Phosphors for Radiation Detectors. Группа авторов

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of PL decay setup."/> Schematic illustration of the common setup of X-ray induced radio luminescence spectrum measurement. Schematic illustration of the common setup of gamma-ray induced scintillation decay curve measurement.

      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 scintillation and PL sometimes show a large discrepancy.

      We now introduce some applications of this pulsed X‐ray tube for scintillation decay measurement. The timing resolution of an X‐ray tube is several tens ps, which is determined by the traveling time of accelerated electrons, and it is enough to measure the rise time of most scintillators [92]. Following these pioneering works, a pulse X‐ray equipped streak camera system was developed, and enabled us to observe the wavelength resolved scintillation decay curve [93]. In this system, the detection part consists of a spectrometer chamber, streak camera, and two‐dimensional CCD, and a two‐dimensional image of wavelength vs. time can be observed. By the streak system, one of the problems of DCM, non‐wavelength resolution, can be solved. Up until the development of the streak camera system, every study used a pulse laser diode as the root of the excitation source, and it was enough to measure fast‐timing events. However, it is not sufficient to measure slow events, especially in the ms time range. One of the solutions for ms measurement is to replace the excitation source to common LED, which has higher power than the pulse‐type laser diode but slower speed. The pulse LED type system was developed in 2014 [94]. This system enabled us to measure slow (ms) scintillation decay as well as the X‐ray induced afterglow. Because the signal intensity per unit time is very low in slower scintillation, the system does not offer the function of wavelength resolution, and the detection part is one PMT. When we compare with typical measurements by DCM, the measurement time dramatically decreases to a few minutes for one sample. At present, no significant difference of excitation energy between 511 keV (DCM) and several tens keV (pulse X‐ray tube) has been found.

      In other ionizing radiations such as α‐ray, measurement is generally difficult. One easy but rough way is to use an oscilloscope for direct observation of the signal output from a detector, consisting of scintillator and photodetector by a self‐trigger mode. Although it may contain noise (e.g., thermal noise of photodetector), we can roughly observe scintillation decay time. Previously, most papers observed decay curves by such an oscilloscope measurement, but recently, a digitizer has been used to accumulate many decay curves. In such a case, we can accumulate a large amount of decay curves, and select correct decay curves by setting correct selection conditions of events. An accurate but technically difficult way is to use DCM by one scintillator sample and two photodetectors. If we take the case of α‐ray irradiation as an example, let us assume one cylindrical scintillator optically coupled with two PMTs by edge surfaces, and α‐ray is irradiated to the side of the scintillator. When the α‐ray is absorbed by the center part of the scintillator, half of the scintillation photons

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