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Comparative study of dosimeter properties of Eu-doped CsBr transparent ceramic and single crystal
Optik 157 (2018) 421–428
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Original research article
Comparative study of dosimeter properties of Eu-doped CsBr transparent ceramic and single crystal Hiromi Kimura ∗ , Fumiya Nakamura, Takumi Kato, Daisuke Nakauchi, Naoki Kawano, Go Okada, Noriaki Kawaguchi, Takayuki Yanagida Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama-cho, Ikoma-shi, Nara 630-0192, Japan
a r t i c l e
i n f o
Article history: Received 6 October 2017 Accepted 19 November 2017 Keywords: Transparent ceramic CsBr Eu OSL TSL
a b s t r a c t We have synthesized Eu-doped CsBr transparent ceramics by spark plasma sintering (SPS) and a single crystal by the vertical Bridgman-Stockbarger method. Subsequently, we have investigated their optical, scintillation and dosimeter properties. In scintillation, both the materials showed a broad emission peaking around 440 nm, and the origin was due to 4f6 5d1 -4f7 transitions of Eu2+ . In the optically-stimulated luminescence (OSL) properties, OSL intensity of the ceramic sample was higher than those of the crystal sample. The dosimetric sensitivities were confirmed as low as 0.01 mGy using TSL while 0.1 mGy using OSL for both of the ceramic and single crystal samples. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction Luminescence materials are often used for ionizing radiation detectors, which are mainly classified into two types. One is scintillators that have a function to convert absorbed energy of ionizing radiation such as X- and ␥-rays into low energy photons immediately. Scintillators have been used in various fields, for examples in medicine [1], security [2] and high energy physics [3]. On the other hand, dosimeters with storage phosphors have a function to record the radiation dose. The absorbed energy is stored in carrier trapping centers. The trapped charges can be released by stimulation of heat or light to emit photons. The resultant emission by the stimulation of heat and light are called thermally-stimulated luminescence (TSL) and optically-stimulated luminescence (OSL), respectively. Dosimeters have been utilized in individual radiation monitoring devices [4] and imaging plates (IPs) [5,6]. So far, phosphors utilizing OSL have been proposed for IPs to achieve high resolution in digital radiography [7]. The OSL materials are required to have efficient X-ray absorption, short lifetime (∼10 s), high luminescence output and linear dose dependence [8]. Since the late 1990s, Eu-doped CsBr has attracted much attention as IPs [9–13] because the Eu-doped CsBr has outstanding properties mentioned above. In the CsBr structure, Eu2+ replaces Cs+ ion, and Eu2+ -VCs isolated dipole centers (IDC) are formed in order to compensate the charge imbalance. The Eu2+ in the IDC shows an emission peak around 440 nm [14]. The emission can be observed as OSL under He-Ne laser (633 nm) stimulation in practice. Thus, luminescence properties of the Eu-doped CsBr have been studied intensively. However, almost all of the studies were done in a form of bulk single crystal [15], bulk opaque ceramics [16] and film by a vacuum deposition technique [17]. There are only a few reports available on luminescence properties of Eu-doped CsBr transparent ceramics [18]. We have previously reported that
∗ Corresponding author. E-mail address: [email protected] (H. Kimura). https://doi.org/10.1016/j.ijleo.2017.11.104 0030-4026/© 2017 Elsevier GmbH. All rights reserved.
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some X-ray induced luminescence properties are improved in transparent ceramics compared with those of single crystals in some common phosphor materials [19–21]. In particular, dosimeter properties of transparent ceramics synthesized by spark plasma sintering (SPS) are enhanced [22–24] since the SPS was performed in a highly reductive environment, which effectively generates defect centers. In this paper, we have synthesized Eu-doped CsBr transparent ceramics using spark plasma sintering (SPS) and examined the optical, and dosimeter properties, in comparison with Eu-doped CsBr single crystal prepared by the vertical BridgmanStockbarger method. In addition to these properties, we have also investigated the scintillation properties since scintillation and dosimeter properties are complementarily related in some materials [25–27]. So, investigations of both of the properties are important to understand the luminescence phenomena induced by ionizing radiations comprehensively. 2. Experiment Eu-doped CsBr transparent ceramic was synthesized by the SPS technique using Sinter Land LabX-100 in a vacuum. Raw powder of CsBr (>99.99%, Furutachi Chemical) and EuCl3 ·6H2 O (>99.9%, Furutachi Chemical) were homogeneously mixed to a molar ratio of 0.994: 0.006. The total mass of the mixture was 1.0 g. The mixture was introduced into a cylindrical graphite die, in which the mixture was held between two graphite punches. During the sintering, the temperature was increased from 20 ◦ C to 450 ◦ C at a rate of 45 ◦ C/min and held for 10 min while applying the pressure of 6 MPa. The temperature was measured using a K-thermocouple attached on the graphite die. After the synthesis, the wide surfaces of the ceramic samples were polished by hand using a sandpaper (3000 grit). On the other hand, Eu-doped CsBr single crystal was grown using the vertical Bridgman-Stockbarger method. The starting compounds were the same as that for the transparent ceramics. The mixture powder was first dried by heating at ∼150 ◦ C for 1 h and ∼536 ◦ C for 2 h in a vacuum. Subsequently, the dried mixture powder was enclosed in a vacuum-sealed quartz ampule, and the sealed ampule was set in a Bridgman furnace (VFK-1800, Crystal Systems Corp.). During the crystal growth, the heater was set to 686 ◦ C, and the ampule was translated downwards at a rate of 1 cm/h. The obtained crystal rod was cut into a rectangle form and then mechanically polished the surfaces by a sandpaper (3000 grit). The sizes of the single crystal and ceramic samples were comparable. The following measurements were carried out by the same manner for each sample. The in-line transmittance spectra were evaluated by using a spectrometer (V670, JASCO) in the spectral range from 200 to 2700 nm with 1 nm intervals. The PL emission and excitation spectra as well as PL quantum yields were measured using a Quantaurus-QY (C11347, Hamamatsu Photonics). The PL decay curves were evaluated with a Hamamatsu Quantaurus- (C11367-04, Hamamatsu). To investigate scintillation properties, X-ray induced scintillation spectra were measured by using our original setup. The radiation source was a conventional X-ray tube operated with a tube voltage and current of 40 kV and 1.2 mA, respectively. The scintillation photons were collected and then guided to a spectrometer (Andor DU-420-BU2 CCD and Shamrock 163 monochromator). The detail of the setup is described elsewhere [28]. Moreover, X-ray induced scintillation decay and afterglow profiles were evaluated by using an afterglow characterization system equipped with a pulse X-ray tube [29]. TSL glow curves were measured using a TSL reader (TL-2000, Nanogray Inc.) [30]. The heating rate was 1 ◦ C/s, and the measurement temperature range was from 50 to 350 ◦ C. To obtain a dose response function, TSL glow curves were measured with different irradiation doses (0.01–3 mGy). The response signal was defined here as an integrated signal from 50 to 350 ◦ C. To measure TSL spectra, the samples were irradiated with X-rays (∼10 Gy) and then heated at a specific temperature on a ceramic heater system (SCR-SHQ-A, Sakaguchi) while measuring the signal using a spectrometer (QEPro, Ocean Optics). OSL spectra were measured after 1 mGy X-ray irradiation by using a spectrofluorometer (FP-8600, JASCO). The stimulation wavelength was 630 nm. In addition, the sample was irradiated by X-rays of a certain dose ranging from 0.01 mGy to 10 mGy and the dose response curves were obtained. 3. Results and discussion 3.1. Sample Fig. 1 shows a photograph of Eu-doped CsBr single crystal and ceramic samples. The thicknesses were 1.03 and 1.06 mm, respectively. It was confirmed that the stripe patterns on the back of the samples were clearly seen through the samples. Although the crystal sample was colorless, the ceramic sample looked brown. The reason is possibly due to a generation of color centers or the difference of the actual Eu concentrations. In-line transmittance spectra of the samples are indicated in Fig. 2. The transmittance of the ceramic sample showed strong absorption at wavelengths shorter than 400 nm. The origin would be typical absorption due to the 4f7 -4f6 5d1 transitions of Eu2+ [31]. However, the single crystal sample did not show such strong absorption in the same range in spite of the same nominal Eu concentration. 3.2. Optical properties Fig. 3 shows PL excitation/emission contour graphs of Eu-doped CsBr transparent ceramic and single crystal samples. The ceramic sample showed a broad emission band peaking around 440 nm under an excitation around 350 nm. The emission origin was the 4f6 5d1 -4f7 transitions of Eu2+ attributed by the former work [15]. In contract, the single crystal sample presented a broad emission band peaking around 370 nm and sharp emissions peaking at 630 and 700 nm under an excitation
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Fig. 1. Synthesized Eu-doped CsBr transparent ceramic (left) and single crystal (right) samples.
Fig. 2. Transmittance spectra of Eu-doped CsBr transparent ceramic and single crystal samples.
Fig. 3. PL excitation/emission map of Eu-doped CsBr transparent ceramic (top) and single crystal (bottom) samples. The horizontal and vertical axes represent excitation and emission wavelengths, respectively.
around 280 nm. The origins of those emission are attributed to oxygen impurities which was observed in undoped CsBr [32], 5 D → 7 F and 5 D → 7 F transitions of Eu3+ [33], respectively. The quantum yields of the ceramic and single crystal samples 0 2 0 4 were 1.6% and 8.6%, respectively. Fig. 4 shows PL decay profiles of the Eu-doped CsBr transparent ceramic and single crystal samples. In the single crystal sample, the decay profiles were measured at two different monitoring wavelengths. First, the decay curve monitored at 370 nm under 280 nm excitation was well-approximated by a sum of two exponential decay functions. The derived decay
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Fig. 4. PL decay profiles of Eu-doped CsBr samples. The top graph shows the decay profile of the ceramic sample monitoring at 440 nm under 340 nm excitation. The middle graph shows the decay profile of the single crystal sample monitoring at 370 nm under 280 excitation. The bottom graph shows the decay profile of the single crystal sample monitoring at 630 nm under 265 excitation.
Fig. 5. X-ray induced scintillation spectra of the Eu-doped CsBr transparent ceramic and single crystal samples.
constants were 0.15 s (90.0%) and 0.41 s (10.0%), which were approximately consistent with the emission due to oxygen impurities [34]. Second, the decay curve monitored at 630 nm under 265 nm excitation was also well-approximated by a sum of two exponential decay functions. The derived decay constants were 18.7 s and 0.21 ms. The faster decay component was attributed to the equipment, and the slower decay component was in a typical time range of 4f-4f transition Eu3+ [33]. In the ceramic sample, the decay curves were well-approximated by a sum of two exponential decay functions. The monitored wavelength was 440 nm, and the excitation wavelength was 340 nm. The derived decay constants were 0.14 s (69.4%) and 0.59 s (30.6%), which were approximately consistent with the decay time constants due to the 4f6 5d1 -4f7 transitions of Eu2+ in CsBr [14]. The only ceramic sample showed the emission due to the 4f6 5d1 -4f7 transitions of Eu2+ because the ceramic sample was synthesized by SPS in a highly reductive environment. In addition, the emission due to oxygen impurities was not observed in the ceramic sample since it had a strong absorption in the wavelength shorter than 400 nm. 3.3. Scintillation properties X-ray induced scintillation spectra of Eu-doped CsBr transparent ceramic and single crystal samples are presented in Fig. 5. The ceramic sample showed an emission band peaking around 440 nm which was consistent with the feature observed in PL, and the emission origin was attributed to the 4f6 5d1 -4f7 transitions of Eu2+ [15]. In the single crystal sample, two emission bands were observed around 400 and 440 nm. The emission band around 400 nm was due to oxygen impurities as for the PL. The origin of emission around 440 nm was due to the 4f6 5d1 -4f7 transitions of Eu2+ [15] although it was not observed in the PL. The difference in PL and scintillation spectra are ascribed to the difference of the energy transportation processes. In PL, we observed luminescence by excitation and relaxation of selective luminescent center while, in scintillation, the excitation energy was so large that it typically induces luminescence of all possible centers in material.
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Fig. 6. Scintillation decay profiles of Eu-doped CsBr transparent ceramics (top) and single crystal (bottom) samples.
Fig. 7. Afterglow profiles of Eu-doped CsBr transparent ceramic and single crystal samples.
Fig. 6 shows X-ray induced scintillation decay time profiles of the Eu-doped CsBr transparent ceramic and single crystal samples. The decay curves were well-approximated by a sum of two exponential decay functions. The faster decay components of the ceramic and single crystal samples were 0.64 and 0.62 s, respectively. Those fast emission components are attributed to Eu2+ . The slower decay components of the ceramic and single crystal samples were 5.63 and 6.96 s, respectively. The origin of those decay time constants are attributed to oxygen impurities from the past results on undoped CsBr [34]. The afterglow curves of Eu-doped CsBr transparent ceramic and single crystal samples are represented in Fig. 7. The derived afterglow levels of the ceramic and single crystal samples were 0.76 and 0.17%, respectively. Here, the afterglow levels (A) is defined as A (%) = 100 × (I2 − IBG )/(I1 − IBG ) where IBG is signal intensity at 2 ms after the irradiation was cut off. The afterglow level of the ceramic sample was larger than that of the single crystal sample. It was suggested that the ceramic sample had higher probability of charge trapping at shallow centers than that of the single crystal. The carriers trapped in shallow trapping centers could be released at room temperature, so the trapping-and-detrapping processes by the stimulation of room temperature were observed as a slow emission. 3.4. Dosimeter properties Fig. 8 represents TSL glow curves of Eu-doped CsBr transparent ceramic and single crystal samples irradiated with Xrays (1 mGy). The ceramic and single crystal samples exhibited primary glow peaks around 160 ◦ C and 180 ◦ C, respectively, which was consistent with the past report [35]. Notice that the glow signal is present even close to room temperature, and the intensity is higher for the ceramic. This is consistent with the result of the afterglow levels (shown in Fig. 7) since the intensity of afterglow corresponds with intensity of TSL at room temperature. The inset shows TSL dose response functions of the Eu-doped CsBr transparent ceramic and single crystal samples. The TSL intensity represents integrated signal over the entre temperature range of measurement. Both the samples showed a good linearity from 0.01 mGy to 3 mGy. However, the sensitivity to low dose is not sufficient by approximately one order of magnitude compared with a commercial TSL dosimeter [36]. Fig. 9 shows TSL spectra of the Eu-doped CsBr transparent ceramic and single crystal samples measured at 110 ◦ C. The samples were irradiated with X-rays (∼10 Gy) before the measurements. The spectral features are very similar to those of scintillation spectra; therefore, the emission origins of TSL should be the same. Fig. 10 shows OSL spectra of the Eu-doped CsBr transparent ceramic and single crystal samples. The optical stimulation wavelength was 630 nm. The samples were irradiated by X-rays (∼10 Gy) prior to the measurement. The inset indicates the relation between the OSL intensity and irradiated X-ray dose (OSL dose response function). The dose response curves of the
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Fig. 8. TSL glow curves of Eu-doped CsBr transparent ceramic and single crystal samples measured after 1 mGy X-ray irradiation. The inset shows TSL dose response curves of Eu-doped CsBr transparent ceramic and single crystal samples from 0.01 mGy to 3 mGy.
Fig. 9. TSL emission spectra of Eu-doped CsBr transparent ceramic and single crystal samples measured at 110 ◦ C after X-ray irradiation (∼10 Gy).
Fig. 10. OSL emission spectra of Eu-doped CsBr transparent ceramic and single crystal samples measured under 630 nm stimulation. Prior to the measurement, the samples were irradiated by X-rays (∼10 Gy). The inset shows OSL dose response curves of the Eu-doped CsBr transparent ceramic and single crystal samples.
ceramic and single crystal samples showed a good linearity from 0.1 mGy to 100 mGy. The intensity of the ceramic sample was significantly higher than that of the single crystal sample by a factor of approximately 8 after 100 mGy X-ray irradiation unlike TSL. It was suggested that the ceramic sample included larger number of trapping centers at deep levels than those of the single crystal sample, and the trapping centers could have been generated during the SPS process since this synthesis technique was highly reductive. The OSL spectra of both samples approximately agree with a previous report [12]. Although TSL and OSL intensities are qualitative values, the ceramic shows lower TSL but higher OSL intensities compared with the crystal. The difference of synthesis routes and material forms cause such a tendency. If ones would like to develop Eu-doped CsBr for OSL, ceramics would be a better choice. For TSL dosimeter, single crystal should be used. These results suggest that the defects created by different synthesis routes differ even if the materials are the same. Although it looks like common sense, the clear experimental evidence is not many. 4. Conclusion We have synthesized Eu-doped CsBr transparent ceramic by SPS and Eu-doped CsBr single crystal by the vertical Bridgman-Stockbarger method. Subsequently, we have investigated the optical, scintillation and dosimeter properties. As
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the optical properties, the samples showed different emissions. The ceramic sample showed a broad emission band peaking around 440 nm under an excitation around 350 nm. The origin was attributed to the 4f6 5d1 -4f7 transitions of Eu2+ . In contrast, the single crystal sample presented a broad emission band peaking around 370 nm and a sharp emission peaking at 630 and 700 nm under an excitation around 280 nm. The origin was attributed to oxygen impurities, 5 D0 → 7 F2 and 5 D0 → 7 F4 transitions of Eu3+ , respectively. Despite the differences in PL, both of the samples showed scintillation with a broad emission peaking around 440 nm due to the 4f6 5d1 → 4f7 transitions of Eu2+ . Both the samples showed a good linearity of TSL response from 0.01 mGy to 3 mGy. In addition, the samples also showed OSL peaking around 450 nm, due to the 4f6 5d1 -4f7 transitions of Eu2+ , by a stimulation of 630 nm. 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Original research article
Comparative study of dosimeter properties of Eu-doped CsBr transparent ceramic and single crystal Hiromi Kimura ∗ , Fumiya Nakamura, Takumi Kato, Daisuke Nakauchi, Naoki Kawano, Go Okada, Noriaki Kawaguchi, Takayuki Yanagida Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama-cho, Ikoma-shi, Nara 630-0192, Japan
a r t i c l e
i n f o
Article history: Received 6 October 2017 Accepted 19 November 2017 Keywords: Transparent ceramic CsBr Eu OSL TSL
a b s t r a c t We have synthesized Eu-doped CsBr transparent ceramics by spark plasma sintering (SPS) and a single crystal by the vertical Bridgman-Stockbarger method. Subsequently, we have investigated their optical, scintillation and dosimeter properties. In scintillation, both the materials showed a broad emission peaking around 440 nm, and the origin was due to 4f6 5d1 -4f7 transitions of Eu2+ . In the optically-stimulated luminescence (OSL) properties, OSL intensity of the ceramic sample was higher than those of the crystal sample. The dosimetric sensitivities were confirmed as low as 0.01 mGy using TSL while 0.1 mGy using OSL for both of the ceramic and single crystal samples. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction Luminescence materials are often used for ionizing radiation detectors, which are mainly classified into two types. One is scintillators that have a function to convert absorbed energy of ionizing radiation such as X- and ␥-rays into low energy photons immediately. Scintillators have been used in various fields, for examples in medicine [1], security [2] and high energy physics [3]. On the other hand, dosimeters with storage phosphors have a function to record the radiation dose. The absorbed energy is stored in carrier trapping centers. The trapped charges can be released by stimulation of heat or light to emit photons. The resultant emission by the stimulation of heat and light are called thermally-stimulated luminescence (TSL) and optically-stimulated luminescence (OSL), respectively. Dosimeters have been utilized in individual radiation monitoring devices [4] and imaging plates (IPs) [5,6]. So far, phosphors utilizing OSL have been proposed for IPs to achieve high resolution in digital radiography [7]. The OSL materials are required to have efficient X-ray absorption, short lifetime (∼10 s), high luminescence output and linear dose dependence [8]. Since the late 1990s, Eu-doped CsBr has attracted much attention as IPs [9–13] because the Eu-doped CsBr has outstanding properties mentioned above. In the CsBr structure, Eu2+ replaces Cs+ ion, and Eu2+ -VCs isolated dipole centers (IDC) are formed in order to compensate the charge imbalance. The Eu2+ in the IDC shows an emission peak around 440 nm [14]. The emission can be observed as OSL under He-Ne laser (633 nm) stimulation in practice. Thus, luminescence properties of the Eu-doped CsBr have been studied intensively. However, almost all of the studies were done in a form of bulk single crystal [15], bulk opaque ceramics [16] and film by a vacuum deposition technique [17]. There are only a few reports available on luminescence properties of Eu-doped CsBr transparent ceramics [18]. We have previously reported that
∗ Corresponding author. E-mail address: [email protected] (H. Kimura). https://doi.org/10.1016/j.ijleo.2017.11.104 0030-4026/© 2017 Elsevier GmbH. All rights reserved.
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some X-ray induced luminescence properties are improved in transparent ceramics compared with those of single crystals in some common phosphor materials [19–21]. In particular, dosimeter properties of transparent ceramics synthesized by spark plasma sintering (SPS) are enhanced [22–24] since the SPS was performed in a highly reductive environment, which effectively generates defect centers. In this paper, we have synthesized Eu-doped CsBr transparent ceramics using spark plasma sintering (SPS) and examined the optical, and dosimeter properties, in comparison with Eu-doped CsBr single crystal prepared by the vertical BridgmanStockbarger method. In addition to these properties, we have also investigated the scintillation properties since scintillation and dosimeter properties are complementarily related in some materials [25–27]. So, investigations of both of the properties are important to understand the luminescence phenomena induced by ionizing radiations comprehensively. 2. Experiment Eu-doped CsBr transparent ceramic was synthesized by the SPS technique using Sinter Land LabX-100 in a vacuum. Raw powder of CsBr (>99.99%, Furutachi Chemical) and EuCl3 ·6H2 O (>99.9%, Furutachi Chemical) were homogeneously mixed to a molar ratio of 0.994: 0.006. The total mass of the mixture was 1.0 g. The mixture was introduced into a cylindrical graphite die, in which the mixture was held between two graphite punches. During the sintering, the temperature was increased from 20 ◦ C to 450 ◦ C at a rate of 45 ◦ C/min and held for 10 min while applying the pressure of 6 MPa. The temperature was measured using a K-thermocouple attached on the graphite die. After the synthesis, the wide surfaces of the ceramic samples were polished by hand using a sandpaper (3000 grit). On the other hand, Eu-doped CsBr single crystal was grown using the vertical Bridgman-Stockbarger method. The starting compounds were the same as that for the transparent ceramics. The mixture powder was first dried by heating at ∼150 ◦ C for 1 h and ∼536 ◦ C for 2 h in a vacuum. Subsequently, the dried mixture powder was enclosed in a vacuum-sealed quartz ampule, and the sealed ampule was set in a Bridgman furnace (VFK-1800, Crystal Systems Corp.). During the crystal growth, the heater was set to 686 ◦ C, and the ampule was translated downwards at a rate of 1 cm/h. The obtained crystal rod was cut into a rectangle form and then mechanically polished the surfaces by a sandpaper (3000 grit). The sizes of the single crystal and ceramic samples were comparable. The following measurements were carried out by the same manner for each sample. The in-line transmittance spectra were evaluated by using a spectrometer (V670, JASCO) in the spectral range from 200 to 2700 nm with 1 nm intervals. The PL emission and excitation spectra as well as PL quantum yields were measured using a Quantaurus-QY (C11347, Hamamatsu Photonics). The PL decay curves were evaluated with a Hamamatsu Quantaurus- (C11367-04, Hamamatsu). To investigate scintillation properties, X-ray induced scintillation spectra were measured by using our original setup. The radiation source was a conventional X-ray tube operated with a tube voltage and current of 40 kV and 1.2 mA, respectively. The scintillation photons were collected and then guided to a spectrometer (Andor DU-420-BU2 CCD and Shamrock 163 monochromator). The detail of the setup is described elsewhere [28]. Moreover, X-ray induced scintillation decay and afterglow profiles were evaluated by using an afterglow characterization system equipped with a pulse X-ray tube [29]. TSL glow curves were measured using a TSL reader (TL-2000, Nanogray Inc.) [30]. The heating rate was 1 ◦ C/s, and the measurement temperature range was from 50 to 350 ◦ C. To obtain a dose response function, TSL glow curves were measured with different irradiation doses (0.01–3 mGy). The response signal was defined here as an integrated signal from 50 to 350 ◦ C. To measure TSL spectra, the samples were irradiated with X-rays (∼10 Gy) and then heated at a specific temperature on a ceramic heater system (SCR-SHQ-A, Sakaguchi) while measuring the signal using a spectrometer (QEPro, Ocean Optics). OSL spectra were measured after 1 mGy X-ray irradiation by using a spectrofluorometer (FP-8600, JASCO). The stimulation wavelength was 630 nm. In addition, the sample was irradiated by X-rays of a certain dose ranging from 0.01 mGy to 10 mGy and the dose response curves were obtained. 3. Results and discussion 3.1. Sample Fig. 1 shows a photograph of Eu-doped CsBr single crystal and ceramic samples. The thicknesses were 1.03 and 1.06 mm, respectively. It was confirmed that the stripe patterns on the back of the samples were clearly seen through the samples. Although the crystal sample was colorless, the ceramic sample looked brown. The reason is possibly due to a generation of color centers or the difference of the actual Eu concentrations. In-line transmittance spectra of the samples are indicated in Fig. 2. The transmittance of the ceramic sample showed strong absorption at wavelengths shorter than 400 nm. The origin would be typical absorption due to the 4f7 -4f6 5d1 transitions of Eu2+ [31]. However, the single crystal sample did not show such strong absorption in the same range in spite of the same nominal Eu concentration. 3.2. Optical properties Fig. 3 shows PL excitation/emission contour graphs of Eu-doped CsBr transparent ceramic and single crystal samples. The ceramic sample showed a broad emission band peaking around 440 nm under an excitation around 350 nm. The emission origin was the 4f6 5d1 -4f7 transitions of Eu2+ attributed by the former work [15]. In contract, the single crystal sample presented a broad emission band peaking around 370 nm and sharp emissions peaking at 630 and 700 nm under an excitation
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Fig. 1. Synthesized Eu-doped CsBr transparent ceramic (left) and single crystal (right) samples.
Fig. 2. Transmittance spectra of Eu-doped CsBr transparent ceramic and single crystal samples.
Fig. 3. PL excitation/emission map of Eu-doped CsBr transparent ceramic (top) and single crystal (bottom) samples. The horizontal and vertical axes represent excitation and emission wavelengths, respectively.
around 280 nm. The origins of those emission are attributed to oxygen impurities which was observed in undoped CsBr [32], 5 D → 7 F and 5 D → 7 F transitions of Eu3+ [33], respectively. The quantum yields of the ceramic and single crystal samples 0 2 0 4 were 1.6% and 8.6%, respectively. Fig. 4 shows PL decay profiles of the Eu-doped CsBr transparent ceramic and single crystal samples. In the single crystal sample, the decay profiles were measured at two different monitoring wavelengths. First, the decay curve monitored at 370 nm under 280 nm excitation was well-approximated by a sum of two exponential decay functions. The derived decay
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Fig. 4. PL decay profiles of Eu-doped CsBr samples. The top graph shows the decay profile of the ceramic sample monitoring at 440 nm under 340 nm excitation. The middle graph shows the decay profile of the single crystal sample monitoring at 370 nm under 280 excitation. The bottom graph shows the decay profile of the single crystal sample monitoring at 630 nm under 265 excitation.
Fig. 5. X-ray induced scintillation spectra of the Eu-doped CsBr transparent ceramic and single crystal samples.
constants were 0.15 s (90.0%) and 0.41 s (10.0%), which were approximately consistent with the emission due to oxygen impurities [34]. Second, the decay curve monitored at 630 nm under 265 nm excitation was also well-approximated by a sum of two exponential decay functions. The derived decay constants were 18.7 s and 0.21 ms. The faster decay component was attributed to the equipment, and the slower decay component was in a typical time range of 4f-4f transition Eu3+ [33]. In the ceramic sample, the decay curves were well-approximated by a sum of two exponential decay functions. The monitored wavelength was 440 nm, and the excitation wavelength was 340 nm. The derived decay constants were 0.14 s (69.4%) and 0.59 s (30.6%), which were approximately consistent with the decay time constants due to the 4f6 5d1 -4f7 transitions of Eu2+ in CsBr [14]. The only ceramic sample showed the emission due to the 4f6 5d1 -4f7 transitions of Eu2+ because the ceramic sample was synthesized by SPS in a highly reductive environment. In addition, the emission due to oxygen impurities was not observed in the ceramic sample since it had a strong absorption in the wavelength shorter than 400 nm. 3.3. Scintillation properties X-ray induced scintillation spectra of Eu-doped CsBr transparent ceramic and single crystal samples are presented in Fig. 5. The ceramic sample showed an emission band peaking around 440 nm which was consistent with the feature observed in PL, and the emission origin was attributed to the 4f6 5d1 -4f7 transitions of Eu2+ [15]. In the single crystal sample, two emission bands were observed around 400 and 440 nm. The emission band around 400 nm was due to oxygen impurities as for the PL. The origin of emission around 440 nm was due to the 4f6 5d1 -4f7 transitions of Eu2+ [15] although it was not observed in the PL. The difference in PL and scintillation spectra are ascribed to the difference of the energy transportation processes. In PL, we observed luminescence by excitation and relaxation of selective luminescent center while, in scintillation, the excitation energy was so large that it typically induces luminescence of all possible centers in material.
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Fig. 6. Scintillation decay profiles of Eu-doped CsBr transparent ceramics (top) and single crystal (bottom) samples.
Fig. 7. Afterglow profiles of Eu-doped CsBr transparent ceramic and single crystal samples.
Fig. 6 shows X-ray induced scintillation decay time profiles of the Eu-doped CsBr transparent ceramic and single crystal samples. The decay curves were well-approximated by a sum of two exponential decay functions. The faster decay components of the ceramic and single crystal samples were 0.64 and 0.62 s, respectively. Those fast emission components are attributed to Eu2+ . The slower decay components of the ceramic and single crystal samples were 5.63 and 6.96 s, respectively. The origin of those decay time constants are attributed to oxygen impurities from the past results on undoped CsBr [34]. The afterglow curves of Eu-doped CsBr transparent ceramic and single crystal samples are represented in Fig. 7. The derived afterglow levels of the ceramic and single crystal samples were 0.76 and 0.17%, respectively. Here, the afterglow levels (A) is defined as A (%) = 100 × (I2 − IBG )/(I1 − IBG ) where IBG is signal intensity at 2 ms after the irradiation was cut off. The afterglow level of the ceramic sample was larger than that of the single crystal sample. It was suggested that the ceramic sample had higher probability of charge trapping at shallow centers than that of the single crystal. The carriers trapped in shallow trapping centers could be released at room temperature, so the trapping-and-detrapping processes by the stimulation of room temperature were observed as a slow emission. 3.4. Dosimeter properties Fig. 8 represents TSL glow curves of Eu-doped CsBr transparent ceramic and single crystal samples irradiated with Xrays (1 mGy). The ceramic and single crystal samples exhibited primary glow peaks around 160 ◦ C and 180 ◦ C, respectively, which was consistent with the past report [35]. Notice that the glow signal is present even close to room temperature, and the intensity is higher for the ceramic. This is consistent with the result of the afterglow levels (shown in Fig. 7) since the intensity of afterglow corresponds with intensity of TSL at room temperature. The inset shows TSL dose response functions of the Eu-doped CsBr transparent ceramic and single crystal samples. The TSL intensity represents integrated signal over the entre temperature range of measurement. Both the samples showed a good linearity from 0.01 mGy to 3 mGy. However, the sensitivity to low dose is not sufficient by approximately one order of magnitude compared with a commercial TSL dosimeter [36]. Fig. 9 shows TSL spectra of the Eu-doped CsBr transparent ceramic and single crystal samples measured at 110 ◦ C. The samples were irradiated with X-rays (∼10 Gy) before the measurements. The spectral features are very similar to those of scintillation spectra; therefore, the emission origins of TSL should be the same. Fig. 10 shows OSL spectra of the Eu-doped CsBr transparent ceramic and single crystal samples. The optical stimulation wavelength was 630 nm. The samples were irradiated by X-rays (∼10 Gy) prior to the measurement. The inset indicates the relation between the OSL intensity and irradiated X-ray dose (OSL dose response function). The dose response curves of the
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Fig. 8. TSL glow curves of Eu-doped CsBr transparent ceramic and single crystal samples measured after 1 mGy X-ray irradiation. The inset shows TSL dose response curves of Eu-doped CsBr transparent ceramic and single crystal samples from 0.01 mGy to 3 mGy.
Fig. 9. TSL emission spectra of Eu-doped CsBr transparent ceramic and single crystal samples measured at 110 ◦ C after X-ray irradiation (∼10 Gy).
Fig. 10. OSL emission spectra of Eu-doped CsBr transparent ceramic and single crystal samples measured under 630 nm stimulation. Prior to the measurement, the samples were irradiated by X-rays (∼10 Gy). The inset shows OSL dose response curves of the Eu-doped CsBr transparent ceramic and single crystal samples.
ceramic and single crystal samples showed a good linearity from 0.1 mGy to 100 mGy. The intensity of the ceramic sample was significantly higher than that of the single crystal sample by a factor of approximately 8 after 100 mGy X-ray irradiation unlike TSL. It was suggested that the ceramic sample included larger number of trapping centers at deep levels than those of the single crystal sample, and the trapping centers could have been generated during the SPS process since this synthesis technique was highly reductive. The OSL spectra of both samples approximately agree with a previous report [12]. Although TSL and OSL intensities are qualitative values, the ceramic shows lower TSL but higher OSL intensities compared with the crystal. The difference of synthesis routes and material forms cause such a tendency. If ones would like to develop Eu-doped CsBr for OSL, ceramics would be a better choice. For TSL dosimeter, single crystal should be used. These results suggest that the defects created by different synthesis routes differ even if the materials are the same. Although it looks like common sense, the clear experimental evidence is not many. 4. Conclusion We have synthesized Eu-doped CsBr transparent ceramic by SPS and Eu-doped CsBr single crystal by the vertical Bridgman-Stockbarger method. Subsequently, we have investigated the optical, scintillation and dosimeter properties. As
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the optical properties, the samples showed different emissions. The ceramic sample showed a broad emission band peaking around 440 nm under an excitation around 350 nm. The origin was attributed to the 4f6 5d1 -4f7 transitions of Eu2+ . In contrast, the single crystal sample presented a broad emission band peaking around 370 nm and a sharp emission peaking at 630 and 700 nm under an excitation around 280 nm. The origin was attributed to oxygen impurities, 5 D0 → 7 F2 and 5 D0 → 7 F4 transitions of Eu3+ , respectively. Despite the differences in PL, both of the samples showed scintillation with a broad emission peaking around 440 nm due to the 4f6 5d1 → 4f7 transitions of Eu2+ . Both the samples showed a good linearity of TSL response from 0.01 mGy to 3 mGy. In addition, the samples also showed OSL peaking around 450 nm, due to the 4f6 5d1 -4f7 transitions of Eu2+ , by a stimulation of 630 nm. 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