Project ResearchProject 6

PR6

Project Research on Nuclear Spectroscopy and Condensed Matter
Physics Using Short-Lived Nuclei

Y. Ohkubo
Research Reactor Institute, Kyoto University
Objective and Participating Research Subjects
The main objectives of this project research are theinvestigation of the nuclear structure of unstable neutronrich nuclei and also the local properties of matters using
short-lived nuclei.
This period is the second year of the project.
Unfortunately, as in the last period, no experiments in all
five research subjects of the project (27P6) were executed
owing to the suspension of the reactor operation. Here, wereport some results which were obtained in previous
periods and have already been published in journals orsubmitted for publication.
The reports presented here are as follows:
Report 1. Thermal Stability of Impurities Ba and La at theSubstitutional Sites in Fe
Report 2. Thermal Stability of Cd and In Impurities inFe3O4
Report 3. Formation Process of Oxygen Vacancy in Zinc
Oxide
Main Points Described in the Reports in the Following
Three Pages
The experimental techniques used to measure hyperfine
fields are in common: ‒-rays time-differential perturbedangular-correlation (TDPAC) technique. The nuclear
probes and matters in which the probes are incorporated
are (Report 1) 140Ce arising from 140Cs through 140Ba andthen 140La (for short, 140Ce(←140La←140Ba←140Cs)) andFe, (Report 2) 111Cd(←111In) and 111Cd(←111mCd) andFe3O4, and (Report 3) 111Cd(←111In) and 100 ppm Aldoped ZnO.
Report 1 describes thermal stability of Ba and La at thesubstitutional sites in Fe. Although neither Ba nor La
atoms (also Ce atoms do not) form a solid solution with Fe(therefore, the only possible method for the incorporationof these atoms in Fe is ion implantation), which implies
that these atoms bond with Fe atoms very weakly, theseatoms at the substitutional sites do not move attemperatures at least as high as 673 K. This result wasobtained in the condition that atomic vacancies producedin the ion implantation are hindered to move and bond withBa or La. The pre-implanted He atoms play a role in
keeping the atomic vacancies to stay.
The aims of implanting 140Cs in Fe using the KUR-ISOL(Isotope Separator On-Line) were the following twothings: the first thing is to measure the nuclear magnetic
moment of the first 4+ excited state of 140Ce having aneutron magic number and to compare it with the nuclear
theory. The second thing is to study the state of impurities
He and Ce in Fe experimentally. This knowledge isimportant in materials science related to nuclear fusion
reactor materials. The results are already reported injournals.
Report 2 describes the locations of Cd and In impurities
in magnetite, Fe3O4, and their thermal stability, the detailsof which will be published in a journal.
About three decades ago, magnetic hyperfine field at111
Cd(←111In) chemically doped in Fe3O4 was measured tostudy the supertransferred magnetic field from themagnetic (Fe) ions via the oxygen ions to the nonmagnetic(Cd) ions. The location of In was assigned to thetetrahedral (A) site. However, quite recently, a paper waspublished stating that the location of In was the octahedral(B) site. W. Sato et al. succeeded in obtaining TDPAC
spectra with damping free for the first time, concluded thatthe location of In is the A site, and moreover obtained asignal indicating that at high temperatures some In ions in
the A sites move to the B sites, which is consistent with adensity functional theory calculation.
Report 3 describes an experimental method to obtain theformation energy of oxygen vacancy in ZnO, which is an
important physical quantity because oxygen vacancies canbring about, for example, ferromagnetism in ZnO. S.
Komatsuda et al. obtained the temperature dependence ofthe rate constant for 111In migration in high-temperature
vacuum from Al aggregations in 100 ppm Al-doped ZnO
to the substitutional defect-free Zn sites. Considering themigration of 111In as being induced by the formation ofoxygen vacancies, they obtained 0.72(6) eV as theformation energy of oxygen vacancy, which is consistent
with theoretical calculations.

27P6- 33 -Thermal Stability of Impurities Ba and La at the Substitutional Sites in Fe

Research Reactor Institute, Kyoto University
1
Graduate School of Science and Engineering, Kagoshima
University
INTRODUCTION: Previously, we projected 100-keV
140Cs ions into Fe foils at KUR-ISOL and observed, using
the time-differential perturbed angular correlation (TDPAC)
technique on the 2083-keV state of 140Ce, that about 35% of140
Ce arising from the  decay of 140Cs through 140Ba andthen 140La occupy the substitutional sites with no latticedefects nearby [1, 2]. The half-lives of 140Cs, 140Ba and140
La are 63.7 s, 12.8 d and 40.3 h, respectively. About aweek after the end of the 140Cs bombardments, when theTDPAC measurements were started, all 140Cs had decayedout and the number ratio of 140Ba to 140La was constant, 0.85,
calculated from the values of their half-lives. We think thatthose 140Ba and 140La leading to 140Ce located at thesubstitutional sites also occupy the same substitutionalsites. Since neither Ba nor La atoms (also Ce atoms do not)form a solid solution with Fe, it might be expected thatthey easily move into different places with increasing
temperature and are finally segregated. This time, wereport on the thermal stability of impurities Ba and La atthe substitutional sites.
EXPERIMENTS: Two Fe samples were prepared: one is
Fe irradiated with 100-keV 140Cs+ at KUR-ISOL and theother is Fe irradiated with 4-keV He+ using an ion beam
gun and subsequently with 100-keV 140Cs+. We name thefirst sample 140CeFe and the second 140CeHe-doped Fe. For
these two samples, 0.5-h annealing in vacuum wasperformed at a same time in a same furnace at 473, 673,773, 873, 973 and 1073 K in this order. Before and after
each annealing, room-temperature TDPAC spectra of140
Ce were obtained with two measurement systems, eachconsisting of standard fast-slow electronic modules and four
BaF2 scintillation detectors. The relevant cascade  rays arethe 329‒487-keV ones and the angles between the cascade rays are 90° and 180°.

140

CeFe. We may consider that with increasing annealing
temperature, parent atoms (Ba and La) at the substitutionalsites move into different places producing various
hyperfine fields at the 140Ce, and/or that with increasing
temperature, atomic vacancies migrate and bond to stillparent atoms of "good" 140Ce, producing various hyperfine
fields at the 140Ce. As explained below, the latter
explanation is adequate at least below a certain
temperature (we call it Tb) between 673 and 773 K.
Fig. 1(b) shows that all "good" 140Ce remain “good” in140
CeHe-doped Fe until at 673 K and about 60% of them
do not at 773 K. This observation means that parent atoms(Ba and La) at the substitutional sites do not move at least
below Tb. We think that the pre-deposited He atoms formHe‒vacancy clusters with vacancies mainly produced in
the He and 140Cs bombardments and that at annealing
temperatures higher than Tb, He and vacancies are released
from those clusters, migrate and bond to parent atoms of"good" 140Ce. This interpretation is consistent withrelevant results of a thermal helium desorption
spectroscopy experiment (ref. 6 in [2]).
A closer look at Fig. 1(b) shows that the oscillation
amplitude increases with annealing temperature up to 673
K: at 673 K it is larger than by about 18% that before
annealing. We need to examine this point by carrying out
additional experiments with various doses of He, which
may also elucidate the reason for the difference in theabundance of “good” 140Ce between the two samples
before annealing.
(a)
-0.10

140CeFe(b)-0.10

no annealing-0.05
0.00

0.00

-0.10

-0.10

annealed at 473 K

-0.05

-0.05

0.00

0.00

-0.10

annealed at 673 K

-0.05
0.00
-0.10

annealed at 473 K

annealed at 673 K-0.05

annealed at 773 K

-0.05
0.00

annealed at 1073 K

-0.10

-0.05

annealed at 1073 K

-0.05

0.00
0-0.10

-0.10

0.00
-0.10

no annealing

0.00

annealed at 773 K-0.05

RESULTS: Fig. 1(a) and 1(b) show the TDPAC spectra
for 140CeFe and 140CeHe-doped Fe, respectively. Theoscillation pattern seen in some of the spectra is due to aunique static magnetic hyperfine interaction at 140Ceoccupying the Fe substitutional site with no lattice defects
nearby [1]. Hereafter, we call these 140Ce "good" 140Ce.
From the oscillation amplitude, "good" 140Ce are about
30% and 40% of the implanted 140Ce, respectively, forunannealed 140CeFe and 140CeHe-doped Fe (“35%” inINTRODUCTION is the average of these two values). We
now look at Fig. 1(a) downwards. The oscillation
amplitude gets reduced by about 20% at the annealing
temperature of 473 K and about 50% at 673 K. At theannealing temperature of 773 K, there is no noticeable
oscillation component, i.e., there is no "good" 140Ce in

140CeHe-doped Fe-0.05

A22G22 (t)Y. Ohkubo, A. Taniguchi, Q. Xu, M. Tanigaki and K. Sato1

A22G22 (t)

PR6-1

0.00
51015t (ns)

2025

05

1015

2025t (ns)Fig. 1. Room-temperature TDPAC spectra for (a) 140CeFe and(b) 140CeHe-doped Fe, subjected to no annealing, 0.5-hannealing in vacuum at 473, 673, 773 and 1073 K (afterannealed at 873 and 973 K).

REFERENCES:[1] Y. Ohkubo, A. Taniguchi, Q. Xu, M. Tanigaki, N. Shimizu
and T. Otsuka, Phys. Rev. C 87, 044324 (2013).
[2] Y. Ohkubo, A. Taniguchi, Q. Xu, M. Tanigaki and K. Sato,
Hyperfine Interact. 230, 187-193 (2015).

27P6-4- 34-Thermal Stability of Cd and In Impurities in Fe3O4

W. Sato, T. Ida, S. Komatsuda1, T. Fujisawa2, S.
Takenaka2 and Y. Ohkubo3
Institute of Science and Engineering, Kanazawa
University1
National Institute of Technology, Ichinoseki College2Graduate School of Natural Science and Technology,Kanazawa University
3Research Reactor Institute, Kyoto University
INTRODUCTION: Magnetite (Fe3O4) is one of themost promising magnetic compounds applicable tospintronic devices due to its excellent electric
conductivity and magnetic property. In addition to theintriguing nature of pure Fe3O4, it has been demonstrated
that the physical properties of the oxide vary with theintroduction of impurity ions, which allows us to expect
wider application of the doped oxide.
In this respect, our interest was directed to functions ofnonmagnetic indium ions (In3+) in Fe3O4. The hyperfine
fields at the impurity site have been reported in many
papers in the past three decades; however, there is still acontroversy even over their residential sites [1-3]. It isthus of importance for a basic study of solid-state physics
of the compound to clarify the site(s) occupied by theimpurity In ions.
In the present work, we succeeded in the preparation of111
In-doped Fe3O4 exhibiting perturbed angularcorrelation (PAC) spectra with distribution-free Larmor
frequencies composed of a unique component, and theimpurity In site was unequivocally determined by acomparative study with the assistance of a different PAC
In addition to the site
probe 111Cd( ← 111mCd).
information, we successfully detected thermal behavior
of In ions at high temperature. The element-dependent
thermal stabilities observed for Cd and In are discussed.
EXPERIMENTS: About 3 mg of cadmium oxide (CdO)
enriched with 110Cd was irradiated with thermal neutrons
at Kyoto University Reactor to produce 111mCd. The
neutron-irradiated CdO powder was then added into
Fe3O4 powder and the homogenized mixture in the formof a disk was sintered at 1173 K for 45 min.
Commercially available 111In HCl solution was added indroplets onto a disk of Fe3O4. It then underwent heat
treatment in a vacuum quartz tube at 1373 K for 3 h forthe diffusion of the radioisotope.
TDPAC measurements of the 111Cd( ← 111mCd) and111
Cd(←111In) probes were performed for the synthesized
samples at various temperatures.
RESULTS: The TDPAC spectra of 111mCd(→111Cd) and111
Cd(←111In) embedded in Fe3O4 are shown in Fig. 1.
The Larmor frequencies ωL observed for both probes atroom temperature show good agreement with each other,signifying that both probes reside in the same site. At
high temperature above TC, however, contrastive patterns
appear in their spectra. For the 111mCd probe, thedirectional anisotropy is unperturbed during the time ofthe present observation. This observation clearly showsthat the probe nucleus is surrounded by a charge
distribution of cubic symmetry; that is, 111mCd ions
initially occupy the tetrahedral A site. This is also trueof the 111In ions because of the same ωL values at R. T.
However, the different observation for the different
probes at high temperature implies that the 111In ionsmigrate into another site. The spectrum in Fig. 1(c) was
well fitted assuming two different components: the major
component is unperturbed and the minority has an axiallysymmetric electric field gradient. Detailed investigationof the temperature dependence of PAC spectra revealed
that the minor component corresponds to the probes atthe substitutional B site [4].
In addition, DFT
calculations suggest that the thermal stability of In ionsare lower at the A site than that of Cd [4]. From theseobservations, we conclude that Cd ions are stabilized in
the A site at least in the present temperature range, while
In ions at the A site jump out of the potential at hightemperature to move into the B site. We believe that thethermally activated displacement of In ions could be an
important clue to materials design for impurity-doped
magnetite.
"#&!"#&'()"#%$',)

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PR6-2!"#&"#&'+)"#%$'-)!"#%$"#%!"#%"#"$!"#"$""!"#"$!"#%"$"%"" %$" &"" &$" *"""#"$"$"%"" %$" &"" &$" *""!!"!#$Fig. 1. PAC spectra (a) of 111Cd(←111mCd) at 873 K, (b) of111
Cd(←111mCd) at R. T., (c) of 111Cd(←111In) at 900 K, and (d)of 111Cd(←111In) at R. T.

REFERENCES:[1] K. Asai et al. J. Phys. Soc. Jpn. 54 (1985) 4325-4330.
[2] Z. Inglot et al. J. Phys. Condens. Matter 3 (1991)
4569-4585.
[3] I. T. Matos et al. J. Appl. Phys. 117 (2015) 17D511(1-4).
[4] W. Sato et al. submitted for publication.

27P6-5- 35 -PR6-3

Formation Process of Oxygen Vacancy in Zinc Oxide

S. Komatsuda, W. Sato1 and Y. Ohkubo2

INTRODUCTION: Defect-induced properties of zincoxide (ZnO) have been attracting much attention toward
their application to functional materials in a wide field ofindustry. Especially, physical properties brought about
by Al ions and/or oxygen vacancies in ZnO are one of themost intriguing topics for the development of future electronic devices. Extrinsic-semiconductor devices such asof Al-doped ZnO are expected to be in use under various
ambient conditions; the states of being of impurity ions in
the matrix are susceptible to change depending on thecondition. For a practical use of Al-doped ZnO device,
therefore, we have investigated the local structures inAl-doped ZnO under various ambient conditions bymeans of the time-differential perturbed angular correlation (TDPAC) method. In one of our previous TDPAC
studies, we observed contrasting atmosphere dependenceof the stability of aggregations of 111In and Al impurities
doped in 100 ppm Al-doped ZnO: (i) Al and In impurities
associate with each other by their thermal diffusion in air,and (ii) the 111In probe is detrapped from the Al aggregations in high-temperature vacuum, resulting in substitution at defect-free Zn sites [1,2]. Detailed investigationof the thermal behavior of the impurities has revealed thatthe dissociation reaction is triggered by the formation ofoxygen vacancies. In the present work, in order to extend quantitative discussion on the kinetics of the formation process of oxygen vacancy, we evaluated theformation energy of oxygen vacancy from the temperature variation of the detrapping process during heat
treatment in vacuum [3].
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
suspension was heated to evaporate the ethanol until
dryness. The powders were pressed into disks and sintered in air at 1273 K for 3 h. Commercially available
111
In HCl solution was added in droplets onto the sintereddisks. The initial concentration of 111In ions doped in
the sample was typically ~100 ppt. After the disks weredried up by heat, they again underwent heat treatment in
air at 1373 K for 2 h. Following the doping of 111In,
each disk was separately sealed in different quartz tubes
in vacuum, and 24-h isochronal annealing was performed
one by one at different temperatures (873~1148 K).
The TDPAC measurements were carried out for the171-245 keV cascade γ rays of the 111Cd(←111In) probewith the intermediate state of I = 5/2 having a half-life of85.0 ns.

RESULTS: It was found from the TDPAC spectra of
the 111In(→111Cd) in 100 ppm Al-doped ZnO heat-treatedat various temperatures that 111In probes come to detrap
from the Al aggregations in high-temperature vacuum bydegrees. It was revealed that this detrapping process iscontrolled by the first-order rate law. For the first-order
reaction of the dissociation process, we obtained the rate
constant k at different temperatures. Their temperature
variation is plotted in Fig. 1. A least-squares fit to the kvalues was then carried out with the following Arrhenius
equation:k = ν0 exp (-Ea / kBT )and the activation energy, Ea, was evaluated to be 0.72
(6) eV. Because the dissociation process of the 111In
probe and Al would be induced by the formation of oxygen vacancies as discussed above, we suggest that theobserved Ea is closely related to the formation energy ofoxygen vacancies in Al-doped ZnO sample. This interpretation is supported by the fact that the present Ea value
shows good agreement with the theoretical ones calculated for the formation energy of oxygen vacancies inZnO (0.8 and 1.0 eV)[4,5].

10-410-5

10-610-7

k (s-1)

Department of General Education, National Institute ofTechnology, Ichinoseki College
1
Institute of Science and Engineering, Kanazawa University
2Research Reactor Institute, Kyoto University

910104/T (K-1)

11Fig. 1. Temperature dependence of therate constant, k. An Arrhenius equation
was used for the fit.
REFERENCES:[1] S. Komatsuda, W. Sato, S. Kawata, and Y. Ohkubo, J.
Phys. Soc. Jpn. 80 (2011) 095001 (1-2).
[2] S. Komatsuda, W. Sato, and Y. Ohkubo, J. Radioanal.
Nucl. Chem. 303 (2015) 1249-1252.
[3] S. Komatsuda, W. Sato, and Y. Ohkubo, J. Appl. Phys.
116 (2014) 183502 (1-5).
[4] S. Lany and A. Zunger, Phys. Rev. Lett. 98 (2007)
045501 (1-4).
[5] F. Oba, A. Togo, and T. Isao, Phys. Rev. B 77 (2008)
245202 (1-6).

27P6-5- 36 -