Crystals, Films and Nanocomposite Scintillators (Volume II)

Dear Colleagues,

Scintillator materials are known as the spectral and energy transformers of high-energy photons from X- or ɣ-ray ranges into a ultraviolet-visible (UV/VIS) light. The accelerated particles (electrons, protons, neutrons, or heavy ions) can also be detected through their energy deposits in scintillator materials, which convert their energy into light. Therefore, the scintillation mechanism can be divided into three consecutive sub-processes: conversion, transport, and luminescence.

The history of bulk single-crystal scintillators started at the end of the 1940s with the development of NaI:Tl and CsI:Tl. NaI:Tl and CsI:Tl crystals, as well as CdWO₄ and Bi₄Ge₃O₁₂ oxide crystals, are still the most widely-used scintillators. Over the last 20 years, considerable effort has been directed to the creation of new scintillation materials for high-energy physics and advanced imaging systems for application in industry, science, biology, and medicine. The majority of new single-crystal scintillators developed during this period were based on Ce³⁺- and Pr³⁺-doped materials due to their fast scintillation response (up to 100 ns) and high light yield, connected with the 5d–4f radiative transitions of these ions.

Despite the general consensus that the best performance is provided by single-crystal scintillators, not all efficient materials can be grown in the form of bulk crystals with sufficiently large dimensions and prices that are viable for practical applications. The high melting temperatures and the presence of phase transitions between the melting point and room temperature or stoichiometry problems resulting in the formation of different types of point and macro-defects are examples of difficulties that can prevent single crystal preparation. For these reasons, optical ceramics have been used as an alternative to single crystals to provide bulk optical elements in cases where crystals cannot be grown, or when transparent or translucent ceramic materials show superior properties in comparison with crystals. The technology of optical ceramics has progressed within the last two decades due to the application of these materials as solid state lasers. However, the applications demand higher quality ceramic as the scintillator with respect to laser ceramics because the point defects and structural irregularities can seriously limit a material’s performance due to the introduction of trapping levels in the material band gap.

The development of optical ceramics for scintillator applications is connected to demands for medical imaging. First, fast optical ceramics, based on Ce- and Pr-doped YAG and LuAG, have been reported. In the last decade, R&D in the field of fast-scintillation ceramics has become a hot topic in the search for new scintillation materials. In addition to classical ceramic technologies, new ones, such as spark plasma or combustion sintering, have been developed.

New X-ray-based imaging applications with submicrometer spatial resolution have required the development of thin-film scintillators with micrometer-scale thicknesses. Liquid phase epitaxy technology is often used for the growth of high-quality single crystalline films onto substrates prepared from well-known optical materials (YAG, YAP, YSO, or sapphire). The limitation of performance of film scintillators in this technology is connected to film–substrate misfits and the influence of flux-related impurities on the scintillation properties.

Modern medical therapies, such as photodynamic therapy, strongly demand the development of nanopowder scintillators. Functionalized nanopowder can be directed by blood flow, e.g., to tumor tissues, and, under excitation by X-rays, a single oxygen is produced from the functionalized surface of the grains, which kills the tumor cells. Lanthanide-doped inorganic nanopowders are also considered for future biomedical applications as luminescent nanoprobes.

Nowadays, nanocomposite materials have also become a hot topic in the field of scintillators, with the aim of preparing bulk transparent materials where scintillation characteristics are defined by a nano-phase dispersed in a suitable host. In principle, these novel materials can include, e.g., organic-inorganic mixtures and offer much higher flexibility in material composition.

In this Special Issue, we aim to introduce and describe in more detail the current status in terms of research and development in bulk, ceramic, film, and nanocomposite scintillators, prepared using different technological methods. Both technological descriptions and the various characterization aspects of scintillation materials, together with application aspects in the abovementioned fields, will be provided.

Prof. Dr. Yuriy Zorenko
Guest Editors

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• scintillators
• crystals, films, ceramics, nanopowders
• melt growth, liquid phase epitaxy, and solid-state reactions
• luminescence
• energy transfer processes
• defects, dopants

• Crystals, Films and Nanocomposite Scintillators Volume III in Crystals (2 articles)
• Crystals, Films and Nanocomposite Scintillators in Crystals (5 articles)