Phosphors for Radiation Detectors. Группа авторов

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stimulation intensity is a function of time, and can be expressed as

      (1.66)equation

      where Φ0 and βΦ are a constant of stimulation and a proportional constant of the stimulation intensity if we assume a linear time dependence. If Φ(t) is constant, it represents a continuous wave OSL (CW‐OSL), and the OSL intensity becomes

      (1.67)equation

      (1.68)equation

      where we assume Φ0 = 0. Generally, we measure OSL in typical PL machines where the environment is free from any other light, and Φ0 = 0 is a valid assumption. In this case, the OSL intensity becomes

      (1.69)equation

      Based on these analytical equations, OSL intensity shows exponential decay under stimulation, and this is an important property to distinguish PL and RPL from OSL. If readers would like to study OSL in more detail, it will be better to read specialized books on OSL (for example, [3]).

      Sections 1.4.2 and 1.4.3 describe common confirmed formulations of TSL and OSL, but up to now, a widely accepted formulation of RPL has not been obtained. RPL is treated as PL after carrier trapping phenomena, and general treatment of PL can be applied.

Schematic illustration of typical emission mechanisms of scintillation, OSL, TSL, and RPL.

      (1.70)equation

      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.

Schematic illustration of the inverse proportional relationship of Ce differently doped CaF2 single crystals on the plane of OSL intensity.

       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

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