Growth and Crystal Luminescence Properties of Cerium Doped Yttrium-Gadolinium Pyrosilicate

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(1.School of Material Science and Engineering, Shanghai University, Shanghai 200444, China; 2.Scool of Physical, Nankai University, Tianjin 300071, China)

Abstract:Novel cerium doped (Gd0.99-xYxCe0.01)2Si2O7 (x=0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7) (GYPS∶Ce) scintillation crystals were grown by optical floating zone method, and phase identifications were carried out by X-ray diffraction (XRD). It belongs to orthorhombic of Pna21. Their photoluminescence (PL) and scintillation properties of as-grown crystals were characterized through vacuum ultra-violet (VUV) excitation and emission spectra, PL decay curves, X-ray excitation luminescence (XEL) spectra, γ-ray pulse height spectra. The emission peaks of VUV and XEL spectra all locate around 360 nm corresponding to 5d→4f transition. The PL decay time of as grown samples are in the range of 29.1 ns to 32.9 ns. The scintillation decay time components are in the range of 28 ns to 68 ns and 256 ns to 583 ns. There has energy transfer behavior of Gd3+→Ce3+ in crystal.

Key words:GYPS∶Ce single crystal; floating zone method; optical property; scintillation crystal; decay time; photoluminescence

0 Introduction

Scintillators are ideal materials for detecting high energy particle such as X-ray, γ-ray[1]. Ce-doped gadolinium pyrosilicate (GPS∶Ce) has drawn extensive concern as a kind of novel inorganic scintillation crystal: compared with the scintillation crystal Ce-doped Gd2SiO5(GSO∶Ce), it shows a much higher light yield, even shorter decay time, and still maintains high luminescence efficiency even at high temperatures[2-7]. Thus it owns good application prospect in the field of medical positron emission tomography, geophysical explorations and well logging applications.

Unfortunately, Gd2Si2O7is a material melting incongruently[8], which caused a lot of difficulties in the preparation of large-size single crystal. Doping with high concentration of Ce will make the peritectic point in Gd2O3-SiO2phase diagram close to Gd2Si2O7composition ratio[9], which can improve the non-uniform melting characteristics of GPS. However, the high concentration of Ce-doping can lead to lower light output because of self-absorption and concentration quenching, resulting in the performance degradation of GPS∶Ce crystal[5].

The La is chosen as the substitution ion for Ce to improve the crystallization behavior without concentration quenching problem[10]and the study of GPS related mixed single crystal scintillator is mainly carried out on this La-GPS∶Ce mixed single crystal scintillator[11-15]. Aim of this paper is to expand the range of proper substitution ions for the single crystal growth of GPS∶Ce. In the crystal growth study of Gd2SiO5∶Ce(GSO∶Ce) single crystal scintillator, Y is chosen as the substitution ion to improve the crystal quality[16]. In our previous work, the polycrystalline samples of (Gd1-xYx)2Si2O7∶0.1% Ce were obtained through solid-state sintering and related PL and scintillation properties were studied[17]. In order to give an detailed insight of the impact on crystal growth, structure and luminescence properties, a series of Ce doped (Gd0.99-xYxCe0.01)2Si2O7(x=0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7) mixed pyrosilicate scintillator samples was obtained by optical floating zone method in the work. Their structure, PL, scintillation characteristics were measured and analyzed. Phase identifications of these samples were measured through X-ray diffraction (XRD). The PL and scintillation properties were investigated through vacuum ultra-violet (VUV) excitation and emission, PL decay, X-ray excited luminescence (XEL), γ-ray multi-channel energy spectra and scintillation decay curves.

In cerium doped pyrosilicate Re2Si2O7∶Ce(Re=Lu, Y, Gd) scintillators, an optimal Ce concentration is usually 0～1% (atomic number fraction)[15]. Starting materials were powders of Gd2O3, Y2O3, CeO2and SiO2with 4N purity (99.99%). They were fully mixed at a stoichiometric ratio of the desired component (Gd0.99-xYxCe0.01)2Si2O7(x=0.1, 0.2,…,0.7). The resulting powder was filled into strip balloon and molded into cylinder with aboutφ10 mm under 100 MPa isostatic pressure. These shaped rods were then calcined at 1 400 ℃ for 6 h under air atmosphere[18].

The calcined rods were then attached to the upper shaft and lower shaft of a High-Temperature Optical Floating Zone Crystal Growth Furnace HKZ. The growth conditions were: growth atmosphere of N2; the crystal growth rate was 5 mm/h; the upper shaft and lower shaft were rotating in the opposite directions in the rates of 20 r/min. As the optical floating zone furnace temperature field was unstable, the crystal cracking was serious. So the large size of the single crystal for polishing were picked out.

Phase identifications by X-ray diffraction from GYPS∶Ce powder samples were carried out using an X-ray diffractometer (18 kW D/MAX 2 500 V+/PC JDX-3500; JEOL) with a Cu target, tube voltage 18 kV, and tube current 20 mA. The VUV excitation and emission spectra of samples were recorded by at 4B8 beam line, Beijing Synchrotron Radiation Facility. The XEL spectrum was recorded on a homemade XEL spectrometer with W target. The γ-ray pulse spectrum was measured under a collimated137Cs source and a PMT R1306 was used to detect the luminescence with 3 000 ns integration time. The PL decay curves were recorded by using the Edinburg FLS 980 time resolved and steady state fluorescence spectrometer. The scintillation decay profile was acquired with an Agilent DSO6104A digital oscilloscope in single shot mode under a137Cs source irradiation. In order to estimate light output, the photoemission peaks of in the pulse height spectrum were compared to BGO single crystal.

1 Phase identifications

A series of GYPS∶Ce single crystal samples were polished and cut into a size of 3 mm×2 mm×0.46 mm. The single crystal samples are transparent without visible inclusions as shown in
Fig.1, which indicates these samples own good quality. During the growth process, the crystals do not undergo cleavage were observed. This result shows GYPS∶Ce with appropriate content of Y2O3can be significantly improved by the crystallization behavior. And the crystal growth rate is favorable for suitability of obtaining the single crystal.

Fig.1 (a) XRD patterns of GYPS∶Ce single crystals; (b) cell volume of these samples

In order to determine the phase of GYPS∶Ce single crystals with different concentrations of Y3+, the XRD patterns were obtained from these single crystal samples. According to the XRD result as shown in
Fig.1 (a), it is found that the peak positions of the seven samples are basically similar. The patterns match well with space groupPna21 (JCPDS 82-0732). It is pointed out that crystal structures of rare-earth pyrosilicate (RE2Si2O7) depend on the mean ionic radius of RE3+[19].r(Y3+)=0.098 nm,r(Gd3+)=0.102 nm, the two elements have similar ion radius, so a continuous solid solution can be formed. The cell volume was calculated according to the XRD data.
Fig.1 (b) shows the cell volume decreases with increasing Y3+concentration. Therefore, it is speculated the cause of this variation is the replacement with Y3+, which has smaller ion radius.

2 PL properties

Fig.2 presents curves of VUV and emission spectra of GYPS∶Ce single crystal samples at 25 K. The excitation spectra were measured under emission of 360 nm and the emission spectra were measured under excitation of 383 nm. It can be concluded that the excitation peaks of Ce3+were about 331 nm, 312 nm, 274 nm, 227 nm and 206 nm, which are corresponding to the electron transitions from ground 4f sublevel to 5d1-5sublevels of Ce3+. It is observed that the sharp peaks mainly around 274 nm, 247 nm and 216 nm of these samples. Based on the report[20]and the work previously, these peaks attributed to the 4f→4f electron transitions of Gd3+∶8S7/2→6DJ(274 nm),8S7/2→6IJ(247 nm),8S7/2→6PJ(216 nm). Overlap of the Ce3+and Gd3+excitation peaks means that energy transfer between Gd3+to Ce3+occurs during the luminescence process. The excitation peak at 162 nm is the absorption band of the host. The emission peaks of these samples are centering at about 360 nm and 383 nm, which is corresponding to 5d→4f emission of Ce3+∶5d(1)→2F5/2(360 nm) and 5d(1)→2F7/2(383 nm)[21]. The energy gap of the two transitions is estimated to be 4.77 eV (5d(1)→2F5/2) and 3.24 eV (5d(1)→2F7/2).

The energy transfer behavior of Gd3+→Ce3+plays an important role in the luminescence characteristic in cerium doped gadolinium base mixed silicate scintillator[22-23]. However, the detailed luminescence properties and energy transfer mechanism of the GYPS∶Ce mixed crystal system haven’t be carried out yet.
Fig.3 shows PL decay curves of GYPS∶Ce single crystal samples at room temperature. The monitoring wavelengths wereλex=334 nm andλem=360 nm. The curves of
Fig.3 were fitted with the single exponential function (1).

I=A0+I0exp(-t/τ)

(1)

WhereI0is the initial spectral intensity,tis decay time,τis the decay time constant,A0is constant.

The decay time of GYPS∶Ce single crystal samples fitted out are about 29.1～32.9 ns, consistent with the previously reported 30 ns[24]. From the fitting results, it is speculated that Ce3+is directly excited under the excitation wavelengthλex=334 nm. The result is characteristic time for Ce3+5d→4f transition. There is no decay slow component in this condition. These results mean that Y3+concentration has little influence on the PL decay time.

Fig.2 Normalized VUV excitation and emission spectra of GYPS∶Ce single crystal samples at 25 K (color online)

Fig.3 PL decay curves of GYPS∶Ce single crystal samples

3 Scintillation properties

The series of GYPS∶Ce single crystal samples were characterized by XEL spectra, as shown in
Fig.4. It can be observed that the XEL emission peaks are about 360 nm, 380 nm respectively. It is consistent with the results of VUV emission measurement. A clear peak around 313 nm is observed when the concentration of Y3+is over 50%, and peak intensity increase with increasing Y3+concentration. This suggests that when the concentration of Y3+reaches a certain value, Gd3+also acts as a luminescent center. The luminescent peak of Gd3+is corresponding to the previous report[25], caused by6P7/2→8S7/2transition in Gd3+ion. The scintillation decay curves are displayed in
Fig.5. The fast components of scintillation decay time were determined to be about 28 ns to 68 ns. And the slow components were determined to be about 256 ns to 583 ns, which caused by energy transformation of Gd3+(6IJ)→Ce3+. The same phenomenon occurs in another similar type of scintillation crystal Gd2SiO5∶Ce[22].

Fig.4 Normalized X-ray excitation luminescence spectra of GYPS∶Ce single crystal samples

Fig.5 Scintillation decay curves of GYPS∶Ce single crystal samples

Pulse-height spectra under 662 keV γ-ray from a137Cs source was measured at room temperature to evaluate the LY of as grown crystal sample as shown in
Fig.6(a). From the spectra, it is noticed that the light output of GYPS∶Ce increase with the increment of Y concentration from 10% to 50%. When the concentration of Y3+is over 50%, the light output decrease. Compared with other samples, the sample of 50% Y3+has the highest LY. Remarkably high scintillation performance is obtained with 50%Y: 2.5 times greater light output than GPS∶1%Ce, and channel width was 7 times greater than BGO under the same test condition, as it is shown in
Fig.6(b).

Fig.6 (a) Pulse height spectra of GYPS∶Ce single crystal samples irradiated under gamma ray; (b) pulse height spectra of (Gd0.49Y0.5Ce0.01)2Si2O7 single crystal compared to (Gd0.99Ce0.01)2Si2O7 and BGO single crystals

4 Conclusion

In summary, a series of GYPS∶Ce crystals were successfully grown by optical floating zone method. Their crystal phases are basically similar with orthorhombic space groupPna21 (JCPDS 82-0732). The VUV emission and XEL spectra are centered around 360 nm, corresponding to the 5d→4f transition of Ce3+. The PL decay time of GYPS∶Ce are 29.1 ns to 32.9 ns, which is characteristic time for Ce3+5d→4f transition. The scintillation decay time components were determined to be about 28 ns to 68 ns and 256 ns to 583 ns. (Gd0.49Y0.5Ce0.01)2Si2O7has 2.5 times greater light output than GPS∶1%Ce under the same test condition.