Journal of Nuclear and Radiochemical Sciences, Vol. 4, No. 1, pp. 15-18, 2003

ArticlesTime-Differential Perturbed Angular Correlation of 140Ce Implanted in Highly Oriented
Pyrolytic Graphite
W. Sato,*, a Y. Kasamatsu,a Y. Ohkubo,b A. Taniguchi,b and A. ShinoharaaaGraduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japanb

Received: May 2, 2003; In Final Form: June 27, 2003
Using an ion implantation method, we introduced cerium nuclei in highly oriented pyrolytic graphite (HOPG), andhave studied their behavior by means of the time-differential perturbed angular correlation technique. The timevariant directional anisotropy for the present pseudo-single crystalline sample is little different from that for polycrystalline graphite. Temperature dependence has been observed on the electric quadrupole interaction between
the probe nuclei and the extranuclear charge distribution: the anisotropy shows gradual attenuation at room temperature, whereas the effect of an electrostatic perturbation is implied in the spectrum at 10 K. For the chemicalstate of the probe atom in HOPG, the trivalent state is suggested from the large magnitude of the electric field gradient at the probe nucleus.

1. Introduction
The physical properties of graphite compounds, represented
by LixC6, have attracted much interest, and the investigations
on their electric conductivity have realized industrial application of the material as an electrode of a rechargeable battery.1
In relation to this study, further investigation on doped
graphite compounds have been made, and some intriguing
properties on the mobility and occupying sites of the doped
atoms have been reported. Zabel et al.2 succeeded in observing
dynamic motion of intercalate alkali-metal atoms (Li, K, Rb,and Cs) in graphite, estimating the diffusion constants and activation energies for each of the atoms. For highly oriented
pyrolytic graphite (HOPG) doped with boron atoms, there is areport that the dopant is substituted for the carbon atom rather
than is intercalated in the layers like the alkali-metal atoms.3 It
is suggested from these observations that behavior of atoms ingraphite indeed depends on the introduced elements havingintrinsic chemical properties. Because doped graphite isexpected to exhibit unique properties on account of the twodimensional (2D) structure, it should be interesting to study thecase for dopants of other types of elements.
In our previous work, we studied the behavior of Ce atoms
implanted in polycrystalline graphite by means of the timedifferential perturbed angular correlation (TDPAC) method,and obtained an implication that the Ce nuclei are dynamically
perturbed from the extranuclear field.4 As the principal axis of
the electric quadrupole interaction is randomly oriented due tothe sample form, however, the experimental results do notallow discussing whether the motion is two dimensional. In
expectation of directional information of the dynamic motion,therefore, Ce atoms were implanted in pseudo-single crystal
HOPG in the present work.
For the observation of the behavior of dopant atoms, theTDPAC method was employed again. In this spectroscopy isapplied a nuclear technique in which one observes the time
variation of the directional anisotropy of a γ−γ cascade fromexcited nuclei. By observing the time-variant anisotropy, one
can obtain direct information concerning the fluctuation of theprincipal axis of the electric field gradient (EFG) produced at

the site of the probe nucleus. In other words, this method
allows evaluating the magnitude of the EFG and/or the reorientational correlation time of the probe nucleus. Taking advantage of this spectroscopy, we discuss the interacting nature
between the probe and the matrix carbon atoms.
2. Experiment
HOPG with a size of approximately 6 mm φ × 1 mm wasstuck on a Teflon-sheet backing of 1 × 10 × 10 mm3, and thenattached to the ion beam collector equipped at the isotope
separator on-line installed at the target chamber (T-1) in Kyoto
University Reactor (KUR-ISOL). The fission products derived
from the enriched 235 U target in the chamber were rapidlytransported into the ion source equipped outside the reactor.
After surface ionization, they were accelerated by the acceleration voltage up to 30 kV and were separated according to theircharge-to-mass ratios by the analyzer magnet. The ion beamof interest, 140Cs, was reaccelerated by the acceleration voltageof 80 kV and was kept implanted in the target HOPG forapproximately 30 h. The number of implanted 140Cs atoms
was estimated to be approximately 1 × 1011 in total. Detailed
description and schematic diagrams of the system of KURISOL appear in References 5−7. The probe nucleus 140Ce isprovided through the disintegration process shown in Figure 1.
Figure 2 shows a typical γ-ray spectrum from a sample after
140

Cs

63.7 sβ−140Ba12.8 dβ−140La

40.3 hβ−3+2412 keV

329 keV+T1/2 = 3.45 ns 42083 keV

487 keV2+1596 keV0+140*Corresponding author. E-mail: wsato@chem.sci.osaka-u.ac.jp.
FAX: 06-6850-5418.

CeGround State

Figure 1. Simplified decay scheme of 140Ce disintegrated from 140Cs.

© 2003 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/30/2003

16SatoJ. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003
-0.2

105

140140
La
La
Ba329 keV 487 keV
163 keV140
Ba537 keV
140140La816 keV

La1596 keV

103-0.1

102-0.05

101

0100

01000

2000

3000

298 K-0.15

4000

R (t )Counts104

140-0.2Channel
Figure 2. γ -ray spectrum measured for a sample 4 days after theimplantation of 140Cs.

10 K-0.15

-0.1

-0.05

0c-axisπ/40.05
0510152025Time/ns

Figure 4. TDPAC spectra of 140Ce implanted in HOPG at 298 and 10 K.

Figure 3. Schematic geometry of the HOPG and BaF2 scintillation
counters.

the implantation. It was confirmed by the spectrum that theprojectile was well isolated in the mass separation process.
After radioactive equilibrium between 140Ba and 140La wasachieved, TDPAC measurements were performed at 298 and
10 K for the probe 140Ce on the 329-487 keV cascade γ rays
with the intermediate state of 4+ having a half-life of 3.5 ns. In
the present work, the directional anisotropy of the angularcorrelations of the cascade γ rays was observed at π/2- and πradian directions with a conventional four-detector system.
The pseudo-single crystalline sample was placed so that the caxis would be parallel to the detector plane and each detector
would be directed to the 2D plane at π/4 radian as illustrated inFigure 3. For the coincidence detection, BaF 2 scintillation
counters with 1.5 in. φ × 1 in. pure crystal were adopted; thetime resolution of the present system was estimated to be 500
to 600 ps based on the full width at half maximum of peaks forprompt coincidence. As regards solid angle correction for theγ-ray detection, this work followed a method developed byLawson and Frauenfelder.8
3. Results and Discussion
TDPAC spectra of 140Ce implanted in HOPG are shown inFigure 4. In the present work, we defined the directional
anisotropy, R(t), which is a function of the time-differential
perturbation factor, G22’(t), by a simple arithmetic operation as
2[N(π, t) – N(π /2, t)]
R(t) =,N(π, t) + 2N(π /2, t)(1)

where t is the time interval between the cascade γ rays andN(θ, t) denotes the number of the coincident events observed at
an angle, θ. As the present sample is a pseudo-single crystal,the observed data are to be dealt with accordingly. It is difficult, however, to see clear difference in the spectra from thoseobserved for polycrystalline graphite investigated in theprevious work. 4 More specifically, the time spectra do notreflect the difference of the sample form, that is, the orientation of the c-axis. For simplicity, therefore, we hereafter treat
the data in the same way as the case for a polycrystalline
sample.
One can see for the time spectrum observed at room temperature that the directional anisotropy shows gradual relaxation
with a slight oscillatory structure at about 3 ns. This relaxation
appears to be an exponential-type attenuation following
G22’(t) = exp(−λ t),(2)which was deduced by Abragam and Pound9, 10 using the diffusion approximation for the typical case that the fluctuation of
the extranuclear field is so fast that the static interaction withthe probe nucleus is hardly kept during the time window of theintermediate state of the cascade. As for the dip at around 3ns, coexistence of another small component experiencing astatic perturbation may be suggested.
The TDPAC spectrum at 10 K shows rather fast attenuationof the directional anisotropy. After the rapid attenuation,
however, the anisotropy maintains an almost constant valueeven until 25 ns. Taking into account the magnitude of thehard core,11 there is a possibility at this low temperature thatthe probe has a static electric quadrupole interaction with theextranuclear charge distribution. In the limited case for anaxially symmetric EFG produced at the probe nucleus by theouter surrounding charge distribution, the perturbation factor
for the present cascade can be expressed by1[331+10cos(3ωQt)+81cos(9ωQt)+180cos(12ωQt)
1155+175cos(15ωQt)+196cos(21ωQt)+126cos(24ωQt)+56cos(36ωQt)], (3)staticG22(t) =in the case of a polycrystalline source. Here, ωQ stands for theTime-Differential Perturbed Angular Correlation of 140Ce ImplantedeQVzz,4I(2I−1)h(4)with the EFG at the site of the probe nucleus, Vzz . In eq 4, Q
represents the nuclear quadrupole moment of the intermediate
state of the cascade (= 0.35 ± 0.07 barn) and I is the nuclear
spin (= 4 for the present case).12 For the interpretation of the10-K spectrum, simulated perturbation patterns are drawn
based on eq 3 in Figure 5. Distributions are assumed for theelectric quadrupole frequency as7

staticG22(t)δω Q = a0 + ∑ ai exp(−i=11 2 2 2δ ω Qi t )cosω Qi t,27(6)i=1-0.2

A 22G 22 (t )

-0.15

-0.1

-0.05

0051015

20−4. Summary

The relative width, δ, is defined by δ = σ /ω 0Q, where σ is thewidth of the distribution and ω 0Q is the centroid of ω Q for eachterm. The simulated line with δ = 0.5 appears to reproduce
fairly well the low temperature spectrum by assuming anothercomponent.
The observations above are reasonably explained. It isconsidered that the probe atoms at low temperature should
occupy certain stable site(s) after the implantation, having astatic interaction with the outer surrounding carbon atoms.
The wide distribution of the electric quadrupole frequency
could be attributed to thermal vibration at the optimum potential well and/or to slight differences in the EFGs due todifferent occupied sites. At room temperature, on the otherhand, the exponentially attenuating component predominatesin the time spectrum; this may imply nuclear spin relaxation
caused by a dynamic perturbation from the extranuclear fluctuation. Considering the thermal motion of alkali-metal atoms ingraphite-intercalated compounds, 2 the present phenomenon
could be interpreted as a thermally activated relative motion of
the probe against ambient carbon atoms. For investigating
whether the motion is two dimensional, sample annealing
following the implantation would be needed.
Based on the assumption that the solid line in Figure 5
reproduces the spectrum at 10 K with an additional component, the EFG at the site of the probe nucleus is as large as

0.05

2(5)

where eq 3 is expressed in a general form as
staticG22(t) = a0 + ∑ ai cosω Qi t.

17Vzz= 1 × 10 V/cm . After β disintegration of a trivalentLa3+, the daughter nuclide should take tetravalent state, Ce4+,
in most cases as reported in References 13−18, and the estimated EFG value for 140Ce4+ is Vzz~1017 V/cm2.19 From thecircumstantial evidence that the difference of the outersurrounding lattice distribution makes little distinction among
the EFGs at 140Ce4+, we propose that the present large EFG be
assigned to the distortion of the extranuclear charge distribution caused by a 4f electron in trivalent Ce3+. At the TDPAC
measurement, the valence state of the probe is considered tohave become trivalent through a prompt rearrangement of theelectronic configuration.
19

electric quadrupole frequency defined by

ωQ =J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003

25Time/ns
Figure 5. Simulated perturbation patterns for the case of an axiallysymmetric electric field gradient for a polycrystalline source. The
electric quadrupole frequency is assumed as ωQ = 5 × 107 rad/s. The
dotted and solid lines correspond to the frequencies with distributionsof δ = 0 and δ = 0.5, respectively. The angular correlation coefficientis assumed to be A22 = -0.125.

The TDPAC method has been applied to the study of Ceimplanted HOPG. The time spectra for the pseudo-single
crystal are little different from those for polycrystalline
graphite, which implies that the principal axis of the EFG ateach probe nucleus is not oriented to the same direction in thesample. At room temperature, the directional anisotropy of thecascade shows exponential-type relaxation in the present time
window, whereas the low-temperature spectrum reflects anelectrostatic interaction with a wide distribution of the EFG.
These observations may suggest the temperature dependenceof the mobility of the implanted Ce atoms. For more detailed
discussion on the direction of the field in HOPG, it is essential
to observe the angular correlation for the sample with the caxis perpendicular to the detector plane as well.
Acknowledgments. The authors are grateful to Prof. K.
Asahi for his cooperation and special interest in this work.
They express their gratitude to Prof. T. Saito and Dr. Y.
Yamaguchi for providing experimental facilities for theTDPAC measurements. They also thank Prof. Y. Kawase forarranging the operation of KUR-ISOL. Dr. Y. Kobayashi generously donated the HOPG sample for the research. The present work was accomplished as part of the Visiting
Researcher’s Program of the Kyoto University Research
Reactor Institute.
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18J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003

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