- Email: [email protected]
Crystal growth and scintillation properties of Pr-doped oxyorthosilicate for different concentration
Nuclear Instruments and Methods in Physics Research A 643 (2011) 64–68
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Crystal growth and scintillation properties of Pr-doped oxyorthosilicate for different concentration Daisuke Totsuka a,b,n, Takayuki Yanagida d, Yutaka Fujimoto a, Jan Pejchal a,c, Yuui Yokota a, Akira Yoshikawa a,d a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan Nihon Kessho Kogaku Co. Ltd, Japan c Institute of Physics AS CR, Cukrovarnicka 10, Prague 6, 162-53, Czech Republic d New Industry Creation Hatchery Center (NICHe) 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 November 2010 Received in revised form 17 February 2011 Accepted 31 March 2011 Available online 14 April 2011
0.05, 0.1 and 0.25 mol% Pr (with respect to Lu) doped Lu2SiO5 (LSO) single crystals were grown by the micro-pulling down (m-PD) method. The grown crystals were transparent, and a slight segregation of Pr3 þ was observed both in the crystal cross-section and growth direction. Transparency in the visible wavelength range was about 80% in all the crystals. Intense absorptions related with the Pr3 þ 4f–5d transitions were observed around 230 and 255 nm, and weak absorptions due to the 4f–4f transitions were detected around 450 nm. In radioluminescence spectra, the Pr3 þ 5d–4f transitions were observed around 275and 310 nm, and emissions due to the 4f–4f transition were observed around 500 nm. In the pulse height analysis using 137Cs gamma-ray excitation, Pr 0.1% doped sample showed the highest light yield of 2,800 ph/MeV. In the decay time measurements using different excitation sources (photoluminescence, X- and gamma-ray), two different processes related to the 5d–4f emission peaks were found. Fast decay component corresponds to direct excitation of Pr3 þ (4–6 ns) and slower component (25 ns) reflects the energy migration process from the host lattice to the emission center. & 2011 Elsevier B.V. All rights reserved.
Keywords: Scintillator Pr3 þ Oxyorthosilicate Crystal growth from the melt Radiation response
1. Introduction Inorganic scintillators have found their application in many fields. Increased needs for better scintillators has triggered the development of new fast and efficient scintillation materials [1–3]. In Positron Emission Tomography (PET) systems, crucial properties such as high light yield, detection efficiency, fast response, easy growth and low cost are required [4,5]. Bi4Ge3O12 (BGO) combines some of the aforementioned requirements and is frequently used. However, the fact that other properties of BGO, such as its relatively slow decay time (300 ns), energy resolution and light yield, make it less suitable for the application in PET. Cerium-doped lutetium oxyorthosilicate (LSO:Ce) is the most important candidate for replacing BGO in recently developed hybrid PET/CT systems [4]. Although LSO:Ce has been established as one of the leading scintillators for PET systems, much superior materials are still expected to be developed. Especially, faster decay time is needed for high timing resolution [6]. Recent studies have shown that Pr3 þ doped materials also exhibit 5d–4f transitions and can be an n Corresponding author at: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-0812, Japan. Tel.: þ81 22 217 5822; fax: þ 81 22 217 5102. E-mail address: [email protected] (D. Totsuka).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.03.063
alternative for the Ce3 þ doped ones. In the case of YAlO3 and Y3Al5O12 [7,8], Lu3Al5O12 [9], Y2SiO5 [10], fast decay time ranging from 7 to 31 ns were reported. Spin and parity-allowed 5d–4f transitions of Pr3 þ ion is interesting due to the higher energies of the 4fn 15d1–4fn transitions with respect to Ce3 þ ones in the same crystal field. Faster decay has been reported as well [Ref]. In the past study, the decay times of LSO:Pr were reported to be 6 ns in photoluminescence and 26 ns under gamma-ray excitation [11]. Time-resolved photoluminescence and gamma-ray excited radioluminescence measurements are normally performed to understand the decay kinetics. However, it is not enough to identify complicated scintillation mechanism with only these methods. Moreover, it is well known that concentration of dopants largely affect scintillation properties. In the present work, LSO:Pr with different Pr concentrations (0.05, 0.1 and 0.25% with respect to Lu) were grown by the micropulling-down method (m-PD), which has been generally used for material research of novel functional single crystals due to the higher growth speed with respect to those of conventional methods such as Czochralski (Cz), Bridgeman–Stockbarger (BS) and Floating Zone (FZ) methods [12]. Such a method is then very suitable for rapid material screening. Optical properties and radiation response were investigated. To study the detailed energy transport mechanism by radiation excitation, not only
D. Totsuka et al. / Nuclear Instruments and Methods in Physics Research A 643 (2011) 64–68
gamma-ray radio isotope but also the picosecond pulse X-ray equipped streak camera was used [13].
65
3. Results and discussion 3.1. Crystal growth
2. Experimental procedure 2.1. Crystal growth Nominal concentration of 0.05, 0.1 and 0.25% Pr (with respect to Lu) doped LSO crystals were grown by the m-PD method. High purity Lu2O3 (99.99%) and SiO2 (99.99%) powders were used as starting materials in a molar ratio of 49:51. The activator Pr6O11 (99.99%) was added in corresponding quantities to reach the above dopant concentrations. These starting materials were well mixed in an agate mortar and charged into a cylindrical-type crucible with a 5 mm square die to control the crystal shape. The crucible was made of an alloy of Ir and Re. The growth furnace chamber was evacuated to 10 1 Torr, then filled with the high purity argon gas. After these processes, the crucible was heated up to the melting temperature of Lu2SiO5. Commercial LSO crystal was used as the seed crystal. The pulling rate was controlled between 0.05 and 0.07 mm/min. After pulling, the grown crystals were cooled down to room temperature in 5–6 h. The phase of the grown crystals was confirmed by X-ray diffraction analysis (RINT2000, Rigaku corporation) in the 101–901 2y range. The X-ray was generated by copper target using the tube voltage of 40 kV and current of 40 mA. The samples were cut from inside of the grown crystals because of pronounced Pr3 þ segregation at the surface, and then mirror was polished to evaluate the optical and scintillation properties.
0.05, 0.1 and 0.25% Pr doped LSO were successfully grown by the m-PD method and the grown crystals are shown in Fig. 1. The physical dimensions of each crystal were 5 mm sq and 25–30 mm in length. The powder XRD results showed that all of the LSO:Pr samples had a crystal structure with space group C2/c (No, 15). There were a few small cracks at the beginning and the end part of the crystals. They were transparent and yellowish in color, which is reported for some Pr3 þ activated crystals [15]. The stronger coloration at the surface of the crystal was probably caused by the Pr segregation. This can be due to the difference of cation radii and the inert atmosphere during the crystal growth. Pr3 þ ions should be distributed between the melt and the solid according to the segregation coefficient. During the solidification process, due to its radius of 100 pm, which is larger than the host element Lu3 þ (86 pm), its concentration in the melt slightly increases. ICP–AES analysis was made on the 0.1% (nominal)
2.2. Optical and scintillation properties Optical transmittance spectra were measured using JASCO V550 spectrophotometer in the wavelength range of 190– 900 nm and the step of 1 nm. Photoluminescence decay time measurement was performed by the Spectrofluorometer FLS920 from Edinburgh Instruments. For the decay time measurement the instrumental response measurement was performed to extract true decay times using the deconvolution procedure. Using this spectrofluorometer, we also evaluated the radioluminescence spectra excited by 241Am because its 5.5 MeV alpha-ray can be easily fully absorbed and its energy deposition is sufficient to generate enough scintillation photons for measurement. Pulse height spectrum measurements were carried out under gamma-ray (137Cs) excitation with a photomultiplier tube R7600U (Hamamatsu Photonics) connected to an ORTEC 113 preamplifier, an ORTEC 572 shaping amplifier with 2 ms shaping time and an Amptek Pocket MCA 8000 A multichannel analyzer for digital signal conversion. The bias voltage of the PMT was supplied at þ700 V (ORTEC 556). Samples were mounted on the PMT with an optical grease (Dupon, Krytox) and covered with several layers of Teflon tape to collect scintillation photons. At the same time, decay time measurement was done using a digital oscilloscope (Tektronix TDS3052B). To evaluate the light yield, BGO crystal was used as a reference [14]. Additionally, the picosecond pulse X-ray equipped streak camera was used for further study of the decay kinetics. This system consists of a laser diode (pulse width is 50 ps and maximum 150 MHz repetition frequency), multi-alkali photocathode of X-ray tube (N5084), tungsten target and streak tube coupled to a microchannel plate. The pulse X-ray was generated by accelerating voltage of 30 kV with 80 ps full width at half maximum (FWHM). Using the same setup, 241Am alpha-ray source was used for excitation to obtain the radioluminescence spectra.
Fig. 1. Photographs of Pr 0.05(top), 0.1(center) and 0.25%(bottom) doped LSO single crystals.
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D. Totsuka et al. / Nuclear Instruments and Methods in Physics Research A 643 (2011) 64–68
Pr3 þ doped LSO. As a result, the concentration was found to be 0.08% (with respect to Lu) that is almost equal to the nominal value. The yellowish coloration of the samples can be most probably caused by the Pr4 þ ion, as reported elsewhere [15]. It is surely contained in the Pr6O11 starting material. The samples for optical and scintillation measurements were prepared from the beginning part of the as-grown crystals and cut into block slices perpendicular to the pulling direction. The samples were finally shaped to the size 4 4 1 mm3.
3.2. Optical properties Fig. 2 shows transmittance spectra of LSO:Pr. Transparency in visible wavelength range was about 80% in the all the samples. Pr3 þ 4f–5d strong absorptions were observed at 215 and 245 nm. The large absorption below 200 nm should be the absorption edge of the host material [16]. Weak absorptions due to 4f–4f transitions were detected around 450 nm. In 270–500 nm range, the sample with the highest Pr concentration exhibited the lowest transparency. This result might be considered due to decreased crystallinity due to inclusions or scattering centers, which are not observable by eyes.
Fig. 2. Transmittance spectra of LSO:Pr. Black, red and green lines represent Pr 0.05, 0.1 and 0.25%, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
3.3. Scintillation characterization Fig. 3 shows the radioluminescence spectra under the alpharay excitation. The Pr3 þ 5d–4f emission peaks are observed at 300 nm and can be assigned to the 5d1–3H4,5,6 transitions. In addition to the 5d–4f emission, transitions from the 3P0,1,2 and 1 D2 4f levels were observed around 500 and 610 nm, respectively. The spectra were normalized at 610 nm. Photo- and radio (gamma; 137Cs)-luminescence decay time measurements of Pr 0.05% LSO are shown in Figs. 4 and 5, respectively. The fastest component in Fig. 5 was noise of the PMT, because we took the data by 512 times averaging. In this measurement, 0.1 and 0.25% samples showed almost the same result as the 0.05% LSO:Pr. As expected, the observed values were well consistent with the previously reported results [10]. Fig. 6 shows the Bremsstrahlung X-ray excited decay curves of the three samples with different Pr concentration. The instrumental response of this measurement range has about 500 ps in width, which is faster enough than almost all the scintillation decay times of the samples studied. Two components exponential function well reproduces the observed results. This points to the fact that that two kinds of emission processes occur after 30 kVp X-ray excitation. The faster component corresponds to a direct excitation of Pr3 þ 4f–5d transition and the other is host processed
Fig. 4. Photoluminescence decay time profiles of Pr3 þ 0.05%, measured at 310 nm and excited at 245 nm.
Quantum Efficiency of PMT
Fig. 3. Alpha-ray (241Am) excited luminescence spectra of LSO:Pr. Black, red and green show Pr 0.05, 0.1 and 0.25%, respectively. Blue line represent quantum efficiency of PMT. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
Fig. 5. Decay time profile of Pr3 þ 0.05%, excited by
137
Cs gamma-ray.
D. Totsuka et al. / Nuclear Instruments and Methods in Physics Research A 643 (2011) 64–68
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Fig. 7. Pulse height spectra of LSO:Pr under 137Cs irradiation. Black, red, green and blue lines. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
Gaussian function and the quantum efficiency of the PMT at the emission peaks in Fig. 3, approximately 25% at 275 nm, 37% at 310 nm, 30% at 500 nm and 7% at 610 nm, in whole sensitive range, a total light output 2800 ph/MeV was deduced. In the total amount of the scintillation photons, Pr3 þ d–f transitions contributed around 1000 ph/MeV. To achieve a higher light yield, it would be necessary to try annealing in a reduced atmosphere. The Pr4 þ broad absorption band can overlap with the Pr3 þ emission and reduce the light yield [15]. The energy resolution, defined as the full width of the half of the maximum of the photo-absorption peak seems to be low [18]. When concentration becomes lower, the energy resolution is improved. The energy resolution could be improved by crystal quality, slightly degraded by the segregation of the dopant.
4. Conclusions
Fig. 6. Decay time profiles of Pr 0.05, 0,10 and 0.25% doped LSO crystals under X-ray (30kVp) excitation. The monitoring range is from 285 to 315 nm.
excitation. The decay time value of the former decay component ranges from 4 to 6 ns, which is similar to the value for the directly excited Pr3 þ emission obtained in the photoluminescence measurement and is slightly faster than that of YAP:Pr [17]. On the other hand, the latter value ( 20 ns) is typical in the gamma-ray excited decay curve and reflects the energy migration from the host to the emission centers. In these measurements, the ratio of the direct f–d excitation is proportional to the Pr concentration and the contribution of the faster component to the total emission ranges from 74% to 89%. It means that the energy migration process depends on the excitation energy. Fig. 7 shows 137Cs gamma-ray pulse height spectra of the three different Pr3 þ concentration crystals and BGO. In all the samples, 662 keV photo-electron absorption peak was observed. Light yields were determined relatively to the BGO, which has the light output of 8000 ph/MeV. A correction factor was taken into account and the quantum efficiency of the photocathode was calculated at the emission peaks in the whole range of sensitivity of the PMT. The spectral response of the PMT used in the light output measurement is shown in Fig. 3. The highest light yield is achieved by the 0.1% doped sample. Based on the fitting by single
We have successfully grown 0.05, 0.1 and 0.25% Pr doped LSO single crystals. Optical properties, radioluminescence, decay time and light yield of these samples were investigated. High transparency of about 80% was observed in the visible wavelength range. Only the 4f–4f absorption lines were observed at 450 nm and Pr3 þ 4f–5d ones were observed at around 215 and 245 nm. In the radioluminescence spectra, the Pr3 þ 5d–4f (300 nm) and 4f–4f (500, 610 nm) emission peaks appeared. Pulse height measurements showed that the 0.1% Pr sample has the highest light yield 1000 ph/MeV for fast, Pr3 þ 5d–4f transitions. In decay time kinetics, two kinds of recombination mechanisms were revealed under in low energy X-ray excitation. This result shows that the decay time kinetics depends on the excitation energy; direct excitation in low energy (photoluminescence), direct and host process in medium energy (X-ray) and host-related process in high energy (gamma-ray). The ratio of direct component is related to concentration of Pr. To improve the scintillation responses, the quality of crystal is important.
Acknowledgments Authors would like to thank to technical services section in IMRAM for designing and making insulators for crystal growth furnace and polishing samples.
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References [1] [2] [3] [4] [5] [6] [7]
C.W.E. van Eijk, Nucl. Instr. and Meth. Phys. Res. A 460 (2001) 1. W.W. Moses, Nucl. Instr. and Meth. Phys. Res. A 487 (2002) 123. M. Moszynski, Nucl. Instr. and Meth. Phys. Res. A 505 (2003) 101. C.W.E. van Eijk, Phys. Med. Biol. 47 (2002) R85. C.W.E. van Eijk, Nucl. Instr. and Meth. A 460 (2001) 1. P.A. Rodnyi., Radiat. Meas. 33 (5) (2001) 605. T. Yanagida, K. Kamada, Y. Fujimoto, M. Sugiyama, Y. Furuya, A. Yamaji, Y. Yokota, A. Yoshikawa, Opt. Spectrosc. Nucl. Instr. and Meth. Phys. Res. 623 (3) (2010) 1020. [8] C.W.E. van Eijk, P. Dorenbos, R. Visser, IEEE Trans. Nucl. Sci. 41 (1994) 738. [9] M. Nikl, H. Ogino, A. Krasnikov, A. Beitlerova, A. Yoshikawa, T. Fukuda, Phys. Status. Solidi A 202 (2005) R4. [10] P. Dorenbos, M. Marsman, S.W.E. van Eijk, M.V. Korzhik, B.I. Minkov, Radiat. Eff. Defect Solids 135 (1995) 325.
[11] J. Pejchal, M. Nikl, E. Mihokova, J.A. Mareˇs, A. Yoshikawa, H. Ogino, ˇ cka, J. Phys. D: K.M. Schillemat, A. Krasnikov, A. Vedda, K. Nejezchleb, V. Mu Appl. Phys. 42 (2009) 055117. [12] A. Yoshikawa, M. Nikl, G. Boulon, T. Fukuda, Opt. Mater. 30 (2007) 6. [13] T. Yanagida, Y. Fujimoto, A. Yoshikawa, Y. Yokota, K. Kamada, N. Jan Pejchal, K. Kawaguchi, K. Fukuda, K. Uchiyama, K. Mori, M. Nikl, K. Kitano, Appl. Phys. Express 3 (2010) 056202. [14] I. Holl, E. Lorenz, G. Mageras., IEEE Trans. Nucl. Sci. NS35 (1988) 105. [15] D. Pawlak, Z. Frukacz, Z. Mierczyk, A. Suchocki, J. Zachara, J. Alloys Comp 275-277 (1998) 361. [16] C. Yan, G. Zhao, Y. Hang, L. Zhang, J. Xu, J. Cryst. Growth 281 (2005) 411. [17] C. Pedrini, D. Bouttet, C. Dujardin, B. Moine, J. Dafinei, P. Lecoq, M. Koseija, K. Blazek, Opt. Mater. 3 (1994) 81. ¨ ¨ [18] C.W.E. van Eijk, P. Dorenbos, E.V.D. van Loef, K. Kramer, H.U. Gudel, Radiat. Meas. 33 (2001) 521.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Crystal growth and scintillation properties of Pr-doped oxyorthosilicate for different concentration Daisuke Totsuka a,b,n, Takayuki Yanagida d, Yutaka Fujimoto a, Jan Pejchal a,c, Yuui Yokota a, Akira Yoshikawa a,d a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan Nihon Kessho Kogaku Co. Ltd, Japan c Institute of Physics AS CR, Cukrovarnicka 10, Prague 6, 162-53, Czech Republic d New Industry Creation Hatchery Center (NICHe) 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 November 2010 Received in revised form 17 February 2011 Accepted 31 March 2011 Available online 14 April 2011
0.05, 0.1 and 0.25 mol% Pr (with respect to Lu) doped Lu2SiO5 (LSO) single crystals were grown by the micro-pulling down (m-PD) method. The grown crystals were transparent, and a slight segregation of Pr3 þ was observed both in the crystal cross-section and growth direction. Transparency in the visible wavelength range was about 80% in all the crystals. Intense absorptions related with the Pr3 þ 4f–5d transitions were observed around 230 and 255 nm, and weak absorptions due to the 4f–4f transitions were detected around 450 nm. In radioluminescence spectra, the Pr3 þ 5d–4f transitions were observed around 275and 310 nm, and emissions due to the 4f–4f transition were observed around 500 nm. In the pulse height analysis using 137Cs gamma-ray excitation, Pr 0.1% doped sample showed the highest light yield of 2,800 ph/MeV. In the decay time measurements using different excitation sources (photoluminescence, X- and gamma-ray), two different processes related to the 5d–4f emission peaks were found. Fast decay component corresponds to direct excitation of Pr3 þ (4–6 ns) and slower component (25 ns) reflects the energy migration process from the host lattice to the emission center. & 2011 Elsevier B.V. All rights reserved.
Keywords: Scintillator Pr3 þ Oxyorthosilicate Crystal growth from the melt Radiation response
1. Introduction Inorganic scintillators have found their application in many fields. Increased needs for better scintillators has triggered the development of new fast and efficient scintillation materials [1–3]. In Positron Emission Tomography (PET) systems, crucial properties such as high light yield, detection efficiency, fast response, easy growth and low cost are required [4,5]. Bi4Ge3O12 (BGO) combines some of the aforementioned requirements and is frequently used. However, the fact that other properties of BGO, such as its relatively slow decay time (300 ns), energy resolution and light yield, make it less suitable for the application in PET. Cerium-doped lutetium oxyorthosilicate (LSO:Ce) is the most important candidate for replacing BGO in recently developed hybrid PET/CT systems [4]. Although LSO:Ce has been established as one of the leading scintillators for PET systems, much superior materials are still expected to be developed. Especially, faster decay time is needed for high timing resolution [6]. Recent studies have shown that Pr3 þ doped materials also exhibit 5d–4f transitions and can be an n Corresponding author at: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-0812, Japan. Tel.: þ81 22 217 5822; fax: þ 81 22 217 5102. E-mail address: [email protected] (D. Totsuka).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.03.063
alternative for the Ce3 þ doped ones. In the case of YAlO3 and Y3Al5O12 [7,8], Lu3Al5O12 [9], Y2SiO5 [10], fast decay time ranging from 7 to 31 ns were reported. Spin and parity-allowed 5d–4f transitions of Pr3 þ ion is interesting due to the higher energies of the 4fn 15d1–4fn transitions with respect to Ce3 þ ones in the same crystal field. Faster decay has been reported as well [Ref]. In the past study, the decay times of LSO:Pr were reported to be 6 ns in photoluminescence and 26 ns under gamma-ray excitation [11]. Time-resolved photoluminescence and gamma-ray excited radioluminescence measurements are normally performed to understand the decay kinetics. However, it is not enough to identify complicated scintillation mechanism with only these methods. Moreover, it is well known that concentration of dopants largely affect scintillation properties. In the present work, LSO:Pr with different Pr concentrations (0.05, 0.1 and 0.25% with respect to Lu) were grown by the micropulling-down method (m-PD), which has been generally used for material research of novel functional single crystals due to the higher growth speed with respect to those of conventional methods such as Czochralski (Cz), Bridgeman–Stockbarger (BS) and Floating Zone (FZ) methods [12]. Such a method is then very suitable for rapid material screening. Optical properties and radiation response were investigated. To study the detailed energy transport mechanism by radiation excitation, not only
D. Totsuka et al. / Nuclear Instruments and Methods in Physics Research A 643 (2011) 64–68
gamma-ray radio isotope but also the picosecond pulse X-ray equipped streak camera was used [13].
65
3. Results and discussion 3.1. Crystal growth
2. Experimental procedure 2.1. Crystal growth Nominal concentration of 0.05, 0.1 and 0.25% Pr (with respect to Lu) doped LSO crystals were grown by the m-PD method. High purity Lu2O3 (99.99%) and SiO2 (99.99%) powders were used as starting materials in a molar ratio of 49:51. The activator Pr6O11 (99.99%) was added in corresponding quantities to reach the above dopant concentrations. These starting materials were well mixed in an agate mortar and charged into a cylindrical-type crucible with a 5 mm square die to control the crystal shape. The crucible was made of an alloy of Ir and Re. The growth furnace chamber was evacuated to 10 1 Torr, then filled with the high purity argon gas. After these processes, the crucible was heated up to the melting temperature of Lu2SiO5. Commercial LSO crystal was used as the seed crystal. The pulling rate was controlled between 0.05 and 0.07 mm/min. After pulling, the grown crystals were cooled down to room temperature in 5–6 h. The phase of the grown crystals was confirmed by X-ray diffraction analysis (RINT2000, Rigaku corporation) in the 101–901 2y range. The X-ray was generated by copper target using the tube voltage of 40 kV and current of 40 mA. The samples were cut from inside of the grown crystals because of pronounced Pr3 þ segregation at the surface, and then mirror was polished to evaluate the optical and scintillation properties.
0.05, 0.1 and 0.25% Pr doped LSO were successfully grown by the m-PD method and the grown crystals are shown in Fig. 1. The physical dimensions of each crystal were 5 mm sq and 25–30 mm in length. The powder XRD results showed that all of the LSO:Pr samples had a crystal structure with space group C2/c (No, 15). There were a few small cracks at the beginning and the end part of the crystals. They were transparent and yellowish in color, which is reported for some Pr3 þ activated crystals [15]. The stronger coloration at the surface of the crystal was probably caused by the Pr segregation. This can be due to the difference of cation radii and the inert atmosphere during the crystal growth. Pr3 þ ions should be distributed between the melt and the solid according to the segregation coefficient. During the solidification process, due to its radius of 100 pm, which is larger than the host element Lu3 þ (86 pm), its concentration in the melt slightly increases. ICP–AES analysis was made on the 0.1% (nominal)
2.2. Optical and scintillation properties Optical transmittance spectra were measured using JASCO V550 spectrophotometer in the wavelength range of 190– 900 nm and the step of 1 nm. Photoluminescence decay time measurement was performed by the Spectrofluorometer FLS920 from Edinburgh Instruments. For the decay time measurement the instrumental response measurement was performed to extract true decay times using the deconvolution procedure. Using this spectrofluorometer, we also evaluated the radioluminescence spectra excited by 241Am because its 5.5 MeV alpha-ray can be easily fully absorbed and its energy deposition is sufficient to generate enough scintillation photons for measurement. Pulse height spectrum measurements were carried out under gamma-ray (137Cs) excitation with a photomultiplier tube R7600U (Hamamatsu Photonics) connected to an ORTEC 113 preamplifier, an ORTEC 572 shaping amplifier with 2 ms shaping time and an Amptek Pocket MCA 8000 A multichannel analyzer for digital signal conversion. The bias voltage of the PMT was supplied at þ700 V (ORTEC 556). Samples were mounted on the PMT with an optical grease (Dupon, Krytox) and covered with several layers of Teflon tape to collect scintillation photons. At the same time, decay time measurement was done using a digital oscilloscope (Tektronix TDS3052B). To evaluate the light yield, BGO crystal was used as a reference [14]. Additionally, the picosecond pulse X-ray equipped streak camera was used for further study of the decay kinetics. This system consists of a laser diode (pulse width is 50 ps and maximum 150 MHz repetition frequency), multi-alkali photocathode of X-ray tube (N5084), tungsten target and streak tube coupled to a microchannel plate. The pulse X-ray was generated by accelerating voltage of 30 kV with 80 ps full width at half maximum (FWHM). Using the same setup, 241Am alpha-ray source was used for excitation to obtain the radioluminescence spectra.
Fig. 1. Photographs of Pr 0.05(top), 0.1(center) and 0.25%(bottom) doped LSO single crystals.
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D. Totsuka et al. / Nuclear Instruments and Methods in Physics Research A 643 (2011) 64–68
Pr3 þ doped LSO. As a result, the concentration was found to be 0.08% (with respect to Lu) that is almost equal to the nominal value. The yellowish coloration of the samples can be most probably caused by the Pr4 þ ion, as reported elsewhere [15]. It is surely contained in the Pr6O11 starting material. The samples for optical and scintillation measurements were prepared from the beginning part of the as-grown crystals and cut into block slices perpendicular to the pulling direction. The samples were finally shaped to the size 4 4 1 mm3.
3.2. Optical properties Fig. 2 shows transmittance spectra of LSO:Pr. Transparency in visible wavelength range was about 80% in the all the samples. Pr3 þ 4f–5d strong absorptions were observed at 215 and 245 nm. The large absorption below 200 nm should be the absorption edge of the host material [16]. Weak absorptions due to 4f–4f transitions were detected around 450 nm. In 270–500 nm range, the sample with the highest Pr concentration exhibited the lowest transparency. This result might be considered due to decreased crystallinity due to inclusions or scattering centers, which are not observable by eyes.
Fig. 2. Transmittance spectra of LSO:Pr. Black, red and green lines represent Pr 0.05, 0.1 and 0.25%, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
3.3. Scintillation characterization Fig. 3 shows the radioluminescence spectra under the alpharay excitation. The Pr3 þ 5d–4f emission peaks are observed at 300 nm and can be assigned to the 5d1–3H4,5,6 transitions. In addition to the 5d–4f emission, transitions from the 3P0,1,2 and 1 D2 4f levels were observed around 500 and 610 nm, respectively. The spectra were normalized at 610 nm. Photo- and radio (gamma; 137Cs)-luminescence decay time measurements of Pr 0.05% LSO are shown in Figs. 4 and 5, respectively. The fastest component in Fig. 5 was noise of the PMT, because we took the data by 512 times averaging. In this measurement, 0.1 and 0.25% samples showed almost the same result as the 0.05% LSO:Pr. As expected, the observed values were well consistent with the previously reported results [10]. Fig. 6 shows the Bremsstrahlung X-ray excited decay curves of the three samples with different Pr concentration. The instrumental response of this measurement range has about 500 ps in width, which is faster enough than almost all the scintillation decay times of the samples studied. Two components exponential function well reproduces the observed results. This points to the fact that that two kinds of emission processes occur after 30 kVp X-ray excitation. The faster component corresponds to a direct excitation of Pr3 þ 4f–5d transition and the other is host processed
Fig. 4. Photoluminescence decay time profiles of Pr3 þ 0.05%, measured at 310 nm and excited at 245 nm.
Quantum Efficiency of PMT
Fig. 3. Alpha-ray (241Am) excited luminescence spectra of LSO:Pr. Black, red and green show Pr 0.05, 0.1 and 0.25%, respectively. Blue line represent quantum efficiency of PMT. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
Fig. 5. Decay time profile of Pr3 þ 0.05%, excited by
137
Cs gamma-ray.
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Fig. 7. Pulse height spectra of LSO:Pr under 137Cs irradiation. Black, red, green and blue lines. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
Gaussian function and the quantum efficiency of the PMT at the emission peaks in Fig. 3, approximately 25% at 275 nm, 37% at 310 nm, 30% at 500 nm and 7% at 610 nm, in whole sensitive range, a total light output 2800 ph/MeV was deduced. In the total amount of the scintillation photons, Pr3 þ d–f transitions contributed around 1000 ph/MeV. To achieve a higher light yield, it would be necessary to try annealing in a reduced atmosphere. The Pr4 þ broad absorption band can overlap with the Pr3 þ emission and reduce the light yield [15]. The energy resolution, defined as the full width of the half of the maximum of the photo-absorption peak seems to be low [18]. When concentration becomes lower, the energy resolution is improved. The energy resolution could be improved by crystal quality, slightly degraded by the segregation of the dopant.
4. Conclusions
Fig. 6. Decay time profiles of Pr 0.05, 0,10 and 0.25% doped LSO crystals under X-ray (30kVp) excitation. The monitoring range is from 285 to 315 nm.
excitation. The decay time value of the former decay component ranges from 4 to 6 ns, which is similar to the value for the directly excited Pr3 þ emission obtained in the photoluminescence measurement and is slightly faster than that of YAP:Pr [17]. On the other hand, the latter value ( 20 ns) is typical in the gamma-ray excited decay curve and reflects the energy migration from the host to the emission centers. In these measurements, the ratio of the direct f–d excitation is proportional to the Pr concentration and the contribution of the faster component to the total emission ranges from 74% to 89%. It means that the energy migration process depends on the excitation energy. Fig. 7 shows 137Cs gamma-ray pulse height spectra of the three different Pr3 þ concentration crystals and BGO. In all the samples, 662 keV photo-electron absorption peak was observed. Light yields were determined relatively to the BGO, which has the light output of 8000 ph/MeV. A correction factor was taken into account and the quantum efficiency of the photocathode was calculated at the emission peaks in the whole range of sensitivity of the PMT. The spectral response of the PMT used in the light output measurement is shown in Fig. 3. The highest light yield is achieved by the 0.1% doped sample. Based on the fitting by single
We have successfully grown 0.05, 0.1 and 0.25% Pr doped LSO single crystals. Optical properties, radioluminescence, decay time and light yield of these samples were investigated. High transparency of about 80% was observed in the visible wavelength range. Only the 4f–4f absorption lines were observed at 450 nm and Pr3 þ 4f–5d ones were observed at around 215 and 245 nm. In the radioluminescence spectra, the Pr3 þ 5d–4f (300 nm) and 4f–4f (500, 610 nm) emission peaks appeared. Pulse height measurements showed that the 0.1% Pr sample has the highest light yield 1000 ph/MeV for fast, Pr3 þ 5d–4f transitions. In decay time kinetics, two kinds of recombination mechanisms were revealed under in low energy X-ray excitation. This result shows that the decay time kinetics depends on the excitation energy; direct excitation in low energy (photoluminescence), direct and host process in medium energy (X-ray) and host-related process in high energy (gamma-ray). The ratio of direct component is related to concentration of Pr. To improve the scintillation responses, the quality of crystal is important.
Acknowledgments Authors would like to thank to technical services section in IMRAM for designing and making insulators for crystal growth furnace and polishing samples.
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