Optical and scintillation properties of LuGd₂Al₂Ga₃O₁₂:Ce, Lu₂GdAl₂Ga₃O₁₂:Ce, and Lu₂YAl₂Ga₃O₁₂:Ce single crystals: A comparative study

Highlights

• Scintillation properties of (Lu,Y,Gd)₃Al₂Ga₃O₁₂:Ce were compared.

• LuGd₂Al₂Ga₃O_12:Ce showed high LY and good energy resolution.

• Lu₂GdAl₂Ga₃O_12:Ce showed lowest proportionality of LY and $\alpha ∕\gamma$ ratio.

• Lu₂YAl₂Ga₃O_12:Ce exhibited superior coincidence time resolution.

Abstract

Optical and scintillation properties of LuGd₂Al₂Ga₃O₁₂:Ce, Lu₂GdAl₂Ga₃O₁₂:Ce, and Lu₂YAl₂Ga₃O₁₂:Ce single crystals are compared. The blue-shift of 5d₁ $\to$ 4f photoluminescence emission (PL) band is observed upon partial substitution of Gd${}^{3+}$ with a smaller Lu${}^{3+}$ (or Y${}^{3+}$) ion. At 662 keV $\gamma$-rays, the light yield of 35,400 ± 1,700 ph/MeV obtained for LuGd₂Al₂Ga₃O₁₂:Ce is higher than that of 29,200 ± 1,500 ph/MeV and 20,200 ± 1,000 ph/MeV for Lu₂GdAl₂Ga₃O₁₂:Ce and Lu₂YAl₂Ga₃O₁₂:Ce, respectively. Scintillation decays are approximated by sum of exponentials with the fast component decay time and relative intensity of 78 ns (42%) for LuGd₂Al₂Ga₃O₁₂:Ce, 48 ns (71%) for Lu₂GdAl₂Ga₃O₁₂:Ce and 45 ns (88%) for Lu₂YAl₂Ga₃O₁₂:Ce, which leads to a superior coincidence time resolution of Lu₂YAl₂Ga₃O₁₂:Ce. PL decay time at 110 K and 290 K, afterglow characteristics, and mass attenuation coefficient at 59.5 keV and 662 keV $\gamma$ - rays are also presented.

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${}_{\mathrm{Al}}$ ) 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${}^{3+}$ 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 Ca^2＋ or Mg${}^{2+}$ 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${}^{4+}$ which provides additional fast radiative recombination pathway in scintillation mechanism [41]. The Ce${}^{4+}$ 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${}^{3+}$ 5d₁ level could be high-energy shifted by partial substitution of Gd${}^{3+}$ with a smaller Lu${}^{3+}$ (or Y${}^{3+}$) 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 ($\tau \sim {\lambda }^3$). Partial substitution of Gd${}^{3+}$ with a smaller Lu${}^{3+}$ (or Y${}^{3+}$) ion could also affect energy migration through the Gd${}^{3+}$ sublattice and an energy transfer to the Ce${}^{3+}$ luminescence centers [50]. The measurements of absorption, photoluminescence emission (PL) and excitation (PLE) spectra, PL decay time, LY values under excitation with $\alpha$- and $\gamma$- rays, scintillation decay time, coincidence time resolution, and afterglow are presented. The mass attenuation coefficient at 59.5 keV and 662 keV $\gamma$ rays is also determined.