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Cs2Li(Na)Gd(1-x)Ce xCl(Br) Crystal Scintillators for Radiation Detection
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Procedia Engineering
ProcediaProcedia Engineering 00 (2012) Engineering 32 000–000 (2012) 53 – 59 www.elsevier.com/locate/procedia
I-SEEC2011
Cs2Li(Na)Gd(1-x)Ce xCl(Br) Crystal Scintillators for Radiation Detection H.J. Kima*, Gul Roohb, Sunghwan Kimc, Sang Jun Kangd a
Department of Physics, Kyungpook National University, Daegu 702-701, Korea b Department of Physics,Abdul Wali Khan University, Mardan 23200, Pakistan c Department of Radiologic Technology, Cheongju University, Cheongju, Chungbuk 360-764, Korea d School of Liberal Art, Semyung University, Jechon 390-711, Korea Elsevier use only: Received 30 September 2011; Revised 10 November 2011; Accepted 25 November 2011.
Abstract We grow Cs2Li(Na)Gd(1-x)CexCl6(Br) crystal scintillators by using Bridgman method for radiation detection (where x = 0.01, 0.1). Grown crystals are all Elpasolite and cubic structure that it is easy to grow even if they are all hygroscopic. X-ray excited luminescence spectra of the subject crystals showed broad emission peaks between 350 and 450 nm, with two overlapping peaks. This emission is caused by transitions from the lowest 5d excited state of Ce3+ to the two spin orbit split 2F5/2 and 2F7/2 ground state levels. Since the transition is favored, we expect fast decay time and high light output. We measured energy resolution and decay time of grown crystals by using gamma radiation source. Since the grown scintillation crystals contained gadolinium (Gd) which has high Z-number and could be used to efficiently detect gamma rays or x-rays in many applications such as computerized tomography (CT), positron emission tomography (PET), single photon emission computed tomography (SPECT) and homeland security. Also lithium (Li) and Gd contained crystals could be promising candidates for neutron detection.
© 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of I-SEEC2011 Keywords: Scintillators; Bridgman; Elpasolite; Decay time; Neutron; Energy resolution
1. Introduction Increasing demand of scintillation materials due to their application in various fields such as medicine, science, and industry activated research into the discovery and growth of new inorganic scintillation materials. The aim behind every new scintillation material is to fulfill the requirements demanded by the application where it will be used [1-3]. Usually, a scintillation crystal should have high density, high Z* Corresponding author. Tel.: +82-53-950-5323; fax: +82-53-952-1739. E-mail address: [email protected].
1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.01.1236
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number, high light yield, short decay time, cost and could be grown easily [4]. Recently, Cedoped/concentrated crystals are grown, which shows promising scintillation properties such as high light yield, short decay time, and high Z-number and density [5, 6]. The possible reason of high light yield and short decay time of Ce-doped/concentrated scintillation crystals are due to the allowed 5d → 4f transition on Ce3+ ion. Incorporation of cerium ions in a host matrix which contained lithium (Li) or gadolinium (Gd) elements make such scintillator ideal for the detection of neutrons detection [7]. Recently, we have reported on the scintillation properties of Cs2LiCeCl6 and Rb2LiCeBr6 scintillation crystal [8, 9]. These scintillators belong to elpasolites family and could be used as neutron-gamma detectors due to the presence of Li in the host matrix. This paper presents report on the scintillation properties of our newly developed cerium doped Cs2Li(Na)Gd(1-x)CexCl6(Br) scintillation crystals. These inorganic scintillators belong to the elpasolites family and are grown by the vertical Bridgman technique. Among the scintillation properties, X-ray induced emission spectra, energy resolution, scintillation light yield, and decay time spectra are presented at room temperature. Due to highly hygroscopic nature of the grown samples, special care is devoted during scintillation characterization of these materials. We expect these scintillators could be used as potential thermal neutron detectors. 2. Experimental Technique 2.1. Crystal growth Single crystals of Cs2Li(Na)Gd(1-x)CexCl6(Br) have been grown by two zone vertical Bridgman technique (where x = 0.01, 0.1) . Stoichiometric amounts of CsCl (Br), Li/Na Cl (Br), and GdCl3 (Br) of 4 ~ 5 N purity from Sigma-Aldrich are loaded in quartz ampoule inside argon purged glove-box. The amount of CeCl3/Br3 (4N) in the feed material is adjusted in order to produce Ce-doped compound. Poly crystalline powder of Cs2Li(Na)Gd(1-x)CexCl6(Br) is loaded in clean, baked, and ultra-dry quartz ampoules under ultra-dry argon atmosphere. These ampoules are sealed with an oxy-propane torch under vacuum of ~ 10-7 Torr. Prior to the growth, melting points of all the samples are measured, and it is found that all materials are congruently melted between 590 oC and 750 oC. Detail of the crystal growth procedure can be found elsewhere in ref. [8]. Figure 1 shows a photograph of the vertical Bridgman technique used for the growth of sample crystals. After the crystal growth process, we obtained transparent samples of Cs2LiGdCl6, Cs2LiGdBr6 and Cs2NaGdBr6 crystals with 1% and 10% Ce-concentrations. The grown crystals parameters such as unit cell dimensions, density (ρ) and effective Z-number (Zeff) are summarized in Table 1. Table 1. Cs2Li(Na)Gd(1-x)CexCl6(Br) single crystals data Structure
Unit cell dimensions (Å)
ρ (g/cm3)
Zeff
Cs2LiGdCl6: Ce3+
Cubic
10.534
3.67
53
[10]
Cs2LiGdBr6: Ce3+
Cubic
11.178
4.31
50
[11, 12]
Cs2NaGdBr6: Ce3+
Cubic
11.37
4.18
50
[11, 12]
Material
Ref.
H.J. – 59 H.J. Kim Kim et et al. al. // Procedia Procedia Engineering Engineering 32 00 (2012) (2012) 53 000–000
3
Fig. 1. A photographic view of the Bridgman growth technique, (a) temperature controller and ampoule lowering motor, (b) vertical furnace
2.2. Equipment The luminescence characteristic of Cs2Li(Na)Gd(1-x)CexCl6(Br) single crystals at room temperature are measured under excitation by X-ray tube from a DRGEM. Co. having a W anode. The power settings are 100 kV and 1 mA. X-rays excitation spectra of the sample crystals are measured with QE65000 fiber optic spectrometer made by Ocean Optics. The energy spectra measurement is carried out with direct coupling of the sample crystals with the photomultiplier tube (PMT) (PhotonisXP2260) using index matching optical grease. Prior to the coupling with the PMT, all faces of the sample crystals except the one facing PMT are wrapped in several layers of 0.1-mm-thick Teflon tape. In order to avoid hydrolysis of the crystals surface, all sample crystals are covered with a cap made of Teflon material. The analog signals from the PMT are shaped with a Tennelec TC 245 spectroscopy amplifier. The output signals are then fed into a 25-MHz flash analog-to-digital converter (FADC) [13, 14]. A software threshold setting is applied to trigger an event by using a self-trigger algorithm on the field programmable gate array (FPGA) chip of the FADC board. The FADC output is recorded into a personal computer by using a USB2 connection, and the recorded data are analyzed with a C++ data analysis program [15]. 3. Results and Discussion 3.1. X-ray induced luminescence Luminescence spectra of Cs2LiGdCl6: 1%Ce3+, Cs2LiGdBr6: 1% Ce3+ and Cs2NaGdBr6: 1% Ce3+ crystals under X-ray excitation at room temperature are shown in Fig. 2. The spectra shows Ce3+ emission with two overlapped peaks caused by transitions from the lowest energy level of the Ce3+ 4f5d configuration to the allowed spin-orbit split 2F5/2 and 2F7/2 levels. The 2F5/2 emission is peaked at 374, 391, and 386 nm for Cs2LiGdCl6: 1%Ce3+, Cs2LiGdBr6: 1% Ce3+ and Cs2NaGdBr6: 1% Ce3+ crystals, respectively. The 2F7/2 emission peaked take placed at 403, 418, and 422 nm for Cs2LiGdCl6: 1%Ce3+, Cs2LiGdBr6: 1% Ce3+ and Cs2NaGdBr6: 1% Ce3+ crystals, respectively. At a longer wavelength side, Cs2LiGdBr6: 1% Ce3+ and Cs2NaGdBr6: 1% Ce3+ crystals shows broad emission bands. These broad emission bands are located between 500 nm and 600 nm for Cs2LiGdBr6: 1% Ce3+, and 500 nm and 700
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nm for Cs2NaGdBr6: 1% Ce3+, respectively. We attributed these bands to the self-trapped exciton (STE) emission [6]. The presence of 312 nm emission peak for all sample crystals in Fig. 3 is due to the Gd3+ 6Pj → 8S7/2 emission. Usually such emission disappears at higher cerium concentration due to energy transfer from the Gd3+ to Ce3+ [16]. The redshift of the Ce3+ emission is observed in the spectra is due to the chemical variation along the halide series. Such redshift is also reported in K2LaX5: 0.1% Ce3+ (X = Cl, Br, I) single crystals [17]. From the scintillation detector point view, the emission spectrum of a scintillator should match to the spectral response curve of the photo-sensor. It is clear from Fig.2 that the emission wavelengths of all the three sample crystals are well matched to the spectral response curve of the modern photomultiplier tubes (PMTs) and other photo-sensors such as large area avalanche photodiode (LAAPD).
Fig. 2. X-ray excited emission spectra at room temperature of (a) Cs2LiGdCl6: 1%Ce3+, (b) Cs2LiGdBr6: 1% Ce3+ and (c) Cs2NaGdBr6: 1% Ce3+
3.2. Pulse height measurements Pulse height spectra of Cs2LiGdCl6: 10%Ce3+, Cs2LiGdBr6: 10% Ce3+ and Cs2NaGdBr6: 10% Ce3+ single crystals under 511-keV 22Na γ-ray excitation recorded with different shaping times (3 - 10 µs) and amplifier gain is shown in Fig. 3. In order to measure the energy resolution of the sample crystals, the obtained 511-keV 22Na photo peaks with the sample crystals are fitted to Gaussian function. We found energy resolutions of 9.5%, 10.5%, and 8.0% (FWHM) for Cs2LiGdCl6: 10%Ce3+, Cs2LiGdBr6: 10% Ce3+ and Cs2NaGdBr6: 10% Ce3+ single crystals, respectively. Inset of Fig. 3 shows the pulse height spectrum of Cs2LiGdBr6: 10% Ce3+ sample crystal. Although the obtained energy resolutions are not very excellent
H.J. – 59 H.J. Kim Kim et et al. al. // Procedia Procedia Engineering Engineering 32 00 (2012) (2012) 53 000–000
5
as compared to the energy resolution of LaBr3 and LaCl3 single crystals, however, these scintillators could find their place in medical imaging applications such as SPECT.
Fig. 3. Pulse height spectra of Cs2LiGdCl6: 10%Ce3+,Cs2LiGdBr6: 10% Ce3+ (see inset) and Cs2NaGdBr6: 10% Ce3+ under 511-keV 22 Na γ-ray excitation
3.3. Decay time profile Decay time spectra of Cs2LiGdCl6: 1% Ce3+, Cs2LiGdBr6: 1% Ce3+, and Cs2NaGdBr6: 1% Ce3+ crystals at room temperature under γ-ray excitation are shown in Fig. 3. For the decay time measurement the sample crystals are attached with the PMT and irradiated by 662-keV γ-ray from a 137Cs source. The generated signals in the PMT are fed into the 400-MHz flash analog-to-digital converter (FADC) and the data is recorded. From the pulse shape information we have calculated the decay time of the sample crystals [18]. From fit to the data we have distinguished three decay time components for all the three sample crystals. For simplicity they are called as short (τ1), intermediate (τ2), and long (τ3) decay time components. Three decay time components and fit to the data of Cs2LiGdCl6: 1% Ce3+ crystal is shown in the inset of Fig. 4. The decay time components and their relative light output intensities are presented in Table 2. It is clear from the Table 2 that the short decay time components of the Bromo-elpasolites are fast as compared to the Cs2LiGdCl6: 1% Ce3+ crystal and more than 60% of the total light is emitted with a fast decay time component. It is observed that further increase of Ce-concentration in the lattice matrix reduces the values of long decay time components of these scintillators (not shown). Which demonstrate that energy transfer rate from the lattice matrix to Ce3+ ion is increases with higher Ce-concentration. Table 2. Decay time components of the sample crystals under 662-keV γ-ray from a 137Cs source measured at room temperature Sample Cs2LiGdCl6: 1% Ce3+ Cs2LiGdBr6: 1% Ce3+ Cs2NaGdBr6: 1% Ce3+
τ1 207 ns (38%) 73 ns (63%) 94 ns (65%)
τ2 713 ns (42%) 542 ns (14%) 423 ns (27%)
τ3 6.7 µs (20%) 3.9 µs (23%) 7.2 µs (8%)
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Fig. 4. Decay time spectra under γ-ray excitation on a double-log scale of Cs2LiGdCl6: 1%Ce3+, Cs2LiGdBr6: 1% Ce3+, and Cs2NaGdBr6: 1% Ce3+ single crystals. The inset shows the decay time spectrum of Cs2LiGdCl6: 1% Ce3+ with the fit to the data and three exponential decay time components, a short (τ1), intermediate (τ2), and long (τ3), respectively
4. Conclusion We studied the scintillation properties of Ce3+ doped Cs2LiGdCl6, Cs2LiGdBr6 and Cs2NaGdBr6 single crystals, grown by vertical Bridgman technique. Due to the cubic structure of these materials, they can be grown easily in large volume. We expect that due to the presence of Li and Gd ions in the lattice matrix these scintillators could be used for the thermal neutron detection. The X-ray induced emission spectra show Ce3+ emission with two overlapped peaks. These emission matches well with the spectral response of the modern photo-sensors. Under 511-kev γ-ray excitation from a 22Na source we obtained best energy resolution of 8.0% (FWHM) for Cs2NaGdBr6: 10% Ce3+ single crystal. For the entire sample crystals three exponential decay time components are observed at room temperature. It is further observed that the short decay time components of Bromo-elpasolites are faster than the Cs2LiGdCl6: 1% Ce3+ crystal. Further investigations such as the detection of thermal neutrons are under way to study the potential of these scintillators as a thermal neutron detector. References [1] S. E. Derenzo, W. W. Moses "Experimental Efforts and Results in finding new heavy Scintillators. In: Heavy Scintillators for scientific and Industrial Applications, Ed. F. De Notaristefani, P. Lecoq, M. Schneegans, Frontieres 1993; 125-136. [2] C.W.E. van Eijk, In: Proceedings of the International Conference on Inorganic Scintillators and their Applications, 1997; 3. [3] C.W.E. van Eijk, Inorganic scintillators in medical imaging detectors. Nucl. Instrum. Meth. Phys. Res. A 2003; 509:17-25. [4] Hongsheng Shi, Laishun Qin, Wenxiang Chai, Jiayu Guo, Qinhua Wei, Guohao Ren, Kangying Shu. The LaBr3:Ce Crystal Growth by Self-Seeding Bridgman Technique and Its Scintillation Properties Cryst. Growth & Design 2010; 10: 4433-4436. [5] E.V.D. Van Loef, P. Dorenbos, C.W.E. Van Eijk, K.W. Kramer, H.U. Gudel, High-energy-resolution scintillator: Ce3+ activated LaBr3 Appl. Phys. Lett 2001;79: 1573-1575. [6] G. Bizarri, P. Dorenbos, Charge carrier and exciton dynamics in LaBr3:Ce3+ scintillators: Experiment and model Phys. Rev. B
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2007; 75: 184302. [7] M. Danang Birowosuto, Pieter Dorenbos, Johan T. M. de Haas, Carel W. E. van Eijk, Karl W. Krämer, Hans U. Güdel, LiBased Thermal Neutron Scintillator Research; Rb2LiYBr6: Ce3+ and Other Elpasolites IEEE Trans. Nucl. Sci. 2008; 55: 1152-1155. [8] Gul Rooh, H. J. Kim, and Sunghwan Kim, Study on Crystal Growth and Scintillation Characteristics of Cs2LiCeCl6 IEEE Trans. Nucl. Sci., 2010; 57: 1255-1259. [9] Gul Rooh, H. J. Kim, and Sunghwan Kim, Luminescence and Scintillation Characteristics of Rb2LiCeBr6 Single Crystal IEEE Trans. Nucl. Sci., 2010; 57: 3836-3840. [10] G. Meyer, G. H. Christian, Kubische und hexagonale elpasolithe vom typ Cs2LiMIIICl6 (MIII = Sc,Y,La-Nd, Sm-Lu,V, Cr, In), Z.Anorg. Allg. Chem., 1978; 445: 147. [11] G. Meyer, and H. – C. Gaebell, Z. Naturforsch., 1978; 33b: 1476. [12] G. Mermant and J. Primot, Mat. Res. Bull., Preparation, cristallogenese et caracterisation des bromures mixtes de lanthanide, de sodium et de cesium Cs2 NaLn (III) Br6 ,1979; 14: 45-50. [13] Notice Korea Co.: http://www.rndkorea.co.kr/. [14] Gul Rooh, Heedong Kang, H. J. Kim, H. Park, and Sunghwan Kim, The growth and scintillation properties of Cs2NaCeCl6 single crystal J. Cryst. Growth 2009; 311: 2470-2473. [15] J. H. So, H. J. Kim, Heedong Kang, H. Park, and Sanghoon Lee, The Proton Energy Response of a LYSO Crystal J. Korean Phys. Soc.,2008; 52: 925-929. [16] Gul Rooh, Heedong Kang, H. J. Kim, H. Park, Sunghwan Kim. Cs2LiGdCl6 (Ce): New scintillation material J. Cryst. Growth 2010; 312: 2243-2246. [17] K.W. Kramer, P. Dorenbos, H.U. Gudel, C.W.E. Van Eijk. Development and characterization of highly efficient new cerium doped rare earth halide scintillator materials. J. Mater. Chem 2006; 16: 2773-2780. [18] Jiang Hua, H.J.Kim, Gul Rooh, H.Park, SunghwanKim, JongKyuCheon. Czochralski growth and scintillation properties of Bi4Si3O12 (BSO) single crystal. Nucl. Instrum. Meth. Phys. Res. A 2011; 648:73-76.
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Procedia Engineering
ProcediaProcedia Engineering 00 (2012) Engineering 32 000–000 (2012) 53 – 59 www.elsevier.com/locate/procedia
I-SEEC2011
Cs2Li(Na)Gd(1-x)Ce xCl(Br) Crystal Scintillators for Radiation Detection H.J. Kima*, Gul Roohb, Sunghwan Kimc, Sang Jun Kangd a
Department of Physics, Kyungpook National University, Daegu 702-701, Korea b Department of Physics,Abdul Wali Khan University, Mardan 23200, Pakistan c Department of Radiologic Technology, Cheongju University, Cheongju, Chungbuk 360-764, Korea d School of Liberal Art, Semyung University, Jechon 390-711, Korea Elsevier use only: Received 30 September 2011; Revised 10 November 2011; Accepted 25 November 2011.
Abstract We grow Cs2Li(Na)Gd(1-x)CexCl6(Br) crystal scintillators by using Bridgman method for radiation detection (where x = 0.01, 0.1). Grown crystals are all Elpasolite and cubic structure that it is easy to grow even if they are all hygroscopic. X-ray excited luminescence spectra of the subject crystals showed broad emission peaks between 350 and 450 nm, with two overlapping peaks. This emission is caused by transitions from the lowest 5d excited state of Ce3+ to the two spin orbit split 2F5/2 and 2F7/2 ground state levels. Since the transition is favored, we expect fast decay time and high light output. We measured energy resolution and decay time of grown crystals by using gamma radiation source. Since the grown scintillation crystals contained gadolinium (Gd) which has high Z-number and could be used to efficiently detect gamma rays or x-rays in many applications such as computerized tomography (CT), positron emission tomography (PET), single photon emission computed tomography (SPECT) and homeland security. Also lithium (Li) and Gd contained crystals could be promising candidates for neutron detection.
© 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of I-SEEC2011 Keywords: Scintillators; Bridgman; Elpasolite; Decay time; Neutron; Energy resolution
1. Introduction Increasing demand of scintillation materials due to their application in various fields such as medicine, science, and industry activated research into the discovery and growth of new inorganic scintillation materials. The aim behind every new scintillation material is to fulfill the requirements demanded by the application where it will be used [1-3]. Usually, a scintillation crystal should have high density, high Z* Corresponding author. Tel.: +82-53-950-5323; fax: +82-53-952-1739. E-mail address: [email protected].
1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.01.1236
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number, high light yield, short decay time, cost and could be grown easily [4]. Recently, Cedoped/concentrated crystals are grown, which shows promising scintillation properties such as high light yield, short decay time, and high Z-number and density [5, 6]. The possible reason of high light yield and short decay time of Ce-doped/concentrated scintillation crystals are due to the allowed 5d → 4f transition on Ce3+ ion. Incorporation of cerium ions in a host matrix which contained lithium (Li) or gadolinium (Gd) elements make such scintillator ideal for the detection of neutrons detection [7]. Recently, we have reported on the scintillation properties of Cs2LiCeCl6 and Rb2LiCeBr6 scintillation crystal [8, 9]. These scintillators belong to elpasolites family and could be used as neutron-gamma detectors due to the presence of Li in the host matrix. This paper presents report on the scintillation properties of our newly developed cerium doped Cs2Li(Na)Gd(1-x)CexCl6(Br) scintillation crystals. These inorganic scintillators belong to the elpasolites family and are grown by the vertical Bridgman technique. Among the scintillation properties, X-ray induced emission spectra, energy resolution, scintillation light yield, and decay time spectra are presented at room temperature. Due to highly hygroscopic nature of the grown samples, special care is devoted during scintillation characterization of these materials. We expect these scintillators could be used as potential thermal neutron detectors. 2. Experimental Technique 2.1. Crystal growth Single crystals of Cs2Li(Na)Gd(1-x)CexCl6(Br) have been grown by two zone vertical Bridgman technique (where x = 0.01, 0.1) . Stoichiometric amounts of CsCl (Br), Li/Na Cl (Br), and GdCl3 (Br) of 4 ~ 5 N purity from Sigma-Aldrich are loaded in quartz ampoule inside argon purged glove-box. The amount of CeCl3/Br3 (4N) in the feed material is adjusted in order to produce Ce-doped compound. Poly crystalline powder of Cs2Li(Na)Gd(1-x)CexCl6(Br) is loaded in clean, baked, and ultra-dry quartz ampoules under ultra-dry argon atmosphere. These ampoules are sealed with an oxy-propane torch under vacuum of ~ 10-7 Torr. Prior to the growth, melting points of all the samples are measured, and it is found that all materials are congruently melted between 590 oC and 750 oC. Detail of the crystal growth procedure can be found elsewhere in ref. [8]. Figure 1 shows a photograph of the vertical Bridgman technique used for the growth of sample crystals. After the crystal growth process, we obtained transparent samples of Cs2LiGdCl6, Cs2LiGdBr6 and Cs2NaGdBr6 crystals with 1% and 10% Ce-concentrations. The grown crystals parameters such as unit cell dimensions, density (ρ) and effective Z-number (Zeff) are summarized in Table 1. Table 1. Cs2Li(Na)Gd(1-x)CexCl6(Br) single crystals data Structure
Unit cell dimensions (Å)
ρ (g/cm3)
Zeff
Cs2LiGdCl6: Ce3+
Cubic
10.534
3.67
53
[10]
Cs2LiGdBr6: Ce3+
Cubic
11.178
4.31
50
[11, 12]
Cs2NaGdBr6: Ce3+
Cubic
11.37
4.18
50
[11, 12]
Material
Ref.
H.J. – 59 H.J. Kim Kim et et al. al. // Procedia Procedia Engineering Engineering 32 00 (2012) (2012) 53 000–000
3
Fig. 1. A photographic view of the Bridgman growth technique, (a) temperature controller and ampoule lowering motor, (b) vertical furnace
2.2. Equipment The luminescence characteristic of Cs2Li(Na)Gd(1-x)CexCl6(Br) single crystals at room temperature are measured under excitation by X-ray tube from a DRGEM. Co. having a W anode. The power settings are 100 kV and 1 mA. X-rays excitation spectra of the sample crystals are measured with QE65000 fiber optic spectrometer made by Ocean Optics. The energy spectra measurement is carried out with direct coupling of the sample crystals with the photomultiplier tube (PMT) (PhotonisXP2260) using index matching optical grease. Prior to the coupling with the PMT, all faces of the sample crystals except the one facing PMT are wrapped in several layers of 0.1-mm-thick Teflon tape. In order to avoid hydrolysis of the crystals surface, all sample crystals are covered with a cap made of Teflon material. The analog signals from the PMT are shaped with a Tennelec TC 245 spectroscopy amplifier. The output signals are then fed into a 25-MHz flash analog-to-digital converter (FADC) [13, 14]. A software threshold setting is applied to trigger an event by using a self-trigger algorithm on the field programmable gate array (FPGA) chip of the FADC board. The FADC output is recorded into a personal computer by using a USB2 connection, and the recorded data are analyzed with a C++ data analysis program [15]. 3. Results and Discussion 3.1. X-ray induced luminescence Luminescence spectra of Cs2LiGdCl6: 1%Ce3+, Cs2LiGdBr6: 1% Ce3+ and Cs2NaGdBr6: 1% Ce3+ crystals under X-ray excitation at room temperature are shown in Fig. 2. The spectra shows Ce3+ emission with two overlapped peaks caused by transitions from the lowest energy level of the Ce3+ 4f5d configuration to the allowed spin-orbit split 2F5/2 and 2F7/2 levels. The 2F5/2 emission is peaked at 374, 391, and 386 nm for Cs2LiGdCl6: 1%Ce3+, Cs2LiGdBr6: 1% Ce3+ and Cs2NaGdBr6: 1% Ce3+ crystals, respectively. The 2F7/2 emission peaked take placed at 403, 418, and 422 nm for Cs2LiGdCl6: 1%Ce3+, Cs2LiGdBr6: 1% Ce3+ and Cs2NaGdBr6: 1% Ce3+ crystals, respectively. At a longer wavelength side, Cs2LiGdBr6: 1% Ce3+ and Cs2NaGdBr6: 1% Ce3+ crystals shows broad emission bands. These broad emission bands are located between 500 nm and 600 nm for Cs2LiGdBr6: 1% Ce3+, and 500 nm and 700
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nm for Cs2NaGdBr6: 1% Ce3+, respectively. We attributed these bands to the self-trapped exciton (STE) emission [6]. The presence of 312 nm emission peak for all sample crystals in Fig. 3 is due to the Gd3+ 6Pj → 8S7/2 emission. Usually such emission disappears at higher cerium concentration due to energy transfer from the Gd3+ to Ce3+ [16]. The redshift of the Ce3+ emission is observed in the spectra is due to the chemical variation along the halide series. Such redshift is also reported in K2LaX5: 0.1% Ce3+ (X = Cl, Br, I) single crystals [17]. From the scintillation detector point view, the emission spectrum of a scintillator should match to the spectral response curve of the photo-sensor. It is clear from Fig.2 that the emission wavelengths of all the three sample crystals are well matched to the spectral response curve of the modern photomultiplier tubes (PMTs) and other photo-sensors such as large area avalanche photodiode (LAAPD).
Fig. 2. X-ray excited emission spectra at room temperature of (a) Cs2LiGdCl6: 1%Ce3+, (b) Cs2LiGdBr6: 1% Ce3+ and (c) Cs2NaGdBr6: 1% Ce3+
3.2. Pulse height measurements Pulse height spectra of Cs2LiGdCl6: 10%Ce3+, Cs2LiGdBr6: 10% Ce3+ and Cs2NaGdBr6: 10% Ce3+ single crystals under 511-keV 22Na γ-ray excitation recorded with different shaping times (3 - 10 µs) and amplifier gain is shown in Fig. 3. In order to measure the energy resolution of the sample crystals, the obtained 511-keV 22Na photo peaks with the sample crystals are fitted to Gaussian function. We found energy resolutions of 9.5%, 10.5%, and 8.0% (FWHM) for Cs2LiGdCl6: 10%Ce3+, Cs2LiGdBr6: 10% Ce3+ and Cs2NaGdBr6: 10% Ce3+ single crystals, respectively. Inset of Fig. 3 shows the pulse height spectrum of Cs2LiGdBr6: 10% Ce3+ sample crystal. Although the obtained energy resolutions are not very excellent
H.J. – 59 H.J. Kim Kim et et al. al. // Procedia Procedia Engineering Engineering 32 00 (2012) (2012) 53 000–000
5
as compared to the energy resolution of LaBr3 and LaCl3 single crystals, however, these scintillators could find their place in medical imaging applications such as SPECT.
Fig. 3. Pulse height spectra of Cs2LiGdCl6: 10%Ce3+,Cs2LiGdBr6: 10% Ce3+ (see inset) and Cs2NaGdBr6: 10% Ce3+ under 511-keV 22 Na γ-ray excitation
3.3. Decay time profile Decay time spectra of Cs2LiGdCl6: 1% Ce3+, Cs2LiGdBr6: 1% Ce3+, and Cs2NaGdBr6: 1% Ce3+ crystals at room temperature under γ-ray excitation are shown in Fig. 3. For the decay time measurement the sample crystals are attached with the PMT and irradiated by 662-keV γ-ray from a 137Cs source. The generated signals in the PMT are fed into the 400-MHz flash analog-to-digital converter (FADC) and the data is recorded. From the pulse shape information we have calculated the decay time of the sample crystals [18]. From fit to the data we have distinguished three decay time components for all the three sample crystals. For simplicity they are called as short (τ1), intermediate (τ2), and long (τ3) decay time components. Three decay time components and fit to the data of Cs2LiGdCl6: 1% Ce3+ crystal is shown in the inset of Fig. 4. The decay time components and their relative light output intensities are presented in Table 2. It is clear from the Table 2 that the short decay time components of the Bromo-elpasolites are fast as compared to the Cs2LiGdCl6: 1% Ce3+ crystal and more than 60% of the total light is emitted with a fast decay time component. It is observed that further increase of Ce-concentration in the lattice matrix reduces the values of long decay time components of these scintillators (not shown). Which demonstrate that energy transfer rate from the lattice matrix to Ce3+ ion is increases with higher Ce-concentration. Table 2. Decay time components of the sample crystals under 662-keV γ-ray from a 137Cs source measured at room temperature Sample Cs2LiGdCl6: 1% Ce3+ Cs2LiGdBr6: 1% Ce3+ Cs2NaGdBr6: 1% Ce3+
τ1 207 ns (38%) 73 ns (63%) 94 ns (65%)
τ2 713 ns (42%) 542 ns (14%) 423 ns (27%)
τ3 6.7 µs (20%) 3.9 µs (23%) 7.2 µs (8%)
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Fig. 4. Decay time spectra under γ-ray excitation on a double-log scale of Cs2LiGdCl6: 1%Ce3+, Cs2LiGdBr6: 1% Ce3+, and Cs2NaGdBr6: 1% Ce3+ single crystals. The inset shows the decay time spectrum of Cs2LiGdCl6: 1% Ce3+ with the fit to the data and three exponential decay time components, a short (τ1), intermediate (τ2), and long (τ3), respectively
4. Conclusion We studied the scintillation properties of Ce3+ doped Cs2LiGdCl6, Cs2LiGdBr6 and Cs2NaGdBr6 single crystals, grown by vertical Bridgman technique. Due to the cubic structure of these materials, they can be grown easily in large volume. We expect that due to the presence of Li and Gd ions in the lattice matrix these scintillators could be used for the thermal neutron detection. The X-ray induced emission spectra show Ce3+ emission with two overlapped peaks. These emission matches well with the spectral response of the modern photo-sensors. Under 511-kev γ-ray excitation from a 22Na source we obtained best energy resolution of 8.0% (FWHM) for Cs2NaGdBr6: 10% Ce3+ single crystal. For the entire sample crystals three exponential decay time components are observed at room temperature. It is further observed that the short decay time components of Bromo-elpasolites are faster than the Cs2LiGdCl6: 1% Ce3+ crystal. Further investigations such as the detection of thermal neutrons are under way to study the potential of these scintillators as a thermal neutron detector. References [1] S. E. Derenzo, W. W. Moses "Experimental Efforts and Results in finding new heavy Scintillators. In: Heavy Scintillators for scientific and Industrial Applications, Ed. F. De Notaristefani, P. Lecoq, M. Schneegans, Frontieres 1993; 125-136. [2] C.W.E. van Eijk, In: Proceedings of the International Conference on Inorganic Scintillators and their Applications, 1997; 3. [3] C.W.E. van Eijk, Inorganic scintillators in medical imaging detectors. Nucl. Instrum. Meth. Phys. Res. A 2003; 509:17-25. [4] Hongsheng Shi, Laishun Qin, Wenxiang Chai, Jiayu Guo, Qinhua Wei, Guohao Ren, Kangying Shu. The LaBr3:Ce Crystal Growth by Self-Seeding Bridgman Technique and Its Scintillation Properties Cryst. Growth & Design 2010; 10: 4433-4436. [5] E.V.D. Van Loef, P. Dorenbos, C.W.E. Van Eijk, K.W. Kramer, H.U. Gudel, High-energy-resolution scintillator: Ce3+ activated LaBr3 Appl. Phys. Lett 2001;79: 1573-1575. [6] G. Bizarri, P. Dorenbos, Charge carrier and exciton dynamics in LaBr3:Ce3+ scintillators: Experiment and model Phys. Rev. B
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