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Figure 1.4 Typical emission mechanisms of scintillation.

      STE is observed in wide‐band‐gap insulator materials, such as undoped alkali halides. Common properties of STE are summarized as follows: a broad emission peak in the emission spectrum, a large Stokes shift, a relatively high light yield in scintillation, a relatively fast decay time from several hundred ns to a few μs, and a large temperature dependence of the scintillation light yield. The most common materials are BaF₂ [8], SrF₂ [19], CaF₂ [20], and their mixed compounds.

      AFL is observed in some materials which have a larger band‐gap energy than the energy between the core and the valence bands [21]. AFL generally shows a very fast luminescence decay (~1 ns) with a short emission wavelength (VUV‐UV). The common materials are BaF₂ [8], BaMgF₄ [22], CsF [23], and Cs₂ZnCl₄ [24]. Up to now, AFL has been observed in halide materials, and a search for other compounds (oxides and nitrides) is an interesting research topic. Conventionally, it was considered that they have no temperature dependence concerning the scintillation light yield [25], but we recently revealed that AFL materials also show temperature dependence [26]. The disadvantage of AFL scintillators is a relatively low scintillation light yield. The details of AFL are described in Chapter 4.

Self‐activation type scintillators are composed only of the host, and this type can be classified as belonging to the intrinsic luminescence‐type materials. The difference from other intrinsic‐type materials is that the host material contains luminescent ions as a main element. Common scintillators of this type are BGO [7], CeF₃ [27], and CeBr₃ [28]. Scintillations from BGO are caused by ns² transitions of Bi³⁺, and those of the latter two materials are due to the 5d‐4f transitions of Ce³⁺. Although the emission mechanism is not fully understood, recent results of Tl‐based crystals, such as TlMgCl₃ and (Tl_1−xA_x)MgCl₃ (where A = alkali metal), show very high scintillation light yields [29, 30], and their emission will relate to Tl⁺. In these materials, the emission properties are governed by the luminescent ions of the host, but this class of materials do not suffer from concentration quenching.

      Then, the topic moves to materials with extrinsic luminescence. The first one is defect‐based luminescent materials, and the most common defect relating to luminescence is the F‐center, which is characterized by one electron captured at an anion vacancy. Materials containing F‐centers sometimes show scintillation. Common materials with an F‐center are simple oxide materials, such as MgO [31] and Al₂O₃ [32]. In addition to F‐centers, there are some other defect‐based luminescence centers, for example, F⁺‐ and F⁻‐centers. Although defects are not exactly activators, defect‐based luminescence is sometimes categorized as extrinsic emission.

      Luminescence due to the ns² ↔ nsnp transition is one of the common electron transitions involved in many scintillators, such as Tl‐doped NaI [6] and Tl‐doped CsI [33], which are very traditional scintillators. These traditional scintillators were discovered around 1940, and ns² ions have become a familiar dopant in this field. The spectral feature of the ns² ↔ nsnp transition is characterized by a broad emission band, and the emission wavelength is from near UV to VIS. Because the decay time is from several hundred ns to several μs, it is acceptable for pulse‐height‐based scintillation detectors. In this type of luminescence, in addition to Tl⁺ and Bi³⁺, In⁺ [34] and Sn²⁺ [35] have shown interesting scintillation properties in recent years.

      In scintillators, transitional metal ions are sometimes activated to obtain a high luminescent efficiency for integration‐type detectors. The spectrum of 3d‐3d and 4d‐4d (d‐d) transitions has a broad luminescence feature, and the decay time is typically of the ms order. Among the transitional metal elements, Mn²⁺ [36] and Cr³⁺ [37] are sometimes selected for scintillation detectors due to a good spectral matching with Si‐PD.

      Electron transitions of the 4f‐4f and 5f‐5f levels are called f‐f transitions, and some scintillators use the 4f‐4f transitions of rare‐earth elements because they typically exhibit a high emission intensity with acceptable decay time of several μs to few ms. The most common scintillator based on the 4f‐4f transition is Pr³⁺‐doped Gd₂O₂S (GOS) ceramic [38], which is often part of the equipment used in medical X‐ray CTs. The Pr‐doped GOS shows a very high scintillation intensity with a decay time in the medium range (several СКАЧАТЬ