VUV spectroscopy and low temperature thermoluminescence of LSO:Ce and YSO:Ce

Journal of Alloys and Compounds 380 (2004) 146–150

VUV spectroscopy and low temperature thermoluminescence of LSO:Ce and YSO:Ce Winicjusz Drozdowski a,∗ , Andrzej J. Wojtowicz a , Dariusz Wi´sniewski a , Piotr Szupryczy´nski a , Sebastian Janus a , Jean-Luc Lefaucheur b , Zhenhui Gou b a

Institute of Physics, Nicolaus Copernicus University, Grudzi˛adzka 5/7, 87-100 Toru´n, Poland b Photonic Materials Ltd., Strathclyde Business Park, Bellshill ML4 3BF, Scotland, UK

Abstract Radioluminescence and UV-excited photoluminescence spectra, photoluminescence time profiles, thermoluminescence glow curves and gamma-excited energy spectra (determining scintillation light yields) of several Lu2 SiO5 :Ce and Y2 SiO5 :Ce samples were recorded. The results are analyzed with attention focused on possible correlations between trap distributions, VUV responses, and light yields. The aspect of two distinct sites occupied by Ce3+ ions is also discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Radioluminescence; Thermoluminescence; VUV spectroscopy; Scintillation light yield

1. Introduction The excellent scintillation properties of Lu2 SiO5 :Ce (LSO:Ce) were first reported by Melcher and Schweitzer [1,2]. The features such as high light yield (LY), short scintillation decay time (∼40 ns), and large density (7.4 g/cm3 ) inspired a study of this crystal as a fast high-energy radiation detector. A cheaper isostructural material, Y2 SiO5 :Ce (YSO:Ce), was studied even earlier, but the results were much less promising than in case of LSO:Ce (lower light yield, longer scintillation decay time, lower density [3,4]). Photoluminescence spectra of Y2 SiO5 :Ce and Gd2 SiO5 :Ce (GSO:Ce) were investigated thoroughly by Suzuki et al. [5,6]. They distinguished two crystallographic sites (denoted as Ce1 and Ce2) occupied by Ce3+ ions and assigned to them two emission bands, peaking in GSO:Ce at 425 and 480 nm, respectively. Both the efficiency and the decay time of the shorter wavelength luminescence (Ce1) were almost temperature-independent between 10 and 300 K. On the contrary, in case of the Ce2 luminescence reduced intensities and shorter decay time constants were observed at higher temperatures. The LSO:Ce and YSO:Ce crystals, however, received less attention than GSO:Ce in the aforementioned

studies [5,6]. Moreover, those measurements were performed at excitation wavelengths longer than 240 nm. Thermoluminescence (TL) of Lu2 SiO5 :Ce was examined predominantly above room temperature. Dorenbos et al. [7] analyzed glow curves recorded in the 300–650 K range, evaluated trap parameters and discussed possible nature of traps. Visser et al. [8] observed an anti-correlation between TL intensities (300–730 K) and light yields of their crystals. Low temperature glow curves of two LSO:Ce samples were first published by Lempicki and Glodo [9]. A similar anti-correlation, i.e. higher TL in the sample showing lower LY, was pointed out in their paper. In this communication, we present some preliminary results of our studies on spectroscopic and scintillation properties of LSO:Ce and YSO:Ce. We confirm the presence of the two distinct cerium sites by UV-excited photoluminescence spectra and time profiles. We also show that besides the TL–LY anti-correlation, there is an apparent correspondence between scintillation light yields and VUV responses of the crystals, i.e. the samples of higher LYs can be much more efficiently excited in the VUV region.

2. Materials and experiments ∗

Corresponding author. Tel.: +48-56-6113319; fax: +48-56-6225397. E-mail address: [email protected] (W. Drozdowski).

0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.03.016

The crystals of Lu2 SiO5 :Ce and Y2 SiO5 :Ce were grown by Photonic Materials Limited, Scotland, using the

W. Drozdowski et al. / Journal of Alloys and Compounds 380 (2004) 146–150

3. Results and discussion Radioluminescence spectra of Lu2 SiO5 :Ce and Y2 SiO5 :Ce are shown in Fig. 1. In both materials the spectra are dominated by strong cerium emission bands (at low temperature the spin-orbit split 2 F5/2 and 2 F7/2 levels of the Ce3+ 4f configuration are resolved). Additionally, at 10 K, weak contributions from LSO and YSO host emissions below 370 nm can be observed. We note that in agreement with earlier measurements by Suzuki et al. [5,6], only the Ce1 centers are efficiently excited by ionizing radiation. To demonstrate the presence of the Ce2 centers in Lu2 SiO5 :Ce and Y2 SiO5 :Ce we chose two emission wavelengths, i.e. 400 and 500 nm for Ce1 and Ce2, respectively,

LSO:Ce excitation spectra lemi = 400 nm (Ce1) lemi = 500 nm (Ce2)

T = 10 K

intensity (arb. units)

Czochralski method. Polished pixel (2 mm×2 mm×10 mm) samples were used in the studies. Synchrotron radiation excited spectra and time profiles were taken at the SUPERLUMI station, an experimental set-up at the high intensity beamline I of HASYLAB (Hamburg, Germany). A detailed description of this station was given by Zimmerer in Ref. [10]. Radioluminescence, thermoluminescence and light yields were measured at Institute of Physics, Nicolaus Copernicus University (Toru´n, Poland). A typical system consisting of an X-ray tube operated at 42 kV and 10 mA, a 0.5 m monochromator (ARC SpectraPro-500), a photomultiplier (Hamamatsu R928), and a closed-cycle helium cooler with a programmable temperature controller, was used to record room and low temperature X-ray excited emission spectra and thermoluminescence glow curves. Relative light yields were determined by analyzing the positions of photopeaks in energy spectra. Details of the experimental set-up and the LY evaluation procedure will be published elsewhere [11].

YSO:Ce excitation spectra lemi = 400 nm (Ce1) lemi = 500 nm (Ce2)

T = 10 K

50

100

150

200

250

300

350

wavelength (nm) Fig. 2. LHeT excitation spectra of the Ce1 and Ce2 emissions in LSO:Ce and YSO:Ce.

and recorded excitation spectra to identify excitation bands belonging to these centers. The low temperature spectra (Fig. 2) reveal a peak at about 320 nm, excitation at which produces a different emission than the Ce1 luminescence displayed in Fig. 1. This emission (Fig. 3) is shifted towards lower energies and can be ascribed to the Ce2 centers. The other bands in Fig. 2 (180, 260, and 290 nm, and also those below 130 nm) are responsible for excitation of the Ce1 luminescence (Fig. 3). We note that the room temperature excitation and emission spectra (not shown) are completely dominated by processes involving the Ce1 sites. The higher intensity of the VUV excitation bands in LSO:Ce than in YSO:Ce will be discussed later on.

LSO:Ce emission spectra lexc = 180 nm (Ce1)

LSO:Ce radioluminescence RT LHeT

lexc = 320 nm (Ce2)

T = 10 K

intensity (arb. units)

intensity (arb. units)

147

YSO:Ce radioluminescence RT LHeT

YSO:Ce emission spectra lexc = 180 nm (Ce1) lexc = 320 nm (Ce2)

T = 10 K

200

250

300

350

400

450

500

550

wavelength (nm) Fig. 1. Radioluminescence spectra of LSO:Ce and YSO:Ce.

600

200

250

300

350

400

450

500

550

600

wavelength (nm) Fig. 3. LHeT photoluminescence spectra of LSO:Ce and YSO:Ce.

W. Drozdowski et al. / Journal of Alloys and Compounds 380 (2004) 146–150

LSO:Ce lexc = 180 nm, lemi = 400 nm

intensity (arb. units)

experiment, T = 300 K experiment, T = 10 K fits

0

20

40

60

80

100

120

140

160

time (ns) Fig. 4. Representative photoluminescence time profiles of the Ce1 and Ce2 emissions in LSO:Ce.

In Fig. 4, we present a number of representative time profiles of both cerium luminescences in LSO:Ce. The Ce1 emission excited at 180 nm and observed at 400 nm decays single-exponentially with a time constant of about 40 ns at low and room temperatures. In case of the 320 nm excitation and the 500 nm detection, a bit slower (∼50 ns) single-exponential decay at 10 K turns into a double-exponential one at 300 K. The short component (∼5 ns) is most probably due to some remnants of the Ce2 luminescence, which is in fact expected to be much faster at room temperature [5,6]. The longer component can be then associated with the Ce1 luminescence, the intensity of which at 500 nm is high enough to contribute to the time profile. Similar upshots were obtained for YSO:Ce. Although the results displayed in Fig. 2 could suggest a stronger VUV response of the Lu2 SiO5 :Ce sample as compared to the Y2 SiO5 :Ce sample, further studies evidence that it is not always the case. We examined three LSO:Ce samples characterized by different light yields in order to compare their emission and excitation spectra. Contrary to the radioand photoluminescence spectra (not shown), which make no distinctions among the three samples, the excitation spectra show significant discrepancies (Fig. 5). Although the band shapes and positions are nearly the same, the relative intensities of the bands are very different. It is evident that the Ce1 luminescence can be excited between 50 and 130 nm much more efficiently in the sample denoted as L01 than

sample L01 sample L02 sample L03 T = 300 K

50

LSO:Ce lexc = 320 nm, lemi = 500 nm experiment, T = 300 K experiment, T = 10 K fits

LSO:Ce excitation spectra lemi = 400 nm

intensity (arb. units)

148

100

150

200

250

300

wavelength (nm) Fig. 5. RT excitation spectra of the Ce1 emission in the three LSO:Ce samples.

in the two other samples, the spectra of which resemble those of YSO:Ce. Interestingly, the L01 sample exhibits the highest scintillation light yield amidst all the LSO:Ce and YSO:Ce samples used in these studies. To test this correlation more precisely, we integrated the excitation spectra in the two spectral ranges (Fig. 5): 50–130 nm (integral I1 ) and 130–320 nm (integral I2 ). Then we compared the I1 /I2 ratios with the corresponding light yields. As shown in Table 1, the correlation is clear. However, a larger set of samples should be examined in order to establish the relation between LY and VUV response. Thermoluminescence glow curves of two Lu2 SiO5 :Ce samples are presented in Fig. 6. In spite of their composite structures, characteristic asymmetry of the major peaks is apparent, indicating that the simple TL model proposed by Randall and Wilkins [12] may be used to analyze the data. To separate the curves into first-order peaks we fitted the following well known Randall–Wilkins formula to experimental points:  
Ei I(T) = n0i s exp − kB T i      T s Ei  × exp − dT (1) exp − β T0 kB T  where I is the TL intensity, T the temperature, β the heating rate, n0 the initial concentration of traps, E the trap depth, s the frequency factor, and kB the Boltzmann constant. Preliminary input values of n0i , Ei , and si for the fitting procedure were chosen by trial and error. As the heating rate Table 1 The correlation between the VUV response of the LSO:Ce samples and their scintillation light yields Sample

I1 (50–130 nm)

I2 (130–320 nm)

I1 /I2

LY (relative units)

L01 L02 L03

87.6 38.2 35.4

88.5 110 112

0.99 0.35 0.32

1.00 0.42 0.33

W. Drozdowski et al. / Journal of Alloys and Compounds 380 (2004) 146–150 2000

2500 LSO:Ce sample L04 thermoluminescence experiment fits

1000

1500

500 0 200

LSO:Ce sample L05 thermoluminescence experiment fits

150

YSO:Ce sample Y01 thermoluminescence experiment fits

2000

intensity (arb. units)

1500

intensity (arb. units)

149

1000 <--------------- x 25 --------------->

500 0 YSO:Ce sample Y02 thermoluminescence experiment fits

400 300

100

200 <---------------------- x 25 ---------------------->

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100

0 50

100

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0

300

50

100

temperature (K)

150

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300

temperature (K)

Fig. 6. Glow curves of the two LSO:Ce samples, recorded at 0.15 K/s following a 10 min X-ray irradiation.

was very low (0.15 K/s), the effect of non-ideal heat transfer [13] was neglected. The results, summarized in Table 2, show that the trap distributions in the L04 and L05 samples are somewhat different. In case of L05, there are less peaks in the glow curve and the corresponding n0 values are generally lower than in L04. In consequence, the entire area under the L05 curve is smaller. Although, unfortunately, no glow curve of the L01 sample was recorded, we note that the LY of the L05 sample is over 10% higher comparing to the L04 one, which confirms the TL–LY anti-correlation in LSO:Ce [9]. An anti-correlation of the same kind is observed in Y2 SiO5 :Ce. The glow curves are displayed in Fig. 7 and the trap parameters are listed in Table 3 (Y01, lower LY; Y02, higher LY).

Fig. 7. Glow curves of the two YSO:Ce samples, recorded at 0.15 K/s following a 10 min X-ray irradiation.

As the room temperature trap lifetimes calculated with the Arrhenius formula:   E 1 (2) τ = exp s kB T using the trap parameters derived from the glow curve fits, are of order of microseconds or longer (the trap no. 1 in Y02 is an exception, but due to the very low initial concentration its significance is negligible), the traps identified in these studies do not affect the scintillation kinetics. Nevertheless, they still may be expected to decrease the light yield of the crystals, as observed.

Table 2 Parameters of traps detected in LSO:Ce (Tmax is the temperature at which the glow curve peaks, E the trap depth, s the frequency factor, n0 the initial trap concentration; n0 is in the same units as TL intensity and s in s−1 )

Table 3 Parameters of traps detected in YSO:Ce (Tmax is the temperature at which the glow curve peaks, E the trap depth, s the frequency factor, n0 the initial trap concentration; n0 is in the same units as TL intensity and s in s−1 )

Sample

Peak no.

Tmax (K)

n0

E (eV)

ln s

Sample

Peak no.

Tmax (K)

n0

E (eV)

ln s

L04 L04 L04 L04 L04 L04 L04 L04 L04 L05 L05 L05 L05 L05 L05

1 2 3 4 5 6 7 8 9 1 2 3 4 5 6

38 61 76 96 129 140 151 186 234 40 67 102 148 186 228

132000 111000 121000 23100 14400 36700 14200 22900 9070 5900 7300 6690 15400 18600 3010

0.023 0.024 0.103 0.061 0.344 0.279 0.176 0.462 0.429 0.066 0.051 0.127 0.332 0.404 0.328

3.65 0.27 12.06 2.90 27.51 19.34 9.16 25.02 17.02 16.18 4.89 10.68 22.38 21.28 12.20

Y01 Y01 Y01 Y01 Y01 Y01 Y01 Y01 Y01 Y02 Y02 Y02 Y02 Y02 Y02

1 2 3 4 5 6 7 8 9 1 2 3 4 5 6

36 97 120 139 167 193 213 235 267 27 79 101 119 149 215

312 115000 233000 17700 2410 257 463 587 953 61.3 53100 5680 396 286 465

0.043 0.108 0.202 0.326 0.328 0.343 0.484 0.771 0.799 0.062 0.074 0.182 0.184 0.303 0.506

11.09 8.99 15.82 23.65 18.83 16.45 22.28 34.31 30.69 24.85 6.98 17.45 14.19 19.88 23.31

150

W. Drozdowski et al. / Journal of Alloys and Compounds 380 (2004) 146–150

4. Conclusions

References

Our measurements provide some additional information on the scintillation properties of Lu2 SiO5 :Ce and Y2 SiO5 :Ce. It is shown that although there are two types of cerium centers (associated with two crystallographic sites) in the crystals, only these denoted as Ce1 are efficiently excited by ionizing radiation. At low and high temperatures under optical excitation into the Ce3+ absorption bands the Ce1 luminescence is reasonably fast (∼40 ns). As the trap lifetimes calculated from the glow curve fits are relatively long at room temperature, practically no trap-related slow components appear in the scintillation time profiles (the presence of such components is a serious deficiency of such scintillators as YAlO3 :Ce and BaF2 :Ce [14,15]) and the scintillation decay time is equal to the optical one. The influence of traps is thus reduced to the lowering of the light yields. Since the samples of higher light yields and weaker thermoluminescence display stronger VUV response, we conclude that both non-radiative recombination as well as free carriers trapping are responsible for some loss of scintillation light.

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Acknowledgements This work was supported by Nicolaus Copernicus University (grant 402-F/2003) and the European Commission (IHP-Contract HPRI-CT-1999-00040/2001-00140). The hospitality and help of Prof. G. Zimmerer and Dr. S. Vielhauer of HASYLAB is also gratefully acknowledged.