with a time scale of ps. In addition to the recombination, some secondary electrons directly excite the electron at the luminescence center, and we can also observe an emission by the relaxation of this excited electron at the luminescence center. Emission caused by these processes is called scintillation, and this final stage is known as the luminescence (or emission) process. The time scale of the luminescence process depends on the emission mechanism or dopant ion. For example, if the dopant shows a 5d‐4f transition, a typical time scale is several tens to hundreds of ns.
Figure 1.3 Typical emission mechanisms of scintillation.
The final process of the scintillation is the same with photoluminescence (PL), and we may have a question about the difference of the scintillation and PL. Although no standard definition has been put forward, we (the authors of this book) consider that the interactions of excited secondary electrons is necessary for scintillation. In PL, generally one UV or Vis photon can excite one electron in the outer orbital, and we observe an emission by the relaxation of this excited electron. In this case, the excited electron does not interact with other electrons. In the case of the direct band‐gap excitation of semiconductor‐type phosphor, the interaction of the excited electron with other electrons may be possible. But typical UV or Vis excitation energy is not so high, and the excited electron does not have enough energy to interact with the other electrons in most cases. Thus, the excitation of multiple electrons by quanta and interactions of these multiple electrons will be the main difference of scintillation with PL. In other words, if we consider typical insulator or semiconductor materials, at least several tens of eV of excitation energy will be required to cause scintillation, and excitation below this energy will be PL. Of course, the threshold energy of scintillation and PL depends on the materials used. In the case of small band‐gap materials, the excitation energy of several eV may be enough to cause scintillation.
1.3.2 Emission Mechanism
Figure 1.4 summarizes typical emission mechanisms in a scintillator, and the same classification is also possible for dosimeter materials. There are two kinds of luminescence in scintillators, namely intrinsic and extrinsic emissions. The intrinsic luminescence represents free‐exciton luminescence, self‐trapped exciton (STE) luminescence, Auger free luminescence, and self‐activation luminescence. The free exciton luminescence is observed in wide band‐gap semiconductor materials having direct transitions. In this type, common examples are ZnO [16] and GaN [17]. Scintillation due to free exciton is characterized by fast scintillation decay (1–10 ns) with a sharp emission peak in the spectrum. Recently, Ga2O3 is found to have fast and very intense scintillation upon γ‐ray irradiation [18]. Although Ga2O3 is still under discussion, whether it is a direct‐ or indirect‐transition type of semiconductor, it shows an attractive scintillation performance.
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 (Tl1−xAx)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
