Scintillation: mechanisms and new crystals

https://doi.org/10.1016/j.nima.2004.03.009Get rights and content

Abstract

The physical mechanisms active in inorganic scintillators used for medical imaging are reviewed briefly. These include relaxation of electronic excitation following initial absorption of high-energy radiation, thermalization of electrons and holes, formation of excitons, charge carrier trapping on defects and self-trapping, transfer of excitation to luminescence centers, and emission of detectable light. Materials include intrinsic and activated insulating crystals and semiconductors involving several different luminescent centers and radiative processes. Fundamental limitations of scintillator performance and nonradiative processes arising from native defects and impurities that can limit scintillation light output are discussed. The properties of several recently reported scintillating crystals are also presented.

Introduction

Scintillation is an example of radioluminescence wherein the absorption of high-energy radiation or particles leads to observable light. Scintillators may be organic and inorganic solids, liquids, and gases; here we limit the discussion to inorganic solid-state detector materials for medical imaging using X-rays and gamma rays. Intrinsic and activated insulating crystals and semiconductors are included.

The past two decades has witnessed an intense research and development effort devoted to discovering improved scintillator materials, prompted in large part by the need for scintillators for precision calorimetry in high-energy physics and for high-light-output scintillators for medical imaging, geophysical exploration, and numerous other scientific and industrial applications. Cerium-activated crystals, in particular, have received considerable attention because of the fast 5d–4f allowed transition of trivalent cerium and the good light output and energy resolution achieved in several dense materials.

The physical processes leading to scintillation in inorganic solids are complex. They include relaxation of the initial electronic excitation, thermalization and trapping of electrons and holes, and eventual excitation of the luminescent center. The radiative processes involved in light emission are numerous. All of these processes are now generally well understood [1], [2], [3], although details for specific materials may still be lacking. The following section presents a brief review of the scintillation mechanisms in inorganic crystals.

Section snippets

Scintillation mechanism

The various stages of the scintillation process may be summarized as beginning with the absorption of a high-energy photon, thereby creating an inner shell hole and an energetic primary electron, followed by radiative decay (secondary X-rays), nonradiative decay (Auger processes and secondary electrons), and inelastic electron–electron scattering in the time domain of ∼10−1510−13s.

When the electron energies become less than the ionization threshold, hot electrons and holes thermalize by

Luminescent species

There are many different luminescent species and radiative processes active in inorganic scintillator materials [6], [7]. Representative examples of various types of scintillator materials and the luminescent transitions involved are shown in Table 1.

Nonradiative processes

Most materials have native defects (vacancies, interstitials, antisites, dislocations, etc.) and contain impurity atoms; both imperfections are difficult to eliminate completely. Defects may promptly trap electrons and holes before they can recombine to form radiative entities or transfer their excitation to activator ions (see, for e.g. [12]), thus reducing the light yield. The overall scintillation efficiency is also affected by the presence of nonradiative recombination centers wherein the

New crystals

The scintillation characteristics of several recently reported inorganic crystals of possible interest for medical imaging are reviewed below (see, also [13]). All results are room temperature values unless noted otherwise.

Concluding remarks

Today we have numerous well-characterized inorganic scintillator materials involving a variety of scintillation mechanisms [28]. First-principles calculations of scintillation conversion efficiencies are possible [29] and cerium- and thallium-activated materials have been found with near-unity transfer and quantum efficiencies and with light yields and energy resolutions approaching theoretical limits [30], [31]. Given these results, one may ask whether there are better scintillators still to

Acknowledgements

It is a pleasure to acknowledge many stimulating and informative discussions with Stephen Derenzo and William Moses. This work was supported by the Director, Office of Science, Office of Biological and Environmental Research, Medical Science Division of the US Department of Energy under Contract No. DE-AC03-76SF00098 and by Public Health Service grant number R01 EB00339 awarded by the National Institute of Biomedical Imaging and Bioengineering, Department of Health and Human Services.

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