Ce-concentration dependence in CaYAl3O7 single crystalline scintillators
Introduction
Scintillators emit numerous photons with a few eV when irradiated by ionizing radiations such as X-, γ-, α-, β-rays and neutron, and they have been widely applied to various fields such as security [1], medical imaging [2] and monitoring nuclear power plants [3]. Recently developed scintillators commonly consist of a host material which absorbs energy from ionizing radiations via interaction, and luminescent centers which emit photons. Scintillation detectors, combined with a scintillator and a photodetector, should be more sensitive and precise. Therefore, scintillators with faster response and higher scintillation light yield (LY) have been diligently developed. A bulk single crystalline form is often adopted to scintillators because of following advantages. Since single crystals exhibit high transmittance in UV and visible regions, emitted photons can be efficiently delivered to photodetectors such as photomultiplier tube (PMT) or photodiode (PD). In addition, a bulk form is very important for X- and γ-ray scintillators because interaction probability between a scintillator and ionizing radiations depends on a size of a scintillator; thus, bulk scintillators feature efficient interaction probability with ionizing radiations. Typical inorganic materials have advantages for scintillator application mentioned above, and this research focuses on melilite single crystals.
Melilite compounds have many kinds of chemical composition, and the basic composition is expressed as (AE)(RE)ZT2O7, where AE and RE stand for alkali metals (e.g., Ca, Sr and Ba), and rare earths (e.g., Y, La and Gd), respectively. In addition, the Z site can be occupied by Mg, Zn and Al, and the T site by Al, Ga and Si [4,5]. Although we generally require scintillators for X- and γ-rays to have high effective atomic number and high density, typical melilite single crystals have medium atomic number and density. For example, the effective atomic number and density of CaYAl3O7 (CYAM) are 28.0 and ~3.61 g/cm3 [6], respectively. However, some of scintillators with medium atomic number and density can be commercially applied if the performances are enough. For instance, Ce-doped YAG [7] or YSO [8] have been used for a small handy-type survey meter for X- and γ-rays [9].
Rare-earth-doped melilite compounds have been developed as phosphors for LEDs [[10], [11], [12], [13]] and long-lasting luminescence materials so far [[14], [15], [16], [17]]. Thus, rare-earth-doped melilite compounds may show good scintillation properties. In our previous work [5,18,19], scintillation properties of some rare-earth-doped melilite single crystals have been reported. Those scintillators showed relatively high LYs, and the emission wavelengths were around 380–500 nm which is suitable for common PMT. In our previous work, we evaluated the Ce-doped Ca(Gd,Y)Al3O7, and among them, Ce:CYAM exhibits the highest LY of 6500 ph/MeV [20]. The radiative decay rate increases with increasing of Ce-concentration. However, over-doping of Ce raises non-radiative decay rate owing to self-absorptions, resulting in decreasing of the quantum yield (QY). Since LY is proportional to the QY, Ce-concentration largely contributes to the scintillation phenomena. To investigate the Ce-concentration dependence in Ce:CYAM scintillators, Ce:CYAM with Ce concentration varied as 0, 0.3, 1.0, 2.0 and 3.0 at.% were synthesized by the optical Floating Zone (FZ) method, and the photoluminescence (PL) and scintillation properties were evaluated.
Section snippets
Experimental
Raw powders of CaO (99.99%, Furuuchi Chemical), Al2O3 (99.99%, Taimei Chemicals), Y2O3 (99.99%, Furuuchi Chemical) and CeO2 (99.99%, Furuuchi Chemical) were uniformly mixed with molar ratio of CaO:Al2O3:Y2O3:CeO2 = 2:3:1:x (x = 0, 0.002, 0.006, 0.02 and 0.06). The mixture was formed into a cylinder rod by applying a hydrostatic pressure. Then, the cylinders were sintered at 1400 °C for 8 h in air. Following the sintering process, crystal growth was conducted in air by the optical FZ method
Results and discussion
Synthesized Ce:CYAM single crystals are shown in Fig. 1. All the samples after mirror-polishing look colorless and transparent under room light. A blue emission is observed from all the samples under 365 nm UV light excitation. Fig. 2 represents powder XRD patterns of all the samples and reference data of CYAM [25]. The patterns indicate that all the obtained samples demonstrate a melilite single-phase with a space group of P21m [4]. Since no impurity phases are observed from all the XRD
Conclusions
Ce-doped CaYAl3O7 single crystals were synthesized by the optical FZ method to examine the Ce-concentration dependency. Broad emissions around 430 nm due to the 5d-4f transition Ce3+ are observed from all the samples except for the undoped one by evaluation of the PL contour plots and the scintillation spectra. The PL and scintillation decay time constants are 30–40 ns and 110–270 ns, and are ascribed to the 5d-4f transitions of Ce3+ and intrinsic luminescence, respectively. Based on the
CRediT authorship contribution statement
Kenta Igashira: Conceptualization, Investigation, Writing - original draft. Daisuke Nakauchi: Investigation, Writing - review & editing. Yutaka Fujimoto: Investigation. Takumi Kato: Investigation. Noriaki Kawaguchi: Supervision. Takayuki Yanagida: Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by Grant-in-Aid for Scientific Research A (17H01375), Scientific Research B (18H03468) and JSPS Research Fellow (17J09488) from JSPS. The Cooperative Research Project of Research Center for Biomedical Engineering, NSG Foundation, Iketani Science and Technology Foundation, Murata foundation and NAIST Foundation are also acknowledged.
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