Characterizations of Ce3+-Doped CaB2O4 Crystalline Scintillator

Article pubs.acs.org/crystal

Characterizations of Ce3+-Doped CaB2O4 Crystalline Scintillator Yutaka Fujimoto,*,† Takayuki Yanagida,‡ Noriaki Kawaguchi,§ Shunsuke Kurosawa,† Kentaro Fukuda,§ Daisuke Totsuka,∥ Kenichi Watanabe,⊥ Atsushi Yamazaki,⊥ Yuui Yokota,† and Akira Yoshikawa†,‡ †

IMR, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan NICHe, Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai, 980-8579, Japan § Tokuyama Corp., 3 Shibuya, Shibuya-ku, Tokyo 150-8383 Japan ∥ Nihon Kessho Kogaku Co.,Ltd., 810-5 Nobe-cho Tatebayashi Gunma, Japan ⊥ Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan ‡

ABSTRACT: Single crystals of undoped and 0.5% Ce3+-doped calcium metaborate CaB2O4 have been grown successfully by micropulling down (μ-PD) technique. The material can find its application in the neutron detection. After the single-phase of CaB2O4 was confirmed using X-ray diffraction analysis, optical and scintillation characteristics were investigated. In the transmittance spectra, Ce3+-doped crystals showed absorption bands around 270 and 320 nm which are ascribed to the transition from 4f ground state to 5d excited state of Ce3+. Under the 241Am 5.5 MeV α-ray excitation, strong emission peak at 370 nm because of the Ce3+ 5d−4f transition was observed for Ce3+-doped crystal, while the undoped crystal showed broad intrinsic emission band around 300− 400 nm which is caused by the lattice defects in the host crystal. The absolute light yield was calculated to be about 2200 photons per neutron under 252Cf neutron irradiation.

1. INTRODUCTION Inorganic scintillation materials are employed in contemporary radiation detectors for medical imaging, industrial inspection, dosimetry, and high-energy physics. They can be used for detection of X-rays, neutrons, and so on. X-rays are scattered by electrons and reveal the position of the electron clouds, while neutrons have no electric charge; they are not deflected by the electron clouds around an atomic nucleus. This means that they can reveal the position of the nucleus itself. Thus neutrons can be used to determine the positions of hydrogen atoms or of light atoms in close proximity to heavy atoms. Beside different isotopes such as hydrogen and deuterium can also readily be distinguished by neutron scattering but not by X-rays. The neutron detectors can be used for homeland security, oil-well logging and gas industry, basic research in nuclear and condensed matter physics, and material research.1 In the case of homeland security, the neutron detectors play an important role in detection of neutrons from radioactive and fissile materials such as plutonium, the material used in nuclear weapons. In the most common thermal neutron detector, 3He gas proportional counters have been used for a long time because of high stopping power for neutrons and low γ-ray background sensitivity. However, due to the large increase of the usage of 3He for security applications, the 3He resources are close to depletion, and can no longer supply for the demand in recent years.2,3 Thus, the alternative neutron detector has been strongly desired. The neutron detection efficiency depends also on the scintillation materials composition. Especially 6Li and 10 B have high cross sections (6Li, 940 barn; 10B, 3836 barn at 25 meV) for thermal neutrons and convert them into ionizing particles according to the following reactions: 6Li(n, α)t, 10 B(n, α)7Li.4,5 The 6Li-doped glass is a well-known commercial © 2011 American Chemical Society

neutron scintillator, however, it has a high sensitivity for gamma-ray background. In addition, the production in a large scale is difficult which increases the cost of the material. Concerning the most recent 6Li-based elpasolite scintillators, despite their excellent scintillation characteristics, the problem of strong hygroscopicity still prevents the materials from practical usage. Despite larger cross section for thermal neutrons when compared to 6Li, the 10Bbased compound scintillators which have higher light yield are less investigated.6,7 In the previous study, we have developed crystalline scintillators based on calcium orthoborate Ca3(BO3)2 and strontium metaborate SrB2O4 activated with Ce3+.8,9 The crystals exhibited satisfactory characteristics which are high enough light yields to detect neutrons, fast response and low sensitivity for gamma-ray backgrounds and so on. Therefore, in this study, we focused attention on another material, the calcium metaborate CaB2O4 It is composed of both alkaline-earth and boron similarly as the Ca3(BO3)2 and SrB2O4 crystals. CaB2O4 has an orthorhombic structure with Pnca space group, and the lattice parameters are as follows: a = 6.216 Å, b = 11.607 Å, c = 4.283 Å, and Z = 4.10 The low density of 2.702 g/cm3 and low effective atomic number of 14 are attractive for γ-ray background sensitivity. In the present work, we developed Ce-doped CaB2O4 crystalline scintillators for thermal neutron detection. After the X-ray diffraction analysis including powder X-ray diffraction (XRD) and X-ray rocking curve (XRC), optical (transmittance and photoluminescence) and scintillation (radioluminescence and pulse height spectra) characteristics were evaluated. Received: July 13, 2011 Revised: November 21, 2011 Published: December 7, 2011 142

dx.doi.org/10.1021/cg200885h | Cryst. Growth Des. 2012, 12, 142−146

Crystal Growth & Design

Article

2. CRYSTAL GROWTH AND X-RAY DIFFRACTION ANALYSIS Undoped and 0.5% Ce-doped CaB2O4 single crystals were grown by the μ-PD technique from a congruent melt. CaCO3 (4N), H3BO3 (4N) and CeO2 (5N) high purity powders were used as starting materials and mixed at a stoichiometric ratio. The mixture was loaded into Pt−Rh crucible and then it was heated to a melting point of CaB2O4 (∼ 1160 °C) by radio frequency (RF) heating system. When the mixture was melted homogeneously, the undoped Ca3(BO3)2 seed crystal physically contacted with the melt at the micronozzle (φ 0.3 mm) at the crucible die. The crystal was pulled out at the pulling rate of 0.05−0.1 mm/min under a nitrogen atmosphere. The power control of the RF generator during the whole growth process was carried out in order to maintain the constant diameter of the crystal monitoring the solid−liquid interface. Figure 1 shows the

Figure 2. XRD patterns of undoped and 0.5% Ce-doped both CaB2O4 crystals in the 2θ range from 10° to 80°.

Figure 3. XRC for (1 3 1) reflection of 0.5% Ce -doped CaB 2O 4 crystal.

Figure 1. Cleaved undoped (left) and 0.5% Ce-doped (right) CaB2O4 single crystals grown by the μ-PD technique.

3. OPTICAL PROPERTIES The transmittance of the polished crystals with thicknesses of 1.0 mm was measured using V-530 UV/vis spectrophotometer (JASCO) at wavelength range from 190 to 900 nm. The results are shown in Figure 4. The cutoff wavelength in the undoped

cleaved undoped and 0.5% Ce-doped CaB2O4 single crystals. The crystals looked clear and colorless, while cleavage cracks corresponding to the (B2O4)n2n− layers parallel to a c-axis were observed along the growth direction.9−12 The concentration of Ce ions in the 0.5% Ce-doped crystal was determined to be about 0.3 mol % by inductively coupled plasma (ICP) method using SII SPS3000 (Seiko Instruments Inc.). To check the obtained phase, the as-grown crystals were examined by powder XRD measurement in the 2θ range from 10° to 80° using a RINT2000 diffractometer (RIGAKU). The X-ray source was CuKα with accelerating voltage of 40 kV, and the tube current of 40 mA. The XRD patterns are typically plotted as the intensity of the diffracted X-rays against the angle 2θ. The peaks will appear in the diffraction pattern at the 2θ values when Bragg’s Law is satisfied. According to the XRD patterns, the crystals present only the CaB2O4 phase, and their patterns matched with the joint committee on powder diffraction standards (JCPDS) record no. 32−0155. From the results, no impurity phase was detected for both the undoped and Ce-doped crystals and we concluded that they formed a single phase, as illustrated in Figure 2. The XRC for (131) reflection of the polished Ce-doped crystal was measured to determine the crystal quality, as shown in Figure 3. The XRC profiles are determined by a 4-bounce Ge (220) channel cut monochromator. From the calculation of the measured data, the full-width at the half-maximum (fwhm) value was about 95 arcsec. The value for the undoped crystal was also equal to that of Ce-doped one. Additionally, the result indicates that the crystalline quality of calcium metaborate is just equal to that of the previously studied calcium orthoborate grown by μ-PD method, and it had a relatively low crystallinity comparable to crystal grown by Czochralski method.8

Figure 4. Optical transmittance spectra of undoped and 0.5% Ce-doped CaB2O4 crystals.

crystal is below 190 nm, while the Ce-doped crystal showed the absorption bands around 270 and 320 nm. We ascribe these absorption bands to the transition from 4f ground state to both 5d (t2g) levels at lower energy and the 5d (e2g) levels at higher energy of Ce3+ because of the octahedral crystal field splitting. The absorption wavelengths are shorter than those for Ce3+:Ca3(BO3)2 crystal,8 because the crystal field is weaker with respect to that of Ca3(BO3)2. In addition, weak absorption bands appeared around 200−300 nm. They may be ascribed to retrapping related to some lattice defect. The defects in this material can be for example the oxygen vacancies (acting as 143



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detector and α-ray source. The crystals were placed directly on the surface of the sealed source, and the scintillation photons from the crystals are detected by the spectrometer. The data were corrected for the instrumental distortions. Figure 7 illustrates the α-ray

electron traps) or calcium vacancies (acting as hole traps), as was recently shown in Yb-doped calcium tetraborate.13 It was suggested that for two Yb3+ ions one Ca2+ vacancy is needed for charge compensation. To examine the luminescence properties of Ce3+ in CaB2O4, we evaluated the excitation and emission spectra using FLS920 spectrometer (Edinburgh Instruments Ltd.) equipped with a Xe-arc lamp as a light source. The spectra were corrected for the instrumental distortions using correction curve. Figure 5

Figure 7. Radioluminescence spectra of undoped and 0.5% Ce-doped CaB2O4 under 241Am α-ray excitation.

excited radioluminescence spectrum of the Ce-doped crystal compared with that of the undoped one. The intense luminescence peak due to the 5d (t2g)-4f transition of Ce3+ was observed for Ce-doped crystal at 375 nm. Meanwhile, the undoped crystal showed broad intrinsic emission peak at 300−400 nm corresponding to the host crystal luminescence, the origin of which is not clear so far and needs further study. It can be related to some kind of self-trapped or trapped exciton emission or defect emission. Moreover, we can also observe weak emission band around 300 nm both for the Ce-doped crystal. It is ascribed to the intrinsic emission distorted by the Ce3+ absorption transitions from the 4f ground state to the t2g and e2g levels.10 The above interpretations are also supported in the decay time profiles using 512 data points taken from the oscilloscope (Tektronix TDS3034B) and a photomultiplier tube (PMT, R7600U), as shown in Figure 8 and Table 1. Using an exponential

Figure 5. Excitation and emission spectra of 0.5% Ce-doped CaB2O4 crystals (λem = 375 nm, λex = 320 nm).

shows the results. In the excitation spectrum, two peaks were observed around 270 and 320 nm for the 375 nm emissions and which are ascribed to the transition between ground 4f state and excited 5d state of Ce3+ and corresponds to the absorption peaks found at the same wavelengths in the transmittance spectrum. The crystal showed a strong emission peak at 370 nm upon excitation at 320 nm. This ultraviolet luminescence was assigned to the transition from the Ce3+ 5d (t2g) state to the 4f ground state levels 2F5/2 and 2F7/2. Additionally, we also evaluated decay time (λex = 320 nm, λem = 370 nm) using the similar spectrometer and nanosecond-flash lamp, as presented in Figure 6. The calculated decay time is obtained

Figure 8. 241Am α-ray excited scintillation decay time profiles of undoped and 0.5% Ce-doped CaB2O4 crystals.

Figure 6. Decay curve of 0.5% Ce-doped CaB2O4 crystal (λem = 375 nm, λex = 320 nm).

with the exponential function fitting and its deconvolution with the instrumental response. The calculated value of main decay component was found to be about 18 ns.

Table 1. Characteristic Components of Scintillation Decay Curves of Undoped and 0.5% Ce-Doped CaB2O4 Crystals

4. SCINTILLATION CHARACTERISTICS According to the emission properties of Ce3+ under photoexcitation, we evaluated the α-ray response which replaces for the 10B(n,α)7Li reaction. In the measurement, the same FLS920 spectrometer and the 241Am sealed radiation source were used as a

sample

decay τ1 [ns]

decay τ2 [ns]

undoped CaB2O4 Ce (0.5%)/CaB2O4

388 62

6115 2916

decay fit (not taking into account the spike peaks at rising edges which are the result of the instrumental error), we found that the decay time is characterized by two components for the Ce3+-doped 144



Figure 9. Schematic representation for setup of the

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252

Cf irradiated pulse height spectra measurement.

crystal. The fast component (∼62 ns) is ascribed to 5d (t2g)-4f transition of Ce3+, while the slow one (∼2906 ns) may be due to the intrinsic emission or retrapping related to some lattice defect. The prolongation of the decay with respect to the photoluminescence one may be caused by the mentioned retrapping of the charge carriers at the defect-related traps during their transport to the Ce3+ luminescence center. The defects may relate to the lattice distortion and charge compensation because the Ce3+ ion resides at the Ca2+ site. They most probably play a role in the above-mentioned host luminescence process.

5. NEUTRON RESPONSE

Figure 10. 252Cf neutron irradiated pulse height spectra of undoped (triangle) and 0.5% Ce-doped (square) CaB 2O4 single crystal compared with GS20 (circle).

To estimate the absolute light yield and consider the capability of the scintillation material for neutron detection, pulse height spectra of undoped and Ce-doped crystals under neutron radiation from a 252Cf sealed source were recorded with a PMT which is connected to a preamplifier (ORTEC 113), shaping amplifier (ORTEC 572), multichannel analyzer (Amptec 8000A), and finally an analytical PC, as illustrated in Figure 9. In these experiments, the crystals were optically coupled onto the PMT window with optical silicon grease (OKEN 6262A). The crystals were covered with a UV reflecting Teflon tape to minimize the light losses. 252Cf neutrons were thermalized by 4.3 cm polystyrene blocks before reaching the crystal. Pb blocks of 50 mm thickness are set between the crystals and the neutron source to reduce gamma-ray background noise also partially originating from the 252Cf source. The GS20 6Li-loaded glass scintillator was also measured together with our crystals as a reference sample. The shaping time was 0.5 μs for GS20 and 10 μs for our crystals as an optimum values, respectively. The absolute light yield which is expressed in photons per neutron (ph/n) was determined by comparing the determined peak channel and quantum efficiency (QE) of PMT with that of GS20 (6000 ph/n).14 In the pulse height spectra, the thermal neutron peak was clearly detected for both crystals. Figure 10 shows the result, where the thermal neutron peaks for the undoped and the Cedoped crystals appeared at 105 and 70 channel,respectively, while that oft he GS20 was observed at 210 channel. The QE at the respective emission wavelengths are about 45% for all the crystals. Thus the light yield of the undoped and the Ce-doped crystals was calculated to be about 3200 and 2200 ph/n, respectively. From this result it follows that the intrinsic emission is dominant in the scintilation process of CaB2O4 crystal since the light yield of the Ce-doped crystal was lower than that of the undoped one most probably because of the

energy losses during the migration from intrinsic trap levels to the 5d excited state of Ce3+.

6. CONCLUSIONS Single crystals of the undoped and the 0.5% Ce-doped CaB2O4 were grown successfully by the μ-PD technique. The grown crystals showed a plane cleavage due to the (B2O4)n2n− layers. From the XRD patterns, no impurity and secondary phases were detected for both the undoped and the Ce-doped crystals. The XRC fwhm values of the polished crystal was about 95 arcsec for (131) reflection. In the transmittance measurements, the undoped crystal showed no absorption band in the 190− 900 nm wavelength range, while the absorption bands due to the Ce3+ 4f-5d transition were observed at 270 and 320 nm for Ce-doped crystal. An intense luminescence was observed under α-ray excitation for Ce-doped crystal. This emission corresponds to the transition from the excited 5d (t2 g) state to the 4f ground states 2F5/2 and 2F7/2 of the Ce3+ ion. In addition, a distorted intrinsic emission influenced by the 4f−5d transition of Ce3+ ion was observed around 300 nm. The decay time is characterized by fast component for Ce3+ 5d (t2g)−4f transition and slow one for the intrinsic emission which can be related to some lattice defect or exciton. The absolute light yield of the undoped and Ce-doped crystals was determined to be about 3200 and 2200 ph/n from the 252Cf neutron-excited pulseheight spectra. In this study, we succeeded in developing of new borate scintilaltors for neutron detection which indicate higher light yield comparing with other borate in such Ce:Ca3(BO3)2 (600−700 ph/n), SrB2O 4 (1500 ph/n) and Ce: SrB 2O 4 (1000 ph/n) crystals. 8,10 145



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Characterizations of Ce3+-Doped CaB2O4 Crystalline Scintillator

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