Photoluminescence, scintillation and thermally-stimulated luminescence properties of Tb-doped 12CaO·7Al2O3 single crystals grown by FZ method

JOURNAL OF RARE EARTHS, Vol. 35, No. 10, Oct. 2017, P. 957

Photoluminescence, scintillation and thermally-stimulated luminescence properties of Tb-doped 12CaOx7Al2O3 single crystals grown by FZ method Narumi Kumamoto, Daisuke Nakauchi, Takumi Kato, Go Okada, Noriaki Kawaguchi, Takayuki Yanagida* (1. Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama-cho, Ikoma-shi, Nara 630-0192, Japan) Received 28 November 2016; revised 31 February 2017

Abstract: Tb-doped 12CaO·7Al2O3(Tb:C12A7) crystals were synthesized by the floating zone (FZ) method and the photoluminescence (PL), scintillation and thermally-stimulated luminescence (TSL) properties were investigated. The photoluminescence (PL) emission spectra and PL decay time profiles were investigated by using Quantaurus-tau (Hamamatsu). The scintillation spectra and decay time profiles were measured by using our laboratory-constructed set-up under X-ray irradiation. Finally, TSL glow curve was measured by using Nanogray TL-2000 with the heating rate of 1 °C/s. In PL and scintillation, emission peaks were observed at 493, 543, 587 and 620 nm due to the 4f-4f transitions of Tb3+. Decay time constants of the emission by PL and scintillation appeared to be different, and the measured values were approximately 2.4 and 1.9 ms, respectively. After X-ray irradiations, the 0.5% (all Tb contents in this paper are in mole fraction) and 1.0% Tb-doped samples showed a single intense and broad glow peak around 100 ºC while 1.2%–1.5% Tb-doped samples showed notable additional peak around 250 ºC. Keywords: C12A7; single crystal; FZ; terbium; scintillation; photoluminescence; TSL; rare earths

Phosphor materials are commonly used in various application fields of radiation detection, and they are typically classified by either scintillator or dosimeter. In both types of device, numerous number of electrons and holes are generated by absorbing incident radiation energy. After this process, scintillator converts the incident ionizing radiation to a large number of low energy photons, such as ultraviolet and visible photons, immediately via charge recombination at emission centers or direct excitation of emission centers by the secondary electrons. Today, scintillators are used in various applications such as high energy physics[1,2], medical imaging[3], security[4] and well-logging[5]. In contrast, dosimeter stores generated electrons as a form of trapping charges at localized centers, typically lattice defects. These charges are then de-trapped via heat (thermally-stimulated luminescence, TSL[6]) or light (optically-stimulated luminescence, OSL[6]) stimulation and recombine at emission centers to emit light. Dosimeters are generally used in medical imaging[7,8] and personal dose monitoring[9,10] applications. Historically, scintillators and dosimeters were studied and developed in different fields of study as they were often used in different applications; however, scintillator and dosimeter share common physical processes. It has been recently pointed out that luminescent properties of scintillator and dosimeter are complementarily related in

some material systems, and the relationship can be explained with a simple energy conservation law[11,12]. Hence, it is important to study both scintillator and dosimeter properties inclusively in one material. Knowing both the properties would extend the understandings of mechanisms involved and help further advancement of the suitable material system in radiation detectors. Rare-earth doped bulk crystalline materials are often used as scintillator and dosimeter. Scintillator typically consists of heavy ions, so it can absorb radiation energy effectively, while dosimeter (in personnel dose monitoring) consists of light elements to be equivalent to biological tissue. Also, luminescent properties are profoundly dependent on the emission center, and rare-earth ions are often used as useful emission center and Tbdoped phosphors are of great interest in radiation measurements. For example, Tb:Gd2O2S (Tb:GOS) shows intense green luminescence due to the 4f-4f (5D4→7F5) transitions of Tb3+ ions with the decay times of 0.2–2 ms and sufficient light yield under X-ray irradiation[13,14]. In addition, Gd3+ and Tb3+ co-doped silica glasses have been studied for UV and X-ray sensors[15] as they exhibit intense luminescence with the decay time of about 2 ms. In this study, we synthesized and characterized Tbdoped 12CaO·7Al2O3 (Tb:C12A7) single crystals with different concentrations of Tb in order to investigate the

Foundation item: Project supported by a Grant-in-Aid for Scientific Research (A) 26249147 from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), JST A-step, Cooperative Research Project of Research Institute of Electronics, Shizuoka University, Inamori foundation, and KRF foundation * Corresponding author: Takayuki Yanagida (E-mail: [email protected]; Tel.: +81-743-72-6144) DOI: 10.1016/S1002-0721(17)60999-2

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possibility of applications in radiation measurements. The unit cell of C12A7 contains twelve cages which have a free space of ~0.4 nm in diameter and host extra-framework O2– ions occupying two different cages[16]. C12A7 crystals are transparent in the near-UV and visible range because they have a wide-band gap of 6.0 eV[17], and replacing O2– ions with electron yields conductive properties. So, they have been intensively studied for applications as optical devices such as transparent display and conductor[18,19]. C12A7 was also studied in the catalyst filed. For example, active anion species of O– and OH– associated with microporous C12A7 powder were reported to convert benzene to phenol as catalyst with a high conversion efficiency and phenol selectivity[20]. C12A7 is one of the components of Portland cement, so it was studied in the hydration of cements[21]. In addition to these applications, the C12A7 would also be interesting for radiation detectors because they consist of common chemical elements in the earth, which would reduce the cost and promote the resource protection. Up until now, a few researches were reported with Tb-doped C12A7; for example, electronic and optoelectronic applications with single crystals[18], optical properties of powder form[22] and TSL of ceramic and powder forms[23]. However, as far as we are aware, no studies were conducted for scintillator and dosimeter applications using Tb-doped C12A7 single crystal.

the crystal rod containing 1.0% of Tb was subject for this measurement.

1 Experimental 1.1 Sample preparation C12A7 crystal rod doped with Tb ion was prepared with the chemical composition of nominally 1.0 mol.% Tb-doped 12CaO-7Al2O3. Non-doped crystal was also grown by the same manner for comparison purpose. The samples were prepared by using high purity CaO (99.99%), Al2O3 (99.99%) and Tb4O7 (99.99%) powders as raw materials. First, the starting compounds were mixed, and the mixture was loaded in a balloon and formed to a cylinder rod by applying the hydrostatic pressure. Next, the rod was sintered at 1100 ºC for 10 h to obtain a ceramic rod. The sintered rod was loaded into the FZ furnace (FZD0192, Canon Machinery Inc.), and the crystal growth was conducted with the pulling down rate of 5 mm/h. The heat source of the FZ furnace was halogen lamp. The obtained crystal rod was cut perpendicular to the direction crystal growth, and 5 pieces of sample were obtained. To each sample piece, X-ray fluorescence spectrometry (XRF, MESA-500W, HORIBA) was carried out in order to investigate the concentration of Tb. Powder X-ray diffraction (XRD) was measured by a diffractometer (Mini Flex 600, RIGAKU) using Cu (Kα) X-ray beam with the tube voltage and current of 40 kV and 15 mA, respectively. A portion of

1.2 Optical characterization By using Quantaurus-τ (C11367, Hamamatsu Photonics), the photoluminescence (PL) emission spectra were investigated under 365 nm excitation. By using the same measurement setup, the PL decay time was also evaluated by monitoring mainly the 543 nm emission band through a band pass filter transmitting 450–620 nm light during excitation at 365 nm by LED. The decay time was deduced by a least-square fitting with a single exponential decay function. 1.3 Scintillation and thermally-stimulated luminescence properties A conventional X-ray tube was used as the X-ray source throughout the research to evaluate scintillation spectrum and TSL. The X-ray tube was equipped with a tungsten anode target and beryllium window (XRB80P& N200X4550, Spellman). The scintillation spectrum was measured by using our laboratory-constructed set-up[24]. During the measurement, the X-ray tube was supplied with a bias voltage of 80 kV and tube current of 2.5 mA. We used a spectrometer of Andor DU-420-BU2 with a Shamrock SR163 monochromator to measure a spectrum from the UV to visible ranges. During the measurement, the detector part was cooled down to 188 K by a Peltier module. Scintillation decay time was investigated by using our original set-up equipped with a pulse X-ray source[25]. In the decay measurements, the wavelength was not resolved. In the scintillation decay time profile, the decay time was deduced by approximating with either 1st- or 2nd-order exponential decay function. TSL glow curve was measured by using Nanogray TL-2000[26] over the temperature range from 50 to 490 ºC. The samples were irradiated with 10 Gy of X-ray prior to the measurement. The heating rate was 1 ºC/s. The X-ray dose was calibrated using an air-filled ionization chamber (TN30013, PTW). Except for the TSL, all the characterizations above were conducted at room temperature.

2 Results and discussion 2.1 Samples The C12A7 crystals grown by the FZ method are shown in Fig. 1. The typical size of as-grown crystal rod was approximately Φ4 mm×35 mm. Despite the transparent appearance of undoped sample, the obtained Tb-doped crystal rod looked brown and the color strength seemed to vary depending on the growth rate adjustment. The as-grown crystal rod was cut into several pieces, and five samples shown in the figure were selected for further characterizations. The indicated values

Narumi Kumamoto et al., Photoluminescence, scintillation and thermally-stimulated luminescence properties of …

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Fig. 1 Undoped and Tb-doped C12A7 crystals synthesized by the FZ method

in the figure are the concentrations of Tb ion measured by XRF. The diameters of these five samples were approximately 4 mm Φ and the thicknesses were 2.9, 3.7, 6.2, 1.4 and 5.8 mm for samples containing 0.5%, 1.0%, 1.2%, 1.4% and 1.5% of Tb. Except for the 1.0% Tb-doped sample, the surface region was strongly colored in brown and the coloration was lighter towards the middle part. Under UV lamp excitation at 365 nm, a bright green emission was seen by naked eyes from the Tb-doped samples. Fig. 2 shows a powder X-ray diffraction pattern of 1.0% Tb-doped C12A7 crystal sample. Each diffraction peak was identified using a reference database of the Joint Committee on Powder Diffraction Standards (JCPDS) (No. 09-0413 of mayenite; C12A7 single crystal, Ca12Al14O33). From this result, we confirmed that the sample did not contain any impurity phases. 2.2 Optical properties Fig. 3 shows the PL emission spectra of Tb:C12A7 crystals under 365 nm excitation. Several peak emissions were observed at 493 (5D4→7F6), 543 (5D4→7F5), 587 (5D4→7F4) and 620 (5D4→7F3) nm. The origins of these emission lines were attributed based on the previous study[13,15,18,22,27–30]. In contrast, the undoped sample did not show any measurable signal. In the present samples, all the emission lines were observed at the same wavelengths in spite of the difference of concentrations. Although the above measurement is a qualitative evaluation, we used the measured intensity to compare the PL efficiency between the samples approximately since the measured sample surface area was approximately con-

Fig. 2 X-ray diffraction pattern of 1% Tb-doped C12A7 crystal

Fig. 3 PL emission spectra of Tb:C12A7 crystals under 365 nm excitation

sistent (φ~4 mm) for each sample. Among the present samples, the optimum concentration of Tb in C12A7 was found to be around 1%. The PL decay time profiles of Tb:C12A7 crystals are shown in Fig. 4. The measurement was carried out by monitoring the 543 nm emission band during excitation at 365 nm. As illustrated in the figure the undoped sample showed a very fast decay time, so the curve fitting was performed for the time range of 2–20 ms to derive the decay time of only emission due to Tb3+. It should be instructive to mention that this fast signal is not due to the instrumental response (or excitation signal) since no signal was detected without sample running the measurement with the same settings. Among the Tb-doped samples, the decay time constants of PL were reasonably consistent regardless of the concentration, and the decay time was approximately 2.4 ms due to the 4f-4f transitions of Tb3+. It seems that the decay time slightly became shorter when the concentration of Tb increased. PL decay time constants of Tb3+ due to the 4f-4f transitions in other phosphor materials are about 1–3 ms, for exam-

Fig. 4 PL decay time profiles of the Tb:C12A7 crystals and undoped C12A7 crystal (1) 1.5% Tb, I=8061exp(–t/2.33); (2) 1.4% Tb, I=6614exp(–t/ 2.32); (3) 1.2% Tb, I=8064exp(–t/2.39); (4) 1.0% Tb, I=9215 exp(–t/2.56); (5) 0.5% Tb, I=7031exp(–t/2.49); (6) Undope, I=1423 exp(–t/0.00772)

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ple in Gd2O3 layer containing Tb3+ coated SiO2 nanoparticle[31], Tb-doped LiBaPO4 (Tb:LiBaPO4) powders[32], Tb-doped Lu2O3 (Tb:Lu2O3) powders[33], Gd2O3 nanoparticles[34] and Tb-doped NaGd(WO4)2 powders[35]. So, the values obtained in this study are similar to those in other Tb-doped phosphors. 2.3 Scintillation and thermally-stimulated luminescence properties Scintillation spectra of Tb:C12A7 crystals under X-ray irradiation are shown in Figs. 5, 6 and 7. As shown in these figures, Tb:C12A7 crystals were confirmed to have a function to emit scintillation light. Since the wavelength resolution of the detector used here was higher than that of PL, we could resolve some weak peaks which were not detected in PL spectra. In the undoped sample, a broad emission peak appeared around 450 nm (see Figs. 5 and 6). This peak can be due to the lattice defects (possibly oxygen vacancy) of C12A7 host. Tb:C12A7 crystals showed intense and weak emission peaks due to the 4f-4f transitions of Tb3+[36–38].These emission peaks were identified as listed in Table 1. As in the case of PL under 365 nm excitation, the strongest peak was 540 nm due to the 5D4→7F5 transition, and the 1% sample showed the highest scintillation intensity. The scintillation decay time profiles of the present

Fig. 7 X-ray induced scintillation spectra expanded in the 640– 693 nm region of Fig. 3 Table 1 Radiative transitions of Tb3+ doped C12A7 crystal and the corresponding emission wavelength measured at R.T. Transition 5

7

D3→ F6 D3→7F5

Wavelength/nm 378

5

412

5

432, 437

5

479, 488

D3→7F4 D4→7F6

5

D4→7F5

540

5

581, 587

D4→7F4

5

D4→7F3

5

7

D4→ F2 D4→7F1

619, 624 651

5

664

5

681

D4→7F0

samples and instrumental response are shown in Fig. 8. The decay time constant of all the Tb-doped samples was consistent regardless of the difference of the concentration of Tb. The measured decay time was around 1.9 ms. This value is slightly shorter than that of PL. In general,

Fig. 5 X-ray induced scintillation spectra of the Tb:C12A7 crystals and undoped C12A7 crystal

Fig. 6 X-ray induced scintillation spectra expanded in the 350–525 nm region of Fig. 3

Fig. 8 Scintillation decay time profiles of the Tb:C12A7 crystals and the instrumental response (1) 1.5% Tb, I=132exp(–t/0.403)+140exp(–t/1.94); (2) 1.4% Tb, I=83.3exp(–t/0.288)+82exp(–t/1.78); (3) 1.2% Tb, I=506exp(–t/ 0.410)+509exp(–t/2.02); (4) 1.0% Tb, I=557exp(–t/0.403)+454 exp(–t/1.94); (5) 0.5% Tb, I=282exp(–t/0.396)+302exp(–t/1.98); (6) Undoped, I=15exp(–t/0.305); (7) Instrumental response I=18exp(–t/0.398)

Narumi Kumamoto et al., Photoluminescence, scintillation and thermally-stimulated luminescence properties of …

PL decay time is shorter than that of scintillation since the scintillation process involves additional energy migration processes[39]. Having shorter decay time in scintillation than PL is typically interpreted as a result of competitions between the energy transfer and quenching due to the interaction among energetic secondary electrons during the scintillation processes. It should be noted that X-rays generate many energetic secondary electrons in a typical interspatial distance of 10–100 nm[39], so the interaction of these excited electrons could not be neglected. In these materials, the quenching processes due to the interaction of the exited secondary electrons would be ascendant. It should be noted here that the measured decay times of PL and scintillation are comparable despite the different instrumental setups. This is because the photodetectors used in the PL and scintillation measurements were the same PMT series manufactured by Hamamatsu Photonics (H7422-01 and H7422P-50, respectively) and these PMTs have the same electrode structures; therefore, it is reasonable to consider that the timing responses were the same. In addition, despite the difference of the spectral sensitivities (9.1% at 540 nm in PL and 11% at 540 nm in scintillation), the detection range of both measurement covered the main four emission lines around 480, 540, 580 and 620 nm for both the PL and scintillation decay measurements. The TSL glow curves of the present samples are shown in Fig. 9. All the samples were irradiated by X-rays (10 Gy) prior to the measurement. In the 1.0% Tb-doped sample, a single intense glow peak appeared around 100 ºC while 1.2%, 1.4% and 1.5% Tb-doped samples showed an additional glow peak at around 250 ºC. The 0.5% Tb-doped sample showed a broad glow peak over the range of 100–300 ºC, and the relative intensity of the peak around 250 ºC is the highest. Since we could not observe any significant signals from the undoped sample, the function of TSL arose by Tb-doping. We interpret that some Ca2+ ions could be substituted with Tb3+ ions because their ionic radii are close (the radius of Tb3+

Fig. 9 TSL glow curves of the undoped and Tb:C12A7 crystals after 10 Gy X-ray exposure (Each glow curve was normalized to the peak value)

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ion is 0.093 nm and that of Ca2+ ion is 0.099 nm[40]), and the difference of their valence charges caused some defects which can work as trapping sites. Hence, Tb-doped samples show meaningful TSL signals. Throughout the present evaluations, Tb-doped C12A7 was confirmed to have a function as a dosimeter material. In the future, it would be interesting to investigate the effect of doping with other trivalent rare earth ions.

3 Conclusions We synthesized undoped and Tb-doped C12A7 single crystals by the FZ method and investigated the PL, scintillation and TSL properties. In the PL and scintillation spectra, emissions due to the 4f-4f transitions of Tb3+ were observed in the visible range. The decay time constants in both PL and scintillation were approximately 2 ms due to the 4f-4f transitions of Tb3+. The TSL characterizations suggested that doping of C12A7 with Tb3+ ion enhanced the TSL properties.

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