APPLIED PHYSICS LETTERS 93, 261901 共2008兲

Subnanosecond time-resolved x-ray measurements using an organic-inorganic perovskite scintillator S. Kishimoto,1,2,a兲 K. Shibuya,2,3 F. Nishikido,2,3 M. Koshimizu,2,4 R. Haruki,5 and Y. Yoda2,5 1

Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan 2 CREST, Japan Science and Technology Agency, Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan 3 Molecular Imaging Center, National Institute of Radiological Sciences, Anagawa, Inage-ku, Chiba 263-8555, Japan 4 Tohoku University, 6-6 Aramaki Aza Aoba, Aoba-ku, Sendai 980-8579, Japan 5 Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan

共Received 31 October 2008; accepted 9 December 2008; published online 29 December 2008兲 We have developed a fast x-ray detector using an organic-inorganic perovskite scintillator of phenethylamine lead bromide 共PhE-PbBr4兲. The scintillator had a dominant light emission with a fast decay time of 9.9 ns. An x-ray detector equipped with a 0.9-mm-thick PhE-PbBr4 crystal was used to detect nuclear resonant scattering in 61Ni 共the first excited level: 67.41 keV; lifetime: 7.6 ns兲 by using synchrotron radiation. With this detector, we could successfully record the decaying gamma rays emitted from 61Ni with a time resolution of 0.7 ns 共full width at half maximum兲 and a relatively high detection efficiency of 24%. © 2008 American Institute of Physics. 关DOI: 10.1063/1.3059562兴 New scintillators with a fast light emission are expected to have high count rate capability, high time resolution, and sufficient detection efficiency in x-ray measurements using synchrotron radiation. For example, plastic scintillators with a decay time of several nanoseconds have been used for high-rate counting of x-ray pulses 共up to 107 s−1兲.1 Since the mid-1990s, silicon avalanche photodiode 共Si-APD兲 detectors, designed for direct detection of x rays, have been used in studies involving synchrotron x rays, especially in nuclear resonant scattering 共NRS兲 experiments. When pulsed synchrotron x rays arrive at a sample, nuclear scattering occurs and nuclear radiation decays with a lifetime of excited state in the NRS experiments. The nuclear radiation events 共⬍101 s−1兲 are usually separated from prompt electronic scattering 共⬎105 s−1兲 by time gating on typically tens of nanoseconds.2 The experiments require the amount of nuclear radiation events and the time evolution of the delayed radiation. The Si-APD detectors satisfied the experimental requirements with large dynamic range of count rates up to 108 s−1 and time resolution 关FWHM 共full width at half maximum兲; ⌬T兴 of less than 1 ns. However, at energies higher than 20 keV, the efficiency of a Si-APD device rapidly decreases to below 10% because of the thin active thickness of ⬍150 ␮m and the small absorption cross sections of silicon.3 On the basis of detection efficiency alone, YAlO3 : Ce 共abbreviated as YAP:Ce兲 and LaBr3 : Ce scintillators, which contain heavy atoms, may be suitable for detecting high-energy x rays.4,5 Unfortunately, their decay times 共⬃30 ns兲 make them unsuitable for the NRS detectors; this is because slow scintillation decay results in a long pulse tail, which distorts successive pulses in the detector outputs. Recently, several scintillators of the lead-halide-based perovskite-type organic-inorganic hybrid compounds have been developed.6 In these scintillators, fast light emission a兲

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was caused by excitons, and further, owing to the quantum confinement effect of the low-dimensional structure consisting of metal-halide layers, a relatively high efficiency of light emission was obtained even at room temperature.7 In experiments using a short-pulsed electron beam, it has been demonstrated that the scintillation decay of the layered organic-inorganic perovskite compounds 共n-CmH2m+1NH3兲2PbX4 共m = 3, 4, 6, 10, etc.; X = Br or I兲 contained a subnanosecond or a several-nanosecond component. However, fabrication of a crystal with a thickness greater than 0.2 mm was difficult; this thickness of the crystal corresponds to an intrinsic efficiency of 5%–6% in the detection of 67 keV x rays, which is considerably lesser than that of a stacked Si-APD detector.8 The 共C6H5C2H4NH3兲2PbBr4 crystal 关bis共phenethylammonium兲 tetrabromoplumbate 共II兲, abbreviated as PhE-PbBr4, phenethylamine lead bromide兴 is known to have two-dimensional structure consisting of leadhalide layers, which are separated by organic layers.9 Our research group was successful in fabricating the crystals with sizes greater than 10 mm2 and thickness of up to 1.7 mm and in measuring their optical and structural properties.10 The maximum wavelength of the scintillation light emitted

FIG. 1. Energy spectrum and energy resolution 共FWHM兲 of PhE-PbBr4 for 67.4 keV x rays. For comparison, those of NE142 and YAP:Ce are also shown.

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© 2008 American Institute of Physics

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FIG. 2. Scintillation decay curve of PhE-PbBr4. The white line shows the least-squares fitting of the decay curve. The inset shows the decay curve up to 35 ns.

FIG. 3. Delayed events of the NRS in 61Ni: data measured at resonance energy 共closed circles兲 and those at off-resonance energy 共open circles兲. The lifetime of 7.4⫾ 0.1 ns was obtained from the fitted decay curve. The inset shows a prompt peak with a FWHM of 0.7 ns.

by these crystals was found to be ⬃440 nm. In this letter, we report on the scintillation properties of PhE-PbBr4 crystals used for x rays and an experiment in which the fast scintillation light from the PhE-PbBr4 crystal was successfully used to observe NRS in 61Ni. The scintillation properties of PhE-PbBr4 were investigated using synchrotron x rays at beamline BL-14A of the Photon Factory 共PF兲. The energy of the x rays was set at 67.4 keV, which is the energy of the first excited level of 61 Ni.11 A single crystal of PhE-PbBr4 共area: ⬃7 ⫻ 8 mm2; thickness: 0.9 mm兲 was mounted on the window of a photomultiplier tube 共PMT兲 共Hamamatsu R7400P兲 by using optical grease and was covered with Teflon tape. An x-ray detector was assembled with the PMT in a cylindrical aluminum casing that had a beryllium window. The pulse-height spectra were measured using this detector, which was connected to a charge-sensitive preamplifier. The spectra of YAP:Ce and 5 wt % lead-loaded plastic scintillator 共NE142兲 共Ref. 12兲 were also obtained using crystals of dimensions, ␾5 ⫻ 1 mm2. The intrinsic efficiency for each crystal was determined by using a ␾0.8 mm beam; the counts detected by these crystals were compared with those detected by a NaI:Tl detector, which was equipped with a 5-mm-thick crystal. The efficiencies of PhE-PbBr4, NE142, and YAP:Ce were found to be 23.7⫾ 0.1%, 2.6⫾ 0.1%, and 50.2⫾ 0.2%, respectively. Figure 1 shows the measured pulse-height spectra. The light yields of PhE-PbBr4 and NE142 were given by 22⫾ 2 and 10⫾ 1, respectively, when the peak channel of YAP:Ce was assumed to be 40 共that of NaI:Tl is 100兲.4 The measurement error of 10% mainly originated from the difference in the optical contact for each crystal mounted on the PMT window. Furthermore, the energy resolutions 共FWHM兲 of PhE-PbBr4, NE142, and YAP:Ce were found to be 34⫾ 1%, 44⫾ 1%, and 19⫾ 1%, respectively. In order to measure the scintillation decay time, another setup was prepared in which each scintillator was placed 38 mm away from the entrance window of the PMT in a vacuum chamber. The detection probability of each scintillation light pulse was much less

than 1 when the solid angle of the PMT was small and a single x ray was absorbed in the scintillator. The decay times were measured during the single-bunch operation of the PF storage ring by using a fast amplifier, a constant-fraction discriminator, and a time-to-amplitude converter 共TAC兲. The signals by the scintillation light were fed into the start of the TAC, and the stop signals were obtained from the accelerator system. The open circles in Fig. 2 denote the scintillation decay of PhE-PbBr4 obtained using the time spectroscopy system. The components of the decay time were found to be 9.9⫾ 0.2 ns 共72⫾ 1% of the total light yield兲, 23⫾ 1 ns 共25⫾ 2%兲, and 94⫾ 3 ns 共2.3⫾ 0.1%兲, and these values were obtained by least-squares fitting using an exponential decay function 共white solid line兲. The inset shows the decay up to 35 ns. The peaks that existed between ⫺2 and 0 ns were due to the direct entry of x rays into the PMT. The scintillation properties of the crystals are listed in Table I. The NRS in 61Ni were measured by using the PhE-PbBr4 x-ray detector at beamline BL09XU of the SPring-8.13 A sample of Ni metal foil 共thickness: 0.38 mm; 61Ni : 95% enriched兲 was attached to a 3-mm-thick aluminum plate in order to absorb fluorescent x rays from the Ni foil. The plate was mounted on the beryllium window of the detector and was positioned at an angle to the incident x ray beam. Only the elastic scattering of the incident x rays and the resonant gamma rays emitted from the 61Ni nuclei could be detected. The intensity of the incident beam was ⬃1 ⫻ 107 photons/ s with energy width of 1 eV. Output pulses were obtained by using a fast amplifier at a PMT voltage of ⫺500 V, of which full width at tenth maximum was less than 20 ns. The count rate of the detector was ⬃3.5⫻ 105 s−1. Figure 3 shows the measured events of the resonant scattering in 61Ni. The closed circles represent the data measured at the resonance energy 共ER兲; the open ones represent the data measured at the off-resonance energy 共Eoff兲, which is 67 eV lower than ER. A prominent peak caused by the intense elastic scattering was observed at 0 ns and is so-called the prompt peak. The detector outputs in the period of up to

TABLE I. Scintillation properties of PhE-PbBr4, NE142, and YAP:Ce measured at 67.4 keV.

PhE-PbBr4 NE142 YAP:Ce

Efficiency 关共%兲; thickness: 1 mm兲兴

Light yield 共NaI:Tl 100兲

Energy resolution 关共%兲; FWHM兲兴

Scintillation decay 共ns兲

23.7⫾ 0.1 共0.9 mmt兲 2.6⫾ 0.1 50.2⫾ 0.2

22⫾ 2 10⫾ 1 404

34⫾ 1 44⫾ 1 19⫾ 1

9.9⫾ 0.2 1.9⫾ 0.1 274

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15 ns were vetoed at the discriminator by using a timing signal from the accelerator; the remaining nuclear decaying part was recorded. The inset in Fig. 3 shows the prompt peak measured at Eoff by using a narrow, low-intensity beam. The time resolution ⌬T was given by 0.71⫾ 0.05 ns. Overlapping of signals occurred because of the intense prompt radiation, which resulted in a tailing of the prompt peak. The fast detector response could extract the decaying structure of 61Ni nuclei after 20 ns in spite of the tailing. The lifetime of 7.4⫾ 0.1 ns was obtained from the fitted decay curve shown in Fig. 3; this value almost corresponds to a reference value of 7.6⫾ 0.2 ns.11 The rate of the delayed counts was approximately 12 s−1. With a fast scintillation light of 9.9 ns decay time, which is nearly one-third of the decay times in Ce3+ emission, the PhE-PbBr4 scintillator could demonstrate the quick response in the NRS measurement on 61Ni. A relatively high efficiency of 24% was also obtained at 67.4 keV. This scintillator can also be widely used in other x-ray experiments in which high-rate counting and high detection efficiency at energies ⬎30 keV are desired. This study was performed with the approval of Japan Synchrotron Radiation Research Institute 共JASRI兲 共Proposal

Nos. 2007A1543 and 2007B1585兲 and of the Photon Factory Advisory Committee 共Proposal Nos. 2005G163 and 2008G104兲. This study was supported by the Japan Science and Technology Agency, CREST. J. L. Radtke, IEEE Trans. Nucl. Sci. 37, 129 共1990兲. A. Q. R. Baron, Hyperfine Interact. 125, 29 共2000兲. 3 XCOM, Photon Cross Sections Database 共http://physics.nist.gov/ PhysRefData/Xcom/Text/XCOM.html兲. 4 V. G. Baryshevsky, M. V. Korzhik, V. I. Moroz, V. B. Pavlenko, A. A. Fyodorov, S. A. Smirnova, O. A. Egorycheva, and V. A. Kachanov, Nucl. Instrum. Methods Phys. Res. B 58, 291 共1991兲. 5 E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. Krämer, and H. U. Güdel, Appl. Phys. Lett. 79, 1573 共2001兲. 6 K. Shibuya, M. Koshimizu, H. Murakami, Y. Muroya, Y. Katsumura, and K. Asai, Jpn. J. Appl. Phys., Part 2 43, L1333 共2004兲. 7 K. Shibuya, M. Koshimizu, K. Asai, and H. Shibata, Appl. Phys. Lett. 84, 4370 共2004兲. 8 I. Sergueev, A. I. Chumakov, T. H. Deschaux Beaume-Dang, R. Rüffer, C. Strohm, and U. van Bürck, Phys. Rev. Lett. 99, 097601 共2007兲. 9 N. Kitazawa and Y. Watanabe, Surf. Coat. Technol. 198, 9 共2005兲. 10 K. Shibuya, F. Nishikido, M. Koshimizu, Y. Takeoka, and S. Kishimoto, Acta Crystallogr., Sect. E: Struct. Rep. Online 共unpublished兲. 11 M. R. Bhat, Nucl. Data Sheets 88, 417 共1999兲. 12 Data sheet of BC452 at http://www.detectors.saint-gobain.com/ 共BC452 is equivalent to NE142兲. 13 Y. Yoda, Nucl. Instrum. Methods Phys. Res. A 467, 715 共2001兲. 1 2

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