heating the diamond to induce from it the emission of light. It is not clear, however, how Boyle energized the diamond in the first place. We now know that the TL phenomenon requires that the material must first absorb energy from an external energy source. The energy thus stored is then released by the application of a second source of energy (heat). As the initial energy is released, some of it is emitted in the form of visible light (thermoluminescence). Without that first energy storage step, no TL can be induced. Boyle may or may not have known that the process he was observing was, in fact, a two-step procedure, but he was vague on how he energized the diamond in his possession; readers are left to speculate how this may have been achieved. Possibilities include natural radioactivity or light, but perhaps the most likely source was physical stimulation (rubbing, scratching, etc.) producing what we now call tribo-thermoluminescence (“tribo-” from the Greek “trī̀bein,” meaning “to rub”). In any case, once heated to release the TL, the material would have to be energized again and energy stored a second time before the TL phenomenon could be seen again during heating.

We may never know in sufficient detail how Boyle treated his diamond to be able to answer this question with certainty – and perhaps we should be satisfied with leaving it as an intriguing mystery. For our purposes here, we can be satisfied that the phenomenon that we discuss in this book was first reported in such vivid and expressive terms as long ago as the mid-seventeenth century, and by such a luminary as Robert Boyle.

McKeever (1985) traces several pre-twentieth century published descriptions of luminescence stimulated by heating and indicates that the term “thermoluminescence” can probably be attributed to Eilhardt Wiedemann (Wiedemann 1889) in his work on the luminescence properties of a wide variety of materials. Following Wiedemann’s work, Wiedemann and Schmidt (1895) studied TL from an extensive series of materials following irradiation with electron beams, while Trowbridge and Burbank (1898), likewise, studied TL of fluorite following excitation by several different energy sources, including x-irradiation. These two early papers are examples where we can see the beginnings of the use of TL in radiation detection since, in each case, a source of radiation was used to provide the initial absorption of energy necessary for ultimate TL production. It is not surprising, therefore, to see the study of TL proceeding alongside the examination of radiation itself, with seminal works by Marie Curie and Ernest Rutherford, among others, including descriptions of thermoluminescence from minerals (Curie 1904; Rutherford 1913). Examinations of the color of the emitted light were also beginning around this time through studies of the spectra of the TL from various minerals (Morse 1905).

A point that should not go without mention is that Wiedemann (1889) and Wiedemann and Schmidt (1895) discussed the mechanism of luminescence in terms an “electric dissociation theory” wherein luminescence phenomena were explained on the basis of the separation and subsequent recombination of positive and negative charged species (specifically, positive and negative molecular ions). Others followed and adopted this initial and innovative suggestion to explain luminescence phenomena in a variety of materials (Nichols and Merritt 1912; Rutherford 1913). While the authors of the period attempted to apply this theory to all forms of luminescence, and while we now know that photoluminescence (i.e., fluorescence), for example, does not involve ionization and charge dissociation, the notion of charge dissociation and recombination nevertheless foreshadows our current understanding of the phenomena of TL, OSL, and phosphorescence. Bearing in mind that these early ideas initially suggested in the 1880s predate the birth of quantum mechanics, band theory, and the concepts of electron and hole generation, it is remarkable that the insight offered by these early pioneers aligns so well with our current understanding of the latter phenomena, which is given in terms of the creation of negative electrons and positive holes, followed by their ultimate recombination.

As described in McKeever (1985), the use of TL in the study of radiation accelerated in the beginning decades of the twentieth century. A key area of research was to examine the relationship between the point defects within the materials studied (e.g. color centers) and their role in localizing (trapping) the electrons and holes ionized from their host atoms during the absorption of radiation. A feature of TL is that the luminescence at first increases and then decreases, forming a series of characteristic TL peaks as the temperature increases. It was realized that the cause of the TL peaks was the thermal release of trapped charge from lattice defects – with the larger the trapping energy, the higher the temperature of the TL peak. In 1930, in Vienna, Austria, Urbach discussed the connection between the energy needed to release the trapped charge and the TL peak position in a series of papers on luminescence from the alkali halides (Urbach 1930). However, it was not until the work of the group at the University of Birmingham in the United Kingdom that the relationship was quantified through the development of mathematical descriptions of the process (Randall and Wilkins 1945a, 1945b; Garlick and Gibson 1948).

Not long afterwards, Farrington Daniels and the research group at the University of Wisconsin, United States of America, discussed several applications in which TL could be a useful research tool. Among them was radiation dosimetry. Daniels and colleagues wrote: “Since in many crystals the intensity of thermoluminescence is nearly proportional to the amount of γ-radiation received, a considerable effort has been devoted to developing a practical means of measuring the exposure to gamma radiation.” (Daniels et al. 1953) – and so was born the field of thermoluminescence dosimetry. These authors specifically highlighted lithium fluoride as being the best crystal for this purpose and their work also initiated the parallel search for other TL dosimetry materials.

The growth of optically stimulated luminescence (OSL) as a method of radiation dosimetry had a similar genesis to that of TL and emerged as a potential dosimetry tool at about the same time. As described by Yukihara and McKeever (2011), the birth of OSL stems from the early work of the Becquerels, father and son, Edmond and Henri (E. Becquerel 1843; H. Becquerel 1883). These and similar studies through the late nineteenth and early twentieth centuries observed that phosphorescence could either be enhanced or quenched by the application of light to an irradiated material, the precise effect being dependent on the wavelength of the stimulating light. Observations of photoconductivity on some of these materials lead to the realization that free electrons were being produced during photostimulation and quenching of the phosphorescence (as discussed by Harvey 1957). Leverenz (1949) noted that when luminescence is enhanced by stimulating with an external light source, the eventual decay of the luminescence is unrelated to the characteristic fluorescence lifetime of the emitting species. As Leverenz (1949) states, for the luminescence to be produced, an “additional activation energy must be supplied to release the trapped electrons … This activation energy may be supplied by heat … or it may be supplied by additional photons.” When the energy is supplied by heat, TL results; when it is supplied by photons, OSL results.

It seems that the name, optically stimulated luminescence (OSL), appeared in the literature only in 1963 with the work of Fowler (Fowler 1963). Earlier names for the phenomenon included photophosphorescence, radiophotostimulation, photostimulation phosphorescence, co-stimulation phosphorescence, and photostimulated emission (see Yukihara and McKeever 2011). Even today, one often sees the phrase photostimulated luminescence (PSL) instead of OSL, the two names being synonymous, referring to the same phenomenon. For clarity, the more popular and most frequently used name of OSL will be used throughout this book.

As with TL, the connection between these optically stimulated effects and the initial absorption of energy from radiation was also established in the mid-twentieth century. The first suggested use of OSL in radiation dosimetry appeared with the work of Antonov-Romanovsky and colleagues (Antonov-Romanovsky et al. 1955). These authors examined infra-red (IR) stimulated luminescence from irradiated sulfides and related the IR-induced luminescence to the initial dose of radiation absorbed. Other similar works followed, but OSL did not emerge as a popular radiation dosimetry tool at this time, primarily because the emphasis of these studies was on sulfide materials and infra-red stimulation. These materials contained defects from which the energy required to release the trapped electrons was quite small (and, hence, the electrons could be released through absorption of low-energy, infra-red light). As a result, room temperature thermal stimulation could also release the trapped charges, which were thus observed not to be stable. As a result, the OSL signal is said to have faded with time since irradiation.

With the advent of studies into wider-band-gap materials (e.g. oxides, alkali halides, and sulfates) it was found that OSL could be stimulated by shorter-wavelength, visible light from defects that required larger activation energies to release the trapped charge. Hence, the OSL signal was more stable and did not fade. Nevertheless, the breakthrough in OSL’s application in radiation dosimetry came not in the radiation dosimetry field itself, but in the related field of geological dating. Huntley and colleagues (Huntley et al. 1985) demonstrated that OSL from quartz deposits in geological sedimentary layers could be used to determine the dose of natural radiation absorbed by the quartz grains since they were deposited in the layer. Analysis of the natural environmental dose rate then leads to a calculation of the age of the sediment (age = dose/dose rate). This paper, more than any other, opened the flood gates for the development of OSL in dosimetry. “Optical dating” is now 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.