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Introduction

1.1 History of Scintillator Developments

A scintillator can convert radiation (alpha-ray, beta-ray, gamma-ray, X-ray, and neutron) into light (photon), and radiation detectors using scintillators and photo acceptance units have been used for various applications in medical, security, environmental, high-energy fields, etc. (Figure 1.1). Photo acceptance units such as photomultiplier tubes (PMT), photodiodes, and multi-pixel photon counters (MPPC) can convert light into electrical signals, which enables detection and measurement (count) of radiation using multi-channel analyzers.

In these applications, there are some required properties of scintillators, and the required performance that is considered important varies greatly depending on the type of application. Typical parameters considered important in scintillator crystals include light yield, energy resolution, decay time, density, effective atomic number, emission wavelength, and afterglow. Furthermore, mass productivity, chemical stability, crystal workability, radiation resistance, etc. are also important for commercial use. For example, high light yield, large density and effective atomic number, and short decay time are required in applications under gamma-ray irradiation with the short measurement time.

One of the most important characteristics of scintillators is the light yield, which is directly related to radiation detection sensitivity. For the emitted light in the scintillator under radiation to efficiently enter the photo acceptance unit, the scintillator must be transparent at the emission wavelength. Especially in the case of high-energy radiation such as gamma-ray, the scintillator becomes large enough to stop the radiation inside, so efficient extraction of the emitted light inside greatly affects the performance of the scintillator. Therefore, many scintillators have been utilized as single crystals with high transparency.

Figure 1.1 Schematic diagram of scintillator and photo acceptance unit in radiation detector.

1.2 Introduction of Conventional Scintillators and Crystal Growth Methods

1.2.1 Conventional Scintillator Single Crystals

Figure 1.2 shows the history of the development of scintillator crystals with the progress of crystal growth method. The development of scintillator single crystals has progressed along with that of crystal growth technology. In order to indicate excellent luminescence and scintillation performance (high light yield, great energy resolution, high transparency etc.), scintillator single crystals are required to be of high quality, and, as a result, progress in crystal growth technology is also highly recommended.

Figure 1.2 History of the development of scintillator crystals with progress of growth method.

First, CsI:Tl and NaI:Tl single crystals were developed as the most familiar and inexpensive scintillator single crystals [1, 2]. They are widely used in application devices because these large bulk single crystals can easily be grown by the conventional melt-growth method and they have relatively low hygroscopicity.

After that, various oxide scintillator single crystals have been developed along with the establishment of crystal growth methods using precious metal crucibles such as iridium (Ir) and platinum (Pt) for oxide single crystals with a high melting point near 2000 °C. Among them, typical oxide scintillator single crystals are the garnet-type Y3Al5O12:Ce and Lu3Al5O12:Pr, perovskite-type YAlO3:Ce and LuAlO3:Ce, and silicate-type Lu2SiO5:Ce, (Lu,Y)2SiO5:Ce, and Gd2SiO5:Ce [3-9]. In addition, as a high-density and effective atomic number scintillator, Bi4Ge3O12, CdWO4 and PbWO4 have been developed for detection of high-energy radiation [10-12].

Around the year 2000, global attention was focused on the high scintillation properties (high light yield and great energy resolution) of halide single crystals. However, many of the halide materials have strong hygroscopicity, and it was difficult to grow high-quality single crystals. Therefore, various techniques for growing single crystals of halide materials with strong hygroscopicity have been developed since then, and the development of halide scintillator single crystals has largely progressed. As a result, halides scintillator single crystals were first developed, centering on binary compounds such as LaBr3:Ce, CeBr3, and SrI2:Eu with high light yield and great energy resolution [13-15]. After that, the material research expanded to complex compounds of halide materials such as Cs2HfCl6 and Cs2LiLaBr6:Ce [16, 17]. In recent years, high-performance chloride scintillator single crystals with relatively low hygroscopicity compared to bromide and iodide scintillator single crystals have been refocused, and they have been actively studied [18, 19].

1.2.2 Feature of Single Crystal

1.2.2.1 Characteristics of Single Crystal

Single crystals consist of a single grain, and they have a lot of special characteristics derived from the structure. Schematic diagrams of single crystal and polycrystalline are shown in Figure 1.3. There is no grain boundary in the single crystal and no grain boundary enables high transparency even in crystal systems with refractive index anisotropy. This high transmittance enables its use in optical applications. In the materials with cubic structure without the refractive index anisotropy, various transparent ceramics with high transparency have been developed by eliminating voids in grains and grain boundaries. However, the transmittance of the transparent ceramics decreases as the thickness increases, while the transmittance of single crystals without voids and cracks doesn't change even if the thickness is changed. Furthermore, the transmittance of transparent ceramics decreases even if voids are eliminated in a material system with refractive index anisotropy. Therefore, some single crystals have been applied for optical devices such as laser, lens, wavelength conversion element, and nonlinear materials and scintillators.

In addition, a single crystal has only "one" crystal orientation because it is composed of a single grain. As a result, it is possible to develop devices using crystal anisotropy of materials, and it enables the use of crystal orientation that indicates the best properties of functional materials. Therefore, single crystals are used in various fields using crystal anisotropy such as semiconductors, substrates, piezoelectric elements, and magnetic and electronic materials. For example, in the piezoelectric and semiconductor crystals, the crystal orientation that maximizes their piezoelectric and electrical properties is used, respectively. In the substrate crystal, the crystal orientation with the lattice constant that best matches that of the deposition material is chosen.

Figure 1.3 Schematic diagrams of single crystal and polycrystalline.

1.2.2.2 Growth Methods of Single Crystal

Single crystals of materials with the congruent composition can be grown from the melts by the unidirectional solidification under a temperature gradient, and it is the "melt-growth method." Various melt-growth methods such as Czochralski (Cz), Bridgman-Stockbarger (BS), and Floating Zone (FZ) have been developed for researches and commercial uses, and of course they have been also used for development of scintillator single crystals. Especially, Cz and BS methods can grow a large bulk single crystal, and they have achieved mass production of the functional single crystals. On the other hand, micro-pulling-down (µ-PD) and laser heating pedestal growth (LHPG) methods have been recently used for material research of functional single crystals because of the faster growth rate than conventional melt-growth methods such as Cz and BS methods. As a result, various novel single crystals have been developed by the µ-PD and LHPG methods. Figure 1.4 displays single crystals of sapphire, scintillator, piezoelectric material, laser material, and magnetic and electrical materials grown by Cz, BS, FZ, and µ-PD methods.

Figure 1.4 Single crystals grown by Cz, BS, FZ, and µ-PD methods.

Many scintillator single crystals represented by NaI:Tl, CsI:Tl, and Lu2SiO5:Ce have been developed by the melt-growth methods, and the developments and applications of radiation detectors equipped with the scintillator single crystals have progressed. On the other hand, novel melt-growth methods have been developed to perform material research in areas that have not been explored so far for various reasons recently. As a result, the development of various novel scintillator single crystals is proceeding with newly developed and modified melt-growth methods.

1.2.2.3 Segregation

Some scintillator single crystals include dopant element as an emission center such as NaI:Tl, CsI:Tl, and Lu2SiO5:Ce, and the solid solution is used for improvements of scintillation properties by controls of the band structure and the crystal field around the emission center such as (Lu,Y)2SiO5:Ce and Gd3(Ga,Al)5O12:Ce. In addition, starting materials contain small amounts of impurities even if they are of high purity, and the amount and distribution of...