Research Achievements - Original paper -

Radioluminescence and photoluminescence properties of Dy-doped 12CaO 7Al<inf>2</inf>O<inf>3</inf> single crystals synthesized by the floating zone method

Narumi Kumamoto, Daisuke Nakauchi, Takumi Kato, Naoki Kawano, Go Okada, Noriaki Kawaguchi, Takayuki Yanagida

Japanese Journal of Applied Physics   57 ( 2 )   2018.02  [Refereed]

Research paper (journal)   Domestic Co-author

© 2018 The Japan Society of Applied Physics. We have synthesized Dy-doped 12CaO 7Al2O3 (Dy:C12A7) single crystals with various Dy concentrations by the floating zone (FZ) method and investigated the photoluminescence (PL) and X-ray induced radioluminescence (RL) properties. The PL emissions are observed around 480-490, 570-590, 650-690, and 750 nm due to the 4f-4f transitions of Dy3+ under excitation around 350-400 and 450 nm. The decay time constant (580 nm emission under 340-390 nm excitation) was approximately 0.54-0.58 ms. The RL also showed line emissions at 487, 580, 668, and 757 nm due to the 4f-4f transitions of Dy3+ while the decay time was approximately 0.49-0.53 ms.

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Radiation-induced luminescence properties of Tb-doped Li<inf>3</inf>PO<inf>4</inf>-B<inf>2</inf>O<inf>3</inf> glasses

Yuya Isokawa, Shotaro Hirano, Naoki Kawano, Go Okada, Noriaki Kawaguchi, Takayuki Yanagida

Optical Materials ( Elsevier B.V. )  76   28 - 33   2018.02  [Refereed]

Research paper (journal)   Domestic Co-author

© 2017 Elsevier B.V. In this study, we developed Li3PO4-B2O3 glasses doped with different concentrations of Tb (0.1, 0.3, 1.0, 3.0, and 10.0%) as well as undoped glass, and then the prepared glasses were studied for the optical, dosimeter and scintillator properties. The Tb-doped samples indicated radioluminescence and photoluminescence (PL) due to the 4f-4f transitions of Tb3+ with sharp spectral features peaking around 375, 410, 435, 480, 540, 590 and 620 nm. The luminescence decay times of radioluminescence and PL were 2.3–2.7 ms and 2.7–2.9 ms, respectively. The shorter radioluminescence decay time than that of PL indicated quenching effect of excited states in radioluminescence. As the concentration of Tb increased, both the radioluminescence intensity and PL quantum yield (QY) increased, and the 10.0% Tb-doped sample showed the highest radioluminescence intensity and QY (54.3%). In addition, thermally-stimulated luminescence (TSL) was observed after irradiating with X-rays. The sensitivity was the highest for the 3.0% Tb-doped sample having a dynamic range from 0.1 mGy to 10 Gy, which was equivalent to commercial dosimeters. The comprehensive studies suggested that X-ray generated charges are captured at TSL-active centers more effectively at lower concentrations of Tb whereas the recombination probability at Tb center during irradiation increases with the concentration of Tb. Consequently, the optimal Tb concentration was 10% as scintillator and 3.0% for TSL dosimeter, among the present samples.

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Photoluminescence, scintillation and TSL properties of Ce-doped Sr(PO<inf>3</inf>)<inf>2</inf> glasses

Hayata Samizo, Takumi Kato, Naoki Kawano, Go Okada, Noriaki Kawaguchi, Takayuki Yanagida

Journal of Materials Science: Materials in Electronics ( Springer New York LLC )  29 ( 3 ) 1985 - 1991   2018.02  [Refereed]

Research paper (journal)   Domestic Co-author

© 2017, Springer Science+Business Media, LLC. In this study, we have synthesized Sr(PO3)2 glasses non-doped and doped with Ce (0.01, 0.05, 0.1, 1.0 and 3.0 mol%) prepared by the conventional melt-quenching method and investigated the optical properties [transmittance spectra, photoluminescence (PL) spectra, PL excitation spectra and PL decay times], scintillation properties with X-rays (scintillation spectra, scintillation decay times) and thermally-stimulated luminescence (TSL) properties (glow curves and dose response functions). The PL spectrum of the Ce-doped samples consist of an intense emission band due to the 5d–4f transitions of Ce3+ ion peaking at 350 nm. The decay time of the corresponding emission was 32–37 ns depending on the Ce concentration. The samples also showed scintillation with X-ray having a spectrum of an intense emission band peaking at 350 nm due to the Ce3+ ions, and decay time was 34–217 ns. The TSL glow peaks were observed at 190 and 450 °C for the Ce-doped glasses, and the dynamic range was confirmed at least over a three orders of magnitude 100–103 mGy.

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Optical and radioluminescence properties of Ce-doped Sr<inf>2</inf>RE<inf>8</inf>(SiO<inf>4</inf>)<inf>6</inf>O<inf>2</inf> (RE = Y, Gd, Lu)

Takuya Igashira, Naoki Kawano, Go Okada, Noriaki Kawaguchi, Takayuki Yanagida

Optik ( Elsevier GmbH )  155   36 - 42   2018.02  [Refereed]

Research paper (journal)   Domestic Co-author

© 2017 Elsevier GmbH Ce-doped Sr2Y8(SiO4)6O2, Sr2Gd8(SiO4)6O2 and Sr2Lu8(SiO4)6O2 (SrYS, SrGS and SrLuS, respectively) apatite crystals were grown by the Floating Zone method, and their optical and radioluminescence (RL) properties were characterized. In the photoluminescence (PL) and RL spectra, we observed an emission band between 400 and 500 nm for all the samples, which were due to the 5d-4f transition of Ce3+. The RL decay time profiles were approximated by a sum of two exponential decay functions, and the origin of faster decay component was due to the 5d-4f transition of Ce3+. Among all the samples, SrGS sample showed the lowest afterglow level. In the pulse height spectrum, all the samples showed a full energy absorption peak under 241Am 5.5 MeV α-ray irradiation. The absolute scintillation light yields of SrYS, SrGS and SrLuS were around 31, 76 and 940 ph/5.5 MeV-α respectively.

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Luminescence properties of organic-inorganic layered perovskite-type compounds under vacuum ultraviolet irradiation

Naoki Kawano, Masanori Koshimizu, Go Okada, Yutaka Fujimoto, Noriaki Kawaguchi, Takayuki Yanagida, Keisuke Asai

Japanese Journal of Applied Physics ( Japan Society of Applied Physics )  57 ( 2 )   2018.02  [Refereed]

Research paper (journal)   Domestic Co-author

© 2018 The Japan Society of Applied Physics. We investigated the luminescence properties of organic-inorganic layered perovskite-type compounds under vacuum ultraviolet irradiation. A crystal of (C6H5C2H4NH3)2PbBr4 was fabricated by the poor-solvent diffusion method. Exciton emissions from the inorganic layer were observed at 410 nm under ultraviolet irradiation (excitation wavelengths: 180 and 300 nm). The rise time behavior observed in the luminescence decay curve showed no difference among the excitation wavelengths of 60-300 nm. In addition, no excitation peak of benzene such as an intense peak at 180 nm (1A1g → 1E1u) in the vacuum ultraviolet region was observed in the excitation spectra measured while monitoring the exciton emissions from the inorganic layer. These results indicate that the effect of energy transfer from the organic layer to the inorganic layer has negligible contribution to the luminescence properties of organic-inorganic layered perovskite-type compounds.

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Effects of Ga substitution in Ce:Tb<inf>3</inf>Ga <inf>x</inf> Al<inf>5-</inf><inf>x</inf> O<inf>12</inf> single crystals for scintillator applications

Daisuke Nakauchi, Go Okada, Naoki Kawano, Noriaki Kawaguchi, Takayuki Yanagida

Japanese Journal of Applied Physics   57 ( 2 )   2018.02  [Refereed]

Research paper (journal)   Domestic Co-author

© 2018 The Japan Society of Applied Physics. Bulk single crystals of Ce-doped Tb3Ga x Al5-x O12 (x = 0-4) were successfully synthesized by the floating zone method. The samples exhibit photoluminescence and scintillation with an intense broad emission due to the 5d-4f transitions of Ce3+ peaking around 550 nm as well as a few sharp peaks due to the 4f-4f transitions of Tb3+. Pulse height spectrum measurements under 137Cs γ-ray irradiation demonstrated a clear photoabsorption peak, in which the scintillation light yields were estimated to be 57,000 (x = 0), 28,000 (x = 1), 19,000 (x = 2), and 10,000 (x = 3) photons/MeV. Afterglow level can be suppressed with an appropriate addition of Ga, in which the optimum concertation is x = 2 leading an afterglow level of 23 ppm.

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Radio-photoluminescence in non-doped K<inf>2</inf>CO<inf>3</inf> ceramics

Fumiya Nakamura, Takumi Kato, Go Okada, Naoki Kawano, Noriaki Kawaguchi, Takayuki Yanagida

Materials Letters ( ELSEVIER SCIENCE BV )  211   100 - 102   2018.01  [Refereed]

Research paper (journal)   Domestic Co-author

© 2017 Elsevier B.V. In this letter, we report a discovery of a new radio-photoluminescence (RPL) material. The RPL appeared in vacuum-sintered non-doped K2CO3 ceramics as an evolution of PL emission band peaking at 440 nm. The RPL signal was confirmed to be sensitive to X-rays and stable at room temperature. The RPL response was observed as low as 1 mGy although the sample was not optimized. We strongly believe that this material should be a promising candidate as RPL dosimeter materials.

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Scintillating organic-inorganic layered perovskite-Type compounds and the gamma-ray detection capabilities

Naoki Kawano, Masanori Koshimizu, Go Okada, Yutaka Fujimoto, Noriaki Kawaguchi, Takayuki Yanagida, Keisuke Asai

Scientific Reports ( NATURE PUBLISHING GROUP )  7 ( 1 )   2017.12  [Refereed]

Research paper (journal)   Domestic Co-author

© 2017 The Author(s). We investigated scintillation properties of organic-inorganic layered perovskite-Type compounds under gamma-ray and X-ray irradiation. A crystal of the hybrid compounds with phenethyl amine (17 × 23 × 4 mm) was successfully fabricated by the poor-solvent diffusion method. The bulk sample showed superior scintillation properties with notably high light yield (14,000 photons per MeV) under gamma-rays and very fast decay time (11 ns). The light yield was about 1.4 time higher than that of common inorganic material (GSO:Ce) confirmed under 137Cs and 57Co gamma-rays. In fact, the scintillation light yield was the highest among the organic-inorganic hybrid scintillators. Moreover, it is suggested that the light yield of the crystal was proportional with the gamma-ray energy across 122-662 keV. In addition, the scintillation from the crystal had a lifetime of 11 ns which was much faster than that of GSO:Ce (48 ns) under X-ray irradiation. These results suggest that organic-inorganic layered perovskite-Type compounds are promising scintillator for gamma-ray detection.

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Optical and scintillation properties of Ce-doped Sr<inf>2</inf>Gd<inf>8</inf>(SiO<inf>4</inf>)<inf>6</inf>O<inf>2</inf> apatite single crystals

Takuya Igashira, Naoki Kawano, Go Okada, Noriaki Kawaguchi, Takayuki Yanagida

Journal of Materials Science: Materials in Electronics ( SPRINGER )  28 ( 24 ) 18630 - 18636   2017.12  [Refereed]

Research paper (journal)   Domestic Co-author

© 2017, Springer Science+Business Media, LLC. We synthesized Sr2Gd8(SiO4)6O2 apatite single crystals with different Ce concentrations by the Floating Zone (FZ) method and evaluated the optical and scintillation properties. We confirmed photoluminescence (PL) and scintillation emissions appearing around 400 nm, which was attributed to the 5d-4f transitions of Ce3+. The PL decay curves were approximated by a sum of two exponential decay functions with the decay constants of 21–24 ns and 78–644 ns, which are due to the host emission and emission by the 5d-4f transition of Ce3+, respectively. The scintillation decays are notably longer than those of PL, and the decay constants decreased with increasing the concentration of Ce. The X-ray induced afterglow was the lowest in the 0.5% Ce-doped sample. The 0.5, 1.6 and 2.1% Ce-doped samples showed a full-energy peak in the pulse height spectrum under 241Am 5.5 MeV α-ray irradiation. Among those samples, 0.5% Ce-doped sample showed the highest scintillation light yield of 76 ph/5.5 MeV-α.

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Comparative study of dosimeter properties between Al<inf>2</inf>O<inf>3</inf> transparent ceramic and single crystal

Takumi Kato, Naoki Kawano, Go Okada, Noriaki Kawaguchi, Takayuki Yanagida

Radiation Measurements ( PERGAMON-ELSEVIER SCIENCE LTD )  107   43 - 47   2017.12  [Refereed]

Research paper (journal)   Domestic Co-author

© 2017 We have synthesized the Al2O3 transparent ceramic by using spark plasma sintering (SPS) and investigated the dosimeter and scintillation properties for X-ray detections, in comparison with the single crystal. Under X-ray irradiation, emissions at 300, 400 and 693 nm were observed in both the samples. To identify the origins of these emissions, scintillation decay times were investigated. Based on the results, emissions at 300, 400 and 693 nm are attributed to F+ centers, F centers and Cr3+ impurity ions, respectively. The thermally-stimulated luminescence (TSL) glow curves of both the samples showed a main peak around 50 °C. In addition, glow peaks were observed at 190, 290 and 360 °C. The TSL spectra had broad emission bands in the 300, 400 and 693 nm, which approximately agreed with the scintillation spectra. The TSL response was confirmed to be linear to the irradiation dose for transparent ceramic over the dose range from 0.3 to 1000 mGy while the sensitivity of the single crystal was lower and the response range confirmed was from 30 to 1000 mGy.

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Optical, scintillation and dosimeter properties of MgO:Tb translucent ceramics synthesized by the SPS method

Naoki Kawano, Takumi Kato, Go Okada, Noriaki Kawaguchi, Takayuki Yanagida

Optical Materials ( ELSEVIER SCIENCE BV )  73   364 - 370   2017.11  [Refereed]

Research paper (journal)   Domestic Co-author

© 2017 Elsevier B.V. MgO translucent ceramics doped with different concentrations of Tb (0.01, 0.05, 0.1, 0.5%) were prepared by the Spark Plasma Sintering (SPS) method. Further, the optical, scintillation, dosimeter properties of were evaluated systematically. In the photoluminescence (PL) and scintillation spectra, sharp emission peaks due to the 4f-4f transitions of Tb3+ were observed. In the PL and scintillation decay curves, the decay time constants were a few ms which were on a typical order of the 4f-4f transitions of Tb3+. The thermally-stimulated luminescence (TSL) glow curves exhibited glow peaks around 80, 160 °C after X ray irradiation of 10 mGy. The intensity of TSL peak at 160 °C exhibited a linear response against X-ray dose over a dose range of 0.1–10 mGy. The optically-stimulated luminescence (OSL) under 590 nm stimulation exhibited strong emissions due to Tb3+ around 385–550 nm after X-ray irradiation. As in TSL, the intensity of OSL peak showed a linear response to X-ray dose, and the dynamic range confirmed was 0.1–1000 mGy.

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Optical and scintillation properties of ScAlMgO<inf>4</inf> crystal grown by the floating zone method

Takayuki Yanagida, Masanori Koshimizu, Naoki Kawano, Go Okada, Noriaki Kawaguchi

Materials Research Bulletin ( PERGAMON-ELSEVIER SCIENCE LTD )  95   409 - 413   2017.11  [Refereed]

Research paper (journal)   Domestic Co-author

© 2017 Elsevier Ltd The undoped ScAlMgO4 crystal was synthesized by the Floating Zone (FZ) method, and we evaluated the optical and scintillation properties. The transmittance spectrum showed the absorption edge around 200 nm. VUV-excited emission/excitation spectrum was characterized at a synchrotron facility (UVSOR). Our experiment revealed that the emission was around 400 nm by 200 nm excitation, and the excitation band was 170–210 nm for the latter emission peak. The decay time characterized by 280 nm excitation was 8.3 and 43 ns. Under X-ray irradiation, an emission was observed around 400 nm as for VUV excitation, and the latter decay time constants were 0.23 and 2.2 μs. In addition, the afterglow level at 20 ms after X-ray irradiation was around 0.1% of the intensity during irradiation.

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Optical and radiation-induced luminescence properties of Ce-doped magnesium aluminoborate glasses

Daisuke Nakauchi, Go Okada, Yutaka Fujimoto, Naoki Kawano, Noriaki Kawaguchi, Takayuki Yanagida

Optical Materials   72   190 - 194   2017.10  [Refereed]

Research paper (journal)   Domestic Co-author

© 2017 Elsevier B.V. Optical, radioluminescence (RL) and thermoluminescence (TL) properties of magnesium aluminoborate glasses were evaluated. The samples were synthesized by the conventional melt-quenching technique using an alumina crucible under ambient atmosphere. After polishing, all the samples show good transparency. In photoluminescence, RL and TL, the emission is predominantly due to the 5d-4f transitions of Ce3+ appearing as a broad feature peaking around 300–500 nm, and the decay time constants are affected by concentration quenching but on the typical range for the 5d-4f transitions of Ce3+. TL glow curves are characterized as a single but broad glow peak over 100–300 °C, and the TL intensity monotonically increases with the X-ray dose irradiated over 1–10000 mGy.

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Comparative studies of optical and scintillation properties between LiGaO<inf>2</inf> and LiAlO<inf>2</inf> crystals

Takayuki Yanagida, Yutaka Fujimoto, Masanori Koshimizu, Naoki Kawano, Go Okada, Noriaki Kawaguchi

Journal of the Physical Society of Japan ( PHYSICAL SOC JAPAN )  86 ( 9 )   2017.09  [Refereed]

Research paper (journal)   Domestic Co-author

©2017 The Physical Society of Japan. We have investigated optical and scintillation properties of LiGaO2 and LiAlO2 crystals. From the transmittance spectra, the absorption edges were found to be around 200 nm but wider band gap was confirmed for LiAlO2 than LiGaO2. Both photoluminescence and scintillation spectra showed a broad emission band peaking at 340 nm in the both crystals. The scintillation decay times of LiGaO2 were 460 ns and 2.8 µs while that of LiAlO2 was 2 µs. Under neutron irradiation from 252Cf, the absolute scintillation light yields of LiGaO2 and LiAlO2 were 3300 ± 300 and 7200 ± 700 ph/n, respectively.

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Scintillation properties of TGG and TSAG crystals for imaging applications

Takayuki Yanagida, Go Okada, Takahiro Kojima, Takeshi Hayashi, Jisaburou Ushizawa, Naoki Kawano, Noriaki Kawaguchi

Physica B: Condensed Matter ( ELSEVIER SCIENCE BV )  518   51 - 55   2017.08  [Refereed]

Research paper (journal)   Domestic Co-author

© 2017 Elsevier B.V. Optical and scintillation properties of TGG (Tb3Ga5O12) and TSAG (Tb3Sc2Al3O12) crystals were investigated, and capabilities to be used as a scintillator screen were demonstrated. In photoluminescence (PL) spectra, some emission lines due to Tb3+4f-4f transitions appeared from 500 to 700 nm. PL quantum yields of TGG and TSAG were 6.5% and 50.9%, respectively. When irradiated by X-rays, these crystals showed intense scintillation, and the emission wavelengths were the same as those in PL spectra. The scintillation decay times of TGG and TSAG were 94 and 678 μs, respectively. Further, we have demonstrated X-ray imaging using both TSGG and TSAG crystal plates and confirmed a capability as scintillator screens.

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Luminescent and scintillation properties of Ce-doped Tb<inf>3</inf>Al<inf>5</inf>O<inf>12</inf> crystal grown from Al-rich composition

Daisuke Nakauchi, Go Okada, Naoki Kawano, Noriaki Kawaguchi, Takayuki Yanagida

Applied Physics Express   10 ( 7 )   2017.07  [Refereed]

Research paper (journal)   Domestic Co-author

© 2017 The Japan Society of Applied Physics. A Ce-doped Tb3Al5O12 single crystal was successfully synthesized via the floating zone method in an Al-rich composition. In both photoluminescence and scintillation, the emissions were predominantly due to the 5d-4f transitions of Ce3+ peaking around 550 nm, but emissions due to defects and the 4f-4f transitions of Tb3+ were revealed by the time-resolved analyses. The crystal quality and luminescence intensity were sufficiently high such that the crystal was applicable for pulse-height spectrum measurement. Under 137Cs γ-ray exposure, a clear photoabsorption peak was observed, and the estimated light yield was 57,000 photons/MeV.

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Eu-doped Ca<inf>2</inf>SiO<inf>4</inf> as a new radio-photoluminescence phosphor

Shota Asada, Go Okada, Takumi Kato, Fumiya Nakamura, Naoki Kawano, Noriaki Kawaguchi, Takayuki Yanagida

Chemistry Letters ( Chemical Society of Japan )  47 ( 1 ) 59 - 61   2017.06  [Refereed]

Research paper (journal)   Domestic Co-author

© 2018 The Chemical Society of Japan. Eu-doped Ca2SiO4 ceramics have been newly found to show radio-photoluminescence (RPL). An as-prepared material shows photoluminescence (PL) only due to Eu3+ whereas an additional broad emission due to Eu2+ appears in the red region as a result of RPL after X-ray irradiation. The RPL is attributed to the intravalence conversion of the Eu ion (Eu3+ ¼ Eu2+), and this response can be detected as low as 10 mGy of X-ray dose.

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Non-doped Na<inf>2</inf>CO<inf>3</inf> ceramics as a new radio-photoluminescence (RPL) material

Fumiya Nakamura, Takumi Kato, Daisuke Nakauchi, Go Okada, Naoki Kawano, Noriaki Kawaguchi, Takayuki Yanagida

Chemistry Letters   46 ( 9 ) 1383 - 1385   2017  [Refereed]

Research paper (journal)   Domestic Co-author

© 2017 The Chemical Society of Japan. In this letter, we report a new radio-photoluminescence (RPL) material. RPL is a phenomenon that new luminescent centers are generated by ionizing radiations. Sintered Na2CO3 ceramics in a vacuum showed PL band peaking around 430 nm under 340 ± 40 nm excitation in which the intensity increases by X-ray irradiation. This evolution indicates a generation of a luminescent center by irradiation, a clear evidence of RPL. The majority of existing RPL materials are typically doped with luminescent ions; however, the present material was not intentionally doped. The RPL signal was observed as low as 0.1 mGy, which is an equivalent sensitivity of a commercial RPL dosimeter. For these reasons, the Na2CO3 ceramic is a promising candidate for RPL dosimeters.

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Luminescence and scintillation properties of LiF:W single crystal for thermal-neutron detection

Noriaki Kawaguchi, Naoki Kawano, Go Okada, Takayuki Yanagida

Sensors and Materials ( MYU, SCIENTIFIC PUBLISHING DIVISION )  29 ( 10 ) 1431 - 1438   2017  [Refereed]

Research paper (journal)   Domestic Co-author

© MYU K.K. The luminescence and scintillation properties of LiF:W were investigated in comparison with those of LiCaAlF6:Eu and GS20 (Ce-doped lithium silicate glass). LiF:W showed photoluminescence and scintillation emission peaking at the wavelength of 440 nm with the decay time of approximately 43 μs, which was shorter than those of LiCaAlF6:Eu and GS20. The light yield of LiF:W estimated by pulse height spectroscopy was approximately 90 photons/neutron.

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