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

Keywords:GarnetSingle crystalScintillatorCerium

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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1. Introduction

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. [1]. 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[111]. 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 [1215].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 [18] which is based on the suppression ofsuch slow delayed recombination processes. Positive role of stableCe4 centres has been proposed in [19] 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 [20].

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. Experimental

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: kamada@imr.tohoku.ac.jp (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 [21]. 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. [22]. 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 [24] 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

3.3. Scintillation properties

The Pulse height spectra of AE co-doped Ce:LuAG excited by662 keV gamma-ray of 137Cs at room temperature and measuredusing the APD are shown in Fig. 5. Mg and Ca codoped samplesshowed higher light yield value compared to that of the nonco-doped one. The light yield of the sample was calibrated fromthe 55Fe direct irradiation peak to APD. Such direct irradiationgenerates 5.9 keV/3.6 eV1640 electronhole pairs. Based on this

value, light yield of Mg co-doped sample was calculated to beapproximately 17,100photon/MeV without correcting quantumefciency (QE) of the APD. After correcting the QE, which was80% at 520 nm, the total LY becomes21,300 photon/MeV, whichis around 190% of light yield of the non co-doped sample andaround 123% of light yield of a reference Ce:LuAG scintillatorproduced by Czochralski (Cz) method. The light yield was calcu-lated by an equation below,

LY(Peal channel of a sample)/(Peak channel of 55Fe directirradiation)1640/0.8/0.662.

Scintillation decay curves were observed by using digitaloscilloscope TD5032B under the excitation by 137Cs radioisotope(662 keV). Scintillation decay curves of AE co-doped Ce:LuAGcrystals were shown in Fig. 6. Under the same experimentalconditions, the scintillation decay amplitude of Mg and Caco-doped sample was 1.8 and 1.6 times higher than that of non-codoped sample, respectively. The scintillation decay curves wereaccelerated by Mg and Ca co-doping and decay time valuedecreased down to 44.4 ns and 44.9 ns, respectively. Table 1 showsthe survey of light yield and scintillation decay time values. Mgco-doped samples showed the highest light yield and the fastestdecay among all the grown AE co-doped samples.

The obtained results can be interpreted in an analogous way asit has been done in the Ce-doped orthosilicate LYSO [18] andaluminum garnet LuAG [19]. Stabilization of the Ce4 center byMg2 and Ca2 co-doping provides an alternative fast radiativede-excitation channel: after the capture of an electron fromconduction band at such a Ce4 center the excited Ce3 ion isformed and scintillation photon in the 520 nm band is emittedimmediately. The Ce4 center thus effectively competes withelectron traps of any kind in the material for the electron captureand contributes to the fastest component of scintillation response.

200 250 300 350 400 450 500 550 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4In

tens

ity (a

rb. u

nit)

wavelength [nm]

Non co-dopedMg-200Ca-200Sr-200Ba-200Increase of Ce

4+ absorption

Fig. 3. Absorption spectra of the AE co-doped Ce:LuAG crystals.

200 250 300 350 400 450 500 550 600 650 7000.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

Inte

nsity

(arb

.uni

t)

wavelength [nm]

non co-dopedMg-200Ca-200Sr-200Ba-200

Fig. 4. Radioluminescence spectra of the AE co-doped Ce:LuAG crystals. Excitationby X-rays, 40 kV, 30 mA, CuK.

0 500 1000 1500 2000 2500 30001

10

100

1000

Cou

nts

MCA Channel

non co-dopedMg-200Ca-200Fe55

Fig. 5. Pulse height spectra of the AE co-doped Ce:LuAG crystals. (excitation662 keV of 137Cs radioisotope).

0 500 1000 150010-3

10-2

10-1

Time / ns

Inte

nsity

/ ar

b. u

nit

noncodopeMg-200Ca-200

Fig. 6. Scintillation decay curves of the AE co-doped Ce:LuAG crystals. (excitation662 keV of 137Cs radioisotope).

Table 1Light yield and scintillation decay time values of the AE co-doped Ce:LuAG crystals.Scintillation decay is approximated by the sum of two exponentials I(t)Aiexp[t/i], i1,2. Ratio (%) is calculated as Aii/(A11A22)100% for i1,2.

Samples Light yield[photon/MeV]

First component decaytime (ns)/intensity (%)

Second componentdecay time (ns)/intensity (%)

Ce:LuAG-PD 11,000 49.5/44 232/56Mg-200 21,300 44.4/56 337/44Ca-200 14,000 44.9/47 315/53Sr-200 9,800 48.7/55 212/45Ba-200 7,700 49.3/49 200/51Ce:LuAG-Cz 17,200 58/42 958/58

K. Kamada et al. / Nuclear Instruments and Methods in Physics Research A 782 (2015) 912 11

4. Conclusion

The 200 ppm Mg, Ca, Sr and Ba co-doped 1%Ce:LuAG singlecrystals were grown by the -PD method and their optical,luminescence and scintillation characteristics were measured.The intensity enhancement of the Ce3 emission band at520 nm in radioluminescence spectra was observed at the Mgand Ca co-doped samples. The scintillation decays were acceler-ated by Mg and Ca co-doping and dominant decay time decreaseddown to 44.4 ns and 44.9 ns, respectively. The Mg co-dopedsamples showed the best light yield of 21,300 photon/MeV. Theseresults indicate that Mg co-doped Ce:LuAG can be promisingscintillator for application which require fast timing resolutionsuch as positron emission tomography. In the near future we willreport about the bulk crystal growth of Mg co-doped Ce:LuAG bythe Cz method. Improvement on scintillation response is expecteddue to much higher quality of Czochralski-grown crystal samples.

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K. Kamada et al. / Nuclear Instruments and Methods in Physics Research A 782 (2015) 91212

Co-doping effects on luminescence and scintillation properties of Ce doped Lu3Al5O12 scintillatorIntroductionExperimentalMicro-pulling down prepared crystals and their structure by X-ray difractometerOptical, luminescence and gamma-ray response measurement procedure

Result and discussionCrystal growthOptical and luminescence propertiesScintillation properties

ConclusionReferences