Optical and Scintillation Properties of Ce-doped 20CsCl–20SrCl 2 –60ZnCl 2 Glasses

Ce-doped 20CsCl–20SrCl 2 –60ZnCl 2 glasses were investigated for their optical and scintillation properties. Under X-ray irradiation, a broad emission peak at approximately 350 nm with a shoulder peak at 375 nm was observed, and its origin was attributed to Ce 3+ . In the pulse height spectrum under 241 Am α-ray, the glass doped with 0.05% Ce exhibited the highest light yield ( LY ) of 25 ph/MeV (138 ph/5.5MeV-α) among zinc-halide-based glasses. The afterglow levels of the samples were superior to that of commercial CsI:Tl.


Introduction
A scintillator is a type of phosphor with the function of converting absorbed radiation energy to low-energy photons.The combination of a scintillator and a photodetector has been utilized as a scintillation detector in radiation detection, such as medicine, (1) security, (2) and resource survey. (3)(19)(20)(21) Efficient X-ray and γ-ray measurements require glass scintillators to contain heavy elements.

Results and Discussion
The prepared samples after polishing are shown in Fig. 1, and no significant differences in appearance are observed for all the samples.The samples have a cloudy and colorless appearance, which is due to the rapid deliquescence of their surface.To confirm that the samples are amorphous, the XRD patterns were evaluated as shown in Fig. 1.In all the samples, broad signals peaking at 12 and 20 degrees are observed.This halo peak suggests the formation of a glass matrix.In addition, no crystalline phase is observed at the detection limit in the XRD  measurements.PL spectra are measured to reveal general phosphor properties.All the samples show similar spectral features, and Fig. 2 shows the spectrum of the 0.05% Ce-doped sample as a representative spectrum.The sample shows a broad emission band peaking at 370 nm, which is almost the same emission wavelength as those of past zinc-halide-based glasses. (37,38)The QY values under excitation at 320 nm are 88.1% for the 0.05% Ce-doped sample, 89.4% for the 0.1% Ce-doped sample, 85.2% for the 0.3% Ce-doped sample, and 81.9% for the 0.5% Ce-doped sample, and QY increases up to the optimal concentration (0.05%) and decreases at higher concentrations.
Figure 3 shows the XRL spectra of the samples.Similar to PL, all the samples show a broad emission peak peaking at ~370 nm.As the Ce concentration increases, the absorption near 350 nm increases, and the spectral features indicate a peak shift; the results are consistent with the PL spectra.To confirm the response speed as a scintillator, XRL decay curves were evaluated as shown in Fig. 4. The least squares approximation shows that the decay curves have two components.The obtained decay values are close to those of zinc-halide-based glasses reported in previous studies, and this result is attributed to the 5d-4f transitions of Ce.The decay time tends to increase with the Ce concentration, and this tendency is also the same as those of other zinc-halide-based glasses.
Figure 6 shows the afterglow profiles observed after 2 ms X-ray irradiation.To compare afterglow levels, the afterglow level at 20 ms after X-ray irradiation was stopped (AG 20ms ) was defined in the same manner as in the previous papers. (43,44)The afterglow levels of the 0.05, 0.1, 0.3, and 0.5% Ce-doped glasses are respectively 245, 230, 138, and 233 ppm, which are slightly superior to that of commercial CsI:Tl (~300 ppm (44) ) and higher than that of Ce-doped CsCl-BaCl 2 -ZnCl 2 glasses (~60 ppm). (37)

Conclusions
Ce-doped 20CsCl-20SrCl 2 -60ZnCl 2 glasses were synthesized to evaluate their scintillation properties.Under UV light or X-ray irradiation, the prepared glasses showed a broad emission band peaking at 370 nm, and peak positions slightly redshifted with increasing Ce concentration.XRL decay curves showed that decay time constants are approximately 20-30 ns, which are   reasonable for the 5d-4f emission of Ce.In the pulse height spectra under 241 Am α-ray irradiation, a full energy peak was observed in all the samples, and the 0.3% doped sample showed the highest LY of 25 ph/MeV (138 ph/5.5MeV-α)under α-rays among the samples.The AG 20ms values of the samples were approximately 100-300 ppm, which were superior to those of commercial CsI:Tl scintillators.Considered comprehensively from LY and AG 20ms , the 0.3% Cedoped glass is the most suitable for scintillator uses among the present glasses.The obtained samples were superior to all reported zinc-halide-based glasses, but were inferior to practical GS20 except for Z eff and QY.Some zinc-halide-based glasses exhibit higher QYs than GS20 but lower LYs than GS20.The cause is considered to be the low energy transport efficiency despite the low phonon energy, but the actual cause is unclear at present.The development of halide glasses to overcome this problem is expected.