Co-doping effects on luminescence and scintillation propertiesof Ce doped Lu3Al5O12 scintillator
Kei Kamada a,b,n, Martin Nikl c, Shunsuke Kurosawa a,d, Alena Beitlerova c, Aya Nagura d,Yasuhiro Shoji b,d, Jan Pejchal a,c, Yuji Ohashi d, Yuui Yokota a, Akira Yoshikawa a,b,d
a Tohoku University, New Industry Creation Hatchery Center, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Miyagi, Japanb C&A Corporation, T-Biz, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Miyagi, Japanc Institute of Physics AS CR, Cukrovarnicka 10, 16253 Prague, Czech Republicd Tohoku University Institute for Material Reseach, 2-1-1 Katahira Aoba-ku, Sendai 980-8577, Miyagi, Japan
a r t i c l e i n f o
Article history:Received 29 September 2014Received in revised form23 January 2015Accepted 29 January 2015Available online 9 February 2015
a b s t r a c t
The Mg, Ca, Sr and Ba 200 ppm co-doped Ce:Lu3Al5O12 single crystals were prepared by micro pullingdown method. Absorption and luminescence spectra were measured together with several otherscintillation characteristics, namely the scintillation decay and light yield to reveal the effect of theco-doping. The scintillation decays were accelerated by both Mg and Ca co-dopants. The Mg co-dopedsamples showed the fastest decay and the highest light yield among the co-doped samples.
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Scintillator materials combined with photo detectors are used todetect high energy photons and particles e.g., in X-ray computedtomography (CT), positron emission tomography (PET) and othermedical imaging techniques, high energy and nuclear physics detec-tors, etc. . In the last two decades, great R&D effort brought severalnew material systems, namely the Ce-doped orthosilicates as Gd2SiO5(GSO), Lu2SiO5 (LSO), (Lu1xYx)2SiO5 (LYSO), pyrosilicates based onRE2Si2O7 (RELu, Y,Gd) and most recently LaX3 (XCl,Br) singlecrystal hosts. Oxide materials based on garnet structure singlecrystals are promising candidates for scintillator applications becauseof well mastered technology developed for laser hosts and otherapplications, optical transparency and easy doping by rare-earthelements. The Ce-doped Lu3Al5O12 (Ce:LuAG) single crystal was shownto be a prospective scintillator material with a relatively high densityof 6.7 g/cm3, a fast scintillation response of about 6080 ns (due to the5d4f radiative transition of Ce3 providing the emission around500550 nm), and light yield of about 1214,000 phot/MeV .In the silicate, perovskite and garnet scintillators the slow tunneling-driven radiative recombination between Ce emission centers and
nearby lying electron traps can deteriorate scintillation performance[16,17]. Ce:LSO and Ce:LYSO single crystals co-doped with Ca2 havebeen recently investigated and improvement in their scintillationcharacteristics, namely afterglow suppression and scintillation decayacceleration, were claimed  which is based on the suppression ofsuch slow delayed recombination processes. Positive role of stableCe4 centres has been proposed in  to explain the improvedscintillation performance of Mg-codoped LuAG:Ce optical ceramics. Itslight yield was enormously enhanced and the presence of Ce4 wasclearly identied by its characteristic charge transfer (CT) absorption inthe near UV range below 350 nm .
The aim of this work is to investigate and compare the divalentalkali earth ions (AE2Mg2 , Ca2 , Sr2 and Ba2) co-dopingeffects on luminescence and scintillation properties of Ce:LuAG singlecrystal scintillator. In this report, AE co-doped Ce:LuAG single crystalswere grown by the micro-pulling down (-PD) method and char-acterized as for the chemical composition. Luminescence and scintilla-tion characteristics were also measured.
2.1. Micro-pulling down prepared crystals and their structure byX-ray difractometer
A stoichiometric mixture of 4 N MgCO3, CaCO3, SrCO3, BaCO3,CeO2, -Al2O3 and Gd2O3, powders (High Purity Chemicals Co.)
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Nuclear Instruments and Methods inPhysics Research A
http://dx.doi.org/10.1016/j.nima.2015.01.1050168-9002/& 2015 Elsevier B.V. All rights reserved.
n Corresponding author at: Tohoku University, New Industry Creation HatcheryCenter, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Miyagi, Japan.Tel.: 81 22 215 2214; fax:81 22 215 2215.
E-mail address: email@example.com (K. Kamada).
Nuclear Instruments and Methods in Physics Research A 782 (2015) 912
was used as starting material. Nominally, starting powders wereprepared according to the formula of (AEx, Cey, Gd1y)3Al5O12(CO3)x. Single crystals of AE (Mg, Ca, Sr and Ba) co-doped and1 atomic% Ce doped LuAG were grown by the -PD method withan RF heating system . The parameter values ywas 0.01 and xswere 0.0002. These samples will furthermore be noted as Mg-200,Ca-200, Sr-200 and Ba-200. A schematic of the -PD growthapparatus is given in Ref. . Typical pulling rates were0.050.07 mm/min and the diameter was around 4 mm. Crystalswere grown from an Ir crucible under the N2 atmosphere. Theseed crystals were 1 1 1 oriented Ce:LuAG crystals. Plates of4 mm1 mm were cut and polished for the absorption andluminescence spectra measurements. Pieces of the grown crystalswere crushed and ground into powders in a mortar. Powder X-raydiffraction analysis was carried out in the 2 range 15751 usingthe RINT Ultima (RIGAKU) diffractometer. The CuK X-ray sourcewas used, the accelerating voltage and current were 40 kV and40 mA, respectively.
2.2. Optical, luminescence and gamma-ray response measurementprocedure
Absorption spectra were measured by the Shimadzu 3101PCspectrometer in the 190800 nm range. At the SR-163 spectro-meter (ANDOR TECHNOLOGY) equipped with the CCD detectorDU920P (ANDOR TECHNOLOGY) the radioluminescence (RL) spec-tra were measured under an X-ray (40 kV, 30 mA) tube. Lightoutput measurements were performed by using photomultipliertube (PMT Hamamatsu R7600U). Sample pieces with dimensionsof 41 mm were cut from the grown single crystal, 4 mmsurfaces were mechanically polished. To determine the light yield,the energy spectra were collected under 662 keV -ray excitation(137Cs source) by using an avalanche photodiode (APD, Hama-matsu, S8664-55). The sample was coupled with the APD by usingsilicone grease (OKEN, 6262A).The sample was covered by usingTeon-tape. The signal was fed into a pre-amplier (CP580K),shaping amplier (CP 4417), pocket multichannel analyzer (pocketMCA, Amptec 8000A), and nally to a personal computer. The biasfor the APD was supplied by a power supplier (CP 6641). Becausethe APD was thermally sensitive, we controlled the ambienttemperature at 2070.5 1C by using a heat bath. Decay time wasmeasured by using a photomultiplier (PMT, Hamamatsu, R7600U)with digital oscilloscope TDS3052 of Tektronix, Inc.
3. Result and discussion
3.1. Crystal growth
Non co-doped and Mg, Ca, Sr, Ba co-doped 1%Ce:LuAG. crystalswere grown by the -PD method. Example photos are shown inFig. 1. The grown crystals were transparent with yellow color and23 mm in diameter, 1530 mm in length. Some of them lookslightly cloudy because of the rough surface coming from thermaletching. However, the inner part of all the crystals is perfectlytransparent.
Powder X-ray diffraction was performed to identify crystalphase of grown crystals. Example results of the powder X-raydiffraction of the grown non co-doped and Mg, Ca, Sr, Ba co-doped1%Ce:LuAG crystals are shown in Fig. 2. All of the grown crystalsshow the single cubic garnet phase.
3.2. Optical and luminescence properties
The absorption spectra of the sample set are presented in Fig. 3.In addition of the 4f5d1,2 absorption bands of Ce3 center at 450
and 340 nm, respectively, the smooth Ce4 CT absorption below350 nm is clearly enhanced by Mg and Ca co-dopant, similarly towhat was observed in [19,20]. Radioluminescence (RL) spectra inFig. 4 show an enhancement of the Ce3 emission band at 520 nmby Mg and Ca co-doping. Intensity of Ce3 emission was slightlydegraded by Sr co-doping. On the other hand, Ba co-dopingdegraded Ce3 emission. Anti-side defects (Lu(Al) or Al(Lu)) relatedemission band at 310 nm were also observed. 310 nm emissionwas decreased by co-doping and disappeared by Mg co-doping.Scintillation performance of Ce-doped garnet is strongly degradedby shallow electron traps which delay energy delivery to the Ce3
emission centers and give rise to intense slow components in thescintillation response [22,23]. These traps were ascribed to theantisite defects in garnet structure  which are typical defects inthe melt-grown garnet crystals of this kind [25,26].
Fig. 1. Appearance of crystals prepared by the -PD method. (For interpretation ofthe references to color in this gure legend, the reader is referred to the webversion of this article.)
Fig. 2. Results of the powder X-ray diffraction of the AE co-doped 1%Ce:LuAGcrystals.
K. Kamada et al. / Nuclear Instruments and Methods in Physics Research A 782 (2015) 91210