Recombination luminescence in lead tungstate scintillating crystals

Radiation Measurements 38 (2004) 381 – 384 www.elsevier.com/locate/radmeas

Recombination luminescence in lead tungstate scintillating crystals G.P. Pazzia;∗ , P. Fabenia , C. Susinia , M. Niklb , P. Bohacekb , E. Mihokovab , A. Veddac , M. Martinic , M. Kobayashid , Y. Usukie a Matter

Structure and Spectroscopy, IFAC (ex IROE)-CNR, Via Panciatichi 64, Florence 50127, Italy b Institute of Physics AS CR, Cukrovarnicka 10, Prague 6 16253, Czech Republic c INFM and Dipartimento di Scienza dei Materiali dell’ Universit3 a di Milano “Bicocca”, Via Cozzi 53, Milano 20 125, Italy d KEK-High Energy Physics Organisation, Tsukuba 305-0801, Japan e Furukawa Co. Ltd., Materials Research Laboratory, Tsukuba 305-0856, Japan Received 26 November 2003; received in revised form 26 November 2003; accepted 26 February 2004

Abstract Room temperature radioluminescence and photoluminescence decay kinetics measurements of Ba-doped PbWO4 crystals were compared with those of undoped and Mo-doped samples. Photoluminescence decay measurements focus on the coexistence of the immediate (fast) decay having a decay time of a few nanoseconds with slower delayed recombination decay processes. The radioluminescence emission peaking at 500 nm in Ba-doped crystals is similar to that observed in Mo-doped samples. However, photoluminescence of the Ba-doped crystals shows much faster decay kinetics with respect to that of PbWO4 : Mo. Wavelength-resolved thermally stimulated luminescence data (10–300 K) provides complementary information about trapping states and is correlated to photoluminescence decay kinetics. c 2004 Elsevier Ltd. All rights reserved.  Keywords: Lead tungstate; BaMo-doping,; Radioluminescence; Photoluminescence; Luminescence time decay; Thermoluminescence

1. Introduction High density (=8:28 g=cm3 ) of PbWO4 (PWO) coupled to its fast intrinsic blue luminescence at ≈ 410 nm with a decay time of few nanoseconds at room temperature, due to the radiative transition of the (WO4 )2− group (Van Loo, 1979a,b), were the key features leading to the choice of this scintillator for high-energy physics experiments despite its low light yield (LY). Another broad emission in the green spectral region (about 500 nm) was ascribed to unwanted Mo impurities (transition within (MoO4 )2− group) (Bohm et al., 1998) or to the radiative transition at intrinsic (WO3 ) defects (Groening and Blasse, 1980). This emission contains very slow components in both the photoluminescence (PL) and scintillation time decays (micro-to-millisecond) which ∗ Corresponding author. Tel.: +39-055-423-5257; fax: +39-055410893. E-mail address: [email protected] (G.P. Pazzi).

c 2004 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter
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aCect the time decay performances of the scintillator (Nikl et al., 1996; Annenkov et al., 1998). Localized trap levels were observed by measurements of thermally stimulated luminescence (TSL) measurements in Mo-doped samples (glow peaks in the 230–270 K region) (Hofstaetter et al., 1978). In general, point defects of extrinsic or/and intrinsic origin were considered to cause a presence of slow decay components, since such levels could temporarily trap free carriers delaying thus their recombination at radiative sites. Application possibilities of this scintillator have been recently enlarged to medical imaging and industrial controls, because of the improvement of LY of PWO achieved by intentionally introducing extrinsic activators like Mo ions. Such activators act as additional radiative recombination sites and are able to compete with non-radiative recombination paths of free carriers. Co-doping with tri- or penta-valent ions was considered as a way to optimize the material characteristics of PWO (Nikl et al., 2000, 2002a,b; Pazzi et al., 2003). The best results were achieved for PWO:Mo,Nb and

G.P. Pazzi et al. / Radiation Measurements 38 (2004) 381 – 384

2. Experimental Undoped, Mo-doped and Ba-doped PWO crystals were grown by the Czochralski method in air, (5N powders and platinum crucibles). Mo and Ba doping in the melt were 1000 and 2000 ppm, respectively. From crystals 40 mm long and 13:5 mm in diameter, optically polished plates 1 mm thick were prepared. Taking into account the Mo segregation coeHcient, 0.8, the true Mo content in the sample was 800 ppm (labelled 800-Mo). Of the Ba-doped crystals, one sample was cut close to seed and the other far from seed. Using inductively coupled plasma analysis the segregation coeHcient of Ba was determined as to be ≈ 1:5 and the true Ba contents in these samples 2630 (close to the seed) and 1210 ppm (far from the seed) was calculated (labelled 2630-Ba and 1210-Ba). Excited radioluminescence spectra (X-ray, 35 kV) were measured at room temperature using a 199S spectroJuorometer (Edinburgh Instruments). Photoluminescence decays in enhanced time and dynamical scales were measured under excimer laser excitation (308 nm XeCl line) with repetition frequency frep = 10 Hz, FWHM of ≈ 10 ns, using a monochromator (Jobin-Yvon Triax 320) followed by a Hamamatsu R1398 photomultiplier and digital scope Tektronix TDS 680B. Wavelength resolved thermally stimulated luminescence measurements were performed after X-ray irradiation at 10 K (Philips 2274 X-ray tube operated at 20 kV). The detection system was a monochromator coupled to a CCD detector (Jobin-Yvon Spectrum One 3000) operating in the 280–710 nm interval (heating rate of 0:1 K=s). Glow curves were obtained after integration of wavelength resolved data (280–710 nm). 3. Results Normalized room temperature RL spectra of all samples are displayed in Fig. 1. The RL spectrum of the undoped crystal is peaking in the blue spectral region at ≈ 410 nm, while the Mo-doped sample features a broad green band centred at 500 nm. The 2630-Ba sample emission was very similar to that of the undoped crystal with a longer wavelength shoulder, while the spectrum of 1210-Ba sample dominated by a band peaking at 500 nm, perfectly superimposed to that of the 800-Mo sample. The absolute integral intensities of

undoped 800-Mo 2630-Ba 1210-Ba

1

0.8

Normalized RL Intensity

PWO:Mo,Nb,Y crystals, where the steady-state radioluminescence (RL) intensity is about 20 times higher than that of undoped PWO. Unfortunately, a presence of slow components in the scintillation decay resulted in a limited improvement of LY in 1
s time gate to about a factor of 3 (Nikl et al., 2002b). We present the Grst encouraging results of RL, PL time decay and TSL measurements for PWO crystals doped with optically inactive Ba2+ ions and compare them with undoped and Mo-doped samples.

0.6

0.4

0.2

0 350

400

450

500

550

600

650

Wavelength [nm] Fig. 1. Normalized room temperature radio-luminescence spectra of undoped, Mo- and Ba-doped PWO crystals. 1

10

Emission Intensity [a.u.]

382

10

-1

-2

10

-3

10

-4

10

10

400 nm -1210-Ba 500 nm-1210-Ba 500 nm - 800-Mo 500 nm-undoped laser

-5

-6

10

-8

10

-7

10

-6

10

-5

Time [ns] Fig. 2. Normalized photoluminescence emission decays of undoped, 800-Mo and 1210-Ba crystals (exc =308 nm). The emission wavelength at which decays were measured are reported in the legend.

the spectra (absolute spectra integrated over the entire range emission wavelengths, always compared to the BGO standard sample) were in the ratio 1:3:2:7 for undoped, 800-Mo, 2630-Ba and 1210-Ba samples. In summary, the RL intensity of doped samples was higher than that of undoped crystal, particularly high value was obtained for 1210-Ba. Emission decays under 308 nm laser excitation are shown in Fig. 2. The decays of the 1210-Ba crystal at 400 and 500 nm are compared with the decays at 500 nm of the 800-Mo sample and of the undoped crystal. The 400 nm decays of the 800-Mo sample and of the undoped crystal, not

G.P. Pazzi et al. / Radiation Measurements 38 (2004) 381 – 384

4. Discussion and conclusions

5

TSL intensity [a.u.]

383

4 1210-Ba 3 2630-Ba

2

800-Mo 1 undoped 0 50

100

150

200

250

300

Temperature [K] Fig. 3. Thermally stimulated luminescence glow curves of undoped, Mo- and Ba-doped PWO crystals after X-ray irradiation at 10 K obtained after integration of wavelength resolved measurements in the 280–710 nm range (curves are shifted in vertical scale for better visualization).

shown, are fast and similar to that of 1210-Ba. The most important feature is that the 500 nm emission decay of 1210-Ba becomes much faster than that of the Mo-doped sample and approaches that of the undoped one. However, much higher RL intensity was obtained in 1210-Ba sample with respect to the undoped one. The shapes of the steady-state RL spectra of 1210-Ba and 800-Mo samples perfectly coincide, while their time decays are signiGcantly diCerent. Luminescence decay of 2630-Ba (not reported) shows an intermediate pattern with the decay at 500 nm lying between those of 800-Mo and 1210-Ba. Thermally stimulated luminescence glow curves, obtained by integrating wavelength-resolved measurements over their entire emission range, are shown in Fig. 3. With respect to the undoped sample, Mo-doping gave rise to a peak at ≈ 15 K and changed the intensitiy ratio of those at 50 and 75 K. However, its major eCect consisted in a new broad and intense glow peak in the 200–250 K region. The major eCect of the Ba-doping showed at T ¡ 100 K a strong increase of the 50 and 65–75 K peaks. In this case, an intense peak at 15 K was detected as well, while at 100 ¡ T ¡ 300 K we observed several very weak glow peaks. No signiGcant differences were noticed by changing the Ba content. Spectral composition of TSL peaks showed an emission at ≈ 450 nm for undoped crystal, and one at ≈ 500 nm for 800-Mo sample (Nikl et al., 2002a) in accordance with their RL spectra. The 2630-Ba sample displayed a blue emission at T ¡ 60 K, followed by recombination at 500 nm at higher temperatures. The 1210-Ba sample features more complex spectrum. Additional high energy ( ¡ 300 nm) and composite low energy (550 ¡  ¡ 650 nm) emission bands were observed specially in the 70 and 230 K regions.

It was found that Ba2+ ions was found to strongly inJuence the optical properties of PWO under both the steady-state and pulsed excitations. Measurements of RL show that an intense emission peaking at 500 nm gradually substitutes for the intrinsic blue emission along the crystal. This fact indicates a progressive formation of new recombination pathway. The position and shape of this emission are very similar to those observed for the Mo-doped sample. It is feasible to suppose that due to the value of BaO segregation coeHcient, higher than 1, the PWO:Ba crystal becomes slightly PbO-deGcient during the process of crystal growth. Recalling previous attributions of the green emission to (WO3 ) defects (Groening and Blasse, 1980), the enhancement of a green emission intensity of purely intrinsic character can be expected towards the crystal end. Furthermore, enhancement of oxygen vacancy concentration may result in more eCective electron transfer to a trace (MoO4 )2− groups as suggested by Nikl et al. (2002a). This fact could also explain the closely similar shapes of emission spectra in 800-Mo and 1210-Ba samples. Very high RL intensity of 1210-Ba, which is 7 times higher than that of the undoped sample, represents an extremely positive result from an application point of view. Moreover, time resolved luminescence measurements revealed that the intensity of the green emission in Mo-doped crystal is considerably faster than that in Ba-doped samples and contains much less slow components in the micro-to-millisecond time scales. Slow decay components were frequently observed at room temperature under sub-band gap excitation. They were explained by thermal disintegration of self-trapped excitons (STE) at T ¿ 150 K (MNurk et al., 1997) and subsequent energy transfer between blue and green emission centres via free charged carriers (Nikl et al., 1998). In our present measurements performed at RT, the optical excitation at 308 nm is perfectly superimposed to the STE excitation (Pazzi et al., 2003). The mechanism of STE excitation, followed by its thermal disintegration and subsequent possibility of carrier trapping at various defect sites before radiative recombination is surely operating. Therefore, in our excitation conditions the time decay features of the radiative transitions are aCected by external trapping–detrapping phenomena which can even become dominant and govern the decay kinetics. Thus, one can suppose that Ba-doped crystals are less affected by the presence of deep localized trap levels than Mo-doped ones. Such an hypothesis is indeed supported by comparison of the TSL glow curves in the 10–300 K interval: Ba doping mainly increases the number of shallow traps, responsible for glow peaks in the 50–70 K interval; the 50 K peak was previously ascribed to (WO4 )3− intrinsic electron traps (Laguta et al.,1998; Martini et al., 1999; Bohm et al., 1999). No intense glow peaks were observed in the 100–300 K region. By contrast, Mo-doping caused an appearance of a broad and composite TSL structure in the 200–250 K region attributed to (MoO4 )3− electron

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traps (Hofstaetter et al., 1978). Such rather deep traps can lie behind an origin of very slow phenomena in 800-Mo PL decay, not observed in Ba-doped samples. In conclusion, doping by Ba2+ can probably drive the internal stoichiometry of PWO towards PbO-deGcient composition and provides a new way to increase the RL output of PWO crystals, by introducing a strong green emission. The decay characteristics of this emission appear more suitable for fast scintillator application with respect to Mo-doped PWO. A comparison between PL time decays and TSL data conGrmed the participation of localized trap levels in energy transport towards the radiative centres in PWO. Acknowledgements Financial support of NATO SfP 973510 and the Grant Agency of the Czech Republic Project N. 202/01/0753 is gratefully acknowledged. References Annenkov, A., AuCray, E., Borisevich, A.E., Drobyshev, G.Yu., Fedorov, A.A., Kondratiev, O.V., Korzhik, M.V., Lecoq, P., Ligun, V.D., Missevitch, O.V., 1998. Slow components and afterglow in PWO crystal scintillations. Nucl. Instrum. Methods A 403, 302–312. Bohm, M., Borsevich, A.E., Drobyshev, G.Yu., Hofstaetter, A., Kondratiev, O.V., Korzhik, M.V., Luh, M., Meyer, B.K., Peigneux, J.P., Scharmann, A., 1998. InJuence of Mo impurity on the spectroscopic and scintillation properties of PbWO4 crystals. Phys. Stat. Sol. A 167, 243–252. Bohm, M., Henecker, F., Hofstaetter, A., Luh, M., Meyer, B.K., Scharmann, A., Kondratiev, O.V., Korzhik, M.V., 1999. Electron traps in the scintillator material PbWO4 and their correlation to thermally stimulated luminescence. Radiat. EC. Defects Solids 150, 21–25. Groening, J.A., Blasse, G., 1980. Some new observations on the luminescence of PbMoO4 and PbWO4 . J. Solid State Chem. 32, 9–20.

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