Radiation hardness of Ce3þ-doped heavy germanate glasses

Shan Wang a,b, Guorong Chen a,b,*, Stefania Baccaro b, Angelica Cecilia b,Yongjuan Du a, Jiaxiang Nie a, Yonghui Zhang a

a Department of Inorganic Materials, East China University of Science and Technology, P.O. Box 306, 130 Meilong Road,

Shanghai 200237, Chinab ENEA-FIS Via Anguillarese 301, 00060 S. Maria di Galeria (Roma), Italy

Received 2 August 2002; received in revised form 6 November 2002

Abstract

In the present work Ce3þ-doped germanate glasses containing Gd2O3 and BaO or La2O3 were developed. The UV

and VIS transmission spectra of these glasses were measured before and after irradiation at doses ranging between 3

and 277 Gy. The radiation induced absorption coefficient l was calculated on the basis of the measured transmissionspectra. From these results the cerium ions doping turns out to be effective in improving the radiation hardness of

glasses with respect to their undoped matrices.

� 2002 Elsevier Science B.V. All rights reserved.

1. Introduction

Ce3þ-doped silicate and phosphate scintillating

glasses have been extensively studied due to their

lower cost, greater ease of production and possi-

bility of adjusting composition in comparison with

other scintillating crystals [1,2]. As it is well

known, for applications in high-energy physics,

the compactness of the scintillating glasses is es-sential in order to reduce the detector volume and

cost. This is achieved by using higher stopping

power materials resulting in a shorter radiation

length [3]. Thus glass matrices with higher densities

are needed. From this point of view, it would be

very interesting to develop a new type of scintil-lating glass on the basis of the heavier glass net-

work former GeO2 with respect to the lighter

formers SiO2 and P2O5. In the present work, Ce-

doped germanate glasses containing the heavy

metal oxides BaO, Gd2O3 and/or La2O3 are pre-

pared and their UV and VIS transmitting property

and irradiation resistance are discussed.

2. Experimental

Four germanate glasses used for the present

study are given in Table 1 where two Ce3þ-free

glass samples are used as reference matrices for

comparison with the Ce3þ-doped glasses. Glass

samples were prepared using reagent grade GeO2,Gd2O3, BaCO3 and/or La2O3 and Ce(NO)3 as

starting materials. They were carefully mixed in

*Corresponding author. Address: Department of Inorganic

Materials, East China University of Science and Technology,

P.O. Box 306, 130 Meilong Road, Shanghai 200237, China.

E-mail address: grchen@online.sh.cn (G. Chen).

0168-583X/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0168-583X(02)01432-5

Nuclear Instruments and Methods in Physics Research B 201 (2003) 475–479

www.elsevier.com/locate/nimb

appropriate proportions and melted in the fused

silica crucibles in an electric furnace at tempera-

tures between 1350 and 1450 �C in an atmosphereof N2 plus graphite in the furnace. The aim of

adding graphite was to consume the remaining O2in the atmosphere producing CO, so as to set up a

stronger reducing atmosphere preventing the for-

mation of Ce4þ in the glass. The glasses prepared

in such an atmosphere are colourless, opposite to

those melted in air or in a pure N2 atmosphere.

They showed yellow or light yellow colours cor-

responding to the existence of Ce4þ ions in the

glasses [4].Irradiation tests were performed at the ‘‘Calli-

ope’’ 60Co irradiation plant (ENEA-Casaccia,

Rome) on all glass samples at absorbed doses (in

air) ranging from 3 to 277 Gy and at a dose rate of

3.1 Gy/h. Before and after each irradiation test,

glasses were submitted to transmission measure-

ments performed by a double ray spectropho-

tometer equipped with an integrating sphere.Before each irradiation test, the samples were

thermally bleached at 600 �C for 3 h in order torestore their initial conditions.

3. Results

Transmission spectra of glasses before irradia-tion treatments are shown in Fig. 1. As can be

seen, the UV absorption edges for glass matrices

(#1 and #3) are located at wavelengths around 340

nm. By comparison, the UV absorption edges of

Ce3þ-doped glasses (#2 and #4) are found to suffer

the shifts towards longer wavelengths.

Transmission spectra of glasses (#1 and #2)

before and after irradiation treatments at differentdoses are compared in Figs. 2 and 3, respectively.

The c irradiation induced a decrease in transmis-sion of glass matrix (#1) that became more pro-

nounced as the dose increased (Fig. 2). On theother hand, the irradiation treatments are found to

0

0.2

0.4

0.6

0.8

1

300 400 500 600 700 800

# 1# 2# 3# 4

Tran

smitt

ance

Wavelength (nm)

Fig. 1. Transmission spectra of glass samples before irradia-

tion.

0

0.2

0.4

0.6

0.8

1

300 400 500 600 700 800

Tran

smitt

ance

Wavelength (nm)

before irradiation20 Gy205 Gy

Fig. 2. Transmission spectra of glass #1 before and after irra-

diation (0, 20, 205 Gy).

Table 1

Compositions of glasses

Name GeO2 (mol%) Gd2O3 (mol%) BaO (mol%) La2O3 (mol%) Ce2O3 (wt%)

#1 60 5 35 – –

#2 60 5 35 – 1.0

#3 60 5 30 5 –

#4 60 5 30 5 1.0

476 S. Wang et al. / Nucl. Instr. and Meth. in Phys. Res. B 201 (2003) 475–479

have less effect on the transmission of Ce3þ-doped

germanate glass (#2) at all the imparted doses

(Fig. 3), inferring much improvement on the irra-

diation hardness of these glasses by Ce3þ-doping.

In order to discuss more thoroughly the effect of

Ce3þ-doping on the radiation hardness of the

analysed samples, the parameter called radiation

induced absorption coefficient (RIAC) l, is intro-duced. It is defined by the following equation:

lðkÞ ¼ ð1=dÞ ln½T0ðkÞ=TirrðkÞ�;where T0 and Tirr stand for the transmittance mea-sured before and after irradiation treatments and dis the length of the light path through the sample

measured. Since the equation includes the length

of the light path through the sample, it is more

precise than transmittance spectra in terms of eval-

uating the effect of irradiation treatment on sam-ples with different lengths. Results of calculations

are illustrated in Figs. 4 and 5 where the RIAC lof the Ce3þ-doped samples suffering irradiation

treatments at the highest doses are shown and

compared with their matrices irradiated by the same

doses. These figures confirm definitely the positive

effect of Ce3þ ions on the irradiation resistance of

germanate glasses by suppressing the strong irra-diation induced absorption in the region above the

cut-off till 850 nm.

4. Discussion

It is known that for the most conventional

oxide glasses containing only lighter cations, the

0

0.2

0.4

0.6

0.8

1

400 500 600 700 800

before irradiation20 Gy205 GyTr

ansm

ittan

ce

Wavelength (nm)

Fig. 3. Transmission spectra of glass #2 before and after irra-

diation (0, 20, 205 Gy).

Fig. 4. Comparison of RIAC l between glasses #1 and #2 after205 Gy radiation.

Fig. 5. Comparison of RIAC l between glasses #3 and #4 after277 Gy radiation.

S. Wang et al. / Nucl. Instr. and Meth. in Phys. Res. B 201 (2003) 475–479 477

conduction band lies at a higher level and the ul-

traviolet absorption edge will be located at a

shorter wavelength [5]. However for the glasses

containing the heavier cations, the energy gap forelectron transfer is lower, resulting in a shift of the

absorption edges toward longer wavelengths. The

exact position of the absorption edge is determined

by the dissolved ions in glasses with the longest

absorption band as well as by the basic structure

of these glasses. So far as the glasses studied in the

present paper are concerned, the absorption edges

of glass matrices are set by the Ge4þ absorbingband, while for the Ce3þ-doped glasses the UV cut-

off positions are controlled by the Ce3þ absorption

band which corresponds to 4f–5d electronic tran-

sitions and is sensitive mainly to the glass matrix

composition [6]. In the present germanate glass

matrices Ce3þ absorption band experienced a red

shift as compared with our previous work on the

Ce3þ-doped phosphate and silicate scintillatingglasses [1]. As we discussed in our previous paper,

such a red shift could be assigned to the higher

optical basicity of the germanate based glasses

than that of silicate and phosphate glasses [7] while

the so-called optical basicity is the expression

showing the electron donor power of the oxides in

the glass [8].

It is well established that when glass is irradi-ated, the most significant radiation damage results

from the formation of colour centers in the bulk

due to the presence of defects induced by irradia-

tion and trapped in the glass network. These col-

our centers may include excited electron centers

(EC) located in anion vacancies and hole centers

(HC) captured by anions near cation vacancies.

The reduction of transmittance of glasses, as weobserved in Figs. 2 and 3, may therefore be related

to the formation of such irradiation induced col-

our centers in the glass bulk and the higher the

irradiation dose, the higher the density of colour

centers. From the spectral point of view EC ab-

sorbs in the ultraviolet region, while HC absorbs

in the visible region [9].

By comparison, the radiation damage on Ce3þ-doped glasses is much smaller, as further demon-

strated in Figs. 4 and 5 in terms of RIAC l afterradiation treatments at doses of 205 and 277 Gy,

respectively. An explanation for such an effect in-

duced by Ce3þ-doping has to be searched in the

electronic structure of the Ce3þ ion (4f1): it has

only one electron above the 4f shell and conse-

quently tends to lose it to take the more stableempty state. This fact determines the possible co-

existence of cerium ions with the two valencies:

Ce3þ and Ce4þ, in the glass network, depending on

conditions [10]. The possible mechanism involved

in the irradiation process includes the capture of

excited holes by Ce3þ ions and afterward the ab-

sorption of the ionised electrons by Ce4þ ions in

glasses, as described by the following equations:

Ce3þ þHC! Ce4þ Ce4þ þ EC! Ce3þ

The consequence of this process is the inhibition or

decreased density of the radiation induced colour

centers in glasses.

5. Conclusions

The red shift of UV cut-off edges is observed forthe present Ce3þ-doped heavy germanate glasses

by comparison with the conventional Ce3þ-doped

silicate and phosphate glasses because of the higher

optical basicity of the glass network former GeO2.

Ce3þ-doping plays the positive role in improving

the radiation hardness of the germanate glasses

with respect to glass matrices. The mechanism in-

volves the capture of the excited holes by Ce3þ ionsand afterward the absorption of the ionised elec-

trons by Ce4þ ions in the glasses due to the special

electronic structure of Ce3þ ions.

Acknowledgement

This work is supported by National ScienceFoundation of China (No. 50242017).

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