Growth and scintillation properties of Pr doped Gd3(Ga,Al)5O12 single crystals
Highlights
► Pr:Gd3(Ga,Al)5O12 single crystals were grown by the m-PD method. ► Pr3+ 4f–5d emission is observed in 340 nm for the first time in this host. ► Pr1%:Gd3Ga3Al2O12 shows highest emission intensity. ► The light yield of Pr:Gd3Ga3Al2O12 sample with 3φ×1 mm size was 4500 photon/MeV. Scintillation decay time was 7.9 ns (0.5%), 46 ns (0.7%) and 214 ns (98.8%).
Introduction
Scintillator materials combined with photodetectors are used to detect high energy photons and accelerated particles in medical imaging techniques, high energy and nuclear physics detectors, high-tech industrial applications and most recently also in the advanced homeland security related techniques.[1] In past decades, scintillators with the best figure-of-merit given by a combination of stopping power, speed and light yield are the recently developed Ce-activated materials [2], [3], [4], [5]. On the other hand, Pr3+ ion also shows the 5d–4f emission with the fast decay time in several host materials and such systems can be another candidate for high figure-of-merit scintillator. Recently our group intensively examined scintillation properties of various Pr-doped compounds [6], [7], [8], [9], [10], [11], [12], [13]. Among those materials, Pr:Lu3Al5O12 (Pr:LuAG) was found with good scintillation properties due to high density (6.7 g/cm3), high light yield (approximately 20,000 photon/MeV), good energy resolution and a very fast 5d–4f emission decay time (20 ns)
The Gd3Al5O12 could be also an attractive host for Ce-doped scintillators because of its relatively high density of 6.2 g/cm3 and absence of intrinsic radioactivity coming from 176Lu isotope with half-life of 3.78×1010 years (2.59% natural abundance). However, Gd3Al5O12 single crystal cannot be grown by the melt growth due to incongruent melting and phase instability [14]. Recently, some groups reported about transparent optical ceramic growth of Ce: (Gd,Y)3Al5O12 and scintillation response of about ∼90 ns at emission around 550 nm, prospective light yield of about 11–16,000 photon/MeV and density of 6.0 g/cm3 [15], [16]. Higher density garnets such as Lu3Ga5O12 (7.4 g/cm3), and Gd3Ga5O12 (7.4 g/cm3) are famous crystal hosts for solid state lasers or substrate materials for epitaxial growth. However Ce3+ 5d–4f luminescence is quenched in these garnets because of the positioning of 5d states of Ce3+ in the host conduction band, which is a well-known problem in other hosts as well [17].
In this report, the Pr:Gd3(Ga,Al)5O12 (Pr:GGAG) single crystals were grown by the micro-pulling down (μ-PD) method. Luminescence and scintillation properties were measured. The substitution phenomenon in the Al3+ sites with Ga3+ in garnet structure has been studied.
Section snippets
Crystal growth procedure
A stoichiometric mixture of 4 N Pr6O11, Gd2O3, β-Ga2O3 and α-Al2O3 powders (High Purity Chemicals Co.) was used as the starting material. More specifically, the Gd3+ site was substituted by Pr3+ according to the formula of (PryGd1–y)3(GaxAl1–x)5O12. Single crystals of Pr:GGAG were grown by the μ-PD method with an RF heating system. The y was 0.01 and additionally 3 mol% of β-Ga2O3 was added to compensate ignition loss. A schematic layout of the μ-PD growth apparatus is given in Refs. [18], [19].
The growth of Pr:GAGG single crystal
Pr1% doped Gd3(GaxAl1–x)5O12 (x=0, 1, 2, 3, 4 and 5) crystals were grown by the μ-PD method. Example photos are shown in Fig. 1. Pr1%:Gd3Al5O12 crystal could not be obtained by the same growth conditions. The grown crystals were transparent with greenish color, 2–3 mm in diameter and 10–30 mm in length. Some of them looked slightly cloudy because of the rough surface caused by gallium oxide evaporation or thermal etching. However, the inner part of all the crystals was perfectly transparent.
Conclusion
Pr doped Gd3(Ga,Al)5O12 single crystals were grown by the μ-PD method with RF heating system. Pr3+ 5d–4f emission in 300–350 nm, Pr3+ 4f–4f emission in 480–650 nm and Gd3+ 4f–4f emission in 310 nm were observed in the grown crystals. The light output of Pr1%:Gd3Ga3Al2O12 sample was around one-fifth of the Cz grown Pr:LuAG standard and around 4500 photon/MeV. Scintillation decay time was 7.9 ns (0.5%), 46 ns (0.7%) and 214 ns (98.8%) using the PMT and digital oscilloscope.
Acknowledgments
This work was mainly supported by the JST Sentan and partially by a Grant in Aid for Young Scientists (B)-15686001, (A)-23686135, and Challenging Exploratory Research 23656584 from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese government (MEXT). Partial financial support from Czech AVM100100910 and GACR 202/08/0893 projects is also gratefully acknowledged.
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