- Email: [email protected]
Scintillation properties of Pr-activated LuAlO3
Optical Materials 28 (2006) 102–105 www.elsevier.com/locate/optmat
Scintillation properties of Pr-activated LuAlO3 Winicjusz Drozdowski a,*, Andrzej J. Wojtowicz a, Dariusz Wis´niewski a, Tadeusz Łukasiewicz b, Jarosław Kisielewski b a
Institute of Physics, Nicolaus Copernicus University, Grudzia˛dzka 5/7, 87-100 Torun´, Poland b Institute of Electronic Materials Technology, Wo´lczyn´ska 133, 01-919 Warsaw, Poland Received 28 September 2004; accepted 30 September 2004 Available online 31 May 2005
Abstract Praseodymium activated LuAlO3 (LuAP) crystals have been grown using the Czochralski method at ITME, Warsaw. In this communication the measurements of radioluminescence (RL), low temperature thermoluminescence (TL), room temperature afterglow (AG), scintillation light yields (LY), and scintillation time profiles (STP), performed on polished 2 · 2 · 10 mm pixels with three Pr concentrations (0.003, 0.04, and 0.08 at.%), are reported. Two sets of samples are compared: (i) ‘‘as grown’’, and (ii) annealed in H2 atmosphere. 2005 Elsevier B.V. All rights reserved. PACS: 29.40.M; 78.60.K; 81.10.F Keywords: Scintillation; Thermoluminescence; Czochralski method
1. Introduction The term ‘‘radioluminescence’’ is used to describe the emission of light (mainly visible or UV) from a sample which is being excited by ionizing radiation. Materials which exhibit radioluminescence are termed ‘‘scintillators’’, and a single flash of light being a response to absorption of a quantum or ionizing particle is called ‘‘scintillation’’. The broad application of scintillators comprises various scientific studies (high energy physics, nuclear physics, astronomy, chemistry), medical equipment (X-ray computer tomographs, positron tomographs), (PET), and industrial uses (product quality control, security and control systems in air transport, oil ledges exploration). Although the number of known scintillators comes up to several hundreds, new and better materials are *
Corresponding author. Tel.: +48 56 611 3319; fax: +48 56 622 5397. E-mail address: [email protected] (W. Drozdowski).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.09.031
still being developed in order to satisfy many requirements dependent on particular applications [1,2]. Not going into details, one can say that a modern, high-performance scintillator should be both fast and efficient. In the last decade, the promising cerium activated crystals such as perovskites, LuAlO3 (LuAP) and YAlO3 (YAP), garnets, Lu3Al5O15 (LuAG) and Y3Al5O12 (YAG), and orthosilicates, Lu2SiO5 (LSO) and Y2SiO5 (YSO), have been investigated thoroughly [3–9]. Following these studies, the same materials activated with others ions, like praseodymium or ytterbium, have also received some attention [10–12]. In this paper we focus on the scintillation properties of LuAP:Pr.
2. Materials and experiment The LuAlO3:Pr crystals have been grown by the Czochralski method using a Cyberstar oxypuller. As a container of the melt an iridium crucible of 50 mm
W. Drozdowski et al. / Optical Materials 28 (2006) 102–105
3. Results and conclusion In our recent paper [12] we have reported the basic scintillation properties of three ‘‘as grown’’ LuAlO3:Pr crystals with different praseodymium concentrations (0.003, 0.04, and 0.08 at.%). We have shown that one of the samples (0.003 at.% Pr) demonstrates an individual behavior (particularly in respect of thermoluminescence); the two other ones (0.04 and 0.08 at.%) are quite similar. For that reason, in the present paper we
limit our attention to the two outermost concentrations, giving place to the aspect of crystal annealing in H2 atmosphere. Radioluminescence spectra of LuAlO3:Pr, measured at 295 and 10 K, are presented in Fig. 1. At higher concentration of the Pr3+ ions (Fig. 1a) they are dominated by the well-known Pr3+ emissions: stronger d–f bands (peaking at about 250 and 280 nm) and weaker f–f lines (490–500 and 610–620 nm). At lower praseodymium content (Fig. 1b) additional bands appear beside the Pr3+ luminescence. The 300 nm band, noted before [16], prevails at room temperature, while the 210 nm band (which we also ascribe to the LuAlO3 host) appears predominantly at 10 K. The process of annealing in hydrogen does not affect the RL spectra. Thermoluminescence of LuAlO3:Pr is shown in Fig. 2. The glow curves of the ‘‘as grown’’ samples have already been analyzed [12]. We have pointed out that a complex spectrum consisting of a number of peaks (Fig. 2a), resembling to some extent a glow curve of undoped or cerium doped LuAlO3 [3], observed at lower Pr concentration (0.003 at.%), is replaced, for higher Pr contents (0.04 and 0.08 at.%), by a spectrum dominated by one peak imposed on a much higher background (Fig. 2b). The curves have been separated into first-order Randall–Wilkins [17] peaks, and the basic trap parameters (depths and frequency factors) have been figured out. The same features are now observed in case of the H2-annealed samples. Their glow curves can be fairly well reproduced with comparable values of parameters. As the steady-state radioluminescence intensity during the irradiation prior to the TL runs has also been
intensity (arb. units)
diameter and 50 mm high has been used. The growth processes controlled by a computer program have been performed in the N2 atmosphere, with the rate of pulling of 1 mm/h and the rate of growing crystal rotation of 15 rpm. In this way, crystals of 20 mm diameter and 50 mm long, free from bubbles, twins and cracks, have been obtained. Additionally, a few samples have been annealed in H2 atmosphere with a view to reducing the concentration of charge traps present in the ‘‘as grown’’ crystals. A typical set-up consisting of an X-ray tube operated at 42 kV and 10 mA, a 0.5 m monochromator (Acton Research Corporation SpectraPro-500), a photomultiplier (Hamamatsu R928), and a closed-cycle helium cooler (APD Cryogenics, Inc.) with a programmable temperature controller (Lake Shore 330), has been used to record room and low temperature X-ray excited emission spectra, afterglow decays, and thermoluminescence glow curves. The spectra have not been corrected for the spectral sensitivity of the detection part. The energy spectra necessary to determine the scintillation light yields have been collected under the 0.511 and 1.274 MeV gamma-ray excitation (22Na source). The pulsed output signal from a photomultiplier (Hamamatsu R2059) has been processed by an integrating preamplifier (Canberra 2005), a spectroscopy amplifier (Canberra 2022), and a multichannel analyzer. Positions of photopeaks (corresponding to full energy scintillations) in energy spectra, depending on photomultiplier and amplifier gains, have been used to evaluate relative light yields of particular samples by comparing them to positions of photopeaks in the spectra of a typical reference crystal (BGO, 2 · 2 · 10 mm; 621 photoelectrons per 1 MeV) [13]. Scintillation time profiles have been measured by a delayed coincidence single photon counting method [14]. The sample has been excited by gamma photons from a 137Cs source and time profiles of subsequent emission pulses have been registered by collecting a large number of scintillation photons and recording their time distribution. Two photomultipliers (Hamamatsu R1104), a time-to-amplitude converter (Canberra TAC/SCA 2145), and a multichannel analyzer have been employed [15].
103
a
LuAP:0.08at%Pr "as grown" radioluminescence RT LHeT
b
LuAP:0.003at%Pr "as grown" radioluminescence
RT LHeT
200
250
300
350
400
450
500
550
wavelength (nm) Fig. 1. Radioluminescence spectra of LuAP:Pr.
600
650
104
W. Drozdowski et al. / Optical Materials 28 (2006) 102–105
LuAP:0.003at%Pr thermoluminescence "as grown" H 2-annealed
steady-state radiolumin. at 10 K
b
LuAP:0.08at%Pr afterglow "as grown" H2 - annealed
intensity (arb.units)
intensity (arb. units)
a
LuAP:0.08at%Pr thermoluminescence "as grown" H2 -annealed
steady-state radiolumin. at 295 K
0
10
20
30
40
50
60
time (min) Fig. 3. Afterglow curves of LuAP:Pr, following a 10 min X-ray irradiation at 295 K.
steady-state radiolumin. at 10 K
10
50
100
150
200
250
300
temperature (K) Fig. 2. Glow curves of LuAP:Pr, recorded at 9 K/min following a 10 min X-ray irradiation at 10 K.
recorded, it is possible to evaluate a ratio (defined as TL/ (TL + ssRL)) indicating the fraction of the total excitation energy that has been accumulated in traps (Table 1). Besides a clear tendency toward higher ratio values for the increasing praseodymium concentration, one can easily notice a drop of these values in the samples annealed in H2 atmosphere. Since traps producing peaks below room temperature are relatively shallow, they can modify scintillation time profiles by reducing fast components and introducing slower ones. This effect may, in turn, result in lowered LY values, when measured in a short timegate. However, light yields can also be decreased by deeper traps, not detectable in low temperature thermoluminescence. To check out the possible presence of deep traps in LuAlO3:Pr, afterglow curves have been recorded at room temperature. Just as in low temperature thermoluminescence, the samples with lower Pr content (0.003 at.%) have revealed different properties comparing to the higher doped ones (0.04 and 0.08 at.%). In the former case (0.003 at.%) no afterglow has been perceived, suggesting that in these crystals the deep trap concentration is rather negligible. The latter case (0.04
and 0.08 at.%) provides us with multi-exponential decays (Fig. 3), indicating the existence of deep traps. Putting the TL and AG results together we suppose that the distribution of charge traps in LuAlO3:Pr is strongly correlated with praseodymium concentration: the higher the Pr content, the deeper the traps. We also note that the value of the AG/(AG + ssRL) ratio, taken from 7-h AG measurements, is about two times smaller for the H2-annealed sample than for the ‘‘as grown’’ one (Table 1). Representative energy spectra of LuAlO3:Pr are presented in Fig. 4. The photopeaks are much better resolved in the spectra of the crystals annealed in hydrogen atmosphere. However, a shift of the photopeak promising a higher scintillation light yield after H2annealing appears only for the samples with the higher praseodymium concentration (0.04 and 0.08 at.%). The values of LY, calculated from series of energy spectra, confirm these observations (Table 1). Although the process of annealing in hydrogen has increased the light output of two LuAP:Pr samples (0.04 and 0.08 at.%) almost twice, the achieved value (57% of the reference BGO LY) is still far away from requirements. This fact suggests a presence of at least two mechanisms quenching the scintillation process: ‘‘extrinsic’’ (nonradiative recombination centers, traps, etc.) and ‘‘intrinsic’’ (e.g. relaxation between the Pr3+ ions). To improve the scintillation performance of LuAlO3:Pr one should weaken the role of both mechanisms.
Table 1 Scintillation properties of the ‘‘as grown’’ and H2-annealed LuAP:Pr crystals Sample
TL/(TL + ssRL)
AG/(AG + ssRL)
LY (rel. to BGO)
Scintillation decay times (ns)
0.003 at.% Pr 0.08 at.% Pr
‘‘As grown’’
0.10 0.23
– 0.016
0.30 0.35
11.7 (84%), 568 (16%) 9.4 (76%), 80 (9%), 550 (15%)
0.003 at.% Pr 0.08 at.% Pr
Annealed in H2
0.07 0.16
– 0.009
0.30 0.57
13.9 (85%), 313 (15%) 8.8 (83%), 81 (17%)
counts
W. Drozdowski et al. / Optical Materials 28 (2006) 102–105
a
LuAP:0.003at%Pr energy spectra "as grown" H 2-annealed
b
LuAP:0.08at%Pr energy spectra "as grown" H 2-annealed
105
centration as well as after H2-annealing (Table 1). The decay constant below 10 ns is a strong point of this material. Summarizing, praseodymium activated LuAlO3 is a fast, but somewhat inefficient scintillator. In the present days a material with a light yield two times lower than BGO cannot be regarded as a promising modern detector of ionizing radiation. Unfortunately, due to the suggested process of ‘‘cross-relaxation’’ between the Pr3+ ions, a substantial improvement of the LuAP:Pr performance seems to be unfeasible.
Acknowledgements This work was supported by the Ministry of Scientific Research and Information Technology of Poland, and Nicolaus Copernicus University (grant no. 402-F/2003).
0
50
100
150
200
250
300
350
400
channel number Fig. 4. Energy spectra of LuAP:Pr (22Na source, PMT voltage: 1500 V, amplifier gain: 18).
LuAP:Pr H2-annealed
intensity (arb. units)
scintillation time profiles 0.003at% 0.08at%
0
200
400
600
800
1000
time (ns) Fig. 5. Scintillation time profiles of LuAP:Pr.
Even if a further reduction of the trap concentration is possible, the ‘‘intrinsic’’ mechanism is rather insuperable. The positions of the Pr3+ 4f5d and 4f2 levels in the orthoaluminate lattice enable a kind of ‘‘cross-relaxation’’ between the Pr3+ ions, which may quench the 4f5d levels and promote the 4f2 ones. The relatively high intensities of the Pr3+ f–f lines in the steady-state radioluminescence of LuAP:Pr confirm this explanation. The scintillation time profiles of LuAlO3:Pr (Fig. 5) can be represented reasonably well by two- or threeexponential decay curves. The scintillation grows faster and the fraction of the total light emitted in the short components tends to increase with praseodymium con-
References [1] S.E. Derenzo, W.W. Moses, J.L. Cahoon, T.A. DeVol, L. Boatner, in: Conference Record of the 1991 IEEE Nuclear Science Symposium and Medical Imaging Conference, New York, 1991, p. 143. [2] C.W.E. van Eijk, in: P.A. Rodnyi, C.W.E. van Eijk (Eds.), Record of the International Workshop on Physical Processes in Fast Scintillators, Delft University of Technology, Delft, 1994, p. 1. [3] A.J. Wojtowicz, P. Szupryczynski, D. Wisniewski, J. Glodo, W. Drozdowski, J. Phys: Condens. Matter 13 (2001) 9599. [4] A.J. Wojtowicz, J. Glodo, A. Lempicki, C. Brecher, J. Phys. Condens. Matter 10 (1998) 8401. [5] A. Lempicki, J. Glodo, Nucl. Instr. Meth. A 416 (1998) 333. [6] K. Wisniewski, Cz. Koepke, A.J. Wojtowicz, W. Drozdowski, M. Grinberg, S.M. Kaczmarek, J. Kisielewski, Acta Phys. Pol. A 95 (1999) 403. [7] C.L. Melcher, J.S. Schweitzer, Nucl. Instr. Meth. A 314 (1992) 212. [8] C.L. Melcher, J.S. Schweitzer, IEEE Trans. Nucl. Sci. NS-39 (1992) 502. [9] W. Drozdowski, A.J. Wojtowicz, D. Wis´niewski, P. Szupryczyn´ski, S. Janus, J.L. Lefaucheur, Z. Gou, J. Alloys Comp. 380 (2004) 146. [10] C. Dujardin, C. Pedrini, J.C. Gacon, A.G. Petrosyan, A.N. Belsky, A.N. VasilÕev, J. Phys. Condens. Matter 9 (1997) 5229. [11] S. Nicolas, E. Descroix, Y. Guyot, M.F. Joubert, C. Pedrini, Z.K.L. Ovanesyan, G.O. Shirinyan, A. Petrosyan, Opt. Mater. 19 (2002) 129. [12] W. Drozdowski, T. Łukasiewicz, A.J. Wojtowicz, D. Wis´niewski, J. Kisielewski, J. Cryst. Growth 275 (2005) 709. [13] A.J. Wojtowicz, W. Drozdowski, J.L. Lefaucheur, Z. Gała˛zka, Z. Gou, Nucl. Instr. Meth. A, to be published. [14] L.M. Bollinger, G.E. Thomas, Rev. Sci. Instr. 32 (1961) 1044. [15] D. Wis´niewski, A.J. Wojtowicz, W. Drozdowski, J.M. Farmer, L.A. Boatner, Cryst. Res. Technol. 38 (2003) 275. [16] D. Wis´niewski, W. Drozdowski, A.J. Wojtowicz, A. Łempicki, P. Dorenbos, J.T.M. de Haas, C.W.E. van Eijk, A.J.J. Bos, Acta Phys. Pol. A 90 (1996) 377. [17] J.T. Randall, M.H.F. Wilkins, Proc. Roy. Soc. London A 184 (1945) 366.
Scintillation properties of Pr-activated LuAlO3 Winicjusz Drozdowski a,*, Andrzej J. Wojtowicz a, Dariusz Wis´niewski a, Tadeusz Łukasiewicz b, Jarosław Kisielewski b a
Institute of Physics, Nicolaus Copernicus University, Grudzia˛dzka 5/7, 87-100 Torun´, Poland b Institute of Electronic Materials Technology, Wo´lczyn´ska 133, 01-919 Warsaw, Poland Received 28 September 2004; accepted 30 September 2004 Available online 31 May 2005
Abstract Praseodymium activated LuAlO3 (LuAP) crystals have been grown using the Czochralski method at ITME, Warsaw. In this communication the measurements of radioluminescence (RL), low temperature thermoluminescence (TL), room temperature afterglow (AG), scintillation light yields (LY), and scintillation time profiles (STP), performed on polished 2 · 2 · 10 mm pixels with three Pr concentrations (0.003, 0.04, and 0.08 at.%), are reported. Two sets of samples are compared: (i) ‘‘as grown’’, and (ii) annealed in H2 atmosphere. 2005 Elsevier B.V. All rights reserved. PACS: 29.40.M; 78.60.K; 81.10.F Keywords: Scintillation; Thermoluminescence; Czochralski method
1. Introduction The term ‘‘radioluminescence’’ is used to describe the emission of light (mainly visible or UV) from a sample which is being excited by ionizing radiation. Materials which exhibit radioluminescence are termed ‘‘scintillators’’, and a single flash of light being a response to absorption of a quantum or ionizing particle is called ‘‘scintillation’’. The broad application of scintillators comprises various scientific studies (high energy physics, nuclear physics, astronomy, chemistry), medical equipment (X-ray computer tomographs, positron tomographs), (PET), and industrial uses (product quality control, security and control systems in air transport, oil ledges exploration). Although the number of known scintillators comes up to several hundreds, new and better materials are *
Corresponding author. Tel.: +48 56 611 3319; fax: +48 56 622 5397. E-mail address: [email protected] (W. Drozdowski).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.09.031
still being developed in order to satisfy many requirements dependent on particular applications [1,2]. Not going into details, one can say that a modern, high-performance scintillator should be both fast and efficient. In the last decade, the promising cerium activated crystals such as perovskites, LuAlO3 (LuAP) and YAlO3 (YAP), garnets, Lu3Al5O15 (LuAG) and Y3Al5O12 (YAG), and orthosilicates, Lu2SiO5 (LSO) and Y2SiO5 (YSO), have been investigated thoroughly [3–9]. Following these studies, the same materials activated with others ions, like praseodymium or ytterbium, have also received some attention [10–12]. In this paper we focus on the scintillation properties of LuAP:Pr.
2. Materials and experiment The LuAlO3:Pr crystals have been grown by the Czochralski method using a Cyberstar oxypuller. As a container of the melt an iridium crucible of 50 mm
W. Drozdowski et al. / Optical Materials 28 (2006) 102–105
3. Results and conclusion In our recent paper [12] we have reported the basic scintillation properties of three ‘‘as grown’’ LuAlO3:Pr crystals with different praseodymium concentrations (0.003, 0.04, and 0.08 at.%). We have shown that one of the samples (0.003 at.% Pr) demonstrates an individual behavior (particularly in respect of thermoluminescence); the two other ones (0.04 and 0.08 at.%) are quite similar. For that reason, in the present paper we
limit our attention to the two outermost concentrations, giving place to the aspect of crystal annealing in H2 atmosphere. Radioluminescence spectra of LuAlO3:Pr, measured at 295 and 10 K, are presented in Fig. 1. At higher concentration of the Pr3+ ions (Fig. 1a) they are dominated by the well-known Pr3+ emissions: stronger d–f bands (peaking at about 250 and 280 nm) and weaker f–f lines (490–500 and 610–620 nm). At lower praseodymium content (Fig. 1b) additional bands appear beside the Pr3+ luminescence. The 300 nm band, noted before [16], prevails at room temperature, while the 210 nm band (which we also ascribe to the LuAlO3 host) appears predominantly at 10 K. The process of annealing in hydrogen does not affect the RL spectra. Thermoluminescence of LuAlO3:Pr is shown in Fig. 2. The glow curves of the ‘‘as grown’’ samples have already been analyzed [12]. We have pointed out that a complex spectrum consisting of a number of peaks (Fig. 2a), resembling to some extent a glow curve of undoped or cerium doped LuAlO3 [3], observed at lower Pr concentration (0.003 at.%), is replaced, for higher Pr contents (0.04 and 0.08 at.%), by a spectrum dominated by one peak imposed on a much higher background (Fig. 2b). The curves have been separated into first-order Randall–Wilkins [17] peaks, and the basic trap parameters (depths and frequency factors) have been figured out. The same features are now observed in case of the H2-annealed samples. Their glow curves can be fairly well reproduced with comparable values of parameters. As the steady-state radioluminescence intensity during the irradiation prior to the TL runs has also been
intensity (arb. units)
diameter and 50 mm high has been used. The growth processes controlled by a computer program have been performed in the N2 atmosphere, with the rate of pulling of 1 mm/h and the rate of growing crystal rotation of 15 rpm. In this way, crystals of 20 mm diameter and 50 mm long, free from bubbles, twins and cracks, have been obtained. Additionally, a few samples have been annealed in H2 atmosphere with a view to reducing the concentration of charge traps present in the ‘‘as grown’’ crystals. A typical set-up consisting of an X-ray tube operated at 42 kV and 10 mA, a 0.5 m monochromator (Acton Research Corporation SpectraPro-500), a photomultiplier (Hamamatsu R928), and a closed-cycle helium cooler (APD Cryogenics, Inc.) with a programmable temperature controller (Lake Shore 330), has been used to record room and low temperature X-ray excited emission spectra, afterglow decays, and thermoluminescence glow curves. The spectra have not been corrected for the spectral sensitivity of the detection part. The energy spectra necessary to determine the scintillation light yields have been collected under the 0.511 and 1.274 MeV gamma-ray excitation (22Na source). The pulsed output signal from a photomultiplier (Hamamatsu R2059) has been processed by an integrating preamplifier (Canberra 2005), a spectroscopy amplifier (Canberra 2022), and a multichannel analyzer. Positions of photopeaks (corresponding to full energy scintillations) in energy spectra, depending on photomultiplier and amplifier gains, have been used to evaluate relative light yields of particular samples by comparing them to positions of photopeaks in the spectra of a typical reference crystal (BGO, 2 · 2 · 10 mm; 621 photoelectrons per 1 MeV) [13]. Scintillation time profiles have been measured by a delayed coincidence single photon counting method [14]. The sample has been excited by gamma photons from a 137Cs source and time profiles of subsequent emission pulses have been registered by collecting a large number of scintillation photons and recording their time distribution. Two photomultipliers (Hamamatsu R1104), a time-to-amplitude converter (Canberra TAC/SCA 2145), and a multichannel analyzer have been employed [15].
103
a
LuAP:0.08at%Pr "as grown" radioluminescence RT LHeT
b
LuAP:0.003at%Pr "as grown" radioluminescence
RT LHeT
200
250
300
350
400
450
500
550
wavelength (nm) Fig. 1. Radioluminescence spectra of LuAP:Pr.
600
650
104
W. Drozdowski et al. / Optical Materials 28 (2006) 102–105
LuAP:0.003at%Pr thermoluminescence "as grown" H 2-annealed
steady-state radiolumin. at 10 K
b
LuAP:0.08at%Pr afterglow "as grown" H2 - annealed
intensity (arb.units)
intensity (arb. units)
a
LuAP:0.08at%Pr thermoluminescence "as grown" H2 -annealed
steady-state radiolumin. at 295 K
0
10
20
30
40
50
60
time (min) Fig. 3. Afterglow curves of LuAP:Pr, following a 10 min X-ray irradiation at 295 K.
steady-state radiolumin. at 10 K
10
50
100
150
200
250
300
temperature (K) Fig. 2. Glow curves of LuAP:Pr, recorded at 9 K/min following a 10 min X-ray irradiation at 10 K.
recorded, it is possible to evaluate a ratio (defined as TL/ (TL + ssRL)) indicating the fraction of the total excitation energy that has been accumulated in traps (Table 1). Besides a clear tendency toward higher ratio values for the increasing praseodymium concentration, one can easily notice a drop of these values in the samples annealed in H2 atmosphere. Since traps producing peaks below room temperature are relatively shallow, they can modify scintillation time profiles by reducing fast components and introducing slower ones. This effect may, in turn, result in lowered LY values, when measured in a short timegate. However, light yields can also be decreased by deeper traps, not detectable in low temperature thermoluminescence. To check out the possible presence of deep traps in LuAlO3:Pr, afterglow curves have been recorded at room temperature. Just as in low temperature thermoluminescence, the samples with lower Pr content (0.003 at.%) have revealed different properties comparing to the higher doped ones (0.04 and 0.08 at.%). In the former case (0.003 at.%) no afterglow has been perceived, suggesting that in these crystals the deep trap concentration is rather negligible. The latter case (0.04
and 0.08 at.%) provides us with multi-exponential decays (Fig. 3), indicating the existence of deep traps. Putting the TL and AG results together we suppose that the distribution of charge traps in LuAlO3:Pr is strongly correlated with praseodymium concentration: the higher the Pr content, the deeper the traps. We also note that the value of the AG/(AG + ssRL) ratio, taken from 7-h AG measurements, is about two times smaller for the H2-annealed sample than for the ‘‘as grown’’ one (Table 1). Representative energy spectra of LuAlO3:Pr are presented in Fig. 4. The photopeaks are much better resolved in the spectra of the crystals annealed in hydrogen atmosphere. However, a shift of the photopeak promising a higher scintillation light yield after H2annealing appears only for the samples with the higher praseodymium concentration (0.04 and 0.08 at.%). The values of LY, calculated from series of energy spectra, confirm these observations (Table 1). Although the process of annealing in hydrogen has increased the light output of two LuAP:Pr samples (0.04 and 0.08 at.%) almost twice, the achieved value (57% of the reference BGO LY) is still far away from requirements. This fact suggests a presence of at least two mechanisms quenching the scintillation process: ‘‘extrinsic’’ (nonradiative recombination centers, traps, etc.) and ‘‘intrinsic’’ (e.g. relaxation between the Pr3+ ions). To improve the scintillation performance of LuAlO3:Pr one should weaken the role of both mechanisms.
Table 1 Scintillation properties of the ‘‘as grown’’ and H2-annealed LuAP:Pr crystals Sample
TL/(TL + ssRL)
AG/(AG + ssRL)
LY (rel. to BGO)
Scintillation decay times (ns)
0.003 at.% Pr 0.08 at.% Pr
‘‘As grown’’
0.10 0.23
– 0.016
0.30 0.35
11.7 (84%), 568 (16%) 9.4 (76%), 80 (9%), 550 (15%)
0.003 at.% Pr 0.08 at.% Pr
Annealed in H2
0.07 0.16
– 0.009
0.30 0.57
13.9 (85%), 313 (15%) 8.8 (83%), 81 (17%)
counts
W. Drozdowski et al. / Optical Materials 28 (2006) 102–105
a
LuAP:0.003at%Pr energy spectra "as grown" H 2-annealed
b
LuAP:0.08at%Pr energy spectra "as grown" H 2-annealed
105
centration as well as after H2-annealing (Table 1). The decay constant below 10 ns is a strong point of this material. Summarizing, praseodymium activated LuAlO3 is a fast, but somewhat inefficient scintillator. In the present days a material with a light yield two times lower than BGO cannot be regarded as a promising modern detector of ionizing radiation. Unfortunately, due to the suggested process of ‘‘cross-relaxation’’ between the Pr3+ ions, a substantial improvement of the LuAP:Pr performance seems to be unfeasible.
Acknowledgements This work was supported by the Ministry of Scientific Research and Information Technology of Poland, and Nicolaus Copernicus University (grant no. 402-F/2003).
0
50
100
150
200
250
300
350
400
channel number Fig. 4. Energy spectra of LuAP:Pr (22Na source, PMT voltage: 1500 V, amplifier gain: 18).
LuAP:Pr H2-annealed
intensity (arb. units)
scintillation time profiles 0.003at% 0.08at%
0
200
400
600
800
1000
time (ns) Fig. 5. Scintillation time profiles of LuAP:Pr.
Even if a further reduction of the trap concentration is possible, the ‘‘intrinsic’’ mechanism is rather insuperable. The positions of the Pr3+ 4f5d and 4f2 levels in the orthoaluminate lattice enable a kind of ‘‘cross-relaxation’’ between the Pr3+ ions, which may quench the 4f5d levels and promote the 4f2 ones. The relatively high intensities of the Pr3+ f–f lines in the steady-state radioluminescence of LuAP:Pr confirm this explanation. The scintillation time profiles of LuAlO3:Pr (Fig. 5) can be represented reasonably well by two- or threeexponential decay curves. The scintillation grows faster and the fraction of the total light emitted in the short components tends to increase with praseodymium con-
References [1] S.E. Derenzo, W.W. Moses, J.L. Cahoon, T.A. DeVol, L. Boatner, in: Conference Record of the 1991 IEEE Nuclear Science Symposium and Medical Imaging Conference, New York, 1991, p. 143. [2] C.W.E. van Eijk, in: P.A. Rodnyi, C.W.E. van Eijk (Eds.), Record of the International Workshop on Physical Processes in Fast Scintillators, Delft University of Technology, Delft, 1994, p. 1. [3] A.J. Wojtowicz, P. Szupryczynski, D. Wisniewski, J. Glodo, W. Drozdowski, J. Phys: Condens. Matter 13 (2001) 9599. [4] A.J. Wojtowicz, J. Glodo, A. Lempicki, C. Brecher, J. Phys. Condens. Matter 10 (1998) 8401. [5] A. Lempicki, J. Glodo, Nucl. Instr. Meth. A 416 (1998) 333. [6] K. Wisniewski, Cz. Koepke, A.J. Wojtowicz, W. Drozdowski, M. Grinberg, S.M. Kaczmarek, J. Kisielewski, Acta Phys. Pol. A 95 (1999) 403. [7] C.L. Melcher, J.S. Schweitzer, Nucl. Instr. Meth. A 314 (1992) 212. [8] C.L. Melcher, J.S. Schweitzer, IEEE Trans. Nucl. Sci. NS-39 (1992) 502. [9] W. Drozdowski, A.J. Wojtowicz, D. Wis´niewski, P. Szupryczyn´ski, S. Janus, J.L. Lefaucheur, Z. Gou, J. Alloys Comp. 380 (2004) 146. [10] C. Dujardin, C. Pedrini, J.C. Gacon, A.G. Petrosyan, A.N. Belsky, A.N. VasilÕev, J. Phys. Condens. Matter 9 (1997) 5229. [11] S. Nicolas, E. Descroix, Y. Guyot, M.F. Joubert, C. Pedrini, Z.K.L. Ovanesyan, G.O. Shirinyan, A. Petrosyan, Opt. Mater. 19 (2002) 129. [12] W. Drozdowski, T. Łukasiewicz, A.J. Wojtowicz, D. Wis´niewski, J. Kisielewski, J. Cryst. Growth 275 (2005) 709. [13] A.J. Wojtowicz, W. Drozdowski, J.L. Lefaucheur, Z. Gała˛zka, Z. Gou, Nucl. Instr. Meth. A, to be published. [14] L.M. Bollinger, G.E. Thomas, Rev. Sci. Instr. 32 (1961) 1044. [15] D. Wis´niewski, A.J. Wojtowicz, W. Drozdowski, J.M. Farmer, L.A. Boatner, Cryst. Res. Technol. 38 (2003) 275. [16] D. Wis´niewski, W. Drozdowski, A.J. Wojtowicz, A. Łempicki, P. Dorenbos, J.T.M. de Haas, C.W.E. van Eijk, A.J.J. Bos, Acta Phys. Pol. A 90 (1996) 377. [17] J.T. Randall, M.H.F. Wilkins, Proc. Roy. Soc. London A 184 (1945) 366.