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

– R. Chen and V. Pagonis 2014

I consider then, that generally speaking, to render a reason of an effect or Phaenomenon, is to deduce it from something else in Nature more known than it self, and that consequently there may be divers kinds of Degrees of Explication of the same thing .

– R. Boyle 1669

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).