Figure 1.11 Schematic drawing of transmittance (absorption) (a) and PL measurement (b) setups.
Figure 1.11b represents a setup of PL excitation and emission spectra by using the integration sphere. Generally, PL can be observed upon UV–VIS excitation from Xe‐lamp, and we can select which luminescence center (or electron transition) can be induced by selecting the excitation wavelength using a grating. If we use the integration sphere, we can collect all the PL photons. If we define A and I as absorption, which is evaluated by the intensity of excitation light with and without the sample and PL emission intensity, respectively, we can deduce Q = I/A experimentally. The detectors for PL photons are generally grating and photodetector. PL quantum yield is a quantitative value, but generally PL is a qualitative evaluation with an arbitrary unit, since PL intensity depends on the geometry of sample setting. In common PL measurement, we do not use the integration sphere.
Figure 1.12 represents a typical experimental configuration of PL decay measurement. The excitation light was emitted from a pulsed light source such as a lamp or an LED. In the lamp, a grating is used to select excitation wavelength, and selected excitation photons are irradiated to the sample. The timing of the excitation is injected as a start signal to time to amplitude converter (TAC). PL photons from the sample are also filtered by the grating to select the target emission, and then collected by the photodetector. The emitted PL photons are reduced to a single photon by filters, pinhole, etc., and this single photon signal is put into the TAC as a stop signal. A timing window is set for the target timing range (e.g., several tens ns for few ns phenomenon), and only the coincidence event within the timing window is recognized as a target signal. The TAC has a function to convert the timing difference of start and stop signals to a pulse signal. After accumulation of pulse signals many times, we can obtain a PL decay curve. Such a technique is called time correlated single photon counting (TCSPC). Generally, we analyze the PL decay curve by an approximation of sum of exponential functions as
(1.71)
where Ai, t, are τi are intensity of each decay component, time, and decay time (lifetime) of each component, respectively. Sometimes, we deconvolute the excitation pulse (instrumental response function, IRF), and it should be noted that perfect deconvolution is generally difficult. After obtaining PL quantum yield Q and decay time τ, we can use convenient equations such as
(1.72)
(1.73)
(1.74)
where kf and knr denote rate constants of radiative and non‐radiative transitions, respectively. In this way, we can evaluate radiative and non‐radiative rate constants quantitatively.
Figure 1.12 Schematic drawing of PL decay setup.
PL is a phenomenon at localized luminescence centers, and except for some cases like the semiconductor scintillator, we do not consider the movement and interactions of carriers. In other words, PL is a phenomenon within a band‐gap. But scintillation is accompanied with interactions of carriers, and sometimes PL and scintillation offer very different emission spectra. As a basic scintillation property, we generally measure radioluminescence spectrum to obtain the scintillation emission intensity to select adequate photodetector for applications. A typical setup is illustrated in Figure 1.13. The most common setup uses an X‐ray generator as an excitation source, and if readers have another radiation source, they can use that instead of the X‐ray generator. We sometimes use α‐rays instead of X‐rays [89] in laboratory‐level experiments, and sometimes use other radiation sources at large facilities [90] by taking our experimental setup there. Scintillation photons are generally collected via an optical fiber, since a direct hit of ionizing radiation causes radiation damage to grating and photodetector, and to avoid this, we generally set them apart from the radiation source. In some literature, light yield is calculated by an integrated area of radioluminescence emission spectrum. It must be noted that such an evaluation is not correct in most cases, as explained in Section 1.3.3. Figure 1.14 shows a geometry of transmission‐type measurement, and in some experiments, we use a reflection type measurement similar to the PL configuration.
Figure 1.13 Common setup of X‐ray induced radioluminescence spectrum measurement.
Figure 1.14 Common setup of γ‐ray induced scintillation decay curve measurement (delayed coincidence method).
Scintillation decay time is generally measured by γ‐ray or X‐ray excitation. Figure 1.14 presents a typical setup for a γ‐ray induced scintillation decay curve measurement. The basic concept is the same as that in PL decay curve measurements but the excitation source is different. Generally, we use the 22Na radioisotope as an excitation source, which emits two 511 keV γ‐rays in 180° opposite directions. One of these two γ‐rays hits a fast scintillator, and creates a start signal through a constant fraction discriminator (CFD). At the opposite side, the other γ‐ray hits the target scintillator for measurement, and emits scintillation photons. Then, the number of these emitted scintillation photons are reduced to a single photon, and a signal pulse corresponding to the single photon is output from photodetector (typically, PMT). After some delay, the signal is injected into TAC as a stop signal through adequate delay and CFD. The process after TAC is the same with PL decay, and the analysis method is also the same. This typical experimental methodology is called delayed coincidence method (DCM). The difference with PL is that DCM generally does not resolve wavelength because of the low signal intensity and frequency. If we resolve the wavelength, a very long measurement time will be required.
Although DCM with a 511 keV γ‐ray source is the most common way to evaluate scintillation decay time, it contains several technical disadvantages. Because the energy of γ‐ray (511 keV) is high and has a high penetrative power of materials, detection efficiency is not high and requires a long time for the measurement to be made. In addition, low detection efficiency makes it difficult to measure a slow component, and generally, a component slower than several μs is difficult to measure. For example, measurements of emissions from Eu3+, Tb3+ and most transition metal ions, are almost impossible, although they are used for integration‐type detectors. The lack of the wavelength resolution is also a problem because identification of the scintillation emission origin is sometimes difficult. In most cases, we can guess the emission origin by PL decay, but decays of