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Phosphors for Radiation Detectors. Группа авторов
Читать онлайн.Название Phosphors for Radiation Detectors
Год выпуска 0
isbn 9781119583387
Автор произведения Группа авторов
Жанр Отраслевые издания
Издательство John Wiley & Sons Limited
(1.70)
where Pscintillation, Pstorage, and Pthermal 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 (=βEg 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.
1.6 Common Characterization Techniques of Ionizing Radiation Induced Luminescence Properties
In this section, common characterization techniques for ionizing radiation induced luminescence phenomena are introduced. Transmittance and absorbance measurements are common characterization techniques for phosphors, because we can obtain the absorption bands of materials by this measurement. The PL spectrum is important when considering the emission origin. Figure 1.11a represents a typical methodology to measure transmittance and absorbance. At first, we measure the light transmission intensity of the sample holder, and we define its intensity as 100% transmittance at measured wavelength. Then, we place a sample on the holder, and measure the light transmission intensity of sample + holder. The ratio of the with‐without sample is the transmittance. The transmittance can be understood as Equation (1.26), and if we can plot it in log‐scale corrected by the thickness of the sample, we can draw the absorbance plot. The absorbance is in arbitrary units, and some literature is confused about the absorbance and absorption coefficient (cm−1). It must be understood that if readers need to evaluate the absorption coefficient quantitatively, reflectance must be measured. Without the correction of reflectance, it is impossible physically to determine the absorption coefficient. In some special cases where the reflectance is close to zero, we can treat the absorbance with the rough absorption coefficient. In the case of undoped materials, the wavelength rapidly drops to transmittance to 0% (or rapid increase of absorbance to infinite value), and we can notice the wavelength (energy) as a band‐gap of each material. Some literature use the term “band‐gap” to absorption bands due to dopant in luminescence center doped materials. Such an expression is wrong, at least in solid state physics, although most readers can understand what authors of the paper mean. The transmittance obtained by Figure 1.11a is called in‐line transmittance, and if we use an integration sphere to collect a non‐straight line of light from the sample, we can measure diffuse transmittance. In ionizing radiation detector uses, diffuse transmittance is close to practical conditions, because we generally use reflectors to collect every photon generated by irradiation to photodetectors.