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Photoluminescence and scintillation properties of Ce-doped Sr2Al2SiO7 crystals
Journal of Luminescence 202 (2018) 409–413
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Photoluminescence and scintillation properties of Ce-doped Sr2Al2SiO7 crystals
T
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Taiki Ogawa , Daisuke Nakauchi, Go Okada, Noriaki Kawaguchi, Takayuki Yanagida Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0192, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Scintillator Scintillation detector Melilite Cerium Phosphor Sr2Al2SiO7
Undoped and Ce-doped Sr2Al2SiO7 bulk crystals with different concentrations of Ce were synthesized by the floating zone method. Under X-ray irradiation, the both undoped and Ce-doped samples show scintillation with broad emission with decay time constants of approximately 30 and 300 ns. By utilizing photoluminescence (PL) techniques, we attributed the origin of the former emission component is due to the 5d-4f transitions of Ce3+ while the one of the latter emission is due the host. The scintillation afterglow levels depends on the concentration of Ce and are on an order of ppm. From the pulse height spectrum measured under 241Am 59.5 keV γray irradiation, the 0.3% Ce-doped Sr2Al2SiO7 crystal was revealed to show the absolute scintillation light yield of 1100 ph/MeV, which is the highest among the present samples. In addition, the Ce-doped samples show TL properties with a glow peak at 250 °C and a shoulder around 130 °C.
1. Introduction
As a candidate of scintillator, we have focused on AE2Al2SiO7 (AE=Ca and Sr) which is one of melilite compounds and has high chemical stability and non-hygroscopicity [16,17]. In our previous work, Ce-doped Ca2Al2SiO7 (a family of AE2Al2SiO7) single crystals were investigated as a scintillator [18], and the scintillation light yield was found to be 16,000 ph/MeV, which is comparable to those of conventional scintillators such as Ce-doped Y3Al5O12 and Ce-doped YAlO3. To extend the research, Ce-doped Sr2Al2SiO7 (SASM) is an interesting candidate as the sensitivity against high energy photons can be enhanced by replacing Ca with Sr since the interaction probability of photoabsorption event depends on ρZeff4 where ρ and Zeff are the density and the effective atomic number, respectively (The theoretical density of SASM is 3.799 g/cm3 [19].). In fact, rare-earth-doped SASM has been studied as phosphors for applications including white light emitting diode (LED) [20,21] and long-lasting phosphor [16,17], and there is only one report on scintillation properties of Ce-doped SASM [22] as far as we are aware. However, no studies have been reported in a bulk crystalline form yet. A bulk crystalline form is important for practical use of scintillator. In this study, for the reasons above, we investigated Ce-doped SASM bulk crystals for scintillator applications. The Ce-doped SASM crystals with various concentrations of Ce were synthesized by the floating zone (FZ) method, and we evaluated the photoluminescence (PL) and scintillation properties in detail. In addition, we studied the TL properties which may be correlated with the scintillation properties.
Inorganic scintillators immediately convert absorbed energy of ionizing radiation into a large number of low energy photons in UV, visible and near-infrared regions [1]. Such materials have been widely used in various fields such as security system [2], well-logging [3], medical imaging [4] personal dose monitoring [5] and astrophysics [6]. In common scintillation mechanism, via interactions between incident ionizing radiations and materials, a large number of secondary electrons are generated in the host matrix, and then some of them are transferred to luminescence centers to emit light (or called scintillation photons) due to recombination of electrons and holes. Over the past decades, various different scintillation materials have been developed because required scintillation properties are different for each application. In general, scintillator materials are designed as a combination of host material and emission center. The host has a function to absorb radiation energy, and the emission center has a function to emit light. In particular, Ce-doped materials (e.g., rare-earth silicates [7–10] and rare-earth aluminates [11–13]) have been intensively studied as they generally show strong emission in the near-UV and visible regions with a fast decay time of several tens of nanoseconds due to the 5d-4f transitions of Ce3+ [14]. Such fast scintillation decay time is advantageous especially in scintillation detectors operated with photon counting mode such as those in positron emission tomography (PET), which requires high counting rate [15].
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Corresponding author. E-mail address: [email protected] (T. Ogawa).
https://doi.org/10.1016/j.jlumin.2018.06.011 Received 1 February 2018; Received in revised form 23 May 2018; Accepted 4 June 2018 Available online 06 June 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.
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2. Experimental
manufactures of integrated-type scintillation detectors. Absolute light yield (LY) was evaluated by measuring pulse height spectrum. For the measurement, we placed a sample on a window of photomultiplier tube (PMT; R7600U-100, Hamamatsu) with optical grease (OKEN 6262 A), and the sample was covered by a Teflon sheet. A bias voltage of 600 V was applied to the PMT by a DC power supply (ORTEC 556), and the electrical signal from the anode of the PMT was amplified by a pre-amplifier (ORTEC 113). Further, the amplified signal was processed by a shaping amplifier (ORTEC 570) with 500 ns shaping time to obtain the light output upon a single γ-ray event, which was then processed by a multichannel analyzer (Pocket MCA 8000 A, Amptek). Finally, the pulse height spectrum was constructed on the computer. We used a sealed radioactive 241Am γ-ray source (59.5 keV) as a radiation source in this measurement. We used the Tl-doped NaI single crystal (40,000 ph/MeV) [26] as a reference to derive LY of present sample. The scintillation spectra of Tl-doped NaI and Ce-doped SASM were similar; therefore, the uncertainty arose by the spectral response of PMT was neglected. In order to characterize shallow trapping centers, TL glow curves were measured by using a TL reader (TL-2000, Nanogray) [27]. In this instrument, emission photons from 300 nm to 500 nm were integrated for detection, and the heating rate was 1 °C/s over the temperature range from 50 to 490°C. All the characterizations were performed at room temperature except for TL measurement.
Undoped SASM crystal as well as 0.03%, 0.1% and 0.3% Ce-doped SASM bulk crystals were synthesized by the FZ method. We used SrCO3 (99.99%), Al2O3 (99.99%), SiO2 (99.99%) and CeO2 (99.99%) powders as raw materials. They were mixed in a stoichiometric ratio homogeneously, and the concentrations of Ce varied as 0.03%, 0.1% and 0.3% with respect to that of Sr. Here, the Ce ion is considered to be embedded at the Sr site. Then, the mixture was formed to a cylinder rod by applying a hydrostatic pressure, and the rod was then sintered at 1200 °C for 6 h to obtain a solid ceramic (The melting point of SASM is 1704 °C [23].). The ceramic rod was loaded into an FZ furnace (FZD0193, Canon Machinery), and the crystal growth was conducted with the pull-down rate of 5 mm/h and rotation rate of 20 rpm in the atmosphere. The actual concentrations of Ce were evaluated by X-ray fluorescence (SEA1000AⅡ, SII) measurements, where the X-ray tube was operated at 50 kV and 100 μA. In addition, X-ray diffraction (XRD) was measured using a diffractometer (MiniFlex600, Rigaku) to identify the crystalline phase. Here, the X-ray tube (CuKα) was operated at 40 kV and 15 mA, and the scanning 2θ range was 5–90 °. The PL excitation and emission spectra of Ce-doped samples were measured by a spectrofluorometer (FP-8600, JASCO). The excitation and emission spectral ranges were 270–370 nm and 350–540 nm, respectively. PL quantum yield (QY) were measured by using QuantaurusQY (C11347, Hamamatsu). The spectral range measured for the excitation and emission were 250–400 nm and 200–950 nm, respectively. The measurement intervals of excitation and emission wavelengths were 10 nm and ~ 0.75 nm, respectively. The absolute QY was defined as QY= Nemit/Nabsorb where Nemit and Nabsorb are the numbers of emitted and absorbed photons, respectively. In this evaluation, Nemit is the number of emitted photons integrated in the 340–600 nm, and Nabsorb is that of absorbed photons of 330 ± 10 nm. We also used Quantaurus-τ (C11367, Hamamatsu) to measure PL spectrum of undoped SASM and PL decay time profiles. The excitation source was LED, and the excitation wavelength was 280 nm for both spectrum and decay curve measurements. For the decay curve measurements, we monitored at two wavelengths (310 and 420 nm) for the undoped sample, and the emission of Ce-doped SASM samples was measured at 390 nm. It is worth mentioning that the excitation wavelength of 280 nm was the shortest wavelength available on the instrument. The pulse repetition rate was 250 kHz, and the decay profiles of all samples were recorded over the range of 1000 ns. As a scintillation property, X-ray-induced scintillation spectrum was evaluated by utilizing our original setup [13]. The irradiation source was an X-ray generator (XRB80N100/CB, Spellman) which is equipped with a conventional X-ray tube (tungsten anode target) and beryllium window. During the measurement, the X-ray tube was supplied with a bias voltage of 60 kV and tube current of 1.2 mA. While the sample was irradiated by X-rays, the scintillation photons from the sample was fed into a CCD-based spectrometer (Andor DU-420-BU2 CCD with Shamrock SR163 monochromator) through a 2 m optical fiber to measure the spectrum. The CCD was cooled down to 188 K by a Peltier module to reduce the thermal noise. Further, we have evaluated the scintillation decay time and afterglow profiles utilizing an afterglow characterization system equipped with a pulse X-ray source [24,25]. For the scintillation decay time measurement, the applied voltage to the pulse Xray source was 30 kV, and the system offers the timing resolution of ~ 1 ns. In addition, the pulse repetition rate was 200 kHz, and the decay profile was recorded over the range of 1000 ns. The afterglow level (A) is defined as A (ppm)= 106 × (I2–I0)/(I1–I0), where I0, I1 and I2 denote the averaged signal intensity before the X-ray irradiation, the averaged signal intensity during irradiation and signal intensity at t = 20 ms after irradiation, respectively. The X-ray pulse width was 2 ms, and the excitation rate was 10 Hz. Although the evaluation manner of afterglow level differs in each manufacture, we referred the methodology by Nihon Kessho Kogaku Co., Ltd. which is one of the well-known
3. Results and discussion 3.1. Samples The SASM crystals nominally doped with undoped, 0.03%, 0.1% and 0.3% Ce were successfully synthesized by the FZ method. The concentration of Ce measured by XRF for the samples nominally doped with 0.03%, 0.1% and 0.3% were 0.03%, 0.05% and 0.09%, respectively. The actual Ce concentrations were close to nominal values, but the discrepancy between the nominal and actual concentrations became larger for higher dopant concentration. The typical size was ~ 6 mm in diameter and ~ 30 mm in length. Fig. 1 shows a photograph of the samples. Each sample was cut from the as-grown crystal rod, and relatively transparent parts were selected. All the samples look white and translucent because many small cracks are included. We selected relatively transparent parts for characterizations. The XRD patterns of all samples are shown in Fig. 2. The observed patterns agreed well with the standard pattern of SASM (ICSD 030698), and no significant impurity phase was observed. 3.2. Photoluminescence properties PL excitation and emission spectra of the 0.3% Ce-doped SASM sample are shown in Fig. 3 as a representative. A broad emission peak is observed around 350 and 390 nm, and the origins are ascribed to the 5d-4f transition of Ce3+ which was often seen in Ce3+-doped materials [28,29], and the emission features were similar to Ce-doped SASM [30] and Ce, Na-doped SASM powder [22]. The excitation bands around 290 and 340 nm are due to the 4f-5d2 and 4f-5d1 transitions of Ce3+, respectively. In addition, the Stokes shift is 3770 cm−1. All the samples
Fig. 1. Synthesized SASM crystal samples undoped and doped with Ce. 410
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Em. 310 nm: I =63exp(-t/171 ns) Intensity [a.u.]
Intensity [a.u.]
undoped 0.03% Ce 0.1% Ce 0.3% Ce
Em. 420 nm: I =981exp(-t/28 ns) + 57exp(-t/257 ns)
ICSD 030698 0
20
40 60 2θ [deg]
0
80
200
400 Time [ns]
600
800
200
Ex. spectrum (Em. 390 nm)
0.03% Ce I =29510exp(-t/31 ns) +143exp(-t/282 ns)
Intensity [a.u.]
Intensity [a.u.]
Fig. 2. XRD patterns of undoped and Ce-doped SASM single crystals as well as the standard card of SASM (ICSD 030698).
Em. spectrum (Ex. 250 nm)
0.1% Ce I =30180exp(-t/31 ns) +48exp(-t/267 ns) 0.3% Ce I =30882exp(-t/31 ns) +115exp(-t/201 ns) 0
300
400 Wavelength
200
400 Time [ns]
600
800
Fig. 5. PL decay time profiles of the undoped SASM sample (top) and Ce-doped SASM samples (bottom). The monitoring wavelengths of the undoped sample were 310 nm and 420 nm while that of Ce-doped sample was 390 nm. The excitation wavelength was 280 nm.
500
Fig. 3. The excitation and emission spectra of the 0.3% Ce-doped SASM.
the monitoring wavelengths are 310 and 420 nm. The decay curve monitoring at 310 nm is approximated by a single exponential decay function while the one monitoring at 420 nm is best-approximated by a sum of two exponential decay functions. The obtained decay time constant monitoring at 310 nm is 171 ns while the obtained decay time constants monitoring at 420 nm are 28 ns (95%) and 257 ns (5%). In contrast, PL decay time curves of the Ce-doped SASM samples are demonstrated in Fig. 5 (bottom). Here, the excitation and monitoring wavelengths were 280 nm and 390 nm, respectively. The decay curves are well-approximated by a sum of two exponential decay functions. The obtained decay time constants are 31 ns (99%) and 282 ns (1%) for the 0.03% Ce-doped samples, 30 ns (99%) and 267 ns (1%) for the 0.1% Ce-doped sample and 30 ns (99%) and 201 ns (1%) for the 0.3% Cedoped sample. From the decay times obtained for the undoped and Cedoped samples, the emission origins were attributed as follows. First, all the Ce-doped samples showed relatively fast decay with the decay time constants of 30–31 ns, which is reasonable for the 5d-4f transitions of Ce3+ [28,29]. In addition, we attribute the origin of the second component (201–282 ns) to the host due to possibly some defects since emission of similar decay time constant was also observed in the undoped sample (171 ns), when monitoring at 310 nm. However, the undoped sample also showed an emission of faster decay time (28 ns) when monitoring at 420 nm. We think the origin of the latter emission is also the 5d-4f transitions of Ce3+ which is unexpectedly contaminated during the synthesis process. Note that the ratio to the host emission is very weak compared to those of the Ce-doped samples. When monitoring at shorter wavelength (310 nm), we only observed emission due to the host which appeared as slower decay. The component due to 5d-4f transition of Ce3+ are similar to those of Ce-doped CASM [18] and Ce, Na-doped SASM powder [22]. In these previous investigations, the emission origin was also ascribed to 5d-4f transition
Intensity [a.u.]
undoped
300
400 500 Wavelength [nm]
600
Fig. 4. PL spectrum of the undoped SASM sample measured under excitation at 280 nm.
show similar excitation/emission features, and QY of the 0.03%, 0.1% and 0.3% samples are 10.9%, 11.5% and 14.7%, respectively. When we compare with Ce-doped CASM [18], the excitation and emission bands shifted to shorter wavelengths, which indicates that the strength of the crystal field of SASM is smaller than that of CASM. The PL QYs of SASM are slightly lower than those of CASM which indicated ~ 20% of PL QY [18]. The undoped sample shows no measurable signals by the same instrument but using Quantaurus-τ. The obtained spectrum is separately shown in Fig. 4. The excitation wavelength was 280 nm. The emission consists of peaks around 340 and 420 nm. The PL decay time curves of the undoped SASM sample are demonstrated in Fig. 5 (top). The excitation wavelength is 280 nm, and 411
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undoped I =505exp(-t/23 ns) +83exp(-t/261 ns) Intensity [a.u.]
Intensity [a.u]
0.03% Ce 0.1% Ce 0.3% Ce undoped
00.3% Ce I =6980exp(-t/38 ns) +511exp(-t/304 ns) 0.1% Ce I =5163exp(-t/30 ns) +301exp(-t/300 ns) 0.3% Ce I =4727exp(-t/31 ns) +402exp(-t/238 ns)
200
300
400
500
600
700
0
200 400 600 Wavelength [nm]
Wavelength [nm] Fig. 6. X-ray induced scintillation spectra of undoped and Ce-doped SASM samples.
800
Fig. 7. X-ray induced scintillation decay time profiles of the undoped and Cedoped samples.
of Ce3+.
Normarized counts [a.u.]
100
3.3. Scintillation properties X-ray induced scintillation spectra of undoped and Ce-doped SASM samples are compared in Fig. 6. Each spectrum is normalized to the highest value. The Ce-doped samples show an intense broad emission peaking around 420 nm, and the emission wavelength of Ce-doped SASM was similar to that of the Ce,Na-doped SASM powder [22]. The emission spectral shape is different from those in PL. The spectral shape consists of a single peak, and the emission wavelength is longer than 30 nm when compared with PL. These is blamed for the measurement geometry, and it does not have a significant meaning. In PL, the measurement was conducted on a reflection mode while the scintillation spectra were measured on a transmission geometry. Since the Ce-doped samples have absorption and emission bands overlapped (see Fig. 3), the measurement of transmittance geometry leads emission wavelength shifted to longer wavelengths due to self-absorption. In addition, when we compare the scintillation spectra with 0.3% Ce-doped CASM [18], the emission peak position of 0.3% SASM was similar, and this is different for the PL when we compare between Ce-doped SASM and Cedoped CASM [18]. The reason is due to transparency of the sample. The 0.3% Ce-CASM was transparent while the 0.3% Ce-doped SASM is translucent. Therefore, 0.3% Ce-doped SASM strongly affected by selfabsorption. The undoped sample shows broad emission bands at 300, 350 and 400 nm. The emission band at 400 nm is not consistent with the spectrum of PL, and the origin should not be the same. These emission wavelengths are preferable for use with PMT, which is one of the advantages of SASM for scintillation detector applications. The X-ray induced scintillation decay time profiles of the undoped and Ce-doped SASM samples are shown in Fig. 7. The decay time profiles of the all samples are well-approximated with a sum of two exponential decay functions. The obtained decay time constants of undoped SASM sample are 23 ns (86%) and 261 ns (14%), and the constants are similar to those in PL monitoring both the emissions by Ce and host at 420 nm. Therefore, we think that the first component is due to the contamination of Ce, and the second component is due to the host matrix. The decay time constants of Ce-doped SASM samples are 30–38 and 238–304 ns. These constants are similar to those of PL by Ce-doped samples. The first components (30–38 ns) are due to the 5d-4f transitions of Ce3+, and the second components (238–304 ns) are due to the host matrix. These results were similar to those in Ce-doped CASM [18]. Afterglow profiles of the Ce-doped SASM samples are shown in Fig. 8. Here, the X-ray pulse width was 2 ms. The afterglow levels evaluated for the 0.03%, 0.1% and 0.3% Ce-doped samples are 2620, 3530 and 990 ppm, respectively. Although the 0.3% Ce-doped sample shows the lowest afterglow level, it is much higher than those of practical scintillators used in integrated-type detectors such as CdWO4
0.03% Ce 0.1% Ce 0.3% Ce
10-1 10-2 10-3 10-4 0
10
20 30 Time [ms]
40
50
Fig. 8. Afterglow curves of the Ce-doped SASM samples after 2 ms X-ray irradiation.
and BGO (~ 10 ppm) [24]. The appearance of such high afterglow level suggests that the 0.03% and 0.1% Ce-doped SASM contain a large number of shallow trapping centers, so some of the charges generated by X-ray are temporarily captured at these traps before recombination to emit light. Pulse height spectrum of 241Am 59.5 keV γ-rays measured using the 0.3% Ce-doped SASM sample as scintillator is presented in Fig. 9. The one measured with Tl-doped NaI is also presented as a reference which is known to have an absolute LY of 40,000 ph/MeV. The 0.3% Ce-doped sample successfully demonstrated a photoabsorption peak, and the absolute scintillation LY was estimated from the peak position measured with respect to that of the Tl-doped NaI. The obtained value was
150 0.3% Ce Intensity [a.u.]
NaI: Tl (gain
1/5)
100
50
0 0
100 Channel
200
Fig. 9. Pulse height spectra of 241Am 59.5 keV γ-rays measured using the 0.3% Ce-doped SASM sample and Tl-doped NaI single crystal. 412
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Acknowledgements
Table 1 Photoabsorption peak channels and LYs of 0.3% Ce-doped SASM sample and Tl:NaI.
0.3% Ce:SASM Tl:NaI
Photoabsorption peak [channel]
Light yield [ph/MeV]
25 180
1100 ± 10% 40,000 ± 10%
This work was supported by Grant-in-Aid for Scientific Research (A) (17H01375), Grant-in-Aid for Young Scientists (B) (17K14911) and Grant-in-Aid for Research Activity Start-up (16H06983) from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese government (MEXT) as well as A-STEP from Japan Science and Technology Agency (JST). The Cooperative Research Project of Research Institute of Electronics, Shizuoka University, Mazda Foundation, Taisei Foundation, Terumo Foundation for Life Sciences and Arts, Izumi Science and Technology Foundation, SEI Group CSR Foundation, Konica Minolta Science and Technology Foundation, NAIST Foundation and TEPCO Memorial Foundation are also acknowledged.
Intensity [a.u.]
after X-ray irradiation (0.1 Gy) 0.03% Ce 0.1% Ce
References [1] P.A. Rodnyi, Physical Processes in Inorganic Scintillators, CRC Press, New York, 1997. [2] J. Glodo, Y. Wang, R. Shawgo, C. Brecher, R.H. Hawrami, J. Tower, K.S. Shah, Phys. Procedia 90 (2017) 285–290, http://dx.doi.org/10.1016/j.phpro.2017.09.012. [3] T. Yanagida, Y. Fujimoto, S. Kurosawa, K. Kamada, H. Takahashi, Y. Fukazawa, M. Nikl, V. Chani, Jpn. J. Appl. Phys. 52 (2013) 076401, http://dx.doi.org/10. 7567/JJAP.52.076401. [4] S. Yamamoto, S. Okumura, N. Kato, J.Y. Yeom, J. Instrum. 10 (2015) T09002. [5] K. Watanabe, T. Yanagida, K. Fukuda, A. Koike, T. Aoki, Sens. Mater. 27 (2015) 269–275. [6] T. Itoh, T. Yanagida, M. Kokubun, M. Sato, R. Miyawaki, K. Makishima, T. Takashima, T. Tanaka, K. Nakazawa, T. Takahashi, N. Shimura, H. Ishibashi, Nucl. Instrum. Methods Phys. Res. A 579 (2007) 239–242, http://dx.doi.org/10. 1016/j.nima.2007.04.144. [7] K. Kawade, K. Fukatsu, Y. Itow, K. Masuda, T. Murakami, T. Sako, K. Suzuki, T. Suzuki, K. Taki, J. Instrum. 6 (2011) 1–12, http://dx.doi.org/10.1088/17480221/6/09/T09004. [8] T. Youichi, J.H. Kaneko, M. Higuchi, S. Nishiyama, H. Ishibashi, Appl. Phys. Express 8 (2015) 62602. [9] L. Pidol, A. Kahn-Harari, B. Viana, E. Virey, B. Ferrand, P. Dorenbos, J.T.M. De Haas, C.W.E. Van Eijk, IEEE Trans. Nucl. Sci. 51 (2004) 1084–1087, http://dx.doi. org/10.1109/TNS.2004.829542. [10] C.L. Melcher, J.S. Schweitzer, IEEE Trans. Nucl. Sci. 39 (1992) 502–505. [11] D. Nakauchi, G. Okada, N. Kawano, N. Kawaguchi, T. Yanagida, Appl. Phys. Express 10 (2017) 072601. [12] M. Mori, J. Xu, G. Okada, T. Yanagida, J. Ueda, S. Tanabe, J. Ceram. Soc. Jpn. 124 (2016) 569–573, http://dx.doi.org/10.2109/jcersj2.15239. [13] T. Yanagida, K. Kamada, Y. Fujimoto, H. Yagi, T. Yanagitani, Opt. Mater. 35 (2013) 2480–2485, http://dx.doi.org/10.1016/j.optmat.2013.07.002. [14] D. Nakauchi, G. Okada, M. Koshimizu, T. Yanagida, Nucl. Instrum. Methods B 377 (2016) 89–93, http://dx.doi.org/10.1016/j.nimb.2016.04.017. [15] M.E. Phelps, E.J. Hoffman, N.A. Mullani, M.M. Ter-Pogossian, J. Nucl. Med. 16 (1975) 210–224. [16] P. Wen, N. Guiling, L. Yuan, Y. Xuefeng, J. Rare Earths 26 (2008) 207–210. [17] Y. Ding, Y. Zhang, Z. Wang, W. Li, D. Mao, H. Han, C. Chang, J. Lumin. 129 (2009) 294–299, http://dx.doi.org/10.1016/j.jlumin.2008.10.009. [18] T. Ogawa, D. Nakauchi, G. Okada, N. Kawano, N. Kawaguchi, T. Yanagida, J. Lumin. 196 (2018) 270–274, http://dx.doi.org/10.1016/j.jlumin.2017.12.009. [19] B. Arun, K.M. Manu, M.T. Sebastian, Int. J. Mod. Phys. Conf. Ser. 22 (2013) 173–178, http://dx.doi.org/10.1142/S2010194513010088. [20] G. Li, M. Li, L. Li, H. Yu, H. Zou, L. Zou, S. Gan, X. Xu, Mater. Lett. 65 (2011) 3418–3420, http://dx.doi.org/10.1016/j.matlet.2011.07.050. [21] H. Zou, D. Peng, Z. Chu, X. Wang, Y. Li, X. Yao, J. Mater. Sci. 48 (2013) 7981–7985, http://dx.doi.org/10.1007/s10853-013-7609-x. [22] L. Lin, R. Shi, R. Zhou, Q. Peng, C. Liu, Y. Tao, Y. Huang, P. Dorenbos, H. Liang, Inorg. Chem. 56 (2017) 12476–12484, http://dx.doi.org/10.1021/acs.inorgchem. 7b01939. [23] S. Adarsh, Development of a Critically Evaluated Thermodynamic Database for the Systems Containing Alkaline-earth Oxides (Ph.D. thesis), Université de Montréal, 2012. [24] T. Yanagida, Y. Fujimoto, T. Ito, K. Uchiyama, K. Mori, Appl. Phys. Express 7 (2014) 062401, http://dx.doi.org/10.7567/APEX.7.062401. [25] T. Yanagida, Y. Fujimoto, A. Yamaji, N. Kawaguchi, K. Kamada, D. Totsuka, K. Fukuda, K. Yamanoi, R. Nishi, S. Kurosawa, T. Shimizu, N. Sarukura, Radiat. Meas. 55 (2013) 99–102, http://dx.doi.org/10.1016/j.radmeas.2012.05.014. [26] W.J. Van Sciver, L. Bogart, IEEE Trans. Nucl. Sci. 5 (1958) 90–92. [27] T. Yanagida, Y. Fujimoto, N. Kawaguchi, S. Yanagida, J. Ceram. Soc. Jpn. 121 (2013) 988–991, http://dx.doi.org/10.2109/jcersj2.121.988. [28] M. Mori, D. Nakauchi, G. Okada, Y. Fujimoto, N. Kawaguchi, M. Koshimizu, T. Yanagida, J. Lumin. 186 (2017) 93–97, http://dx.doi.org/10.1016/j.jlumin. 2017.01.035. [29] S. Chen, X. Zhao, H. Xie, J. Liu, L. Duan, X. Ba, J. Zhao, Appl. Surf. Sci. 258 (2011) 3255–3259, http://dx.doi.org/10.1016/j.apsusc.2011.11.077. [30] M. Kolte, V.B. Pawade, S.J. Dhoble, Appl. Phys. A 122 (2016) 59.
0.3% Ce 100
200 300 400 Temperature [°C]
Fig. 10. TL glow curves of undoped and Ce-doped SASM samples measured after 0.1 Gy X-ray irradiation.
1100 ph/MeV (typical error is ± 10%). The measured peak positions and derived LY are summarized in Table 1. It is worth mentioning that the LY of the undoped SASM sample and those doped with 0.03% and 0.1% Ce are not sufficiently high that a photoabosorption peak is detected in the present aperture. Compared with Ce-doped CASM [18], the scintillation LY is lower. 3.4. TL properties TL glow curves measured after X-ray irradiation are shown in Fig. 10. Each glow curve is normalized to the highest intensity. The irradiation dose was 0.1 Gy. The undoped SASM sample does not show measurable TL signals, and the Ce-doped SASM samples show TL with a glow peak at 250 °C and a shoulder around 130 °C. A similar signal at 130 °C was reported in Eu:SASM and Eu,Dy:SASM powders [17]; therefore, we attribute the origin of this shoulder to host defects. In addition, the peak around 250 °C can be attributed to Ce-doping. The Ce-doping makes some defects due to charge imbalance between Sr2+ and Ce3+. In addition, the peak shape and position were similar to those of Ce-doped CASM [18,31], and the origin is considered to be the same. 4. Summary We investigated the PL, scintillation and TL properties of the undoped and Ce-doepd SASM crystals synthesized by the FZ method. Under X-ray irradiation, the Ce-doped samples show strong scintillation with a broad emission around 420 nm with decay time constants of approximately 30 and 300 ns. From the results of undoped sample and PL characterizations, we attributed the origins of the former decay component is the 5d-4f transitions of Ce3+ while the one of the latter emission is due to the host. The afterglow level is the lowest for the 0.3% Ce-doped sample (990 ppm). The LY is the highest for the 0.3% Ce-doped sample indicating 1100 ph/MeV under 241Am γ-ray (59.5 keV). In addition, the Ce-doped samples show TL properties with a glow peak at 250 °C and a shoulder around 130 °C.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence and scintillation properties of Ce-doped Sr2Al2SiO7 crystals
T
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Taiki Ogawa , Daisuke Nakauchi, Go Okada, Noriaki Kawaguchi, Takayuki Yanagida Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0192, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Scintillator Scintillation detector Melilite Cerium Phosphor Sr2Al2SiO7
Undoped and Ce-doped Sr2Al2SiO7 bulk crystals with different concentrations of Ce were synthesized by the floating zone method. Under X-ray irradiation, the both undoped and Ce-doped samples show scintillation with broad emission with decay time constants of approximately 30 and 300 ns. By utilizing photoluminescence (PL) techniques, we attributed the origin of the former emission component is due to the 5d-4f transitions of Ce3+ while the one of the latter emission is due the host. The scintillation afterglow levels depends on the concentration of Ce and are on an order of ppm. From the pulse height spectrum measured under 241Am 59.5 keV γray irradiation, the 0.3% Ce-doped Sr2Al2SiO7 crystal was revealed to show the absolute scintillation light yield of 1100 ph/MeV, which is the highest among the present samples. In addition, the Ce-doped samples show TL properties with a glow peak at 250 °C and a shoulder around 130 °C.
1. Introduction
As a candidate of scintillator, we have focused on AE2Al2SiO7 (AE=Ca and Sr) which is one of melilite compounds and has high chemical stability and non-hygroscopicity [16,17]. In our previous work, Ce-doped Ca2Al2SiO7 (a family of AE2Al2SiO7) single crystals were investigated as a scintillator [18], and the scintillation light yield was found to be 16,000 ph/MeV, which is comparable to those of conventional scintillators such as Ce-doped Y3Al5O12 and Ce-doped YAlO3. To extend the research, Ce-doped Sr2Al2SiO7 (SASM) is an interesting candidate as the sensitivity against high energy photons can be enhanced by replacing Ca with Sr since the interaction probability of photoabsorption event depends on ρZeff4 where ρ and Zeff are the density and the effective atomic number, respectively (The theoretical density of SASM is 3.799 g/cm3 [19].). In fact, rare-earth-doped SASM has been studied as phosphors for applications including white light emitting diode (LED) [20,21] and long-lasting phosphor [16,17], and there is only one report on scintillation properties of Ce-doped SASM [22] as far as we are aware. However, no studies have been reported in a bulk crystalline form yet. A bulk crystalline form is important for practical use of scintillator. In this study, for the reasons above, we investigated Ce-doped SASM bulk crystals for scintillator applications. The Ce-doped SASM crystals with various concentrations of Ce were synthesized by the floating zone (FZ) method, and we evaluated the photoluminescence (PL) and scintillation properties in detail. In addition, we studied the TL properties which may be correlated with the scintillation properties.
Inorganic scintillators immediately convert absorbed energy of ionizing radiation into a large number of low energy photons in UV, visible and near-infrared regions [1]. Such materials have been widely used in various fields such as security system [2], well-logging [3], medical imaging [4] personal dose monitoring [5] and astrophysics [6]. In common scintillation mechanism, via interactions between incident ionizing radiations and materials, a large number of secondary electrons are generated in the host matrix, and then some of them are transferred to luminescence centers to emit light (or called scintillation photons) due to recombination of electrons and holes. Over the past decades, various different scintillation materials have been developed because required scintillation properties are different for each application. In general, scintillator materials are designed as a combination of host material and emission center. The host has a function to absorb radiation energy, and the emission center has a function to emit light. In particular, Ce-doped materials (e.g., rare-earth silicates [7–10] and rare-earth aluminates [11–13]) have been intensively studied as they generally show strong emission in the near-UV and visible regions with a fast decay time of several tens of nanoseconds due to the 5d-4f transitions of Ce3+ [14]. Such fast scintillation decay time is advantageous especially in scintillation detectors operated with photon counting mode such as those in positron emission tomography (PET), which requires high counting rate [15].
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Corresponding author. E-mail address: [email protected] (T. Ogawa).
https://doi.org/10.1016/j.jlumin.2018.06.011 Received 1 February 2018; Received in revised form 23 May 2018; Accepted 4 June 2018 Available online 06 June 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.
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2. Experimental
manufactures of integrated-type scintillation detectors. Absolute light yield (LY) was evaluated by measuring pulse height spectrum. For the measurement, we placed a sample on a window of photomultiplier tube (PMT; R7600U-100, Hamamatsu) with optical grease (OKEN 6262 A), and the sample was covered by a Teflon sheet. A bias voltage of 600 V was applied to the PMT by a DC power supply (ORTEC 556), and the electrical signal from the anode of the PMT was amplified by a pre-amplifier (ORTEC 113). Further, the amplified signal was processed by a shaping amplifier (ORTEC 570) with 500 ns shaping time to obtain the light output upon a single γ-ray event, which was then processed by a multichannel analyzer (Pocket MCA 8000 A, Amptek). Finally, the pulse height spectrum was constructed on the computer. We used a sealed radioactive 241Am γ-ray source (59.5 keV) as a radiation source in this measurement. We used the Tl-doped NaI single crystal (40,000 ph/MeV) [26] as a reference to derive LY of present sample. The scintillation spectra of Tl-doped NaI and Ce-doped SASM were similar; therefore, the uncertainty arose by the spectral response of PMT was neglected. In order to characterize shallow trapping centers, TL glow curves were measured by using a TL reader (TL-2000, Nanogray) [27]. In this instrument, emission photons from 300 nm to 500 nm were integrated for detection, and the heating rate was 1 °C/s over the temperature range from 50 to 490°C. All the characterizations were performed at room temperature except for TL measurement.
Undoped SASM crystal as well as 0.03%, 0.1% and 0.3% Ce-doped SASM bulk crystals were synthesized by the FZ method. We used SrCO3 (99.99%), Al2O3 (99.99%), SiO2 (99.99%) and CeO2 (99.99%) powders as raw materials. They were mixed in a stoichiometric ratio homogeneously, and the concentrations of Ce varied as 0.03%, 0.1% and 0.3% with respect to that of Sr. Here, the Ce ion is considered to be embedded at the Sr site. Then, the mixture was formed to a cylinder rod by applying a hydrostatic pressure, and the rod was then sintered at 1200 °C for 6 h to obtain a solid ceramic (The melting point of SASM is 1704 °C [23].). The ceramic rod was loaded into an FZ furnace (FZD0193, Canon Machinery), and the crystal growth was conducted with the pull-down rate of 5 mm/h and rotation rate of 20 rpm in the atmosphere. The actual concentrations of Ce were evaluated by X-ray fluorescence (SEA1000AⅡ, SII) measurements, where the X-ray tube was operated at 50 kV and 100 μA. In addition, X-ray diffraction (XRD) was measured using a diffractometer (MiniFlex600, Rigaku) to identify the crystalline phase. Here, the X-ray tube (CuKα) was operated at 40 kV and 15 mA, and the scanning 2θ range was 5–90 °. The PL excitation and emission spectra of Ce-doped samples were measured by a spectrofluorometer (FP-8600, JASCO). The excitation and emission spectral ranges were 270–370 nm and 350–540 nm, respectively. PL quantum yield (QY) were measured by using QuantaurusQY (C11347, Hamamatsu). The spectral range measured for the excitation and emission were 250–400 nm and 200–950 nm, respectively. The measurement intervals of excitation and emission wavelengths were 10 nm and ~ 0.75 nm, respectively. The absolute QY was defined as QY= Nemit/Nabsorb where Nemit and Nabsorb are the numbers of emitted and absorbed photons, respectively. In this evaluation, Nemit is the number of emitted photons integrated in the 340–600 nm, and Nabsorb is that of absorbed photons of 330 ± 10 nm. We also used Quantaurus-τ (C11367, Hamamatsu) to measure PL spectrum of undoped SASM and PL decay time profiles. The excitation source was LED, and the excitation wavelength was 280 nm for both spectrum and decay curve measurements. For the decay curve measurements, we monitored at two wavelengths (310 and 420 nm) for the undoped sample, and the emission of Ce-doped SASM samples was measured at 390 nm. It is worth mentioning that the excitation wavelength of 280 nm was the shortest wavelength available on the instrument. The pulse repetition rate was 250 kHz, and the decay profiles of all samples were recorded over the range of 1000 ns. As a scintillation property, X-ray-induced scintillation spectrum was evaluated by utilizing our original setup [13]. The irradiation source was an X-ray generator (XRB80N100/CB, Spellman) which is equipped with a conventional X-ray tube (tungsten anode target) and beryllium window. During the measurement, the X-ray tube was supplied with a bias voltage of 60 kV and tube current of 1.2 mA. While the sample was irradiated by X-rays, the scintillation photons from the sample was fed into a CCD-based spectrometer (Andor DU-420-BU2 CCD with Shamrock SR163 monochromator) through a 2 m optical fiber to measure the spectrum. The CCD was cooled down to 188 K by a Peltier module to reduce the thermal noise. Further, we have evaluated the scintillation decay time and afterglow profiles utilizing an afterglow characterization system equipped with a pulse X-ray source [24,25]. For the scintillation decay time measurement, the applied voltage to the pulse Xray source was 30 kV, and the system offers the timing resolution of ~ 1 ns. In addition, the pulse repetition rate was 200 kHz, and the decay profile was recorded over the range of 1000 ns. The afterglow level (A) is defined as A (ppm)= 106 × (I2–I0)/(I1–I0), where I0, I1 and I2 denote the averaged signal intensity before the X-ray irradiation, the averaged signal intensity during irradiation and signal intensity at t = 20 ms after irradiation, respectively. The X-ray pulse width was 2 ms, and the excitation rate was 10 Hz. Although the evaluation manner of afterglow level differs in each manufacture, we referred the methodology by Nihon Kessho Kogaku Co., Ltd. which is one of the well-known
3. Results and discussion 3.1. Samples The SASM crystals nominally doped with undoped, 0.03%, 0.1% and 0.3% Ce were successfully synthesized by the FZ method. The concentration of Ce measured by XRF for the samples nominally doped with 0.03%, 0.1% and 0.3% were 0.03%, 0.05% and 0.09%, respectively. The actual Ce concentrations were close to nominal values, but the discrepancy between the nominal and actual concentrations became larger for higher dopant concentration. The typical size was ~ 6 mm in diameter and ~ 30 mm in length. Fig. 1 shows a photograph of the samples. Each sample was cut from the as-grown crystal rod, and relatively transparent parts were selected. All the samples look white and translucent because many small cracks are included. We selected relatively transparent parts for characterizations. The XRD patterns of all samples are shown in Fig. 2. The observed patterns agreed well with the standard pattern of SASM (ICSD 030698), and no significant impurity phase was observed. 3.2. Photoluminescence properties PL excitation and emission spectra of the 0.3% Ce-doped SASM sample are shown in Fig. 3 as a representative. A broad emission peak is observed around 350 and 390 nm, and the origins are ascribed to the 5d-4f transition of Ce3+ which was often seen in Ce3+-doped materials [28,29], and the emission features were similar to Ce-doped SASM [30] and Ce, Na-doped SASM powder [22]. The excitation bands around 290 and 340 nm are due to the 4f-5d2 and 4f-5d1 transitions of Ce3+, respectively. In addition, the Stokes shift is 3770 cm−1. All the samples
Fig. 1. Synthesized SASM crystal samples undoped and doped with Ce. 410
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Em. 310 nm: I =63exp(-t/171 ns) Intensity [a.u.]
Intensity [a.u.]
undoped 0.03% Ce 0.1% Ce 0.3% Ce
Em. 420 nm: I =981exp(-t/28 ns) + 57exp(-t/257 ns)
ICSD 030698 0
20
40 60 2θ [deg]
0
80
200
400 Time [ns]
600
800
200
Ex. spectrum (Em. 390 nm)
0.03% Ce I =29510exp(-t/31 ns) +143exp(-t/282 ns)
Intensity [a.u.]
Intensity [a.u.]
Fig. 2. XRD patterns of undoped and Ce-doped SASM single crystals as well as the standard card of SASM (ICSD 030698).
Em. spectrum (Ex. 250 nm)
0.1% Ce I =30180exp(-t/31 ns) +48exp(-t/267 ns) 0.3% Ce I =30882exp(-t/31 ns) +115exp(-t/201 ns) 0
300
400 Wavelength
200
400 Time [ns]
600
800
Fig. 5. PL decay time profiles of the undoped SASM sample (top) and Ce-doped SASM samples (bottom). The monitoring wavelengths of the undoped sample were 310 nm and 420 nm while that of Ce-doped sample was 390 nm. The excitation wavelength was 280 nm.
500
Fig. 3. The excitation and emission spectra of the 0.3% Ce-doped SASM.
the monitoring wavelengths are 310 and 420 nm. The decay curve monitoring at 310 nm is approximated by a single exponential decay function while the one monitoring at 420 nm is best-approximated by a sum of two exponential decay functions. The obtained decay time constant monitoring at 310 nm is 171 ns while the obtained decay time constants monitoring at 420 nm are 28 ns (95%) and 257 ns (5%). In contrast, PL decay time curves of the Ce-doped SASM samples are demonstrated in Fig. 5 (bottom). Here, the excitation and monitoring wavelengths were 280 nm and 390 nm, respectively. The decay curves are well-approximated by a sum of two exponential decay functions. The obtained decay time constants are 31 ns (99%) and 282 ns (1%) for the 0.03% Ce-doped samples, 30 ns (99%) and 267 ns (1%) for the 0.1% Ce-doped sample and 30 ns (99%) and 201 ns (1%) for the 0.3% Cedoped sample. From the decay times obtained for the undoped and Cedoped samples, the emission origins were attributed as follows. First, all the Ce-doped samples showed relatively fast decay with the decay time constants of 30–31 ns, which is reasonable for the 5d-4f transitions of Ce3+ [28,29]. In addition, we attribute the origin of the second component (201–282 ns) to the host due to possibly some defects since emission of similar decay time constant was also observed in the undoped sample (171 ns), when monitoring at 310 nm. However, the undoped sample also showed an emission of faster decay time (28 ns) when monitoring at 420 nm. We think the origin of the latter emission is also the 5d-4f transitions of Ce3+ which is unexpectedly contaminated during the synthesis process. Note that the ratio to the host emission is very weak compared to those of the Ce-doped samples. When monitoring at shorter wavelength (310 nm), we only observed emission due to the host which appeared as slower decay. The component due to 5d-4f transition of Ce3+ are similar to those of Ce-doped CASM [18] and Ce, Na-doped SASM powder [22]. In these previous investigations, the emission origin was also ascribed to 5d-4f transition
Intensity [a.u.]
undoped
300
400 500 Wavelength [nm]
600
Fig. 4. PL spectrum of the undoped SASM sample measured under excitation at 280 nm.
show similar excitation/emission features, and QY of the 0.03%, 0.1% and 0.3% samples are 10.9%, 11.5% and 14.7%, respectively. When we compare with Ce-doped CASM [18], the excitation and emission bands shifted to shorter wavelengths, which indicates that the strength of the crystal field of SASM is smaller than that of CASM. The PL QYs of SASM are slightly lower than those of CASM which indicated ~ 20% of PL QY [18]. The undoped sample shows no measurable signals by the same instrument but using Quantaurus-τ. The obtained spectrum is separately shown in Fig. 4. The excitation wavelength was 280 nm. The emission consists of peaks around 340 and 420 nm. The PL decay time curves of the undoped SASM sample are demonstrated in Fig. 5 (top). The excitation wavelength is 280 nm, and 411
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undoped I =505exp(-t/23 ns) +83exp(-t/261 ns) Intensity [a.u.]
Intensity [a.u]
0.03% Ce 0.1% Ce 0.3% Ce undoped
00.3% Ce I =6980exp(-t/38 ns) +511exp(-t/304 ns) 0.1% Ce I =5163exp(-t/30 ns) +301exp(-t/300 ns) 0.3% Ce I =4727exp(-t/31 ns) +402exp(-t/238 ns)
200
300
400
500
600
700
0
200 400 600 Wavelength [nm]
Wavelength [nm] Fig. 6. X-ray induced scintillation spectra of undoped and Ce-doped SASM samples.
800
Fig. 7. X-ray induced scintillation decay time profiles of the undoped and Cedoped samples.
of Ce3+.
Normarized counts [a.u.]
100
3.3. Scintillation properties X-ray induced scintillation spectra of undoped and Ce-doped SASM samples are compared in Fig. 6. Each spectrum is normalized to the highest value. The Ce-doped samples show an intense broad emission peaking around 420 nm, and the emission wavelength of Ce-doped SASM was similar to that of the Ce,Na-doped SASM powder [22]. The emission spectral shape is different from those in PL. The spectral shape consists of a single peak, and the emission wavelength is longer than 30 nm when compared with PL. These is blamed for the measurement geometry, and it does not have a significant meaning. In PL, the measurement was conducted on a reflection mode while the scintillation spectra were measured on a transmission geometry. Since the Ce-doped samples have absorption and emission bands overlapped (see Fig. 3), the measurement of transmittance geometry leads emission wavelength shifted to longer wavelengths due to self-absorption. In addition, when we compare the scintillation spectra with 0.3% Ce-doped CASM [18], the emission peak position of 0.3% SASM was similar, and this is different for the PL when we compare between Ce-doped SASM and Cedoped CASM [18]. The reason is due to transparency of the sample. The 0.3% Ce-CASM was transparent while the 0.3% Ce-doped SASM is translucent. Therefore, 0.3% Ce-doped SASM strongly affected by selfabsorption. The undoped sample shows broad emission bands at 300, 350 and 400 nm. The emission band at 400 nm is not consistent with the spectrum of PL, and the origin should not be the same. These emission wavelengths are preferable for use with PMT, which is one of the advantages of SASM for scintillation detector applications. The X-ray induced scintillation decay time profiles of the undoped and Ce-doped SASM samples are shown in Fig. 7. The decay time profiles of the all samples are well-approximated with a sum of two exponential decay functions. The obtained decay time constants of undoped SASM sample are 23 ns (86%) and 261 ns (14%), and the constants are similar to those in PL monitoring both the emissions by Ce and host at 420 nm. Therefore, we think that the first component is due to the contamination of Ce, and the second component is due to the host matrix. The decay time constants of Ce-doped SASM samples are 30–38 and 238–304 ns. These constants are similar to those of PL by Ce-doped samples. The first components (30–38 ns) are due to the 5d-4f transitions of Ce3+, and the second components (238–304 ns) are due to the host matrix. These results were similar to those in Ce-doped CASM [18]. Afterglow profiles of the Ce-doped SASM samples are shown in Fig. 8. Here, the X-ray pulse width was 2 ms. The afterglow levels evaluated for the 0.03%, 0.1% and 0.3% Ce-doped samples are 2620, 3530 and 990 ppm, respectively. Although the 0.3% Ce-doped sample shows the lowest afterglow level, it is much higher than those of practical scintillators used in integrated-type detectors such as CdWO4
0.03% Ce 0.1% Ce 0.3% Ce
10-1 10-2 10-3 10-4 0
10
20 30 Time [ms]
40
50
Fig. 8. Afterglow curves of the Ce-doped SASM samples after 2 ms X-ray irradiation.
and BGO (~ 10 ppm) [24]. The appearance of such high afterglow level suggests that the 0.03% and 0.1% Ce-doped SASM contain a large number of shallow trapping centers, so some of the charges generated by X-ray are temporarily captured at these traps before recombination to emit light. Pulse height spectrum of 241Am 59.5 keV γ-rays measured using the 0.3% Ce-doped SASM sample as scintillator is presented in Fig. 9. The one measured with Tl-doped NaI is also presented as a reference which is known to have an absolute LY of 40,000 ph/MeV. The 0.3% Ce-doped sample successfully demonstrated a photoabsorption peak, and the absolute scintillation LY was estimated from the peak position measured with respect to that of the Tl-doped NaI. The obtained value was
150 0.3% Ce Intensity [a.u.]
NaI: Tl (gain
1/5)
100
50
0 0
100 Channel
200
Fig. 9. Pulse height spectra of 241Am 59.5 keV γ-rays measured using the 0.3% Ce-doped SASM sample and Tl-doped NaI single crystal. 412
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Acknowledgements
Table 1 Photoabsorption peak channels and LYs of 0.3% Ce-doped SASM sample and Tl:NaI.
0.3% Ce:SASM Tl:NaI
Photoabsorption peak [channel]
Light yield [ph/MeV]
25 180
1100 ± 10% 40,000 ± 10%
This work was supported by Grant-in-Aid for Scientific Research (A) (17H01375), Grant-in-Aid for Young Scientists (B) (17K14911) and Grant-in-Aid for Research Activity Start-up (16H06983) from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese government (MEXT) as well as A-STEP from Japan Science and Technology Agency (JST). The Cooperative Research Project of Research Institute of Electronics, Shizuoka University, Mazda Foundation, Taisei Foundation, Terumo Foundation for Life Sciences and Arts, Izumi Science and Technology Foundation, SEI Group CSR Foundation, Konica Minolta Science and Technology Foundation, NAIST Foundation and TEPCO Memorial Foundation are also acknowledged.
Intensity [a.u.]
after X-ray irradiation (0.1 Gy) 0.03% Ce 0.1% Ce
References [1] P.A. Rodnyi, Physical Processes in Inorganic Scintillators, CRC Press, New York, 1997. [2] J. Glodo, Y. Wang, R. Shawgo, C. Brecher, R.H. Hawrami, J. Tower, K.S. Shah, Phys. Procedia 90 (2017) 285–290, http://dx.doi.org/10.1016/j.phpro.2017.09.012. [3] T. Yanagida, Y. Fujimoto, S. Kurosawa, K. Kamada, H. Takahashi, Y. Fukazawa, M. Nikl, V. Chani, Jpn. J. Appl. Phys. 52 (2013) 076401, http://dx.doi.org/10. 7567/JJAP.52.076401. [4] S. Yamamoto, S. Okumura, N. Kato, J.Y. Yeom, J. Instrum. 10 (2015) T09002. [5] K. Watanabe, T. Yanagida, K. Fukuda, A. Koike, T. Aoki, Sens. Mater. 27 (2015) 269–275. [6] T. Itoh, T. Yanagida, M. Kokubun, M. Sato, R. Miyawaki, K. Makishima, T. Takashima, T. Tanaka, K. Nakazawa, T. Takahashi, N. Shimura, H. Ishibashi, Nucl. Instrum. Methods Phys. Res. A 579 (2007) 239–242, http://dx.doi.org/10. 1016/j.nima.2007.04.144. [7] K. Kawade, K. Fukatsu, Y. Itow, K. Masuda, T. Murakami, T. Sako, K. Suzuki, T. Suzuki, K. Taki, J. Instrum. 6 (2011) 1–12, http://dx.doi.org/10.1088/17480221/6/09/T09004. [8] T. Youichi, J.H. Kaneko, M. Higuchi, S. Nishiyama, H. Ishibashi, Appl. Phys. Express 8 (2015) 62602. [9] L. Pidol, A. Kahn-Harari, B. Viana, E. Virey, B. Ferrand, P. Dorenbos, J.T.M. De Haas, C.W.E. Van Eijk, IEEE Trans. Nucl. Sci. 51 (2004) 1084–1087, http://dx.doi. org/10.1109/TNS.2004.829542. [10] C.L. Melcher, J.S. Schweitzer, IEEE Trans. Nucl. Sci. 39 (1992) 502–505. [11] D. Nakauchi, G. Okada, N. Kawano, N. Kawaguchi, T. Yanagida, Appl. Phys. Express 10 (2017) 072601. [12] M. Mori, J. Xu, G. Okada, T. Yanagida, J. Ueda, S. Tanabe, J. Ceram. Soc. Jpn. 124 (2016) 569–573, http://dx.doi.org/10.2109/jcersj2.15239. [13] T. Yanagida, K. Kamada, Y. Fujimoto, H. Yagi, T. Yanagitani, Opt. Mater. 35 (2013) 2480–2485, http://dx.doi.org/10.1016/j.optmat.2013.07.002. [14] D. Nakauchi, G. Okada, M. Koshimizu, T. Yanagida, Nucl. Instrum. Methods B 377 (2016) 89–93, http://dx.doi.org/10.1016/j.nimb.2016.04.017. [15] M.E. Phelps, E.J. Hoffman, N.A. Mullani, M.M. Ter-Pogossian, J. Nucl. Med. 16 (1975) 210–224. [16] P. Wen, N. Guiling, L. Yuan, Y. Xuefeng, J. Rare Earths 26 (2008) 207–210. [17] Y. Ding, Y. Zhang, Z. Wang, W. Li, D. Mao, H. Han, C. Chang, J. Lumin. 129 (2009) 294–299, http://dx.doi.org/10.1016/j.jlumin.2008.10.009. [18] T. Ogawa, D. Nakauchi, G. Okada, N. Kawano, N. Kawaguchi, T. Yanagida, J. Lumin. 196 (2018) 270–274, http://dx.doi.org/10.1016/j.jlumin.2017.12.009. [19] B. Arun, K.M. Manu, M.T. Sebastian, Int. J. Mod. Phys. Conf. Ser. 22 (2013) 173–178, http://dx.doi.org/10.1142/S2010194513010088. [20] G. Li, M. Li, L. Li, H. Yu, H. Zou, L. Zou, S. Gan, X. Xu, Mater. Lett. 65 (2011) 3418–3420, http://dx.doi.org/10.1016/j.matlet.2011.07.050. [21] H. Zou, D. Peng, Z. Chu, X. Wang, Y. Li, X. Yao, J. Mater. Sci. 48 (2013) 7981–7985, http://dx.doi.org/10.1007/s10853-013-7609-x. [22] L. Lin, R. Shi, R. Zhou, Q. Peng, C. Liu, Y. Tao, Y. Huang, P. Dorenbos, H. Liang, Inorg. Chem. 56 (2017) 12476–12484, http://dx.doi.org/10.1021/acs.inorgchem. 7b01939. [23] S. Adarsh, Development of a Critically Evaluated Thermodynamic Database for the Systems Containing Alkaline-earth Oxides (Ph.D. thesis), Université de Montréal, 2012. [24] T. Yanagida, Y. Fujimoto, T. Ito, K. Uchiyama, K. Mori, Appl. Phys. Express 7 (2014) 062401, http://dx.doi.org/10.7567/APEX.7.062401. [25] T. Yanagida, Y. Fujimoto, A. Yamaji, N. Kawaguchi, K. Kamada, D. Totsuka, K. Fukuda, K. Yamanoi, R. Nishi, S. Kurosawa, T. Shimizu, N. Sarukura, Radiat. Meas. 55 (2013) 99–102, http://dx.doi.org/10.1016/j.radmeas.2012.05.014. [26] W.J. Van Sciver, L. Bogart, IEEE Trans. Nucl. Sci. 5 (1958) 90–92. [27] T. Yanagida, Y. Fujimoto, N. Kawaguchi, S. Yanagida, J. Ceram. Soc. Jpn. 121 (2013) 988–991, http://dx.doi.org/10.2109/jcersj2.121.988. [28] M. Mori, D. Nakauchi, G. Okada, Y. Fujimoto, N. Kawaguchi, M. Koshimizu, T. Yanagida, J. Lumin. 186 (2017) 93–97, http://dx.doi.org/10.1016/j.jlumin. 2017.01.035. [29] S. Chen, X. Zhao, H. Xie, J. Liu, L. Duan, X. Ba, J. Zhao, Appl. Surf. Sci. 258 (2011) 3255–3259, http://dx.doi.org/10.1016/j.apsusc.2011.11.077. [30] M. Kolte, V.B. Pawade, S.J. Dhoble, Appl. Phys. A 122 (2016) 59.
0.3% Ce 100
200 300 400 Temperature [°C]
Fig. 10. TL glow curves of undoped and Ce-doped SASM samples measured after 0.1 Gy X-ray irradiation.
1100 ph/MeV (typical error is ± 10%). The measured peak positions and derived LY are summarized in Table 1. It is worth mentioning that the LY of the undoped SASM sample and those doped with 0.03% and 0.1% Ce are not sufficiently high that a photoabosorption peak is detected in the present aperture. Compared with Ce-doped CASM [18], the scintillation LY is lower. 3.4. TL properties TL glow curves measured after X-ray irradiation are shown in Fig. 10. Each glow curve is normalized to the highest intensity. The irradiation dose was 0.1 Gy. The undoped SASM sample does not show measurable TL signals, and the Ce-doped SASM samples show TL with a glow peak at 250 °C and a shoulder around 130 °C. A similar signal at 130 °C was reported in Eu:SASM and Eu,Dy:SASM powders [17]; therefore, we attribute the origin of this shoulder to host defects. In addition, the peak around 250 °C can be attributed to Ce-doping. The Ce-doping makes some defects due to charge imbalance between Sr2+ and Ce3+. In addition, the peak shape and position were similar to those of Ce-doped CASM [18,31], and the origin is considered to be the same. 4. Summary We investigated the PL, scintillation and TL properties of the undoped and Ce-doepd SASM crystals synthesized by the FZ method. Under X-ray irradiation, the Ce-doped samples show strong scintillation with a broad emission around 420 nm with decay time constants of approximately 30 and 300 ns. From the results of undoped sample and PL characterizations, we attributed the origins of the former decay component is the 5d-4f transitions of Ce3+ while the one of the latter emission is due to the host. The afterglow level is the lowest for the 0.3% Ce-doped sample (990 ppm). The LY is the highest for the 0.3% Ce-doped sample indicating 1100 ph/MeV under 241Am γ-ray (59.5 keV). In addition, the Ce-doped samples show TL properties with a glow peak at 250 °C and a shoulder around 130 °C.
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