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Luminescence and photothermally stimulated defects creation processes in PbWO4:La3+, Y3+ (PWO II) crystals
Journal of Luminescence 168 (2015) 256–260
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Luminescence and photothermally stimulated defects creation processes in PbWO4:La3 þ , Y3 þ (PWO II) crystals E. Auffray a, M. Korjik b, S. Zazubovich c,n a
CERN, Geneva 23, Geneva, Switzerland Institute for Nuclear Problems, 11 Bobruiskaya, 220020 Minsk, Belarus c Institute of Physics, University of Tartu, Ravila 14 c, 50411 Tartu, Estonia b
art ic l e i nf o
a b s t r a c t
Article history: Received 27 April 2015 Received in revised form 24 July 2015 Accepted 14 August 2015 Available online 22 August 2015
Photoluminescence and thermally stimulated luminescence (TSL) are studied for a PbWO4 crystal grown by the Czochralski method at Bogoroditsk Technical Chemical Plant, Russia from the melt with a precise tuning of the stoichiometry and co-doped with La3 þ and Y3 þ ions (the PWO II crystal). Photothermally stimulated processes of electron and hole centers creation under selective UV irradiation of this crystal in the 3.5–5.0 eV energy range and the 85–205 K temperature range are clarified and the optically created electron and hole centers are identified. The electrons in PWO II are mainly trapped at the (WO4)2 groups located close to single La3 þ and Y3 þ ions, producing the electron {(WO4)3 –La3 þ } and {(WO4)3 –Y3 þ } centers. The holes are mainly trapped at the regular oxygen ions O2 located close to La3 þ and Y3 þ ions associated with lead vacancies, producing the hole O (I)-type centers. No evidence of single-vacancy-related centers has been observed in PWO II. The data obtained indicate that excellent scintillation characteristics of the PWO II crystal can be explained by a negligible concentration of single (non-compensated) oxygen and lead vacancies as the traps for electrons and holes, respectively. & 2015 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Thermoluminescence Defects PbWO4:La3 þ Y3 þ (PWO II) crystal
1. Introduction Single crystals of lead tungstate (PbWO4) became a subject of renewed interest about 20 years ago when their favorable characteristics for scintillation detection were reported [1–4]. Since this time, luminescence, energy transfer and defects creation processes in PbWO4 crystals were considered in a huge number of papers (see, e.g., reviews [3–7] and references therein). PbWO4 (PWO) crystals are widely used as a scintillation material for electromagnetic calorimeters in many high-energy physics projects due to its compactness, fast response, short decay time, high resolution in a wide energy range, and good radiation hardness (see, e.g., Refs. [3,4]). For example, these crystals have been successfully applied for the construction of the electromagnetic calorimeter of the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC) built in CERN, Geneva – a particle accelerator where the Higgs boson has recently been discovered [8,9]. PWO has been chosen also for several other projects, e.g., for ALICE project at CERN and for electromagnetic calorimeter of the PANDA detector at the future Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, where hadron physics in antiproton annihilation processes n
Corresponding author. E-mail address: [email protected] (S. Zazubovich).
http://dx.doi.org/10.1016/j.jlumin.2015.08.028 0022-2313/& 2015 Elsevier B.V. All rights reserved.
will be investigated [10]. However, due to thermal quenching of the PWO luminescence, the relatively low (E10 ph.e./MeV at 295 K) scintillation light yield became a limiting factor for low energy measurement applications. Considerably improved PWO crystals (denoted as PWO II) were prepared by the simultaneous doping with La3 þ and Y3 þ ions with a total concentration of less than 50 ppm and improvement of the purity and structural perfection of the crystals [11]. The study of the scintillation characteristics of these crystals showed that they have about two times higher light yield as compared with the standard PWO crystals used in the LHC, reaching at room temperature (RT) a value near 20 ph. e./MeV [11,12]. In these crystals, 97% of light is integrated within a 100 ns time window at RT [13]. At 25 °C, the light yield of the PWO II crystals becomes well above 60 ph.e./MeV [11–13]. The significantly improved light yield was obtained without loosing the fast response, excellent energy resolution, and good radiation hardness. Optical transmission of PWO II was even better as compared with the standard PWO [12] used in LHC, indicating the reduced concentration of various intrinsic defects. Just the PWO II crystals are used for the construction of the electromagnetic target calorimeter of PANDA [10]. Further, the peculiarities of the radiation damage and recovery processes at lower temperatures ( 25 °C) and other characteristics (e.g., light yield, optical transmittance, scintillation kinetics, radiation hardness, homogeneity, etc), related to the
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application of the PWO II crystals in PANDA detectors, were studied for the improved PWO II crystals (see, e.g., Refs. [14–18] and references therein). However, photoluminescence characteristics, photothermally stimulated defects creation processes and the origin of various intrinsic defects in these crystals were not studied in detail. Previous detailed studies of PbWO4 crystals of different origin, prepared in different conditions and containing different impurity ions, by the photoluminescence, thermally stimulated luminescence (TSL) and electron spin resonance (ESR) methods have shown that their characteristics can be strongly different. In many cases, the reasons of these differences have been understood, and the origin of the defects responsible for different TSL peaks has been clarified and confirmed also by the ESR data (see, e.g., Refs. [5,6,19–33] and references therein). In this work, we have tried to use this knowledge and experience at the investigation of the above-mentioned improved PWO II crystals. Namely, we have carried out the detailed study of the photoluminescence and photothermally stimulated defects creation processes in the PWO II crystals and compared the results with those obtained for the other, earlier investigated PbWO4 crystals. Our main aim was to understand the possible reason of excellent scintillation characteristics of the PWO II crystals reported in Refs. [11–13]. The investigations carried out in the present work have shown that the reason is in negligible concentration of single (non-compensated) lead and oxygen vacancies in the PWO II crystals as compared with the other PbWO4 crystals, even with the crystals doped with trivalent rare-earth ions studied for about 20 years and reported, e.g., in Refs. [3–7,22,23,26,27,34–52].
2. Experimental procedure Single crystals of PbWO4:La3 þ , Y3 þ (PWO II) were grown by the Czochralski method at Bogoroditsk Technical Chemical Plant (Bogoroditsk, Russia) from the melt with a precise tuning of the stoichiometry and contain trivalent Y and La ions with a total concentration up to 40 ppm. The steady-state emission and excitation spectra in the 85–400 K temperature range were measured using a setup, consisting of the deuterium DDS-400 lamp, two monochromators (SF-4 and SPM-1), and a vacuum nitrogen cryostat. The luminescence was detected by a photomultiplier (FEU-39 or FEU-79) with an amplifier and recorder. The spectra were corrected for the spectral distribution of the excitation light, the transmission and dispersion of the monochromators, and the spectral sensitivity of the detectors. Thermally stimulated luminescence (TSL) glow curves were measured with a heating rate of 0.2 K/s after selective irradiation of the crystals at different irradiation temperatures Tirr (85–205 K) with different irradiation energies Eirr (3.5–5.0 eV) and irradiation durations tirr (1–60 min) (for more details, see Refs. [19,20]). A crystal located in the nitrogen cryostat was irradiated with the well-focused radiation of the LOT-ORIEL xenon lamp (150 W) through a monochromator SF-4. The spectral width of the monochromator slit did not exceed 5 nm. Due to a small TSL intensity, the TSL glow curves were measured without a monochromator or filters. The integrated emission of the crystal was detected with the photomultiplier FEU-39. From these measurements, the TSL peaks creation spectra – ITSL(Eirr), activation energies Ea and dose dependences – ITSL(tirr) were obtained. To determine the depth Et of the traps corresponding to each TSL peak, the partial heating method was used (for more details, see Ref. [21] and references therein): the crystal, irradiated at the temperature Tirr, was cooled down to 85 K, heated up to a temperature Tstop, then quickly cooled down to 85 K, heated again up to the next temperature Tstop, etc. From the slope of the ln ITSL(1/T) dependence, the Et
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value was calculated. Due to a small TSL intensity, the Et values were determined also from the higher-temperature slopes of the TSL glow curves measured after irradiation at different Tirr.
3. Experimental results Under excitation at 85 K in the band-to-band transitions region, the B emission band of PWO II is located at 2.72 eV, while under excitation in the exciton band maximum, at 2.79 eV (Fig. 1, curves 1 and 2, respectively). The lowest-energy excitation band of this emission is located at 4.13 eV (curve 1′). Under 3.8 eV excitation, a weak green G(I) emission, peaking at 2.27 eV, and a weak ultraviolet (UV) emission, peaking at about 3.1 eV and investigated in Refs. [22,23], are also observed (curve 3). In the exciton and bandto-band absorption regions, the excitation spectrum measured for Eem ¼3.15 eV coincides with the excitation spectrum measured for the B emission (curve 1′), but a weak band appears around 3.9 eV in the former spectrum (curve 3′). The B emission intensity remains constant up to 150 K and then decreases (twice at 177 K, see the inset). At RT, the B emission band is located at 2.9 eV and its lowest-energy excitation band, at 3.95 eV. No 2.5 eV emission with the characteristics described in Refs. [6,20,24] (the G(II) emission), which arises from tunneling recombinations of electron and hole centers connected with single oxygen and lead vacancies [25], is observed in this crystal. The integrated TSL intensity in the PWO II crystal is about 50– 100 times weaker as compared with the PWO crystals singledoped with La3 þ or Y3 þ ions studied in Ref. [23]. After UV irradiation of PWO II at 85 K in the band-to-band transitions region, the TSL glow curve consists of a broad peak located around 122 K and a weaker peak around 170 K (Fig. 2). Both these peaks are of complex structure. No separate peaks around 100–110 K, arising 3þ 3þ } and { WO3− } centers (see, e.g., from electron { WO3− 4 –La 4 –Y Ref. [22] and references therein), which, according to Ref. [13], should surely exist in the PWO II crystal, appear at the TSL glow curve. Most probably, this is caused by too high irradiation temperature (Tirr ¼ 85 K), due to which these centers are partly thermally destroyed. The increase of Tirr results in the reduction of the TSL intensity under this irradiation in the whole temperature range. Therefore, we suggest that the centers responsible for the TSL peaks at 122 K and 170 K can arise from the re-trapping of electrons, released at the thermal destruction of the electron
Fig. 1. Normalized emission spectra of the PWO II crystal measured at 85 K under Eexc ¼5.0 eV (curve 1), Eexc ¼ 4.12 eV (curve 2), and Eexc ¼ 3.8 eV (curve 3). Normalized excitation spectra measured for Eem ¼ 2.8 eV or Eem ¼3.15 eV (curve 1′) and Eem ¼ 2.2 eV (curve 2′); for Eem ¼ 3.15 eV, the intensity around 3.9 eV is increased 10 times (curve 3′). In the inset, temperature dependence of the maximum intensity of the B emission.
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Fig. 2. TSL glow curve measured after irradiation of the PWO II crystal for 1 h at 85 K with Eirr ¼ 4.7 eV.
3þ 3þ { WO3− } and { WO3− } centers, at some deeper traps. This 4 –La 4 –Y means that the observed TSL peaks are of an electron origin. After irradiation at 140 K, the measurable TSL intensity is observed only after irradiation in the exciton and defect-related absorption region. From the comparison of these data with the results of previous studies (see, e.g., Refs. [6,19,20,25]) it can be concluded that the corresponding electron and hole centers in the PWO II crystal are also created at the disintegration of the localized exciton and defect-related states. The shape of the TSL glow curve strongly depends on the irradiation energy (Fig. 3a). The main TSL glow curve peaks are located at about 178 K and 192 K. Additional weak higher-temperature TSL peaks appear more clearly after irradiation at higher temperatures (Fig. 3b, curves 2 and 3). The creation spectra of the main E178 K and E192 K peaks have the maximum at about 4.0 eV and 3.95 eV, respectively (Fig. 4a). The comparison of the TSL peaks creation spectra with the excitation spectra of the B and G(I) emission measured at the same conditions (Fig. 4b) indicate that at Tirr ¼140 K, the recombining electron and hole centers are most effectively created in the exciton- and defect-related absorption regions. The 178 K peak (Fig. 4a, filled circles) is more effectively created in the band-to-band and exciton regions as compared with the 192 K peak (empty circles). The intensity of the 192 K peak reaches the saturation after about 20 min of the irradiation at this temperature (see the inset in Fig. 3a). From the ln ITSL(1/Tirr) dependence shown in the inset of Fig. 4a, the activation energy for the 192 K peak creation at Eirr ¼3.95 eV is found to be Ea ¼0.147 0.05 eV. In Fig. 5, the dependence of the trap depth values Et on the partial heating (Tstop) or irradiation (Tirr) temperature is presented. The values of Et in the lowest temperature range do not exceed 0.14–0.17 eV (triangles). They should arise from the electron 3þ 3þ { WO3− } and { WO3− } centers surely existing in the PWO 4 –La 4 –Y II crystals [13] but partly destroyed due to too high irradiation temperature Tirr used in this work. For the E178 K and E192 K peaks, Et ¼0.3170.02 eV and Et ¼0.4370.03 eV, respectively (circles). Due to a very small TSL intensity of the TSL glow curve peaks appearing at T 4210 K (Fig. 3), their detailed investigation is impossible.
4. Discussion The self-trapped electrons ({ WO3− 4 } centers), electrons trapped 3þ 3þ } and {WO3− } close to trivalent rare-earth ions (e.g., {WO3− 4 –La 4 –Y centers), and single oxygen vacancy (VO) related electron centers (e.g., {Pb þ –VO}) were detected by ESR in PbWO4 crystals [5,26–29].
Fig. 3. TSL glow curves measured for the PWO II crystal after irradiation (a) at Tirr ¼ 140 K with Eirr ¼4.2 eV (curve 1), Eirr ¼ 3.95 eV (curve 2), and Eirr ¼3.75 eV (curve 3); (b) with Eirr ¼3.95 eV at Tirr ¼ 140 K (curve 1), Tirr ¼ 172 K (curve 2), and Tirr ¼ 183 K (curve 3). The TSL peak intensities at 192–194 K are normalized. In the inset, the dose dependence measured for the 192 K peak at Tirr ¼ 140 K under irradiation with Eirr ¼ 3.95 eV.
Recently [30,31], the O (I)-type hole centers (a hole trapped at an oxygen ion close to a La3 þ or Y3 þ ion and a lead vacancy VPb) were also found and studied. An existence of non-paramagnetic hole centers (two holes trapped at two oxygen ions in the vicinity of a single lead vacancy VPb) was suggested in [32]. Tunneling recombination in the pairs of the vacancy-related electron and hole centers was found to result in the appearance of the G(II) emission [6,20,24,25]. A small intensity of the G(II) luminescence in the PWO II crystals indicates that the concentration of single (non-compensated) oxygen and lead vacancies as the traps for electrons and holes, respectively, in these crystals are small. Therefore, the appearance in these crystals of the shallow electron centers of 3þ 3þ 3− the types of WO3− } and { WO3− } and the holes 4 , { WO 4 –La 4 –Y 3þ 3þ or Y ions associated trapped at oxygen ions close to the La with VPb vacancies (the hole O (I)-type centers) is mainly expected. Indeed, the TSL glow peaks at about 195 K, arising from the thermal destruction of the single oxygen vacancy related electron {Pb þ –VO} centers with the trap depth Et ¼0.56 eV [19,25,27], is not detected in these crystals. Due to a negligible concentration of single oxygen vacancies, the concentration of single lead vacancies should be negligible as well. Therefore, the peak at 225–230 K cannot be of the same origin as that observed in Ref. [25] and suggested to arise from non-paramagnetic hole centers of the type of {2O –VPb}, i.e. two holes trapped at the oxygen ions located close to a single lead vacancy VPb. The peak around 190 K with Et ¼0.43 7 0.03 eV, most probably, arises from the thermal destruction of the hole O (I)-type centers [30]. This
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5. Conclusions The electrons, optically created in the PbWO4:La3 þ , Y3 þ (PWO II) crystals studied in this work, are mainly trapped at the (WO4)2 groups located close to the La3 þ and Y3 þ ions as well as at some unidentified defects. The optically created holes are mainly trapped at the oxygen ions, located close to La3 þ and Y3 þ ions and lead vacancies VPb, producing the O (I)-type hole centers. The PWO II crystals are found to contain extremely small concentrations of electron and hole centers connected with oxygen and lead vacancies. Due to that the tunneling recombination G(II) luminescence in these crystals is very weak, and, consequently, the slow components in their green luminescence decay should be extremally weak as well. Thus, the obtained results indicate that the excellent scintillation characteristics of the PWO II crystal reported in Refs. [11–13] can be explained by a negligible concentration of single (non-compensated) oxygen and lead vacancies as the traps for electrons and holes, respectively. The data obtained in this work could be useful for further development and improvement of PbWO4-based scintillation materials.
Acknowledgments The work is supported by the Institutional Research Funding IUT02-26 of the Estonian Ministry of Education and Research.
Fig. 4. (a) Creation spectra of the TSL glow curve peaks located at 178 K (filled circles) and 192 K (empty circles) measured for the PWO II crystal at Tirr ¼ 142 K; (b) Excitation spectra of the B (curve 1) and G(I) (curve 2) emissions measured at the same conditions. The spectra are not corrected on the excitation intensity. In the inset, the ln ITSL(1/Tirr) dependence is presented for the 192 K peak under Eirr ¼ 3.95 eV.
Fig. 5. Dependence of trap depth value Et on the temperature Tirr or Tstop measured after irradiation of the PWO II crystal with Eirr ¼ 4.7 eV (triangles) and Eirr ¼ 3.95 eV (circles).
conclusion is confirmed by our recent studies of the PbWO4:Mo, La, Y (PWO III) crystal prepared with the use of the same technology and having similar TSL characteristics, where the hole O (I)-type centers have been observed by ESR [33]. The activation energy Ea ¼0.14 7 0.05 eV obtained for the 190 K peak creation allows the suggestion that the hole O (I)-type centers can be created as a result of the disintegration of the excitons localized near La3 þ or Y3 þ ions associated with VPb. The origin of the other strongly overlapping TSL peaks, connected with the traps with Et E 0.3 eV, remains uncertain.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence and photothermally stimulated defects creation processes in PbWO4:La3 þ , Y3 þ (PWO II) crystals E. Auffray a, M. Korjik b, S. Zazubovich c,n a
CERN, Geneva 23, Geneva, Switzerland Institute for Nuclear Problems, 11 Bobruiskaya, 220020 Minsk, Belarus c Institute of Physics, University of Tartu, Ravila 14 c, 50411 Tartu, Estonia b
art ic l e i nf o
a b s t r a c t
Article history: Received 27 April 2015 Received in revised form 24 July 2015 Accepted 14 August 2015 Available online 22 August 2015
Photoluminescence and thermally stimulated luminescence (TSL) are studied for a PbWO4 crystal grown by the Czochralski method at Bogoroditsk Technical Chemical Plant, Russia from the melt with a precise tuning of the stoichiometry and co-doped with La3 þ and Y3 þ ions (the PWO II crystal). Photothermally stimulated processes of electron and hole centers creation under selective UV irradiation of this crystal in the 3.5–5.0 eV energy range and the 85–205 K temperature range are clarified and the optically created electron and hole centers are identified. The electrons in PWO II are mainly trapped at the (WO4)2 groups located close to single La3 þ and Y3 þ ions, producing the electron {(WO4)3 –La3 þ } and {(WO4)3 –Y3 þ } centers. The holes are mainly trapped at the regular oxygen ions O2 located close to La3 þ and Y3 þ ions associated with lead vacancies, producing the hole O (I)-type centers. No evidence of single-vacancy-related centers has been observed in PWO II. The data obtained indicate that excellent scintillation characteristics of the PWO II crystal can be explained by a negligible concentration of single (non-compensated) oxygen and lead vacancies as the traps for electrons and holes, respectively. & 2015 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Thermoluminescence Defects PbWO4:La3 þ Y3 þ (PWO II) crystal
1. Introduction Single crystals of lead tungstate (PbWO4) became a subject of renewed interest about 20 years ago when their favorable characteristics for scintillation detection were reported [1–4]. Since this time, luminescence, energy transfer and defects creation processes in PbWO4 crystals were considered in a huge number of papers (see, e.g., reviews [3–7] and references therein). PbWO4 (PWO) crystals are widely used as a scintillation material for electromagnetic calorimeters in many high-energy physics projects due to its compactness, fast response, short decay time, high resolution in a wide energy range, and good radiation hardness (see, e.g., Refs. [3,4]). For example, these crystals have been successfully applied for the construction of the electromagnetic calorimeter of the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC) built in CERN, Geneva – a particle accelerator where the Higgs boson has recently been discovered [8,9]. PWO has been chosen also for several other projects, e.g., for ALICE project at CERN and for electromagnetic calorimeter of the PANDA detector at the future Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, where hadron physics in antiproton annihilation processes n
Corresponding author. E-mail address: [email protected] (S. Zazubovich).
http://dx.doi.org/10.1016/j.jlumin.2015.08.028 0022-2313/& 2015 Elsevier B.V. All rights reserved.
will be investigated [10]. However, due to thermal quenching of the PWO luminescence, the relatively low (E10 ph.e./MeV at 295 K) scintillation light yield became a limiting factor for low energy measurement applications. Considerably improved PWO crystals (denoted as PWO II) were prepared by the simultaneous doping with La3 þ and Y3 þ ions with a total concentration of less than 50 ppm and improvement of the purity and structural perfection of the crystals [11]. The study of the scintillation characteristics of these crystals showed that they have about two times higher light yield as compared with the standard PWO crystals used in the LHC, reaching at room temperature (RT) a value near 20 ph. e./MeV [11,12]. In these crystals, 97% of light is integrated within a 100 ns time window at RT [13]. At 25 °C, the light yield of the PWO II crystals becomes well above 60 ph.e./MeV [11–13]. The significantly improved light yield was obtained without loosing the fast response, excellent energy resolution, and good radiation hardness. Optical transmission of PWO II was even better as compared with the standard PWO [12] used in LHC, indicating the reduced concentration of various intrinsic defects. Just the PWO II crystals are used for the construction of the electromagnetic target calorimeter of PANDA [10]. Further, the peculiarities of the radiation damage and recovery processes at lower temperatures ( 25 °C) and other characteristics (e.g., light yield, optical transmittance, scintillation kinetics, radiation hardness, homogeneity, etc), related to the
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application of the PWO II crystals in PANDA detectors, were studied for the improved PWO II crystals (see, e.g., Refs. [14–18] and references therein). However, photoluminescence characteristics, photothermally stimulated defects creation processes and the origin of various intrinsic defects in these crystals were not studied in detail. Previous detailed studies of PbWO4 crystals of different origin, prepared in different conditions and containing different impurity ions, by the photoluminescence, thermally stimulated luminescence (TSL) and electron spin resonance (ESR) methods have shown that their characteristics can be strongly different. In many cases, the reasons of these differences have been understood, and the origin of the defects responsible for different TSL peaks has been clarified and confirmed also by the ESR data (see, e.g., Refs. [5,6,19–33] and references therein). In this work, we have tried to use this knowledge and experience at the investigation of the above-mentioned improved PWO II crystals. Namely, we have carried out the detailed study of the photoluminescence and photothermally stimulated defects creation processes in the PWO II crystals and compared the results with those obtained for the other, earlier investigated PbWO4 crystals. Our main aim was to understand the possible reason of excellent scintillation characteristics of the PWO II crystals reported in Refs. [11–13]. The investigations carried out in the present work have shown that the reason is in negligible concentration of single (non-compensated) lead and oxygen vacancies in the PWO II crystals as compared with the other PbWO4 crystals, even with the crystals doped with trivalent rare-earth ions studied for about 20 years and reported, e.g., in Refs. [3–7,22,23,26,27,34–52].
2. Experimental procedure Single crystals of PbWO4:La3 þ , Y3 þ (PWO II) were grown by the Czochralski method at Bogoroditsk Technical Chemical Plant (Bogoroditsk, Russia) from the melt with a precise tuning of the stoichiometry and contain trivalent Y and La ions with a total concentration up to 40 ppm. The steady-state emission and excitation spectra in the 85–400 K temperature range were measured using a setup, consisting of the deuterium DDS-400 lamp, two monochromators (SF-4 and SPM-1), and a vacuum nitrogen cryostat. The luminescence was detected by a photomultiplier (FEU-39 or FEU-79) with an amplifier and recorder. The spectra were corrected for the spectral distribution of the excitation light, the transmission and dispersion of the monochromators, and the spectral sensitivity of the detectors. Thermally stimulated luminescence (TSL) glow curves were measured with a heating rate of 0.2 K/s after selective irradiation of the crystals at different irradiation temperatures Tirr (85–205 K) with different irradiation energies Eirr (3.5–5.0 eV) and irradiation durations tirr (1–60 min) (for more details, see Refs. [19,20]). A crystal located in the nitrogen cryostat was irradiated with the well-focused radiation of the LOT-ORIEL xenon lamp (150 W) through a monochromator SF-4. The spectral width of the monochromator slit did not exceed 5 nm. Due to a small TSL intensity, the TSL glow curves were measured without a monochromator or filters. The integrated emission of the crystal was detected with the photomultiplier FEU-39. From these measurements, the TSL peaks creation spectra – ITSL(Eirr), activation energies Ea and dose dependences – ITSL(tirr) were obtained. To determine the depth Et of the traps corresponding to each TSL peak, the partial heating method was used (for more details, see Ref. [21] and references therein): the crystal, irradiated at the temperature Tirr, was cooled down to 85 K, heated up to a temperature Tstop, then quickly cooled down to 85 K, heated again up to the next temperature Tstop, etc. From the slope of the ln ITSL(1/T) dependence, the Et
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value was calculated. Due to a small TSL intensity, the Et values were determined also from the higher-temperature slopes of the TSL glow curves measured after irradiation at different Tirr.
3. Experimental results Under excitation at 85 K in the band-to-band transitions region, the B emission band of PWO II is located at 2.72 eV, while under excitation in the exciton band maximum, at 2.79 eV (Fig. 1, curves 1 and 2, respectively). The lowest-energy excitation band of this emission is located at 4.13 eV (curve 1′). Under 3.8 eV excitation, a weak green G(I) emission, peaking at 2.27 eV, and a weak ultraviolet (UV) emission, peaking at about 3.1 eV and investigated in Refs. [22,23], are also observed (curve 3). In the exciton and bandto-band absorption regions, the excitation spectrum measured for Eem ¼3.15 eV coincides with the excitation spectrum measured for the B emission (curve 1′), but a weak band appears around 3.9 eV in the former spectrum (curve 3′). The B emission intensity remains constant up to 150 K and then decreases (twice at 177 K, see the inset). At RT, the B emission band is located at 2.9 eV and its lowest-energy excitation band, at 3.95 eV. No 2.5 eV emission with the characteristics described in Refs. [6,20,24] (the G(II) emission), which arises from tunneling recombinations of electron and hole centers connected with single oxygen and lead vacancies [25], is observed in this crystal. The integrated TSL intensity in the PWO II crystal is about 50– 100 times weaker as compared with the PWO crystals singledoped with La3 þ or Y3 þ ions studied in Ref. [23]. After UV irradiation of PWO II at 85 K in the band-to-band transitions region, the TSL glow curve consists of a broad peak located around 122 K and a weaker peak around 170 K (Fig. 2). Both these peaks are of complex structure. No separate peaks around 100–110 K, arising 3þ 3þ } and { WO3− } centers (see, e.g., from electron { WO3− 4 –La 4 –Y Ref. [22] and references therein), which, according to Ref. [13], should surely exist in the PWO II crystal, appear at the TSL glow curve. Most probably, this is caused by too high irradiation temperature (Tirr ¼ 85 K), due to which these centers are partly thermally destroyed. The increase of Tirr results in the reduction of the TSL intensity under this irradiation in the whole temperature range. Therefore, we suggest that the centers responsible for the TSL peaks at 122 K and 170 K can arise from the re-trapping of electrons, released at the thermal destruction of the electron
Fig. 1. Normalized emission spectra of the PWO II crystal measured at 85 K under Eexc ¼5.0 eV (curve 1), Eexc ¼ 4.12 eV (curve 2), and Eexc ¼ 3.8 eV (curve 3). Normalized excitation spectra measured for Eem ¼ 2.8 eV or Eem ¼3.15 eV (curve 1′) and Eem ¼ 2.2 eV (curve 2′); for Eem ¼ 3.15 eV, the intensity around 3.9 eV is increased 10 times (curve 3′). In the inset, temperature dependence of the maximum intensity of the B emission.
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Fig. 2. TSL glow curve measured after irradiation of the PWO II crystal for 1 h at 85 K with Eirr ¼ 4.7 eV.
3þ 3þ { WO3− } and { WO3− } centers, at some deeper traps. This 4 –La 4 –Y means that the observed TSL peaks are of an electron origin. After irradiation at 140 K, the measurable TSL intensity is observed only after irradiation in the exciton and defect-related absorption region. From the comparison of these data with the results of previous studies (see, e.g., Refs. [6,19,20,25]) it can be concluded that the corresponding electron and hole centers in the PWO II crystal are also created at the disintegration of the localized exciton and defect-related states. The shape of the TSL glow curve strongly depends on the irradiation energy (Fig. 3a). The main TSL glow curve peaks are located at about 178 K and 192 K. Additional weak higher-temperature TSL peaks appear more clearly after irradiation at higher temperatures (Fig. 3b, curves 2 and 3). The creation spectra of the main E178 K and E192 K peaks have the maximum at about 4.0 eV and 3.95 eV, respectively (Fig. 4a). The comparison of the TSL peaks creation spectra with the excitation spectra of the B and G(I) emission measured at the same conditions (Fig. 4b) indicate that at Tirr ¼140 K, the recombining electron and hole centers are most effectively created in the exciton- and defect-related absorption regions. The 178 K peak (Fig. 4a, filled circles) is more effectively created in the band-to-band and exciton regions as compared with the 192 K peak (empty circles). The intensity of the 192 K peak reaches the saturation after about 20 min of the irradiation at this temperature (see the inset in Fig. 3a). From the ln ITSL(1/Tirr) dependence shown in the inset of Fig. 4a, the activation energy for the 192 K peak creation at Eirr ¼3.95 eV is found to be Ea ¼0.147 0.05 eV. In Fig. 5, the dependence of the trap depth values Et on the partial heating (Tstop) or irradiation (Tirr) temperature is presented. The values of Et in the lowest temperature range do not exceed 0.14–0.17 eV (triangles). They should arise from the electron 3þ 3þ { WO3− } and { WO3− } centers surely existing in the PWO 4 –La 4 –Y II crystals [13] but partly destroyed due to too high irradiation temperature Tirr used in this work. For the E178 K and E192 K peaks, Et ¼0.3170.02 eV and Et ¼0.4370.03 eV, respectively (circles). Due to a very small TSL intensity of the TSL glow curve peaks appearing at T 4210 K (Fig. 3), their detailed investigation is impossible.
4. Discussion The self-trapped electrons ({ WO3− 4 } centers), electrons trapped 3þ 3þ } and {WO3− } close to trivalent rare-earth ions (e.g., {WO3− 4 –La 4 –Y centers), and single oxygen vacancy (VO) related electron centers (e.g., {Pb þ –VO}) were detected by ESR in PbWO4 crystals [5,26–29].
Fig. 3. TSL glow curves measured for the PWO II crystal after irradiation (a) at Tirr ¼ 140 K with Eirr ¼4.2 eV (curve 1), Eirr ¼ 3.95 eV (curve 2), and Eirr ¼3.75 eV (curve 3); (b) with Eirr ¼3.95 eV at Tirr ¼ 140 K (curve 1), Tirr ¼ 172 K (curve 2), and Tirr ¼ 183 K (curve 3). The TSL peak intensities at 192–194 K are normalized. In the inset, the dose dependence measured for the 192 K peak at Tirr ¼ 140 K under irradiation with Eirr ¼ 3.95 eV.
Recently [30,31], the O (I)-type hole centers (a hole trapped at an oxygen ion close to a La3 þ or Y3 þ ion and a lead vacancy VPb) were also found and studied. An existence of non-paramagnetic hole centers (two holes trapped at two oxygen ions in the vicinity of a single lead vacancy VPb) was suggested in [32]. Tunneling recombination in the pairs of the vacancy-related electron and hole centers was found to result in the appearance of the G(II) emission [6,20,24,25]. A small intensity of the G(II) luminescence in the PWO II crystals indicates that the concentration of single (non-compensated) oxygen and lead vacancies as the traps for electrons and holes, respectively, in these crystals are small. Therefore, the appearance in these crystals of the shallow electron centers of 3þ 3þ 3− the types of WO3− } and { WO3− } and the holes 4 , { WO 4 –La 4 –Y 3þ 3þ or Y ions associated trapped at oxygen ions close to the La with VPb vacancies (the hole O (I)-type centers) is mainly expected. Indeed, the TSL glow peaks at about 195 K, arising from the thermal destruction of the single oxygen vacancy related electron {Pb þ –VO} centers with the trap depth Et ¼0.56 eV [19,25,27], is not detected in these crystals. Due to a negligible concentration of single oxygen vacancies, the concentration of single lead vacancies should be negligible as well. Therefore, the peak at 225–230 K cannot be of the same origin as that observed in Ref. [25] and suggested to arise from non-paramagnetic hole centers of the type of {2O –VPb}, i.e. two holes trapped at the oxygen ions located close to a single lead vacancy VPb. The peak around 190 K with Et ¼0.43 7 0.03 eV, most probably, arises from the thermal destruction of the hole O (I)-type centers [30]. This
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5. Conclusions The electrons, optically created in the PbWO4:La3 þ , Y3 þ (PWO II) crystals studied in this work, are mainly trapped at the (WO4)2 groups located close to the La3 þ and Y3 þ ions as well as at some unidentified defects. The optically created holes are mainly trapped at the oxygen ions, located close to La3 þ and Y3 þ ions and lead vacancies VPb, producing the O (I)-type hole centers. The PWO II crystals are found to contain extremely small concentrations of electron and hole centers connected with oxygen and lead vacancies. Due to that the tunneling recombination G(II) luminescence in these crystals is very weak, and, consequently, the slow components in their green luminescence decay should be extremally weak as well. Thus, the obtained results indicate that the excellent scintillation characteristics of the PWO II crystal reported in Refs. [11–13] can be explained by a negligible concentration of single (non-compensated) oxygen and lead vacancies as the traps for electrons and holes, respectively. The data obtained in this work could be useful for further development and improvement of PbWO4-based scintillation materials.
Acknowledgments The work is supported by the Institutional Research Funding IUT02-26 of the Estonian Ministry of Education and Research.
Fig. 4. (a) Creation spectra of the TSL glow curve peaks located at 178 K (filled circles) and 192 K (empty circles) measured for the PWO II crystal at Tirr ¼ 142 K; (b) Excitation spectra of the B (curve 1) and G(I) (curve 2) emissions measured at the same conditions. The spectra are not corrected on the excitation intensity. In the inset, the ln ITSL(1/Tirr) dependence is presented for the 192 K peak under Eirr ¼ 3.95 eV.
Fig. 5. Dependence of trap depth value Et on the temperature Tirr or Tstop measured after irradiation of the PWO II crystal with Eirr ¼ 4.7 eV (triangles) and Eirr ¼ 3.95 eV (circles).
conclusion is confirmed by our recent studies of the PbWO4:Mo, La, Y (PWO III) crystal prepared with the use of the same technology and having similar TSL characteristics, where the hole O (I)-type centers have been observed by ESR [33]. The activation energy Ea ¼0.14 7 0.05 eV obtained for the 190 K peak creation allows the suggestion that the hole O (I)-type centers can be created as a result of the disintegration of the excitons localized near La3 þ or Y3 þ ions associated with VPb. The origin of the other strongly overlapping TSL peaks, connected with the traps with Et E 0.3 eV, remains uncertain.
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