Investigation on crystal growth and spectra property of NaBi(W04)2Jing Sun,Jianli Li,Jinghe Liu, Changli Li, Yanhong Li,Liang Zhang

College of Material and Chemical Engineering of Changchun University of Science and

Technology, Changchun China

ABSTRACT

In this work, the reason of flat interface crystal growth was analyzed and rational technological parameters were

designed and high quality sodium bismuth tungstate (molecular formula :NaBi(W04)2,abbreviated as NBW) single

crystal of 45mm in diameter and 35mm in length was grown by Czochralski method. X-ray diffraction ( XRD)

spectra showed that the NBW single crystal had cubic system and the space group was 141/a; Transmission spectra of

NBW indicated that its anti-radiation damaging capability was strong ; Its emission spectra analysis showed that the

emission peak was in the range of visible light—green light wave band

Key words: Scintillation crystal, NaBi(W04)2 single-crystal, Czochralski method

1. INTRODUCTION

In recent years, with the rapid development of nuclear physics, nuclear medicine and high energy physics, there were

more and more requirement for quality and performance of scintillation crystal. We used new rapid scintillation

crystal substitute old scintillation crystal, then it would enhance test and differentiate efficiency. Sodium bismuth

tungstate was a kind of scintillation crystal material with high performance, belongs to duality tungstate system. In

the end of 80s, its scintillation performance was found when study on NBW's powder31. It has many virtue461 such

as high density ( 7.57g/cm3 ) , shortdamping time ( 3ns) , high anti-radiation damaging capability ( 1.0 X lO7rad) , high

chemical stability. This kind of crystal could be used in advanced medical equipment, the technique of positron

emission photograph and mining, it had vast using prospect. In this work, we got large size NBW crystal by

Czochralski method, and researched its structure and spectra performance.

2. EXPERIMENTAL

2.1 Preparation of NBW polycrystalline materialWe use Bi203 with 99.99% purify, W03 with 99.99% purify and benchmark level Na2CO3 as raw material, batch ratio

was Bi703 :W03 :Na2CO3 =1 : 4: 1(mole ratio), through many times mixed equally,and adopted secondary meltingmaterial skill, cooled it in atmosphere, at last, the NBW polycrystalline material was obtained.

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U . IiiiFig.2 NBW X-ray diffraction analysis

2.2 NBW Crystal growthNBW Crystal Growth required single crystal stove, kryptol as heating element, use DWT-702 controlling temperature

instrument(with microcomputer control program) controlled critically temperature, the crucible was the platinumcrucible of 60mm in diameter and 30mm deep. In order to enhance the precision of controlling temperature,

thermocouple lied at the bottom of crucible, closing to the heat body.

NBW crystal was grown by Czochralski method. Temperature field designing: the differential temperature in 10mm

fluctuation of liquid surface was 0.8'CImm, the radial temperature gradient should be small, commonly it was not more

than 0.3tlmm. Pulling speed was 2-4mm/h, rotation speed was l0-15r/min, temperature dropping speed was 25'CIh,the size of crystal we got was 45mm in diameter and 35mm in length(refer to Fig. I).

3. RESULTS AND DISCUSSION

3.1 The factor of affecting flat interface growth of crystal

In the usual process of crystal growth, we found that some crystal were not grown as flat interface, possibly as

protruding interface, or concave interface. There are two reasons which affected the shape of crystal interface: one was

natural convection, the other was forced convection. The natural convection comprised heat convection and solute

convection. The driving force of heat convection was temperature gradient, and it was affected by these factors: the

shape of container, the relative orientation between the container and the heat current, the gravity field and the heat

current, melt and its boundary character. The state of melt could be described by nondimensional value when natural

convection existed. We abbreviated it as NRa its expression was as (1)1 11:

agl3 dTNR= (1)a ydz

In this expression, a was the melt's coefficient of thermal expansion, g was acceleration of gravity, 1 was the

Fig.l NBW crystal grown by Cz method

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container's geometry parameter, dT/dz was the melt's longitudinal temperature gradient, 7 was the melt's

kinematic viscosity coefficient, IC was the melt's thermal conductivity.

The natural convection became strong when NRa increased, and when the melt's NRa increased to a certain critical

value, the melt generated unstable convection, the unstable heat convection would arose the melt's temperature

surge,And it interfered crystal growth, brought growth stripe, then decreased the crystal's optical uniformity.The forcedconvection was created by the drive of crystal or the fluid's rotation around the crystal. It could be described by a state

function NRC which was nondimensional value. Its expression was as (2)

NRe 712 (2)

In this expression, U) was the crystal's rotation speed, d was the diameter of crystal, 7 was the melt's kinematic

viscosity coefficient.When NRe went beyong a certain critical value,crystal growth became unstable, the forced

convection became predominance, then arose the overturn of the interface. According to expression (2), in the process

of the crystal growth, the allowable maximum rotation speed was as the expression (3):

wc < 2 (NRe )c 7/'2 (3)

In this expression, (NRe )c was critical value of NRe Considering all influencing factors of crystal growth interface,

rational technological parameters had been designed in 1 .2section,the results showed that crystal interface was

flat,moreover, the crystal was large-size and high quality.

3.2 X-ray diffraction analysisWe got the X-ray diffraction figure of NBW crystal (refer to Fig.2) by D/max-YA type rotate target X-ray diffractioninstrument, scanned in the range of 1O°-70°. After analyzed the NBW diffraction curve, we got the result that NBW

crystal belonged to cubic crystal system and the space group was 141/a, according to formula of cubic crystal system, we

could get the crystal lattice parameters, and they were a=b=5.30A, c=ll.58A. They accorded with the value given by

the reference 4J

3.3 Transmission rate and the anti-radiation damaging capability analysis

The sample we used in the test was the cylinder of 10mm in diameter and 10mm in length (sample 1).. 10mm in

diameter and 20mm in length (sample 2), the result was shown in Fig.3, in this figure, the first curve showed the

transmission rate curve of the first sample, and the second shows the transmission rate curve of the second sample, the

third one showed the transmission rate curve of the first sample which was irradiated by the radioactive source with

lMrad dose Cj'°,and the fourth curve showed the transmission rate curve of the second sample which was irradiated by

the radioactive source with lMrad dose C060.From the figure we could see that all the samples' transmission rate

became flat after 480nm, even reach the maximun value. Its cut-off absorption peak was located about the 360nm, and

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we also found that the transmission rate of the sample which was irradiated by the radioactive source decreased no more

than 3%. It showed that the sample's anti-radiation damaging capability was strong.

Transmission before/after Co6° irradiation

Fig.3 Transmission spectra of NBW

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0.5 —

400 500 600 700 n.m

Fig.4 Emission spectrum of NBW

3.4 Emission Spectrum of NBW Crystal

The sample's emission spectrum was tested by spectrophotofluorometer(refer to Fig.4), we could see from the figure 4

that NBW crystal's emission apex was about 560nm, it was in the green light wave band of visible light, so it could be

matched perfectly with photomultiplier tube.

4. CONCLUSIONS

During analyzing the reason of flat interface crystal growth we designed a rational temperature field, and grow the good

quality crystal with 45mm in diameter and 35mm in length by Czochralski method. The X-ray diffraction analysis

showed that NBW crystal belonged to cubic crystal system and the space group was I41/a; The performance testing

result showed that NBW crystal's anti-radiation damaging capability was strong, and the emission apex was in the green

light wave band of visible light.

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contact author:Sun Jing

phone:+86-043 1-53 155 19(H)or+86-043 1-5384537(0)

E-mail:sunjing9401@sohu.comAddress: College of Material and Chemical Engineering of Changchun University

of Science and Technology, Wei Xing Road NO.7, Changchun, China

post code: 130022

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