PR12 Project Research on Nuclear and Using Short-Lived Nuclei

Y. Ohkubo annealed in vacuum at various temperatures. They observed no clear signal of such complexes in the Research Reactor Institute, Kyoto University TDPAC spectra. However, the TDPAC spectra indicate that Ce and He form complexes having a variety of geometrical structures. Comparison with TDPAC results Objective and Participating Research Subjects reported by another research group on 111Cd arising from The main objectives of this project research are the 111In in He-doped stainless steel shows that the parent investigation of the nuclear structure of unstable atoms (La and Ba) of 140Ce trap He atoms more neutron-rich nuclei and also the local properties of efficiently than In atoms do, indicating stronger bonding materials using short-lived nuclei. of He to the former atoms, while different from the This period is the first year of the project. present case, 111Cd (In)‒He complexes form a unique Unfortunately, no experiments in all six research subjects geometrical structure. A part of the results are described of the project (26P12) were executed owing to the in the following second page. suspension of the reactor operation. Here, we report some S. Komatsuda et al. (PRS-3) investigated thermal results which were obtained in previous periods and have behavior and interacting nature of 100-ppm Al and already been published in journals. ~100-ppt In impurities doped in a semiconductor ZnO The research subjects (PRS) reported here are as by means of the TDPAC technique with the 111In(→ follows: 111Cd) probe. They observed the following contrasting interactions between Al and In impurities depending on PRS-1  Decay of 150Ce to 150Pr different atmospheric conditions: (1) in air, Al and In PRS-2 Interaction between He and Elements with A = impurities irreversibly associate with each other in the 140 in Fe process of thermal diffusion of these two species, but (2) PRS-3 Atmosphere Dependence of Stability of Local in vacuum, these bound states formed in air dissociate by Fields in Al-Doped ZnO heat treatment at temperature higher than 873 K, and the PRS-4 Extranuclear Dynamic Motion of 111Cd(← 111Ag) dissociation reaction is enhanced with increasing Doped in AgI Nanoparticles temperature. Considering that the dissociation is triggered by the oxygen vacancy formation near the locally Main Results and Contents of This Report associated In‒Al structure and using the TDPAC data obtained, they estimated the activation energy of the At KUR-ISOL (the on-line isotope separator installed oxygen‒vacancy formation to be 0.72(6) eV. A part of at the Kyoto University Research Reactor), Y. Kojima et the results are described in the following third page. al. (PRS-1) studied the  decay of 150Ce having a Superionic conductivity observed for AgI emerges only half-life of 6.05(7) s produced by the thermal-neutron- at the high-temperature (≥ 420 K)  phase because of induced fission of 235U. From -ray singles and ‒ temperature-dependent crystal structures, which is a coincidence measurements, they newly found 9 excited barrier to the practical applications of this compound. states and 37-rays and constructed a decay scheme Recently, this problem was attacked by a novel technique: containing 18 excited states and 55 -rays which include powder AgI coated with poly-N-vinyl-2-pyrrolidone the newly found excited states and -rays. They (PVP) can drastically enhance the ionic transport property concluded that the dominant -feeding at the 110-keV at room temperature. W. Sato et al. (PRS-4) studied 150  150 dynamic behavior of the extranuclear field relative to the excited state of Pr arising from the  decay of Ce to 111 111 be an allowed unhindered spin-flip transition. They Cd(← Ag) probe nucleus incorporated in AgI, using furthermore proposed the Nilsson configurations of the the TDPAC method. For PVP-coated AgI nanoparticles, 150 they observed nuclear spin relaxation of the probe at ground state and the 110-keV excited state of Pr. A + part of the results are described in the next page. room temperature, which indicates that Ag ions in the In order to examine whether Ce (or rather, La and Ba) PVP-coated sample make hopping motion from site to and He form complexes having a definite geometrical site at this low temperature. From the TDPAC data structure in Fe as suggested in first-principles density obtained, they estimated the activation energy for the functional theory calculations, Y. Ohkubo et al. (PRS-2) dynamic motion to be 46(10) meV. A part of the results projected 100-keV 140Cs+ at KUR-ISOL and then 4-keV are described in the following fourth page. He+ using an ion beam gun into an Fe foil and took time-differential perturbed-angular-correlation (TDPAC) spectra at room-temperature of 140Ce arising through 140Ba-140La from 140Cs in He-doped Fe, unannealed and

採択課題番号 26P12 短寿命 RI を用いた核分光と核物性研究 プロジェクト （京大・原子炉）大久保嘉高

70 PR 12-1  decay of 150Ce to 150Pr

Y. Kojima, K. Kosuga1, Y. Shima2, A. Taniguchi3, sition were deduced from peak counts observed in the H. Hayashi4 and M. Shibata singles spectrum or in coincident spectrum. Multipolari- ties of three -rays were also estimated from the intensity Radioisotope Research Center, Nagoya University ratio between KX and -ray peaks observed in the coin- 1 School of Engineering, Nagoya University cident spectrum: M1/E2 for the 103- and 154-keV -rays, 2 Graduate School of Engineering, Nagoya University and E1 for the 110-keV -ray. 3Research Reactor Institute, Kyoto University 4 Finally, the I and log-ft values were calculated from the Institute of Biomedical Science, The University of To- transition intensity ( and conversion electron) imbalance kushima Graduate School for each excited level. A small log-ft value of 4.9 was

observed for the 110-keV level. It leads to spin-parity J + = 1 for this level. INTRODUCTION: Decay data provide valuable in- DISCUSSION: The very intense -feeding to the formation about unstable nuclides. In particular, the - 110-keV level is an allowed transition, and should be branching ratio I is one the most important properties  explained by selection rules for -decays of deformed from the nuclear structure viewpoint. For even-even 150Ce, nuclei [6]. Near the Nilsson orbitals observed in the Fogelberg et al. briefly reported an intense -transition to 150 neighboring nuclides, only the spin-flip transition be- the 110-keV level in Pr [1]. If this -feeding is an al- tween 3/2[532] and 5/2[532] satisfies the selection lowed unhindered transition, it will lead to the assign- rules. Thus, this -transition is interpreted as one of the ment of the Nilsson orbital for 150Pr. However, it is un- coupled neutron in 3/2[532] orbital in 150Ce changing to certain whether this transition is firmly an allowed un- a proton in the vacant 5/2[532] orbital in 150Pr. This con- hindered transition because of the following reasons.  + First, ref. [1] did not provide detailed experimental data figuration yields J = 1 , and agrees with our spin-parity 150 150 assignment. The configuration of 5/2+[413]-3/2[521] on Ce. Second, the other published data for Ce are 150 very scarce and partly inconsistent with each other [2,3]. was also proposed for the ground state of Pr from the In the present work, decay studies on 150Ce have been systematics of the neighboring nuclides and from cou- pling rules. performed using the on-line isotope separator at KUR 150 (KUR-ISOL) [4] to investigate low-energy level struc- CONCLUSIONS: A detailed decay scheme of Ce, tures in150Pr. Note that this work was recently published which includes 37 new -rays and 9 new excited levels, [5]. This report is an outline of the paper. was proposed from -ray measurements. On the basis of EXPERIMENTS: The 150Ce nuclides were produced the systematics and observation of the allowed spin-flip by the thermal neutron induced fission of 235U. The transition, the Nilsson configurations were newly pro- 150 mass-separated source was periodically moved to a de- posed for the ground and 110-keV level in Pr. tector port with time intervals of 12.2 s. Time-dependent -ray singles and  coincidence meas- 2000

urements were performed with two Ge detectors in a KX 100 measurement period of 46 h. The energy and efficiency 180 231 1000 789

calibrations were made using standard -ray sources. 688 106 558 62, 63

169 x5 200 279.2 Counts RESULTS: The half-life and coincidence relationships 380 with the Pr KX-ray were analyzed to assign the parent 0 0 2000 4000 6000 nuclide of each -ray observed in this work. From these Channel number (0.125 keV/ch) analyses, 57 -rays including 39 new -transitions were 150 Fig. 1. A -ray spectrum gated on the 110-keV found to be due to the decay of Ce. 150 -transition. Coincident -rays are indicated with Energy spectra gated on the-rays from Ce were ana- their energies in units of keV. lyzed to establish -ray cascade relations. For example, Fig. 1 is a spectrum gated on the 110-keV -ray. This REFERENCES: spectrum shows that the 110-keV -ray is coincident with [1] B. Fogelberg et al., Nucl. Phys. A 453 (1986) 15-25. 13 -rays. From these cascade relations and also from [2] K. Yamauchi et al., KURRI-KR-3 (1996), pp.51-53 energy sum rules, the excited levels in 150Pr and the -ray (in Japanese). placements were unambiguously proposed. The decay [3] K. S. Basu et al., Nucl. Data Sheets 114 (2013) 435. scheme includes 18 excited levels. Nine levels of them [4] A. Taniguchi et al., Nucl . Instr. and Meth. A 351 were newly proposed. The 717-keV level seen in ref. [3] (1994) 378-382. was not observed in this work. [5] Y. Kojima et al., J. Phys. Soc. Jpn. 84 (2015) The -ray intensities relative to that of the 110-keV tran- 054201/1-8. [6] G. Alaga, Phys. Rev. 100 (1955) 432-433.

採択課題番号 26P12-3 核分裂生成物に対する線直線偏光度測定を中心とした プロジェクト 核分光実験 （名大・RI）小島康明、柴田理尋（名大院・工）嶋 洋佑、常少亮太 （名大・工）小菅数人 （徳島大・医）林 裕晃 （京大・原子炉）谷口秋洋

71 PR12-2 Interaction between He and Elements with A = 140 in Fe

Y. Ohkubo, A. Taniguchi, Q. Xu, M. Tanigaki and K. He-doped(1)140CeFe (4 × 1013/cm2 He corresponding to Sato only 0.0003 dpa), it is unlikely for He ions to directly displace them. We think that He atoms diffused and Research Reactor Institute, Kyoto University bonded to the parent atoms of "good" 140Ce (interstitial He atoms diffuse much more easily than vacancies: the corresponding calculated activation energies are 0.06 and INTRODUCTION: As far as we know, the state of 0.68 eV, respectively). As the dose of He increases, the impurities He and Ce in Fe has not been studied number of the parent atoms of "good" 140Ce decreases. If experimentally until our recent work [1]. This knowledge 140Ce‒He complexes take one or a few definite is important in related to nuclear fusion geometrical structures, these would be reflected in the reactor materials. With a nuclear spectroscopic technique, TDPAC spectra and it may be possible to determine the time-differential perturbed angular correlation (TDPAC) whether He atoms are at substitutional or interstitial sites. technique, we obtained the result that the parent atoms The observation that the oscillation amplitude is only (La and Ba) of 140Ce very efficiently trap He and they reduced but not damped with time under the He form complexes in Fe, which do not have a definite implantation indicates that 140Ce‒He complexes take a geometrical structure. The present report describes the variety of geometrical structures, accordingly these 140Ce latter result. See [1] for the full description. bonded by He feeling various electric field gradients due to He. It is natural to think that many He atoms surround EXPERIMENTS: 100-keV 140Cs+ (~1.4 × 1012/0.2 cm2) the parent atom and that there is a distribution of the were projected into an Fe foil at KUR-ISOL. 140Cs decays number of He atoms attached to it. through 140Ba and then 140La to 140Ce. We name this sample 140CeFe. Three additional samples were prepared 140 using the CeFe. The first, second and third were -0.10 no He prepared by projecting 4-keV He+ of ~4 × 1013, ~5 × 1014 [1.93(1)Grad/s, -0.042(3)] and ~2 × 1016/cm2 in this sequence into the 140CeFe. We -0.05 name them He-doped(1)140CeFe, He-doped(2)140CeFe and He-doped(3)140CeFe, respectively. The profile of the 0.00 140 -0.10 He concentration in Fe overlaps that for Cs. 1st He irrad. 140 Room-temperature TDPAC spectra of Ce were taken [1.90(1), -0.020(3)] for these four samples. -0.05

0.00 (t)

RESULTS: Fig. 1 (from top to bottom) shows the 22 -0.10 140 G 2nd He irrad.

TDPAC spectra obtained at room temperature for CeFe, 22 [1.91(1), -0.013(3)] 140 140 A He-doped(1) CeFe, He-doped(2) CeFe and He-doped -0.05 (3)140CeFe, all four with no annealing. The top spectrum is the one appearing in [2]. As explained there, the 0.00 oscillation pattern seen in the spectrum is due to a unique -0.10 140 static magnetic hyperfine interaction at Ce occupying 3rd He irrad. the Fe substitutional site with no lattice defects nearby -0.05 (the Larmor frequency equals 1.927(7) Grad/s). Hereafter, we call these 140Ce "good" 140Ce. From the value of the 0.00 amplitude of the oscillation, "good" 140Ce are only about 0 5 10 15 20 25 30% of the 140Ce implanted in Fe. In Fig. 1 are also t (ns) shown the characteristic values for each oscillation Fig. 1. Room-temperature TDPAC spectra for 140CeFe before component seen in the TDPAC spectra. As the dose of He He irradiation, He-doped(1)140CeFe, He-doped(2)140CeFe, and 140 increases to ~5 × 1014/cm2, the oscillation amplitude gets He-doped(3) CeFe, all with no annealing. The two values in the square brackets in each panel are the Larmor frequency reduced more and more (note that the amplitude is not (Grad/s)and theamplitude ofthe oscillation component. ~ 16 2 damped with time), and at the dose of 2 × 10 /cm (He-doped(3)140CeFe) no clear oscillation is observed. 16 2 REFERENCES: The dose of 2 × 10 /cm corresponds to 0.15 dpa [1] Y. Ohkubo, A. Taniguchi, Q. Xu, M. Tanigaki and K. (displacement per atom) at the maximum. This large Sato, Phil. Mag. Lett. 94, 470-477 (2014). number may imply that He ions directly displace the 140 [2] Y. Ohkubo, A. Taniguchi, Q. Xu, M. Tanigaki, N. parent atoms of "good" Ce from the substitutional sites. Shimizu and T. Otsuka, Phys. Rev. C 87, 044324 However, based on the fact that the oscillation amplitude (2013). is reduced by as much as ~50% even for the

採択課題番号 26P12-4 鉄中での不純物原子と 140Ce 不純物原子核との相互作用 プロジェクト （京大・原子炉）大久保嘉高、谷口秋洋、谷垣 実、徐 虬（京大院理）常山正幸、谷口良徳

72 PR 12-3 Atmosphere Dependence of Stability of Local Fields in Al-Doped ZnO

1 2 S. Komatsuda, W. Sato and Y. Ohkubo G22 t the time-differential perturbation factor as a func- tion of the time interval, t, between the relevant cascade Department of General Education, National Institute of γ-ray emissions, and N(θ,t) the number of the coincidence Technology, Ichinoseki College events observed at angle, θ. The spectrum in Fig.1(a) 1 Institute of Science and Engineering, Kanazawa Univer- shows that 100 ppm Al and 100 ppt 111In are local- sity ly-associated in ZnO matrix, suggesting that there is a 2 Research Reactor Institute, Kyoto University strong attractive force between Al and In in ZnO [1,2]. For the spectrum in Fig. 1(b), two different components were observed: one is that appearing in the spectrum in INTRODUCTION: Impurity-induced properties Fig. 1(a) and the other is that observed for undoped emerging in ZnO has been attracting increasing attention ZnO[3]. This observation implies that the probe gradually toward its application to functional materials in a wide resides solely at the substitutional Zn site after annealing field of industry. For a practical use of ZnO as a conduc- under argon gas in the same way as the case for the sam- tion-controlling device, it is of great importance to study ple heat-treated in vacuum. These observations suggest the physical and chemical states of doped impurities that the drastic change of local structure formed by Al and/or oxygen vacancies. From this point of view, we and In ions is closely related to the presence of O2 gas. In have investigated the factors determining the local struc- oxide compounds, in general, oxygen vacancies are likely tures and their stability of Al-doped ZnO samples synthe- to be formed during annealing process at a lower partial sized on different conditions by means of the pressure of atmospheric oxygen. Therefore, it is suggest- time-differential perturbed angular correlation (TDPAC) ed that the local association of Al and 111In becomes un- method. In a series of our TDPAC studies, drastic change stable in anaerobic atmosphere due to the formation of of the local structure was observed for Al-doped ZnO oxygen vacancies in the sample. For further information sample heat-treated in vacuum [1,2]. In order to provide on the stability of the local structure, dependence of par- further insight into the above atmosphere dependence, in tial pressure of oxygen for the dissociation process needs the present work, we performed an additional TDPAC to be investigated. measurement for 100 ppm Al-doped ZnO annealed under argon gas flow.

EXPERIMENTS: For the synthesis of 100 ppm Al-doped ZnO, stoichiometric amounts of Al(NO3)3・ 9H2O and ZnO powder were mixed in ethanol. The sus- pension was heated to evaporate the ethanol until dryness. The powder was pressed into two disks and sintered in air at 1273 K for 3 h. For TDPAC measurements, commer- cially available 111In solution was added in droplets onto the sintered disks at the concentration of 100 ppt. The disks again underwent heat treatment in air at 1373 K for 2 h. After that, a TDPAC measurement was performed for one of these samples prepared in air. The other sam- ple was ground into powder and annealed under argon gas flow at 1373 K for 24 h. A TDPAC measurement was carried out for the 111In(→111Cd) probe on the 171-245 keV cascade γ rays with the intermediate state of I = 5/2 having a half-life of 85.0 ns.

RESULTS: Fig. 1 shows the TDPAC spectra of 111In(→ 111Cd) probe in 100 ppm Al-doped ZnO annealed (a) in air and (b) in argon gas. The directional anisotropy on the Fig. 1. TDPAC spectra of 111In( → 111Cd) in 100 ppm ordinate, A22G22 t , was deduced by the following rela- Al-doped ZnO annealed (a) in air and (b) in argon gas. The tion for delayed coincidence events of the cascade: TDPAC measurements were performed at room temperature.

REFERENCES:

－ ππ tNtN )],2/(),([2 [1] S. Komatsuda et al., J. Appl. Phys., 116 (2014) 2222 tGA )( = . 183502/1-5. + ππ tNtN ),2/(2),( [2] S. Komatsuda et al., J. Radioanal. Nucl. Chem., 303 Here, A denotes the angular correlation coefficient, (2015) 1249-1252. 22 [3] W. Sato et al., Phys. Rev. B, 78 (2008) 045319/1-5.

採択課題番号 26P12-5 金属酸化物中不純物サイトの局所構造観察 プロジェクト （金沢大・理工研究域）佐藤 渉（金沢大院・自然）小松田沙也加（京大・原子炉）大久保嘉高

73 PR12-4 Extranuclear Dynamic Motion of 111Cd(← 111Ag) Doped in AgI Nanoparticles

W. Sato, R. Mizuuchi1, N. Irioka2, S. Komatsuda3, S. were performed for the synthesized sample at various 4 5 Kawata , A. Taoka and Y. Ohkubo temperatures. The directional anisotropy, A22G22(t), was deduced with the following relation: Institute of Science and Engineering, Kanazawa 2[N (π,t)− N (π /2,t)] University A22G22(t) = . (1) 1Graduate School of Natural Science and Technology, N(π,t) +2N (π /2,t) Kanazawa University Here, A denotes the angular correlation coefficient, 2School of Chemistry, Kanazawa University 22 3 G22(t) the time-differential perturbation factor as a Department of General Education, National Institute of function of the time interval, t, between the relevant Technology, Ichinoseki College 4 cascade γ-ray emissions, and N(θ, t) the number of the Department of Chemistry, Faculty of Science, Fukuoka delayed coincidence events observed at an angle, θ. University 5 RESULTS: It was found that TDPAC spectra of the Research Reactor Institute, Kyoto University 111Cd( ← 111Ag) probe introduced in PVP-coated AgI measured at different temperatures show exponential relaxation of the directional anisotropy, which signifies INTRODUCTION: Silver iodide, AgI, is a promising that the probe is dynamically perturbed by the metal halide showing superionic conductivity as in its α extranuclear field. This phenomenon is understandable phase, and its application to functional materials such as for the spectra of the sample as in the α phase (> 420 K). solid electrolytes is thus highly expected. The What is to be noted here is that the spectral relaxation conducting phenomenon, however, only emerges at was also observed even at room temperature. It was elevated temperature (> 420 K) because the crystal found from this observation that Ag+ ions surrounding structure depends on temperature; this has indeed been a the probe nuclei exhibit site-to-site hopping motion in the barrier to the practical applications of this compound. PVP-coated AgI nanoparticle even at this low Recently, an epoch-making technique has broken temperature. Fig. 1 shows the temperature dependence through this situation: powder AgI coated with of the relaxation constants. From a least-squares fit to poly-N-vinyl-2-pyrrolidone (PVP) can drastically the data with an Arrhenius-type equation, the activation enhance the ionic transport property at room temperature, 2 -1 1 energy, Ea, of the hopping motion was successfully recording the conductivity of 1.5 × 10− Ω cm− [1]. estimated to be Ea = 46 (10) meV. This value shows According to the report, this achievement is due to fair agreement with that obtained for the superionic successful control of the particle size as small as conductivity: Ea = 60 meV [1]. The present result is € nanoscale. In addition to the property in the bulk, it is significant in that superionic conduction can be probed of great importance to obtain complementary information + with local dynamics observed through nuclear spin on the site-to-site hopping motion of Ag ions on an relaxation. atomic scale for a detailed understanding of ionic 10 conductivity of this binary solid. For that purpose, in the present work, we have applied the time-differential perturbed angular correlation (TDPAC) technique with the 111Cd(← 111Ag) probe nuclei to observe dynamic + ) behavior of Ag ions. We here report successful -1 s

6 evaluation of the activation energy of the dynamic

+ (10

motion of Ag ions [2,3]. E a = 46 (10) meV EXPERIMENTS: Pd foil was irradiated with thermal neutrons in Kyoto University Reactor to produce the TDPAC probe. After radioequilibrium was achieved between 111Pd and 111Ag, the Pd foil was dissolved in 1 111 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 HNO aq. solution, and carrier-free Ag was isolated by 3 3 1/T (10 K-1) an anion-exchange chromatography. The separated 111 Fig. 1. Temperature dependence of the relaxation constant. Ag was incorporated together in PVP-coated AgI The line is the result of a least-squares fit with an sample when the powder sample was synthesized by Arrhenius-type equation. precipitation of their raw materials. We confirmed by REFERENCE: transmission electron microscopy that microscopic [1] R. Makiura et al. Nature Mater. 8 (2009) 476-480. particles with sizes of 10-100 nm were expectedly 111 [2] W. Sato et al. Chem. Phys. Lett. 609 (2014) 104-107. synthesized for Ag-free PVP-coated AgI. [3] W. Sato et al. Hyperfine Interact. 231 (2015) 107-113. TDPAC measurements of the 111Cd(←111Ag) probe

採択課題番号 26P12-6 超イオン伝導体ヨウ化銀中 111Ag の核スピン緩和 プロジェクト （金沢大・理工研究域）佐藤 渉（金沢大院・自然）藤澤 照功（京大・原子炉）大久保 嘉高

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