in the nondestructive study of humans (e.g., dental application) or objects. The storage phosphors used for such applications are mainly classified into three types by emission mechanisms, namely optically stimulated luminescence (OSL) [3], thermally stimulated luminescence (TSL) [4], and radiophotoluminescence (RPL) [5] materials. Hereafter, the two kinds of luminescent materials (scintillators and storage phosphors) are focused. Although the emission mechanisms of storage phosphors are different from those of scintillators, the readout system of dosimeters is similar to that of scintillation detectors. Their emissions are typically read out by PMT under photo‐ or thermal‐stimulation (excitation). Details are described in later chapters.
Typically, luminescent materials for ionizing radiation measurements consist of the host material and the dopant for luminescence centers. For example, Tl‐doped NaI is the most common scintillator [6], NaI is the host which is generally an insulator or semiconductor, and Tl is the dopant with a typically very low concentration. In these kind of materials, the host has a role to absorb the target ionizing radiation efficiently, and the dopant has a role to emit photons whose number is proportional to the incident radiation energy or amount. The combination of the host and the dopant is one of the more recent trends of R&D in this field. The same trend is true, not only for scintillators, but also for storage phosphors for dosimeters.
In addition to host‐dopant type materials, undoped materials have also been investigated for the measurement of radiation. Although we can control the detection properties of luminescent materials by doping, some difficulties in the synthesis, such as a uniformity of dopant distribution, are always problematic. To avoid such problems, continuous effort has been paid to the study of undoped materials. The most common undoped materials for radiation measurement are semiconductor scintillators, and in addition to these, self‐activated scintillators such as Bi4Ge3O12 (BGO) [7] and Auger‐free luminescence (AFL) scintillators (e.g., BaF2 [8]) are also common. Undoped materials have been also investigated for dosimeter applications, and a common material used is BeO [9].
1.2 Interactions of Ionizing Radiation with Matter
Interaction probabilities of ionizing radiation depend on the species of ionizing radiation and matter under investigation. Figure 1.2 demonstrates a very common intuitive illustration of the interaction of ionizing radiation with matter. Although there are numerous species of ionizing radiations, the species included in Figure 1.2 are the most common.
Figure 1.2 Interaction of typical ionizing radiations with matter.
α‐rays (He nucleus) are common charged particle and have a high interaction probability with many materials, and a thin sheet of paper is enough to absorb these rays. In practical applications, a thin plate with a thickness of a few to several tens of μm scintillators are used as α‐ray detectors, and the most common scintillator for this use is Ag‐doped ZnS [10]. The weak point of the conventional Ag‐doped ZnS powder is poor energy resolution without a clear full‐energy deposited peak in pulse height spectrum. Generally, experimental environments of α‐ray detection contain so much X‐ and γ‐ray background, that detectors with Ag‐doped ZnS have difficulty in discriminating the level of signal and noise. In order to solve this problem, some new approaches were recently proposed [11, 12].
β‐rays are electrons that have higher penetrative power than α‐rays, but they can be absorbed by thin metals such as aluminum plate. For β‐rays detection, organic (plastic or liquid) scintillators are used [13], and organic scintillators are described in Chapter 2. In addition to organic scintillators, some inorganic scintillators are used by the electron detector in scanning electron microscopes (SEM) [14]. In β‐ray detectors, a fast response (rise and decay times) is often required. Because charged particles such as α‐ and β‐rays are easily stopped by detector components, dosimeters for these particles are not common. Recently, the muon has been used for the non‐destructive study of both artificial and natural objects [15], and plastic scintillators are used for muon detectors.
High energy photons, such as X‐ and γ‐rays, are the most useful ionizing radiation in practical applications, and one of their characteristics is a high penetrative power. In order to absorb X‐ and γ‐rays efficiently, we must prepare enough large and heavy materials such as Pb and Fe block. Generally, we use the density (ρ) and the effective atomic number (Zeff) to evaluate the detection efficiency of X‐ and γ‐ray detectors. Here, Zeff is defined as
(1.1)
where wi is the weight ratio of the i‐th element of the detector material, and Zi is the atomic number (Z) of the i‐th element. The power in the formula depends on its application. In the case of the scintillation detector, generally we evaluate the detection efficiency by Zeff4, since the interaction probability of the photoelectric absorption event is proportional to ~ρ Zeff4, while we use the power of ~3 (sometimes 2.94) for individual dosimeter applications. It must be noted that Zeff in scintillator and dosimeter fields can vary. In scintillators for X‐ and γ‐ray detectors, generally, high ρ and Zeff are preferable. On the other hand, light materials with Zeff close to 7.1 are preferable for individual dosimeter applications, because Zeff of the human body is around 7.
In the case of neutrons, the tendency is largely that of differently charged particles and photons. Although neutrons have a high penetrative power when applied to paper, thin metal plate, and heavy block, they have a high interaction probability with H, so H2O is an effective material to interact with neutrons. In neutrons, some specific elements, such as H, 3He, 6Li, and 10B, have a high interaction probability, and in order to detect neutrons efficiently, and detector materials should contain these elements. Especially, a 3He‐filled gas proportional counter is the standard detector for thermal neutron detectors.
1.3 Scintillation
1.3.1 Energy Conversion Mechanism
Figure 1.3 shows typical emission mechanisms of a scintillator assuming a combination of the host and the dopant for the luminescence center. When the incident ionizing radiation is absorbed by the ionizing radiation‐induced luminescent materials, many secondary electrons (i.e., δ‐rays, or carriers in a semiconductor field) are generated via electromagnetic interaction such as Coulomb scattering. In the case of X‐ or γ‐ray irradiation, one primary electron is generated by one of the three interactions, including photoelectric absorption, Compton scattering, and pair creation, and the primary electron can excite many secondary electrons. In spite of the species of the incident ionizing radiation, many secondary electrons are generated, and the number of them is proportional to the incident energy of the ionizing radiation if we assume the irradiation of a single quantum. This first process is called the conversion process, and the typical time scale of this process is extremely fast (as ~fs). After the generation of a large number of secondary electrons, these electrons dissipate their kinetic energy via the interactions with each other or the host lattice. If some electrons can directly reach the dopant sites (luminescence centers), they can recombine with the holes, and we can observe an emission.
