- Email: [email protected]
Crystal growth and scintillation characterizations of Tl2LiYCl6: Ce3+
Author’s Accepted Manuscript Crystal Growth and Scintillation Characterizations of Tl2LiYCl6: Ce3+ Gul Rooh, H.J. Kim, H. Park, Sunghwan Kim
www.elsevier.com/locate/jcrysgro
PII: DOI: Reference:
S0022-0248(16)30849-1 http://dx.doi.org/10.1016/j.jcrysgro.2016.11.129 CRYS23857
To appear in: Journal of Crystal Growth Received date: 29 June 2016 Revised date: 21 November 2016 Accepted date: 30 November 2016 Cite this article as: Gul Rooh, H.J. Kim, H. Park and Sunghwan Kim, Crystal Growth and Scintillation Characterizations of Tl2LiYCl6: Ce3+, Journal of Crystal Growth, http://dx.doi.org/10.1016/j.jcrysgro.2016.11.129 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Crystal Growth and Scintillation Characterizations of Tl2LiYCl6: Ce3+ Gul Rooha, H. J. Kimb*, H. Parkb, Sunghwan Kimc a
Department of Physics, Abdul Wali Khan University, Mardan,23200, Pakistan Department of Physics, Kyungpook National University, Daegu 702-701, Korea c Department of Radiological Science, Cheongju University, Cheongju 360-764, Korea b
*
Corresponding author. Tel.:+82-53-950-5323; fax: +82-53-956-1739. E-mail address: [email protected] (H.J. Kim). Abstract Single crystals of Tl2LiYCl6 with different mole% of Ce-concentration (0.5 and 5 mole%) were presented. This material belongs to Chloro-elpasolite crystal family and was grown by two zone vertical Bridgman technique. X-ray diffraction pattern of the Tl2LiYCl6 single crystal was measured. X-ray diffraction analysis confirmed tetragonal crystal structure. The melting point of the sample was measured by DSC. Typical Ce3+ ion emission spectra were observed when samples were excited by an X-ray source at room temperature. The observed emission spectra were located between 350 nm and 530 nm wavelength range and peaking around 430 nm. Pulse height spectra of the samples under 662 keV γ-rays excitation shows improvement in energy resolution with the increase of Ce-concentration in the host lattice. The energy resolutions were obtained to be 10.6% and 7.2% (FWHM) for 0.5% and 5% Ce3+ concentration, respectively. Under γ-ray excitation, a maximum light yield of 23,800±2400 ph/MeV was observed for 5% Ce-concentration. Studied samples of Tl2LiYCl6: Ce3+ showed three exponential decay time components under γ-ray excitation. From the measured scintillation properties, we consider this material is a promising scintillator for radiation detection. Key words: A1. X-ray, A1. Decay Time, B2. Light yield, A1. Energy resolution, A1. Elpasolite PACS codes: (i) 29.30. Kv, (ii) 78.70. En, (iii) 78.70. Ps, (iv) 81.10. Fq
Introduction Inorganic scintillators are widely used materials for the detection of different radiations in the field of medical imaging, security inspection, high-energy-physics, and well-logging for oil and
gas exploration [1]. In recent years, more attention is devoted in the discovery of new scintillators characterized by excellent energy resolution and high light yield under γ-ray excitation, fast decay time, high density and high Z-number [2, 3]. Inorganic halide scintillators activated by rare-earth ion are currently very attractive for the scientific community owing to their wide range of applications. Specially Ce-doped elpasolites scintillators showed excellent scintillation performance and the research on different elpasolites materials have also shown that this rare earth could be a good doping element for high efficient scintillation materials, the well-known examples are Cs2LiYCl6: Ce3+, Cs2LiLaCl6: Ce3+ and Tl2LiGdCl6: Ce3+ [4-7]. Compared with other compounds, more than thousand elpasolites compounds are reported in the literature [8, 9], this make the choice of selection of scintillation materials wider for various applications. Therefore, it is expected that elpasolites crystal family has the potential to provide excellent scintillation material which can fulfill the requirement of an ideal scintillator. Present research is focused to discover new scintillator belonging to this crystal family. This study is the continuation of our research work on Tl2LiYCl6 (TLYC) single crystals doped with different Ce-concentrations. A detailed description of the crystal growth procedure was presented. X-ray diffraction and DSC were performed for the phase confirmation and melting point. Scintillation properties were studied under X- and γ-ray excitation at room temperature. Some of the scintillation properties of TLYC doped with 1 and 10 % Ce was presented in ref [10].
Experimental Technique
A. Crystal growth Cerium activated Tl2LiYCl6 (0.5 and 5 mole % Ce) were grown via the two zone vertical Bridgman technique. Raw materials of 5N purity powders (TlCl, LiCl, YCl3 and CeCl3) from Sigma-Aldrich or Alfa-Aesar were weighing in ultra-dry argon purged glove box having moisture and oxygen level less than 10 ppm. Quartz ampules cleaned with ethanol, acetone and water were baked in an oven, residual moisture in the quartz ampules was removed by using diffusion pump and Oxy-propane torch. Already weighing powder was loaded in the ampules inside the glove box. In order to remove moisture from the powders, all ampules loaded with
powders were baked in vacuum at ~200 oC for several hours and sealed with an oxy-propane torch under dynamic vacuum of ~10-7 Torr. In order to find the melting point of this new material, a vertical transparent chamber made of quartz tube was used. A thermocouple was directly attached with the ampoule loaded with raw material for the observation of the melting point of the material and was lowered in the transparent chamber. Temperature of the transparent chamber was raised very slowly (i.e. 100 oC/h) until the whole material was completely melted. The melting point of the material was found to be 490 oC. Such melting point was also verified in the TGA/DSC study of this material. After the melting point measurements, ampoules of TLYC: Ce3+ samples were lowered into two zone vertical Bridgman chamber with the help of a synchronous motor. Temperature of the furnace was raised to 530 oC, which was 40 oC higher than the melting point i.e. 490 oC. In addition, the upper and lower zone temperatures were adjusted to maintain a temperature gradient of 10 oC/cm. During the crystal growth process, the rate of growth was maintained at 10-12 mm/day. After the growth, the ampoules were allowed to cool to room temperature at a rate of 10 oC/h. Although, the cooling rate was very slow, however many cracks were observed in the samples. All ampoules were carefully cut with a dry diamond coated stainless steel wire loop and small samples were obtained for different scintillation characterization.
B. Equipment The structure of the TLYC: Ce3+ crystal in powder form was examined by X-ray diffraction (XRD) technique. For powder XRD, Philips, XPERT-PRO X-ray diffractometer equipped with Cu Kα (λ = 1.54056 Å) was used at room temperature. The XRD analysis was carried out in the 2θ range from 10o to 80o with step size of 0.02o and scanning rate of 1.2o/min at 40 kV and 30 mA. To study the phase transition and melting point of the grown sample, Differential Scanning Calorimetry (DSC) and Thermogravimetric analysis (TGA) were performed. DSC measurements were performed by using TA instruments model SDT Q600 in the temperature range from 50 to 1200oC. The X-ray induced luminescence spectra of TLYC: Ce3+ crystals were obtained at room temperature. Investigated sample were excited with an X-ray tube (DRGEM. Co.) using a tungsten anode operating at 50 kV and 1 mA. The emission spectra were measured by utilizing a spectrometer (QE65000 fiber optic spectrometer) made by Ocean Optics. Pulse height spectra
were measured with a Hamamatsu R6233 photomultiplier tube (PMT) at room temperature. Polished sample crystals were wrapped in several layers of 0.1-mm-thick UV reflecting Teflon tape and coupled directly to the entrance window of the PMT using index matching optical grease. After irradiation with γ-rays, the analog signals generated in the crystal were shaped with a Tennelec TC 245 spectroscopy amplifier. The output signals were then fed into a 25-MHz flash analog-to-digital converter (FADC). A software threshold was set to trigger an event by using a self-trigger algorithm on the field programmable gate array (FPGA) chip on the FADC board. The FADC output was recorded into a personal computer by using a USB2 connection, and the recorded data are analyzed with a C++ data analysis program [11]. Decay time spectra was measured by optically coupled crystals of TLYC: Ce3+ with the PMT (Hamamatsu R6233) and was excited by 662 keV γ-rays from a
137
Cs source. Signals produced
in the PMT were fed into a 400-MHz FADC, this FADC module is fabricated to sample the pulse every 2.5 ns for duration up to 64 µs so that one can fully reconstruct each photoelectron pulse [12]. Finally, the decay time of TLYC: Ce3+ was measured from the recorded pulse shape information.
Scintillation properties A. Crystal analysis X-ray diffraction (XRD) pattern of the TLYC: Ce3+ single crystal is shown in Fig. 1. XRD data obtained from the crushed single crystal of TLYC indicated that major peaks of these samples were consistent with the tetragonal elpasolite structure. TLYC has P4/nbm space group with lattice parameters of a = 10.1260 Å and c = 10.1248 Å. The volume and density of the unit cell were obtained as 1038.2 Å3 and 4.58 g/cm3, respectively. The hump appeared between 2θ = 1330o is due to the Kapton tape which is used to protect the powder sample from the moisture. Figure 2 shows the thermograme of the as grown TLYC: Ce3+ single crystal. From the DSC thermograme, the melting point is obtained to be 490 oC, in addition, no obvious phase transition is observed up to the melting point of the grown sample. B. X-ray excited luminescence Figure 3 shows emission spectra of 0.5% and 5% Ce doped TLYC crystals under x-ray excitation at room temperature. Emission spectra contained broad bands between 350 and 550 nm peaking at 425 nm. The observed emission was attributed to the parity allowed 5d → 4f transition of the
Ce3+ ion, i.e. transition from the lowest 5d level to 2F5/2 and 2F7/2 levels of the 4f1 configuration [13]. The emission spectra of the grown samples revealed that this material is an attractive for γray spectroscopy, since it matches well with the response function of the photomultiplier tubes and silicon photo-diodes.
C. Pulse height spectrum and light yield Pulse height spectra of 0.5% and 5% Ce doped samples were obtained under γ-ray excitation from a
137
Cs source. The energy resolutions of the samples were calculated by applying a
Gaussian fit to the photopeaks after irradiation with γ-ray source. Best energy resolution of 7.2 % (FWHM) was found TLYC: 5% Ce3+ crystal while 0.5% Ce sample shows an energy resolution of 10.6% (FWHM) (not shown). Figure 4 shows the pulse height spectrum of TLYC: 5% Ce3+ crystal under 662 keV γ-ray excitation from a
137
Cs source. A satellite peak observed at low
energy in the pulse height spectrum is attributed to the Tl K-X-ray escape peak. The light yield (LY) of the 0.5% and 5% Ce doped TLYC crystals were measured at room temperature. LYSO scintillator having a light yield of 33,000 ph/MeV was used as a standard crystal for the comparison of LY of TLYC samples [14]. LYSO and TLYC: Ce3+ crystals were optically coupled with a windowed large area avalanche photodiode (LAAPD) (630-70-74-501, Advanced Photonix, Inc.) and irradiated with 662 keV photons from a
137
Cs source. During
measurements, similar conditions of the LAAPD bias, shaping time and amplifier gain was used. Electronic signals of the LAAPD were amplified with Tennelec TC 245 shaping amplifier and digitized with a 25 MHz flash analog-to-digital converter (FADC). The recorded the pulse height spectra obtained from LYSO and TLYC: Ce3+ crystals were compared. Based on the comparison, we obtained highest LY of 23,800±2400 ph/MeV for 5% Ce doped TLYC crystal using shaping time of 6μs. Figure 5 shows the pulse height spectra of TLYC: 5% Ce3+ and LYSO crystals, the channel number corresponds to the LY.
Decay times Figure 6 shows the decay time spectra of the 0.5 % and 5% Ce doped TLYC crystals. Decay time measurement of the grown samples is performed under γ-ray excitation using
137
Cs source at
room temperature. The decay time spectra contained more than one decay constant, therefore, multi-exponential decay functions are applied to the decay curves. The decay time constants of
0.5% and 5% Ce doped crystals along with their relative contributions to the total light yield are listed in Table 1. Explanation of the origin of scintillation decay time constants is beyond the scope of this study, however we speculate that the fast and intermediate decay time constants may be due to the delayed transfer of excitation energy (electron-hole pairs) and binary electronhole recombination on Ce3+ ions, respectively since the probability of direct recombination of electron hole pairs at the luminescence center in the elpasolites is very less [15-18]. The long decay time constants are attributed to the combination of self-trapped exciton (STE) emission and 5d–4f emission due to the delayed transfer from STEs to Ce3+ ions [18, 19].
Conclusions This study summarizes our investigations of the cerium doped TLYC: Ce3+ single crystals. This material exhibits tetragonal crystal structure with an effective Z-number 69 and density 4.58 g/cm3. From the γ-ray detection point of view, this material is capable to detect X- and γ-rays efficiently due to higher Z-number. Energy resolution and scintillation light yield of 7.2% (FWHM) and 23,800±2400 ph/MeV, respectively, are obtained for 5% Ce doped TLYC crystal under γ-ray excitation. Since TLYC: Ce3+ also contains lithium ion, it can be used to detect thermal neutrons. Under X-ray excitation, broad emission bands are observed due to Ce3+ ion. Times profile of 0.5% and 5% Ce-doped samples under γ-rays excitation shows three decay components at room temperature. Higher Ce concentration leads to increase the values of the scintillation decay time constants, which is assigned to the competition between STE and Ce3+ emission. Overall, this study shows that TLYC: Ce3+ is a promising scintillation material for radiation detection.
Table I. Scintillation characteristics of TLYC: Ce3+ crystals at room temperature. Ce-concentration (mole%)
Energy resolution at 662 keV ΔE/E (FWHM) %
Scintillation Light Yield (photons/MeV) 1.5 µs
4µs
6 µs
Decay Time
0.5
10.6
18,000±1800
19,500±2000
19,500±2000
43 ns (2%), 317 ns (42%), 680 ns (56%)
5
7.1
22,000±2200
23,800±2400
23,800±2400
60 ns (4%), 450 ns (64%), 1.1 µs (6%)
Acknowledgment
These investigations have been supported by the National Research Foundation of Korea (NRF) funded
by
the
Ministry
of
Science
and
Technology,
Korea
(MEST)
(No.
2015R1A2A1A13001843).
References [1]
M. Nikl, and A. Yoshikawa, Adv. Opt. Mater., 3 (2015) 463.
[2]
A. Bessiere, P.Dorenbos, C. W. E Van Eijk, K. W. Kramer, H. U. Gudel, Nucl. Instrum. Meth. Phys. Res. A, 537 (2005) 242.
[3]
M. Nikl, P. Bruza, D. Panek, M. Vrbova, E. Mihokova, J. A. Mares, A. Beitlerova, N. Kawaguchi, K. Fukuda and A. Yoshikawa, Appl. Phys. Lett., 2013, 102, 161907.
[4]
H. Yang et al., CrystEngComm, 16 (2014) 2480.
[5]
D. W. Lee, L. C. Stonehill, A. Klimenko, J. R.Terry, S. R. Tornga, Nucl. Instrum. Meth. Phys. Res. A, 664 (2012) 1.
[6]
P. A. Rodny, V. B. Mikhalik, G. B. Stryganyuk, A. S. Voloshinovskii, C. W. E Van Eijk, G. F. Zimmerer, J. Lumin. 86 (2000) 161.
[7]
H. J. Kim, G. Rooh, H. Park, S. Kim, J. Lumin. 164 (2015) 86.
[8]
K. Biswas, M.-Hua Du, Phys. Rev. B 86 (2012) 014102.
[9]
G. Meyer, Prog. Solid State Chem. 14 (1982) 141.
[10] H. J. Kim, Gul Rooh, H. Park, and Sunghwan Kim, IEEE Trans. Nucl. Sci., 63 (2016) 439442. [11]
J. H. So, H. J. Kim, H. Kang, H. Park, and S. Lee, J. Korean Phys. Soc. 52(3) (2008) 925.
[12]
H. J. Kim et al. IEEE Trans. Nucl. Sci., 57 (3) (2010) 1475.
[13]
K. Mori, and M. Nakayama, Phys. Rev. B 67 (2003)165206.
[14]
G. Rooh, H. J. Kim, H. Park, S. Kim, J. Cryst. Growth 377 (2013) 28.
[15]
E.V.D. van Loef, P. Dorenbos, C.W.E. van Eijk, K.W. Kramer, H.U. Gudel, J. Phys. Condens. Matter 14 (2002) 8481.
[16]
C.M. Combes, P. Dorenbos, C.W.E. van Eijk, K.W. Kramer, H.U. Gudel, J. Lumin. 82 (1999) 299.
[17]
P. Dorenbos, Phys. Stat. Sol. A 202 (2005) 195.
[18]
G. Rooh, H. J. Kim, H. Park, S. Kim, J. Lumin. 132 (2012) 713.
[19]
M.D. Birowosuto, P. Dorenbos, C.W.E. van Eijk, K.W. Kramer, H.U. Gudel, J. Phys, Condens. Matter 18 (2006) 6133.
Figure 1. Powder XRD pattern of the TLYC single crystal.
Figure 2. DSC and TGA curves of the TLYC single crystal showing the melting point at 490 oC.
Figure 3. X-ray excited emission spectra at room temperature of TLYC: x Ce3+ (x= 0.5% and 5%).
Figure 4. The scintillation pulse height spectrum of TLYC: 5% Ce3+ irradiated with γ-rays from a 137
Cs source.
Figure 5. Pulse height spectra of TLYC: 5% Ce3+ and LYSO: Ce3+crystals excited with γ-rays from a 137Cs source. The photopeak position is proportional to the light yield.
Figure 6. Time profiles of TLYC crystals with 0.5% and 5% Ce concentrations measured under γ-rays excitation at room temperature.
Highlights Scintillation characterization of Tl2LiYCl6:Ce3+ were presented. Tl2LiYCl6:Ce3+ were grown by two zone vertical Bridgman technique. It has high Zeff and could be a potential candidate for medical imaging technique. Energy resolution of 7.2% and light yield of 23,800±2400 ph/MeV were obtained. Three decay time components were observed under γ-ray excitation.
www.elsevier.com/locate/jcrysgro
PII: DOI: Reference:
S0022-0248(16)30849-1 http://dx.doi.org/10.1016/j.jcrysgro.2016.11.129 CRYS23857
To appear in: Journal of Crystal Growth Received date: 29 June 2016 Revised date: 21 November 2016 Accepted date: 30 November 2016 Cite this article as: Gul Rooh, H.J. Kim, H. Park and Sunghwan Kim, Crystal Growth and Scintillation Characterizations of Tl2LiYCl6: Ce3+, Journal of Crystal Growth, http://dx.doi.org/10.1016/j.jcrysgro.2016.11.129 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Crystal Growth and Scintillation Characterizations of Tl2LiYCl6: Ce3+ Gul Rooha, H. J. Kimb*, H. Parkb, Sunghwan Kimc a
Department of Physics, Abdul Wali Khan University, Mardan,23200, Pakistan Department of Physics, Kyungpook National University, Daegu 702-701, Korea c Department of Radiological Science, Cheongju University, Cheongju 360-764, Korea b
*
Corresponding author. Tel.:+82-53-950-5323; fax: +82-53-956-1739. E-mail address: [email protected] (H.J. Kim). Abstract Single crystals of Tl2LiYCl6 with different mole% of Ce-concentration (0.5 and 5 mole%) were presented. This material belongs to Chloro-elpasolite crystal family and was grown by two zone vertical Bridgman technique. X-ray diffraction pattern of the Tl2LiYCl6 single crystal was measured. X-ray diffraction analysis confirmed tetragonal crystal structure. The melting point of the sample was measured by DSC. Typical Ce3+ ion emission spectra were observed when samples were excited by an X-ray source at room temperature. The observed emission spectra were located between 350 nm and 530 nm wavelength range and peaking around 430 nm. Pulse height spectra of the samples under 662 keV γ-rays excitation shows improvement in energy resolution with the increase of Ce-concentration in the host lattice. The energy resolutions were obtained to be 10.6% and 7.2% (FWHM) for 0.5% and 5% Ce3+ concentration, respectively. Under γ-ray excitation, a maximum light yield of 23,800±2400 ph/MeV was observed for 5% Ce-concentration. Studied samples of Tl2LiYCl6: Ce3+ showed three exponential decay time components under γ-ray excitation. From the measured scintillation properties, we consider this material is a promising scintillator for radiation detection. Key words: A1. X-ray, A1. Decay Time, B2. Light yield, A1. Energy resolution, A1. Elpasolite PACS codes: (i) 29.30. Kv, (ii) 78.70. En, (iii) 78.70. Ps, (iv) 81.10. Fq
Introduction Inorganic scintillators are widely used materials for the detection of different radiations in the field of medical imaging, security inspection, high-energy-physics, and well-logging for oil and
gas exploration [1]. In recent years, more attention is devoted in the discovery of new scintillators characterized by excellent energy resolution and high light yield under γ-ray excitation, fast decay time, high density and high Z-number [2, 3]. Inorganic halide scintillators activated by rare-earth ion are currently very attractive for the scientific community owing to their wide range of applications. Specially Ce-doped elpasolites scintillators showed excellent scintillation performance and the research on different elpasolites materials have also shown that this rare earth could be a good doping element for high efficient scintillation materials, the well-known examples are Cs2LiYCl6: Ce3+, Cs2LiLaCl6: Ce3+ and Tl2LiGdCl6: Ce3+ [4-7]. Compared with other compounds, more than thousand elpasolites compounds are reported in the literature [8, 9], this make the choice of selection of scintillation materials wider for various applications. Therefore, it is expected that elpasolites crystal family has the potential to provide excellent scintillation material which can fulfill the requirement of an ideal scintillator. Present research is focused to discover new scintillator belonging to this crystal family. This study is the continuation of our research work on Tl2LiYCl6 (TLYC) single crystals doped with different Ce-concentrations. A detailed description of the crystal growth procedure was presented. X-ray diffraction and DSC were performed for the phase confirmation and melting point. Scintillation properties were studied under X- and γ-ray excitation at room temperature. Some of the scintillation properties of TLYC doped with 1 and 10 % Ce was presented in ref [10].
Experimental Technique
A. Crystal growth Cerium activated Tl2LiYCl6 (0.5 and 5 mole % Ce) were grown via the two zone vertical Bridgman technique. Raw materials of 5N purity powders (TlCl, LiCl, YCl3 and CeCl3) from Sigma-Aldrich or Alfa-Aesar were weighing in ultra-dry argon purged glove box having moisture and oxygen level less than 10 ppm. Quartz ampules cleaned with ethanol, acetone and water were baked in an oven, residual moisture in the quartz ampules was removed by using diffusion pump and Oxy-propane torch. Already weighing powder was loaded in the ampules inside the glove box. In order to remove moisture from the powders, all ampules loaded with
powders were baked in vacuum at ~200 oC for several hours and sealed with an oxy-propane torch under dynamic vacuum of ~10-7 Torr. In order to find the melting point of this new material, a vertical transparent chamber made of quartz tube was used. A thermocouple was directly attached with the ampoule loaded with raw material for the observation of the melting point of the material and was lowered in the transparent chamber. Temperature of the transparent chamber was raised very slowly (i.e. 100 oC/h) until the whole material was completely melted. The melting point of the material was found to be 490 oC. Such melting point was also verified in the TGA/DSC study of this material. After the melting point measurements, ampoules of TLYC: Ce3+ samples were lowered into two zone vertical Bridgman chamber with the help of a synchronous motor. Temperature of the furnace was raised to 530 oC, which was 40 oC higher than the melting point i.e. 490 oC. In addition, the upper and lower zone temperatures were adjusted to maintain a temperature gradient of 10 oC/cm. During the crystal growth process, the rate of growth was maintained at 10-12 mm/day. After the growth, the ampoules were allowed to cool to room temperature at a rate of 10 oC/h. Although, the cooling rate was very slow, however many cracks were observed in the samples. All ampoules were carefully cut with a dry diamond coated stainless steel wire loop and small samples were obtained for different scintillation characterization.
B. Equipment The structure of the TLYC: Ce3+ crystal in powder form was examined by X-ray diffraction (XRD) technique. For powder XRD, Philips, XPERT-PRO X-ray diffractometer equipped with Cu Kα (λ = 1.54056 Å) was used at room temperature. The XRD analysis was carried out in the 2θ range from 10o to 80o with step size of 0.02o and scanning rate of 1.2o/min at 40 kV and 30 mA. To study the phase transition and melting point of the grown sample, Differential Scanning Calorimetry (DSC) and Thermogravimetric analysis (TGA) were performed. DSC measurements were performed by using TA instruments model SDT Q600 in the temperature range from 50 to 1200oC. The X-ray induced luminescence spectra of TLYC: Ce3+ crystals were obtained at room temperature. Investigated sample were excited with an X-ray tube (DRGEM. Co.) using a tungsten anode operating at 50 kV and 1 mA. The emission spectra were measured by utilizing a spectrometer (QE65000 fiber optic spectrometer) made by Ocean Optics. Pulse height spectra
were measured with a Hamamatsu R6233 photomultiplier tube (PMT) at room temperature. Polished sample crystals were wrapped in several layers of 0.1-mm-thick UV reflecting Teflon tape and coupled directly to the entrance window of the PMT using index matching optical grease. After irradiation with γ-rays, the analog signals generated in the crystal were shaped with a Tennelec TC 245 spectroscopy amplifier. The output signals were then fed into a 25-MHz flash analog-to-digital converter (FADC). A software threshold was set to trigger an event by using a self-trigger algorithm on the field programmable gate array (FPGA) chip on the FADC board. The FADC output was recorded into a personal computer by using a USB2 connection, and the recorded data are analyzed with a C++ data analysis program [11]. Decay time spectra was measured by optically coupled crystals of TLYC: Ce3+ with the PMT (Hamamatsu R6233) and was excited by 662 keV γ-rays from a
137
Cs source. Signals produced
in the PMT were fed into a 400-MHz FADC, this FADC module is fabricated to sample the pulse every 2.5 ns for duration up to 64 µs so that one can fully reconstruct each photoelectron pulse [12]. Finally, the decay time of TLYC: Ce3+ was measured from the recorded pulse shape information.
Scintillation properties A. Crystal analysis X-ray diffraction (XRD) pattern of the TLYC: Ce3+ single crystal is shown in Fig. 1. XRD data obtained from the crushed single crystal of TLYC indicated that major peaks of these samples were consistent with the tetragonal elpasolite structure. TLYC has P4/nbm space group with lattice parameters of a = 10.1260 Å and c = 10.1248 Å. The volume and density of the unit cell were obtained as 1038.2 Å3 and 4.58 g/cm3, respectively. The hump appeared between 2θ = 1330o is due to the Kapton tape which is used to protect the powder sample from the moisture. Figure 2 shows the thermograme of the as grown TLYC: Ce3+ single crystal. From the DSC thermograme, the melting point is obtained to be 490 oC, in addition, no obvious phase transition is observed up to the melting point of the grown sample. B. X-ray excited luminescence Figure 3 shows emission spectra of 0.5% and 5% Ce doped TLYC crystals under x-ray excitation at room temperature. Emission spectra contained broad bands between 350 and 550 nm peaking at 425 nm. The observed emission was attributed to the parity allowed 5d → 4f transition of the
Ce3+ ion, i.e. transition from the lowest 5d level to 2F5/2 and 2F7/2 levels of the 4f1 configuration [13]. The emission spectra of the grown samples revealed that this material is an attractive for γray spectroscopy, since it matches well with the response function of the photomultiplier tubes and silicon photo-diodes.
C. Pulse height spectrum and light yield Pulse height spectra of 0.5% and 5% Ce doped samples were obtained under γ-ray excitation from a
137
Cs source. The energy resolutions of the samples were calculated by applying a
Gaussian fit to the photopeaks after irradiation with γ-ray source. Best energy resolution of 7.2 % (FWHM) was found TLYC: 5% Ce3+ crystal while 0.5% Ce sample shows an energy resolution of 10.6% (FWHM) (not shown). Figure 4 shows the pulse height spectrum of TLYC: 5% Ce3+ crystal under 662 keV γ-ray excitation from a
137
Cs source. A satellite peak observed at low
energy in the pulse height spectrum is attributed to the Tl K-X-ray escape peak. The light yield (LY) of the 0.5% and 5% Ce doped TLYC crystals were measured at room temperature. LYSO scintillator having a light yield of 33,000 ph/MeV was used as a standard crystal for the comparison of LY of TLYC samples [14]. LYSO and TLYC: Ce3+ crystals were optically coupled with a windowed large area avalanche photodiode (LAAPD) (630-70-74-501, Advanced Photonix, Inc.) and irradiated with 662 keV photons from a
137
Cs source. During
measurements, similar conditions of the LAAPD bias, shaping time and amplifier gain was used. Electronic signals of the LAAPD were amplified with Tennelec TC 245 shaping amplifier and digitized with a 25 MHz flash analog-to-digital converter (FADC). The recorded the pulse height spectra obtained from LYSO and TLYC: Ce3+ crystals were compared. Based on the comparison, we obtained highest LY of 23,800±2400 ph/MeV for 5% Ce doped TLYC crystal using shaping time of 6μs. Figure 5 shows the pulse height spectra of TLYC: 5% Ce3+ and LYSO crystals, the channel number corresponds to the LY.
Decay times Figure 6 shows the decay time spectra of the 0.5 % and 5% Ce doped TLYC crystals. Decay time measurement of the grown samples is performed under γ-ray excitation using
137
Cs source at
room temperature. The decay time spectra contained more than one decay constant, therefore, multi-exponential decay functions are applied to the decay curves. The decay time constants of
0.5% and 5% Ce doped crystals along with their relative contributions to the total light yield are listed in Table 1. Explanation of the origin of scintillation decay time constants is beyond the scope of this study, however we speculate that the fast and intermediate decay time constants may be due to the delayed transfer of excitation energy (electron-hole pairs) and binary electronhole recombination on Ce3+ ions, respectively since the probability of direct recombination of electron hole pairs at the luminescence center in the elpasolites is very less [15-18]. The long decay time constants are attributed to the combination of self-trapped exciton (STE) emission and 5d–4f emission due to the delayed transfer from STEs to Ce3+ ions [18, 19].
Conclusions This study summarizes our investigations of the cerium doped TLYC: Ce3+ single crystals. This material exhibits tetragonal crystal structure with an effective Z-number 69 and density 4.58 g/cm3. From the γ-ray detection point of view, this material is capable to detect X- and γ-rays efficiently due to higher Z-number. Energy resolution and scintillation light yield of 7.2% (FWHM) and 23,800±2400 ph/MeV, respectively, are obtained for 5% Ce doped TLYC crystal under γ-ray excitation. Since TLYC: Ce3+ also contains lithium ion, it can be used to detect thermal neutrons. Under X-ray excitation, broad emission bands are observed due to Ce3+ ion. Times profile of 0.5% and 5% Ce-doped samples under γ-rays excitation shows three decay components at room temperature. Higher Ce concentration leads to increase the values of the scintillation decay time constants, which is assigned to the competition between STE and Ce3+ emission. Overall, this study shows that TLYC: Ce3+ is a promising scintillation material for radiation detection.
Table I. Scintillation characteristics of TLYC: Ce3+ crystals at room temperature. Ce-concentration (mole%)
Energy resolution at 662 keV ΔE/E (FWHM) %
Scintillation Light Yield (photons/MeV) 1.5 µs
4µs
6 µs
Decay Time
0.5
10.6
18,000±1800
19,500±2000
19,500±2000
43 ns (2%), 317 ns (42%), 680 ns (56%)
5
7.1
22,000±2200
23,800±2400
23,800±2400
60 ns (4%), 450 ns (64%), 1.1 µs (6%)
Acknowledgment
These investigations have been supported by the National Research Foundation of Korea (NRF) funded
by
the
Ministry
of
Science
and
Technology,
Korea
(MEST)
(No.
2015R1A2A1A13001843).
References [1]
M. Nikl, and A. Yoshikawa, Adv. Opt. Mater., 3 (2015) 463.
[2]
A. Bessiere, P.Dorenbos, C. W. E Van Eijk, K. W. Kramer, H. U. Gudel, Nucl. Instrum. Meth. Phys. Res. A, 537 (2005) 242.
[3]
M. Nikl, P. Bruza, D. Panek, M. Vrbova, E. Mihokova, J. A. Mares, A. Beitlerova, N. Kawaguchi, K. Fukuda and A. Yoshikawa, Appl. Phys. Lett., 2013, 102, 161907.
[4]
H. Yang et al., CrystEngComm, 16 (2014) 2480.
[5]
D. W. Lee, L. C. Stonehill, A. Klimenko, J. R.Terry, S. R. Tornga, Nucl. Instrum. Meth. Phys. Res. A, 664 (2012) 1.
[6]
P. A. Rodny, V. B. Mikhalik, G. B. Stryganyuk, A. S. Voloshinovskii, C. W. E Van Eijk, G. F. Zimmerer, J. Lumin. 86 (2000) 161.
[7]
H. J. Kim, G. Rooh, H. Park, S. Kim, J. Lumin. 164 (2015) 86.
[8]
K. Biswas, M.-Hua Du, Phys. Rev. B 86 (2012) 014102.
[9]
G. Meyer, Prog. Solid State Chem. 14 (1982) 141.
[10] H. J. Kim, Gul Rooh, H. Park, and Sunghwan Kim, IEEE Trans. Nucl. Sci., 63 (2016) 439442. [11]
J. H. So, H. J. Kim, H. Kang, H. Park, and S. Lee, J. Korean Phys. Soc. 52(3) (2008) 925.
[12]
H. J. Kim et al. IEEE Trans. Nucl. Sci., 57 (3) (2010) 1475.
[13]
K. Mori, and M. Nakayama, Phys. Rev. B 67 (2003)165206.
[14]
G. Rooh, H. J. Kim, H. Park, S. Kim, J. Cryst. Growth 377 (2013) 28.
[15]
E.V.D. van Loef, P. Dorenbos, C.W.E. van Eijk, K.W. Kramer, H.U. Gudel, J. Phys. Condens. Matter 14 (2002) 8481.
[16]
C.M. Combes, P. Dorenbos, C.W.E. van Eijk, K.W. Kramer, H.U. Gudel, J. Lumin. 82 (1999) 299.
[17]
P. Dorenbos, Phys. Stat. Sol. A 202 (2005) 195.
[18]
G. Rooh, H. J. Kim, H. Park, S. Kim, J. Lumin. 132 (2012) 713.
[19]
M.D. Birowosuto, P. Dorenbos, C.W.E. van Eijk, K.W. Kramer, H.U. Gudel, J. Phys, Condens. Matter 18 (2006) 6133.
Figure 1. Powder XRD pattern of the TLYC single crystal.
Figure 2. DSC and TGA curves of the TLYC single crystal showing the melting point at 490 oC.
Figure 3. X-ray excited emission spectra at room temperature of TLYC: x Ce3+ (x= 0.5% and 5%).
Figure 4. The scintillation pulse height spectrum of TLYC: 5% Ce3+ irradiated with γ-rays from a 137
Cs source.
Figure 5. Pulse height spectra of TLYC: 5% Ce3+ and LYSO: Ce3+crystals excited with γ-rays from a 137Cs source. The photopeak position is proportional to the light yield.
Figure 6. Time profiles of TLYC crystals with 0.5% and 5% Ce concentrations measured under γ-rays excitation at room temperature.
Highlights Scintillation characterization of Tl2LiYCl6:Ce3+ were presented. Tl2LiYCl6:Ce3+ were grown by two zone vertical Bridgman technique. It has high Zeff and could be a potential candidate for medical imaging technique. Energy resolution of 7.2% and light yield of 23,800±2400 ph/MeV were obtained. Three decay time components were observed under γ-ray excitation.