Читать книгу Phosphors for Radiation Detectors - Группа авторов - Страница 25

Up to now, scintillation (1.3) and ionizing radiation induced storage luminescence (1.4) have been carried out in different fields and by different people. However, these phenomena occur at the same time, and Figure 1.9 describes the situation relating to luminescence by combining Figures 1.3 and 1.8.

Figure 1.9 Typical emission mechanisms of scintillation, OSL, TSL, and RPL.

In addition to the luminescence phenomena described in these figures, some of the secondary electrons lose their energy thermally, and in such a case, we obtain no emissions. In the model of scintillation described in Section 1.3, it sometimes contains thermal energy dissipation as in Equation (1.15), but it does not consider that a significant amount of carrier is stored at trapping centers. In storage luminescence, such as TSL and OSL explained in Section 1.4, analysis is done by the rate equation, which does not consider how many carriers can reach the trapping centers, and scintillation is never assumed. The rate equation is only considered after the trapping (or generally luminescence) sites are occupied, so it cannot be used before the occupation. As commented above, scintillation and storage luminescence occur at the same time after the absorption of ionizing radiation. Therefore, in ionizing radiation induced luminescent materials, branching ratio of scintillation, storage luminescence, and thermal loss is important, and the branching probability of these phenomena can be expressed as

(1.70)

where P_{scintillation}, P_storage, and P_thermal are the branching probabilities of secondary electrons to scintillation, storage luminescence, and thermal loss, respectively. It should be noted that the thermal energy loss is a very rough concept, and contains energy loss during any diffusion processes and relaxation processes at localized trapping and luminescence centers. Although the above equation is quite natural if we assume energy conservation, such a relation has not been considered historically because scintillator and dosimeters with storage phosphors have been investigated in different scientific fields.

The relationship among scintillation, storage luminescence, and thermal loss has been experimentally investigated recently by the author [82, 86–88]. In materials recognized as a phosphor, the ratio of thermal loss is relatively small, and in such a case, scintillation and storage luminescence should show an inverse proportional relationship. In order to confirm the validity of our assumption, we investigated the relationship of scintillation and storage luminescence intensities of materials which have different dopant concentrations. Figure 1.10 shows a representative of the inverse proportional relationship of Ce differently doped CaF₂ on OSL vs. scintillation. A clear inverse proportionality is observed, and confirms the validity of the assumption. A similar relationship has been confirmed in many material forms, including single crystal, glass, and ceramics in scintillation vs. OSL/TSL. In most luminous materials, we can observe such a relationship, but in the case of Ce‐doped LiCaAlF₆, we observed a positive proportional relationship [87]. Ce‐doped LiCaAlF₆ is not luminous both in scintillation and storage luminescence, so we consider most of the absorbed energy will be converted to thermal loss. If we neglect the effect of thermal loss, a low temperature experiment will be helpful.

Figure 1.10 The inverse proportional relationship of Ce differently doped CaF2 single crystals on the plane of OSL intensity as a PL quantum yield (%) and scintillation light yield (ph/5.5 MeV‐α).

Source: The data taken from [86].

This experimental result presents some important problems to conventional understanding of ionizing radiation induced luminescence fields. For example, from the viewpoint of scintillation, the degradation of light yield in higher dopant concentration has been interpreted as concentration quenching. However, Figure 1.10 shows that most energy in highly Ce‐doped samples is not converted to thermal loss but to energy storage. Another point is the ε‐value, which is the average energy to generate one electron–hole pair in solid state materials. The most common example is the Si semiconductor detector, and the value of Si is known to be ~3.6 eV (=βE_g of Si in conventional understanding based on Section 1.3), which also relates to the theoretical limit of Si solar cell of <30% ~ 1/3.6 ~ 28%. The ε‐value is also defined for scintillators, and has been considered as 10–20 eV. This calculation is based on the scintillation light yield from a pulse height spectrum, and does not take into account storage luminescence. If we consider the ε‐value from the definition, such an estimation in scintillators is not correct, because we do not count the contribution from the carrier storage.

As described above, we think ionizing radiation induced luminescence can be treated as one form of unified physics, and this is why we describe these topics in one book, although they have often been treated as different scientific fields. In this case, the base of the consideration is the energy conservation law, and it strongly assumes the integration of energy in infinite time, which is a standard strategy in astrophysics because the real‐time (time‐derivative) observation is impossible. The author (Yanagida) studied astrophysics, and the consideration depends on the field of the origin. Recently, another author (Koshimizu), whose field of origin is in solid state physics, proposed a real‐time observation on the energy transportation (carrier diffusion) process, which is a key process to understanding S in the equations presented in the previous sections. Such an observation is enabled by transient absorption spectroscopy, i.e., optical absorption spectroscopy as a function of time after excitation by pulsed electron beams. Because the excited states are probed with optical absorption, their real‐time dynamics prior to scintillation can be analyzed. Actually, slow decay of the transient absorption correlates with low scintillation intensity, and is consistent with observation results based on energy conservation. Such transient spectroscopy has long been used with pulsed light, from flash lamps to laser instruments, as excitation sources to elucidate the excited states dynamics. For ionizing radiation, pulsed electron beams can be used as excitation sources. Such a measurement technique based on pulsed electron beams has also long been used to analyze the chemical reaction dynamics in radiation chemistry and is called “pulse radiolysis.” This technique can also be applied to ionizing radiation‐induced luminescence materials and gives information on energy transfer, carrier trapping, and quenching. Thus, to understand S, both energy conservation and temporal analysis‐based experiments have been used recently.