Optical and scintillation properties of LuGd2Al2Ga3O12:Ce, Lu2GdAl2Ga3O12:Ce, and Lu2YAl2Ga3O12:Ce single crystals: A comparative study
Introduction
Cerium-doped Lu3Al5O12 (LuAG:Ce) single crystal scintillator became systematically studied at the very beginning of new millennium.
The growth of variously doped crystals [1], [2], [3], [4], theoretical calculations focused on the defect creation in YAG and LuAG garnet structures [5], [6], [7], [8] and a number of characterization studies focused on specific optical, luminescence and scintillation properties, e.g. [9], [10], [11] and structure–property relations [12] have been published, see also the review [13]. LuAG:Ce single crystal scintillator is a competitive material in the family of heavy scintillators due to its density of 6.7 g/cm3 and fast scintillation decay time of 60–80 ns [14]. However, its high theoretical light yield (LY) of about 60,000 ph/MeV [15] has never been achieved as the value measured for the best LuAG:Ce crystal was about 26,000 ph/MeV [16]. The reason for such a LY deterioration was explained by the presence of antisite (Lu ) defect-related shallow electron traps [17] which delay the energy delivery to emission centers and give rise to a high content of slow components in the scintillation decay time profile [18], [19]. The Ga - admixture in the aluminum garnet, which is compatible with the melt-growth technologies, diminishes the trapping effect of shallow electron traps and speed-up the scintillation response [20], [21] by lowering the bottom of the conduction band to bury the electron traps [22], [23]. Some increase of LY value was obtained for Ga content up to 20 at.% in Lu3(Al,Ga)5O12:Ce as well [24]. Finally, the balanced admixture of Gd and Ga into LuAG structure with a general formula (Lu,Gd)3(Ga,Al)5O12:Ce led to dramatic LY increase above 40,000 ph/MeV and a novel family of so called multicomponent garnet scintillators was reported in 2011 [25]. The Czochralski-grown Gd3Al2Ga3O12:Ce single crystals have been reported up to four-inch diameter [26] with high LY above 53,000 ph/MeV and leading decay time of 76 ns in scintillation decay. The effect of Gd and Ga admixture on the band-gap value and band edge positioning [27] and on the Ce luminescence quenching [28] has been investigated further. Positive effect of post-growth annealing in air was reported as well [29]. Temperature dependence of light yield was reported in [30]. The development of multicomponent garnet phosphors and scintillators within last decade has been also recently reviewed [31]. The effect of Ga/Al ratio in Gd3(Al,Ga)5O12:Ce crystals was studied as for LY, energy resolution and scintillation speed: high LY value up to 58,000 ph/MeV, energy resolution down to 4.2% at 662 keV and decay times of fast component around 90–120 ns were reported [32], [33], [34]. Multicomponent garnet scintillators have been further optimized by targeted codoping following the strategy adopted in orthosilicates [35]. Timing characteristics of Gd3Al2Ga3O12:Ce were further improved by codoping with Ca2+ or Mg ions [36], [37], [38] and the Czochralski-grown Gd3Al2Ga3O12:Ce,Mg crystal with a fast scintillation decay time of 45 ns (58%) has been recently reported [39]. Further improvement was achieved by the admixture of Y into Gd sublattice [40]. Generally accepted explanation of positive effect of such divalent ion codoping in cerium doped garnet scintillators is based on stabilization of Ce which provides additional fast radiative recombination pathway in scintillation mechanism [41]. The Ce presence was proved also by X-ray Absorption Near Edge Spectroscopy [42]. It is worth noting that also other codoping strategies based on Li [43] and B [44], [45] ions have been reported as well.
The aim of this work is to investigate the scintillation properties of LuGd2Al2Ga3O12:Ce, Lu2GdAl2Ga3O12:Ce and Lu2YAl2Ga3O12:Ce single crystals for fast scintillation detector application with respect to Gd3Al2Ga3O12:Ce one. The position of the Ce 5d1 level could be high-energy shifted by partial substitution of Gd with a smaller Lu (or Y) ion at dodecahedral site due to the decrease of crystal field splitting of the 5d levels [46], [47], [48], [49]. The decay time shortening could be expected resulting from a blue-shift of emission wavelength (). Partial substitution of Gd with a smaller Lu (or Y) ion could also affect energy migration through the Gd sublattice and an energy transfer to the Ce luminescence centers [50]. The measurements of absorption, photoluminescence emission (PL) and excitation (PLE) spectra, PL decay time, LY values under excitation with - and - rays, scintillation decay time, coincidence time resolution, and afterglow are presented. The mass attenuation coefficient at 59.5 keV and 662 keV rays is also determined.
Section snippets
Experimental details
LuGdAl2Ga3O12:Ce (x = 1, 2) and Lu2YAl2Ga3O12:Ce single crystals with cerium concentration of 1 at.% were grown by the Czochralski method from an Ir crucible under Ar + 2% O2 atmosphere, for details see [51]. Polished crystal samples with size of 5 × 5 × 1 mm3 were used for all measurements.
The absorption spectra in UV–Visible region were recorded using a Perkin Elmer (Lambda 35 UV–Vis) spectrophotometer. PLE and PL spectra were measured using a Hitachi F-2500 fluorescence
Absorption and photoluminescence characteristics
The absorption spectra of LuGdAl2Ga3O12:Ce (x = 1, 2) and Lu2YAl2Ga3O12:Ce crystals are displayed in Fig. 1. Two dominant absorption bands are related to the 4f5d1 and 4f5d2 transitions of the Ce ions. Absorption lines at 275 and 312 nm are due to the 8S 6I and 8S 6P transitions of Gd ions, respectively. The normalized PL spectra of studied crystals are displayed in Fig. 2 together with an PLE spectrum of a LuGd2Al2Ga3O12:Ce. The PL emission bands (450–630 nm) related to
Conclusion
In this work, the scintillation characteristics of Czochralski-grown LuGd2Al2Ga3O12:Ce, Lu2GdAl2Ga3O12:Ce and Lu2YAl2Ga3O12:Ce single crystals are investigated for -ray detection. LuGd2Al2Ga3O12:Ce exhibits a high LY value of 35,400 ph/MeV with an E/E of 7.7% as measured at 662 keV -rays. Instead of a lower LY value, Lu2YAl2Ga3O12:Ce shows a faster scintillation decay time () of 45 ns (88%) and superior time resolution () of 490 ps. The timing performance of both Lu2YAl2Ga3O12:Ce and Lu2
CRediT authorship contribution statement
Warut Chewpraditkul: Photoluminescence and Spectrometry measurements, Manuscript preparation. Nakarin Pattanaboonmee: Conceptualization. Weeeapong Chewpraditkul: Supervisor. Tomasz Szczesniak: Timing measurements. Marek Moszynski: Conceptualization. Kei Kamada: Sample preparation. Akira Yoshikawa: Sample preparation. Romana Kucerkova: Photoluminescence decay measurements. Martin Nikl: Supervisor.
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 is supported by the King Mongkut’s University of Technology Thonburi, Thailand under Post-doctoral Research Project 2021 (Warut Chewpraditkul). Partial supports by the Institute for Materials Research (IMRT), Tohoku University Japan and by the Czech Ministry of Education, Youth and Sports under Project SOLID21 CZ.02.1. 01/0.0/0.0/16_019/0000760 are gratefully acknowledged.
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Optical and scintillation characteristics of Lu<inf>2</inf>Y(Al<inf>5-x</inf>Ga<inf>x</inf>)O<inf>12</inf>:Ce,Mg multicomponent garnet crystals
2022, Optical MaterialsCitation Excerpt :Polished plates of ∅3 mm × 1.35 mm cut from parent crystal rods were used for all measurements. Polished plates (5 × 5 × 1 mm3) of Lu2YAl2Ga3O12:Ce [48] and GAGG:Ce,Mg [40] crystals grown by CZ method at the C&A Corporation in Japan were used for comparison in some experiments. Absorption spectra in the UV–Visible region were recorded using a Shimadzu 3101 PC spectrophotometer.