Scintillation from Eu21 in Nanocrystallized Glass

Jie Fuw

R&D Department, Ohara Inc., Sagamihara-shi, Kanagawa 229-1186, Japan

Masaaki Kobayashi and Shojiro Sugimoto

High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan

John M. Parker

Department of Engineering Materials, The University of Sheffield, Sheffield S1 3JD, UK

A Eu21-doped SiO2–Al2O3–CaO–CaF2 glass was prepared

and converted into a transparent glass ceramic by heat treat-ment. The crystalline phase and its size were determined byX-ray diffraction and a transmission electron microscopy.The scintillation of Eu

21ions in both glass and ceramic under

X-ray excitation was investigated and compared with that in asingle-crystal scintillator.

I. Introduction

RARE earth ion-activated scintillators have been widely ex-plored for application in high-energy physics and in X-CT

for industrial and medical imaging.1,2 In comparison with singlecrystals and ceramic scintillators, glassy scintillators have advan-tages in low-cost production and ease of manufacture in differentsizes and shapes, such as fibers,3 but they also have a disadvan-tage in the light yield, which usually is low and hence limits theirapplications. Most of the rare earth ions are known to fluoresceefficiently in various glass hosts under UV excitation, but onlyCe31, Tb31, and Eu31 were reported to scintillate,4 and the effi-ciency of the scintillation in many cases is o20% of the lightyield of a single crystal.5 Transparent glass ceramics have almostthe same advantages as glasses, and previous research has shownthat fluorescence of the dopant rare earth ions under UV exci-tation in glass ceramics could be enhanced considerably if a crys-tal phase that can act as a sink for the dopant is precipitated.6–8

To the present authors’ knowledge, however, there are no reportson whether rare earth ions scintillate in a transparent glassceramic.

In this work, we prepared a Eu21-doped transparent glassceramic. Scintillation properties has been investigated and com-pared with the corresponding glass and a CaF2:Eu

21 single-crystal.

II. Experimental Procedure

A glass with the molar composition 45SiO2–20Al2O3–10CaO–25CaF2–0.1Eu2O3 was prepared. Reagent grade SiO2, Al(OH)3,CaCO3, CaF2, and Eu2O3 were used as starting materials. Thebatch was placed in a vitreous quartz crucible and melted at14001C for 2 h. To obtain the europium ions in the divalentstate, a 97%N2/3%H2 atmosphere was applied during the glass

melting process. It has been confirmed from the spectral mea-surements that no emission from Eu31 ions was detectable, in-dicating that europium is present mainly as a divalent state inthe glass. Composition changes, due to the dissolution of thequartz crucible and the volatilization of fluorine, could takeplace during the melting but have not been taken into account inthe present work. The melt was cast onto a preheated, stainlesssteel plate, and subsequently annealed and cooled to room tem-perature. The glass was cut and polished to a thickness of 2 mmand then converted to glass ceramic by heat-treatment.

Luminescence under UV excitation from a Xe lamp as theexcitation source was recorded by a JASCO LP-750 spectro-meter (JASCO Corporation, Hachioji, Japan). The radiolumi-nescence (RL) spectra were measured using an X-ray generator(50 kV, 30 mA, Cu target) and a spectrometer (Photonic Mul-tichannel Analyzer PMA-11, Hamamatsu, Japan). The PMA-11consisted of a grating monochromator, an image intensifier, anda linear CCD array, calibrated in the wavelength range of 200–860 nm. The RL was taken via a silica optical fiber from theinjected surface of the samples at 901 to the X-ray direction sothat the sample thickness (transmission) should not be impor-tant. The RL was integrated offline between 250 and 750 nm forsamples and 200–700 nm for CaF2:Eu

21 single-crystal so that allthe emission peaks were covered. The relative light yield of theglass samples was estimated by comparing the integrated RLintensity of the glasses with that of a CaF2:Eu

21 single-crystal.The single-crystal scintillator CaF2:Eu

21 used in this study wasproduced by OKEN Co. Ltd., (Tokyo, Japan) All the abovemeasurements were made at room temperature.

III. Results

Glass ceramic was fabricated by heat treating the glass at 7601Cfor 4h. The glass ceramic maintained a high transparency in thevisible region. Bulk X-ray diffraction patterns for the glass andglass ceramic are shown in Fig. 1. For the glass, only an amor-phous halo pattern is observed, but for the glass ceramic, threeextra sharp diffraction peaks appeared. All the peaks are attrib-utable to CaF2 crystals, indicating that CaF2 is the only crys-talline phase present in the glass ceramic. Transmission electronmicroscope observation, as shown in Fig. 2, revealed the crys-tallite size of CaF2 was about 35 nm. Luminescence spectra un-der UV excitation for the glass and glass ceramic are shown inFig. 3, where the spectrum of a CaF2:Eu

21 single-crystal is alsoincluded for comparison. The observed emission peaks are at-tributed to the 5d–4f transitions of Eu21. For the glass, twopeaks at 420 and 460 nm are clear, but for the glass ceramic, theformer peak has become more dominant. These results indicatethat the Eu21 ions are located in two sites in the glass but mainly

M. Ferrari—contributing editor

wAuthor to whom correspondence should be addressed. e-mail: fu@ohara-inc.co.jpManuscript No. 25841. Received March 1, 2009; approved Mach 31, 2009.

Journal

J. Am. Ceram. Soc., 92 [9] 2119–2121 (2009)

DOI: 10.1111/j.1551-2916.2009.03143.x

r 2009 The American Ceramic Society

2119

in just one site in the glass ceramic. It is clear that the emission ofEu21 in the glass ceramic is similar to that in the single-crystalCaF2:Eu

21, suggesting that Eu21 ions in the glass ceramic arepresent predominantly in the CaF2 crystalline phase. RL spectrafor the glass, glass ceramic, and CaF2:Eu

21 single-crystal areshown in Fig. 4. The glass showed little scintillation, but theglass ceramic scintillated strongly. The emission peak of the ce-ramic is consistent with that of the CaF2:Eu

21 single-crystal.The light yield estimated from the emission peaks relative to theCaF2:Eu

21 single-crystal is about 30%. Excitation spectra ofEu21 ions are shown in Fig. 5. The excitation bands betweenglass and glass ceramic are very different; there is a dominateband at 370 nm in the glass ceramic, and this band has almostthe same position as that of CaF2:Eu

21 single-crystal. These re-sults strongly suggest a change in the environment around Eu21

ions migrating from the amorphous phase into the CaF2 crys-talline phase. Therefore, we believe that the largely enhancedscintillation in the glass ceramic must result from the Eu21 ionspresent in the CaF2 crystalline phase.

In conclusion, Eu21-doped glass ceramic containing nano-sized CaF2 crystals has been fabricated. Eu21 ions were prob-ably present mainly in the CaF2 crystalline phase, and for this

10 20 30 40 50 602 � (Degree)

Inte

nsity

(a.

u.)

(a) : glass

(b): glass ceramic

(a)

(b)

Fig. 1. Bulk X-ray diffraction patterns of the glass and glass ceramic.

100 nm

Fig. 2. Transmission electron microscope image of the glass ceramic.

300 350 400 450 500 550 600Wavelength (nm)

Inte

nsity

(a.

u.)

glassglass ceramicCaF2:Eu single crystal

Fig. 3. Luminescence spectra under UV excitation for the glass, glassceramic, and CaF2:Eu

21 single-crystal.

300 350 400 450 500 550 600Wavelength (nm)

Inte

nsity

(a.

u.)

glassglass ceramic(c): CaF2:Eu single crystal

Fig. 4. Radioluminescence spectra under X-ray excitation for the glass,glass ceramic, and CaF2:Eu

21 single-crystal.

200 250 300 350 400 450Wavelength (nm)

Inte

nsity

(a.

u.)

glassglass ceramicCaF2:Eu single crystal

Fig. 5. Excitation spectra for the glass, glass ceramic, and CaF2:Eu21

single-crystal.

2120 Communications of the American Ceramic Society Vol. 92, No. 9

reason a large enhancement of scintillation has been observed inthe glass ceramic. The light yield obtained in the glass ceramic isrelatively high, up to 30% of that of CaF2:Eu

21 single-crystal.To our knowledge, this is the first observation that Eu21 ionsscintillated clearly in transparent glass ceramic materials. Futurework will focus on improving the light yield by optimizing thesize and volume fraction of the crystallites, and the concentra-tions of the Eu21 ions.

References

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