Journal of Nuclear and Radiochemical Sciences, Vol. 8, No. 2, pp. 79-84, 2007

TDPAC and Its Application to Chemistry
Y. Ohkubo,*,a Y. Murakami,b W. Sato,c and A. YokoyamadaResearch Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan
Graduate School of Science, Kyoto University, Kyoto 606-8502, Japanc
Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japand
Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa, Ishikawa 920-1192,Japanb

Received: October 25, 2006; In Final Form: December 26, 2006
Time-differential perturbed angular correlation (TDPAC) is a powerful tool in material science for measuring the
local electromagnetic fields at probe nuclei through hyperfine interactions. We have been applying this techniquewith reactor-made radioactive nuclei to several different material systems. In this paper, following a brief description of the TDPAC technique, our recent results are reviewed.

1. Introduction→

This paper aims to fulfill two objectives. The first one is tooutline the theory and technique of the time-differential perturbed-angular-correlation (TDPAC) method, which is a
nuclear spectroscopic technique for investigating local structures, electronic states, etc., in materials via hyperfine interactions. A detailed account of the perturbed-angular-correlation
theory is given by Frauenfelder and Steffen.1 The second is to
give a brief account of TDPAC experiments using the Kyoto
University research reactor (KUR). Our recent results arethose on the ferroelectric phase transition of LiTaO3, 2 local
magnetic fields in the Mo layer of Mo/Fe multilayer,3 hopping
motion of Ce in graphite,4 and hyperfine fields in a protein,mavicyanin.5→

W(p1, p2, t) =

Σ A (1)A (2)G

k1k2 N1 N2

k1

k2

N1N2
–1/2
k1k2 (t)[(2k1+1)(2k2+1)]

× Y Nk11 * (θ1,ϕ1)Y Nk22 (θ2,ϕ2),
→

(2)

→

where p1 and p2 are the momenta of the first and second γ rays,
respectively, and θi and ϕi (i = 1, 2) in the spherical harmonics
are their spherical coordinates. Like Ak (1) and Ak (2) in eq 1,
Ak1 (1) and Ak2 (2) are quantities concerning the first and second γtransitions, respectively, determined only by the nuclear properties. The parameters ki (i = 1, 2) are non-negative even integers
and Ni are integers satisfying –ki ≤ Ni ≤ ki. The perturbation factor G Nk11kN2 2 (t) contains information on the interaction of thenucleus in its intermediate state having spin I with extranuclear
fields and is written as

2. Outline of the TDPAC Theory
The two γ rays in a cascade process emitted from an excitednucleus exhibit the angular correlation between them, which
depends primarily on nuclear properties such as the multipolarities of the γ rays and the spins of the relevant levels. The
angular correlation W(θ) is expressed as follows; W(θ)dΩ isthe relative probability that the second γ ray is emitted into thesolid angle dΩ at an angle θ with respect to the emission direction of the first γ ray.
W(θ) =Σ A (1)A (2) P (cosθ),
k

k

k

k

(1)

where the summation index k takes non-negative even integers
with the upper limit determined by the values of the multipolarities of the two γ rays and the spin of the intermediate
nuclear level, and Pk is the k th -order Legendre polynomial.
Ak (1) and Ak (2) are quantities concerning the first and second γtransitions, respectively, determined only by the nuclear properties.
When a nucleus in its intermediate level interacts with an
extranuclear field, the angular correlation is perturbed. The
general expression for the perturbed angular correlation is asfollows:

G Nk11kN2 2 (t) =Σ (–1)

2I+ma +mb

 I I k1 [(2k1+1)(2k2+1)]1/2  m' –m N 
a

m amb

 I I k × m' –m N2 〈mbΛ(t)ma〉 〈m'bΛ(t)m'a〉∗, b b 2

a

1

(3)

where the eigenvalues such as ma in the 3-j symbols are referred
to an arbitrarily chosen z-axis. The quantity Λ(t) reflects theinteraction of the nucleus with extranuclear fields. In the caseof static interactions, Λ(t) is written as exp(−iHintt/h), where Hint
is the time-independent interaction Hamiltonian. If there is noeffective extranuclear field, i.e., Λ(t) = 1, G Nk11kN2 2 (t) is equal toδN1N2 δk1k2 and eq 2 is thus reduced to eq 1, as it should be. Using
the eigenvector n〉 of Hint with the eigenvalue En, eq 3 is rewritten as
G Nk11kN2 2 (t) =Σ Σ (–1)

2I+m a +m b

mamb n,n'[(2k1+1)(2k2+1)]1/2 e–(i/h )(En–En')t I I k   I I k2 × 〈nmb〉∗〈nma〉 〈n'm'b〉 〈n'm'a〉∗ m' –m N1. (4) a a 1   m'b –mbN2 
If the perturbing field is of axial symmetry and the z-axis ischosen parallel to the symmetry axis,*Corresponding author. E-mail: ohkubo@rri.kyoto-u.ac.jp. FAX:
+81-72-451-2625.

© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/19/2007

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OhkuboJ. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007
G NN
k1k2 (t) =Σ[(2k1+1)(2k2+1)]1/2

m I I k1   I I k2 ×  m'–mN   m'–mN  e–(i/h )(Em–Em')t .

(5)

When the sample is a powder source, i.e., an ensemble of randomly oriented microcrystals, the perturbation factor must be
averaged over the Euler angles which transform the laboratory
coordinate system to the coordinate system of each microcrystal. As a result, the perturbation factor does not depend on theparameters N1 and N2:
Gkk (t) =Σ Σ (–1)

2I+ma+mb

N,ma,mb n,n'

 I I k   I I k  –(i/h)(En–En')te m'a –maN   m'b –mbN × 〈nmb〉∗〈nma〉 〈n'm'b〉 〈n'm'a〉∗.
→

→

→

k

→

→

kk

Σ  n'I–nIn–n'k  e

k

n,n'

k

k

k

(10)

L

N(θ,t) = exp(–t ln2/t1/2)W(θ,t),(7)

R(t) = [N(180°,t) – N(90°,t)] / [N(180°,t) + N(90°,t)]≈ 3 A2 (1)A2 (2)cos(2ωLt),
4

(8)

.

(11)

where t1/2 is the half-life of the intermediate state of the probe
nucleus. Assuming that the maximum value of k is 2, the ratio
R(t) obtained with the coincidence counts at θ = 180° and 90°
becomes2–(i/h)(En–En')tΣ A (1)A (2)P [cos(θ–ω t)],k=0where θ is the angle between p1 and p2 . Since in this form thedifferences from the expression for the unperturbed angularcorrelation, eq 1, are only the factors G kk (t), they are particularly called the attenuation factors. When each microcrystal isof axial symmetry, Gkk (t) has the simpler form
Gkk (t) =k max

taking θ1 = θ2 = 90° and ϕ2 − ϕ1 = θ. Coincidence counts N(θ, t)of the cascade γ rays are given byΣ A (1)A (2)G (t)P (cosθ),k(9)where k1 = k 2 = k. Substituting eq 9 into eq 2 leads to theexpression

(6)

→

k

G NN
kk (t) = exp(–iNωLt),

W(θ,t) =The angular correlation W(p1, p2 , t) has the formW(p1, p2, t) = W(θ,t) =where a uniform static magnetic field is applied perpendicular tothe plane on which two γ detectors and a sample→containing
TDPAC nuclei are situated. The magnetic field (B) is axiallysymmetric, and the relevant
expression is eq 5 with Em = mh ωL→(z-axis is taken parallel to B). Here, ωL is the Larmor frequency,
and using the nuclear g-factor and nuclear magneton µN, it isdefined as ωL = –gµNB/h. The exponential factor depends on N
= m–m' and thus can be moved outside the summation symbol.
Using the orthogonality relation of the 3j-symbols, the perturbation factor is written as

Note that the perturbed angular correlation is observable evenfor polycrystals.
Here, as a simple example we give the expression for the case

(12)

From the measurements of the angular correlation, ωL can beobtained. Once ω L is obtained and when either of the twoquantities g or B is known, the other quantity can be derived.
TDPAC technique gives the product of such two quantities.
One is concerned with nuclear properties such as g-factor

TABLE 1: Characteristics of γ -γ cascades appropriate for TDPAC
Decay mode
Parent → Probe
Half-lifeβ–
99
Mo → 99Tc
2.7 d
EC
99
Rh → 99Ru
16 d
EC
100
Pd → 100Rh
3.6 d
β–
111
Ag → 111Cd
7.5 d
IT
111m
Cd → 111Cd
49 minEC
111
In → 111Cd
2.8 d
β–
117
Cd → 117In
2.5 h
β–
140
La → 140Ce
1.7 d
β–
181
Hf → 181Ta
42 d








Iπ

Mean life / ns

µ / nm

5/2+

5.2+3.291

3/2+

29.6–0.2842+309+4.324

5/2+

123–0.7656Q/b+0.231+0.77

γ1 / keV

γ2 / keV

A22

740

181

+0.10

528
353

90
90

–0.19
–0.15

84

75

+0.17

96

245

–0.13

151

245

+0.18

171

245

–0.18

3/2+

77.3

+0.938(–)0.59

90

344

–0.36

4+

5.0

+4.35

0.35

329

487

–0.13

5/2+

15.6

+3.29

(+)2.35

133

482

–0.20

TDPAC and Its Application to Chemistry
AMP1
Det 1

Source
AMP2

Det 4

Det 2
Det 3

AMP3

AMP4

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

SCA1a
SCA1b
SCA2a
SCA2b

Coin. Matrix

SCA3a
SCA3b
Router

SCA4a
SCA4b

Gate
LG

ADC

TAC

PHA

Start
Delay
Stop

Figure 1. Block diagram of the TDPAC measurement system withfour BaF2 detectors.

(a)

(b)

PS

Figure 2. The (a) ferroelectric and (b) paraelectric structures of the
LiNbO3 -LiTaO3 system. The arrow indicates the direction of the
spontaneous polarization (Ps) in the ferroelectric phase. Li is represented by the hatched, Nb (Ta) by the dotted circles. The horizontal
lines stand for the oxygen planes.

which nuclear physicists are interested in and the other withmaterial properties which chemists are interested in.
The expressions for the cases where the perturbing fields are
static electric fields and dynamic ones are described in sec. 5
together with our recent TDPAC results.
3. TDPAC Probes
Conventional TDPAC probes are 99 Ru(←99 Rh), 100 Rh(←Pd), 111Cd(←111Ag), 111Cd(←111mCd), 111Cd(←111In), 117 In(←
117Cd), 140Ce(←140La), and 181Ta(←181Hf). The relevant quantities for these probes are summarized in Table 1, together withthose for 99Tc(←99Mo) used in our recent study on Mo/Fe multilayers. As seen from the half-life in Table 1, the time range
during which interactions of nuclei with their surroundings areobserved is the order of 10–1000 ns. 99Ru and 100Rh are especially useful for probing materials containing 4d transition elements. Likewise, 14 0 Ce is good for studying materials
comprising rare-earth elements. 111Cd can be produced fromthree different elements, Ag, Cd, and In. When these elements
may occupy distinct sites in materials, corresponding 111CdTDPAC measurements can bring more information than a single measurement using only one parent element. 111Cd(←
111mCd) and 117In(←117Cd) are an interesting pair. Both parent
elements are the same, i.e., they occupy the same sites, but
111
Cd- and 117 In-TDPAC spectra may indicate that Cd and In
behave differently (an example is given in sec. 5.1). Recently,
19
F arising from 19O was used as a new TDPAC probe.6100

4. TDPAC Measurement System
A measurement system can be constructed with standard fast-81

slow electronic modules and two γ detectors. However, it isoften that four γ detectors are used and placed at right angles
with each other on a plane. By doing this, 4 sets of N(180°, t)
and those of N(90°, t) can be obtained at one measurement, sothat a high measurement efficiency is gained. Moreover, thisarrangement can avoid the unwanted effects owing to different
detection efficiencies (which include the effect of different
source-to-detector distances). For γ detection, scintillation
detectors with high Z elements and good timing response areused. BaF2 is such a scintillator. Figure 1 displays the block
diagram of our TDPAC measurement system using four BaF2
detectors.
5. Recent TDPAC Experiments Using the Kyoto University
Research Reactor
5.1. Ferroelectric transitions in LiNbO3 and LiTaO3 . 2
Lithium niobate, LiNbO3, and isostructural lithium tantalate,
LiTaO3, are ferroelectrics and are known as, for example, important non-linear optic materials. LiNbO3 and LiTaO3 adopt anilmenite (FeTiO3)-related structure (not the perovskite structure)
because of the similar ionic sizes of Li+ (76 pm), Nb5+ (64 pm),and Ta 5+ (64 pm). These oxides undergo a structural phasetransition (corresponding to a ferroelectric-to-paraelectricphase transition) at temperatures (TC ) of 1483 K and 938 K,
respectively. The ferroelectric structure belongs to the R3cspace group. The atomic arrangement consists of oxygen octahedra sharing faces along the polar trigonal axis. When the
spontaneous polarization direction is downward, the Nb (Ta) ionon this axis is displaced downward from the center of its octahedron [Figure 2(a)]. The next octahedron above contains a Li ion
displaced upward from its center and the one more above has no−
metal ion inside it. The paraelectric structure belongs to the R3cspace group. The Nb (Ta) ion is located at the center of its octahedron [Figure 2(b)]. The average Li position is on the uppertriangle oxygen base of the octahedron; the Li ion is disordered
and occupies the previously empty octahedron half the time
(dynamic disorder) or in half the unit cells (static disorder). In
order to examine whether dynamic disorder takes place or static
one does, TDPAC measurements using 111Cd(←111m Cd) and117
In(←117Cd) are effective. From a PIXE/channeling experiment, it is known that Cd (and also In) occupy the Li sites. If
dynamic disorder is the right phenomenon, the temperaturedependence of the quadrupole frequencies for 117In and 111Cd
can be quite different, since the ionic radius of Cd2+ (95 pm) is
so large that local dynamic disorder may be difficult to takeplace. On the other hand, the ionic radius of In3+ (80 pm) is
comparable to that of Li+ (76 pm), so that local dynamic disorder is not hampered. Since the bonding of In3+ with oxygen isstronger due to the higher charge, local dynamic disorder mighttake place at lower temperature than TC, as is suggested by thefact that replacing a part of Li with In reduces TC (e.g., TC of
Li0.8In0.067TaO3 is 818 K,7 120 K reduction).
Typical TDPAC spectra of 117In and 111Cd in polycrystalline
LiTaO3 are shown in Figures 3(a) and 3(b), respectively. The
ordinate A22G22(t) was obtained as follows:
A22 G 22 (t) = 2

N(π,t) – N(π /2,t).
N(π,t) + 2N(π /2,t)(13)

The quantity A22 is the same as A2 (1)A2 (2) in eq 7. The patternin each spectrum is modulated in a manner characteristic of asingle electric quadrupole interaction in polycrystalline samples, depending on the spin value of the intermediate nuclear
state. Since the Li site is of axial symmetry, the expected
expressions of G 22 (t) for 117In (I = 3/2) and 111Cd (I = 5/2) are,respectively,

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J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

(a)

117

OhkuboIn(←117Cd)

-0.4

6.5

4.2 K

6.0
Vzz(lattice)/10 Vm

−2

-0.2

20

A22G22 (t)

0.0

0.2

-0.4 1073 K
-0.2
0.0

0
111

(b)

Cd(←

20
111m

40

Cd)

t/ns

60

80

100

4.2 K

0.10

117

In(←117Cd)

5.5
7

5.0

Li

×4

4.5
4.0

0.2

938 K

0

111

Cd(←111mCd)

400
800
Temperature/K

1200

Figure 4. Temperature dependences of the lattice part of Vzz at 117In(closed squares) and at 111Cd (open squares) in LiTaO3 with TC = 938
K, together with that for 7 Li in LiTaO3 (Reference 8) (solid line),
which is multiplied by a factor of 4.

0.05

0.00
A22G22 (t)-0.05

1073 K
0.05

0.00

-0.05

-0.10
0

50

100
t/ns

150

200

Figure 3. TDPAC spectra A 22 G 22 (t) of (a) 117In(←117Cd) and of (b)111Cd(←111mCd) in LiTaO3 at 4.2 and 1073 K with the solid curves representing the least-squares fits of eqs 14 and 15 in the text taking into
account of ωQ distributions.

G 22 (t) =1[1 + 4cos(6ωQt)],
5

(14)

and
G 22(t) =

1
13
10
[1 +
cos(6ωQt) +cos(12ωQt)
5
7
7
5
+
cos(18ωQt)],
7(15)

from eq 8 with En = [3n2 – I(I + 1)]h ωQ, where ωQ is the quadrupole frequency and is defined, using the nuclear electric
quadrupole moment of the intermediate state Q and the largest
component of the electric field gradient Vzz in the principal axis
system, as h ωQ = eQVzz /[4I(2I – 1)]. In the actual data analyses, G22(t) expressions without the assumption of axial symmetry
were used and the results were consistent with the case of axial
symmetry. Figure 4 displays the temperature dependences ofVzz (lattice) at 117In and 111Cd derived from dividing the obtained
Vzz by a factor (1 – γ∞) for each probe, where γ∞ is the so-called
Sternheimer antishielding factor. For comparison, also shownin Figure 4 is the temperature dependence of Vzz (lattice) at 7Li
obtained from 7Li-NMR.8
As can be seen in Figure 4, the increase rate of Vzz (lattice) at
117
In is very large, comparable to the case of 7 Li, while in thecase of 111Cd it is small. In a certain temperature range above
TC, Vzz (lattice) at 117In is constant, which is also true for the case

Figure 5. The local structure around a Li ion in the paraelectricphase of the LiNbO 3 -LiTaO 3 system. Li is represented by thehatched, and O by the open circles. The two upper oxygen triangles
represent a single one as being dynamically disordered.

of 7Li, while in the case of 111Cd there is no such constant region.
The existence of the constant part means that the local system
consisting of Li or In and oxygen ions is extremely stable, despite
the high temperatures. These features fit very well the idea ofdynamic disorder: Li or In of small ionic size comes closer tothe center of three oxygen ions surrounding it together with otherthree oxygen ions, as the temperature increases; Above TC, since
there is enough space among the three oxygen ions due to thermal expansion, Li or In goes back and forth through this hole.
This state is considered to be a sort of resonance state of highstability. On the contrary, Cd of large ionic size cannot move
much toward the center of the three oxygen ions and thus cannot
form a resonance state. Since the lowest temperature of the constant part of Vzz (lattice) at 117In coincides with TC , ions which
actually disorder must be oxygen, not Li or In (Figure 5).
Although we did not measure the TDPAC spectra for LiNbO3 attemperatures near TC because of high TC, we expect that the similar dynamic disorder of oxygen ions takes place also in thismaterial.
5.2. Local magnetic fields in the Mo layer of Mo/Fe multilayer.3 Metallic multilayers consisting of ferromagnetic layers and nonmagnetic layers in alternate sequence display
interesting magnetic phenomena. Oscillatory interlayer
exchange coupling is an important example. As the nonmagnetic-layer thickness is varied, the coupling between two ferromagnetic layers sandwiching a nonmagnetic layer periodically

TDPAC and Its Application to Chemistry

TABLE 2: Hyperfine magnetic fields (Bi, i = 1–4) at 99Tc in(A) Fe(10 nm)/[Mo(1.1 nm)/Fe(2.0 nm)] 120 and (B) Fe(10nm)/[Mo(1.3 nm)/Fe(2.0 nm)] 120 at room temperature
obtained with TDPAC

0.15

R(t)

0.10
0.05

Sample A

Sample B(a) Fe(10nm)/[Mo(1.1nm)/Fe(2.0nm)]120
-0.05
0

20

15

10
t/ns

5

0.15

(b) Fe(10nm)/[Mo(1.3nm)/Fe(2.0nm)]120
0.10
R(t)

Bi / T
Fraction

0.00

0.05
0.00

-0.05
0

10
t/ns

5

15

20

Figure 6. TDPAC spectra R(t) at room temperature of 99Tc(←99Mo)
in (a) Fe(10 nm)/[Mo(1.1 nm)/Fe(2.0 nm)]120 and in (b) Fe(10 nm)/[Mo(1.3 nm)/Fe(2.0 nm)]120 with the solid curves representing theleast-squares fits of eq 16.

changes from being ferromagnetic to antiferromagenetic.
Oscillatory interlayer exchange coupling is basically understood as due to the spin polarization of conduction electrons in
the nonmagnetic layer, but local information about magnetic
structure in the nonmagnetic layer is valuable.
Mo/Fe multilayer samples were prepared with the electronbeam evaporation technique: Fe(10 nm)/[Mo(d nm)/Fe(2.0nm)]120 (designed values of the Mo layer thickness d are 0.4,
0.7, 0.9, 1.1, and 1.3), the 10-nm Fe being a buffer layer, where
the subscript indicates the number of the bilayers. These samples have bcc(110) texture and two neighboring Fe layers areferromagnetically coupled with each other. The parent nuclei
99
Mo of the TDPAC probe 99Tc were obtained by irradiating thesamples with thermal neutrons. Figure 6 displays the roomtemperature TDPAC spectra for Mo(d = 1.1 nm) and Mo(d =1.3 nm). Assuming the perturbing fields are magnetic hyperfine fields, the TDPAC spectra were analyzed with the following expression:R(t) = [N(π,t) – N(π /2,t)]/2–t/t1/2=

83

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

3
A22
4

Σ P [a + a cos(ω t) + a cos(2ω t)].
i

i

i0

i
L

i1

i2

i
L

(16)

Here, t1/2 is the half-life of the intermediate state of the TDPAC
probe, Pi is the fraction of the probe nuclei perturbed by thehyperfine magnetic fields B i , and the values of a ij ( j = 0–2)
depends on the orientation of the hyperfine field with respect
to the detector plane, satisfying
aij = 1. ω iL is the LarmorΣjfrequency corresponding to Bi. Different from the case of theuniform magnetic field (see eq 1), here the term of cos(ωL t)
appears in addition to cos(2ωLt), because in the present casethe magnetic field is not axially symmetric. The solid curves

Bi / T
Fraction

i=1

2

3

4

13.6(3)7.6(10)5.5(6)

1.3(1)

0.2

0.2

0.2

0.4

14.9(4)9.6(4)

7.0(8)

1.4(3)

0.2

0.2

0.4

0.2

in Figure 6 represent the results of the fit, which are summarized in Table 2. We consider that the different hyperfine fields
roughly correspond to different Mo monolayers in a Mo layer,
and that the monolayers for which the hyperfine fields are relatively large are those near the Mo/Fe interfaces. In Table 2, wenote that especially the value of the hyperfine field near the
Mo/Fe interface, B1, for Mo(d = 1.3 nm) is larger than that forMo(d = 1.1 nm).
It is expected that the magnetization, M, in the Mo layer is asuperposition of the magnetic profiles induced by the two interfaces [M(x) = m(x) + m(d − x), where x is the distance from oneof the interfaces]. For large nonmagnetic layer thicknesses (d~ 10 monolayers), the magnetization caused by spin polarization from one side of the interface can be regarded as oscillating as9m(x) = Cx−α sin(2πx/Λ + φ),(17)

where Λ, φ, and α are, respectively, the oscillation period ofinterlayer coupling, a constant given by the electronic structureof the interlayer material, and the attenuation parameter. The
thickness d affects coherence in the superposition of the spatially oscillating magnetizations arising from the two interfaces. Based on this model, the difference in the magnitude of
the hyperfine field between the two Mo/Fe multilayer samples
can be ascribed to the difference of the Mo layer thicknesses.
For the present Mo(110)/Fe(110) multilayers, Λ is 1.2 nm and φis 0 or π.10 Assuming that the x dependence of B1 is the sameas that of M(x), the value of the attenuation parameter α wasderived from the dependence of B1 on d to be about 2. This isin agreement with a theoretical value based on the RKKY
mechanism for Cu/Fe and Ag/Fe.11 We thus tentatively conclude that interlayer exchange coupling is due to RKKY interactions. Details of the experiments will be publishedelsewhere.
5.3. Hopping motion of Ce in graphite. 4 Related to
TDPAC studies on Ce inside the cages of fullerenes, C82 andC80,12,13 it is natural to extend the study to the cases of the otherallotropes, graphite, and diamond. When Ce exists as Ce3+, it
has one 4f electron, and when Ce exists as Ce 4 + , it has no
unpaired electron. It is interesting to examine the state of Ce
in these materials. Both samples containing 140 Ce were prepared by implanting 100-keV 140 Cs ions into highly oriented
pyrolytic graphite (HOPG) and diamond samples using the isotope separator installed at KUR. The nucleus 140 Cs decays to
140
Ba, then 140La, and finally 140Ce.
The TDPAC spectra for diamond show that the relevantinteraction is a weak static quadrupole one, indicating that Ce
exists as Ce 4+ and remains rest making chemical bonds withneighboring carbons.13 However, the TDPAC spectra for HOPG
are essentially of the exponential decay type, implying that Ceions are hopping from one of its minimum energy sites toanother. That Ce is moving in HOPG is similar to the case offullerenes, although for fullerenes Ce is confined in the cage.

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OhkuboJ. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

In order to examine the valence of Ce in graphite, a TDPAC
spectrum was obtained under an external magnetic field of
0.84 T. If Ce exists as Ce3+ and has one 4f electron, the magnetic moment tends to be aligned with the direction of theapplied magnetic field, so that the effective magnetic field at
the Ce nucleus will be different from the external one, whereas
if Ce exists as Ce4+ and has no unpaired electron, such a thing
will not take place. The magnitude of a parameter of the socalled paramagnetic correction factor deduced from the
TDPAC spectrum clearly indicates that Ce exists as Ce3+. For
both cases of HOPG and fullerenes, it was thus concluded that
Ce exists as Ce3+, and it is considered that this is related to thefact that both materials are electron donors.
5.4. Hyperfine fields in a protein, mavicyanin. 5 The
structure around the metal site of mavicyanin, a relatively small
protein molecule with a single copper site, has been investigated with 117In- or 111Cd-TDPAC. The main problem so far isthat the parent nuclei, 117Cd or 111mCd, are hard to locate at thecopper site within a time comparable to their short half-lives(see Table 1). Preparing mavicyanin being also not an easytask, it is difficult to obtain TDPAC spectra with good statistics. We are planning to use 111Ag as a parent nucleus to produce 111Cd.
The preliminary results so far obtained are the electric field
gradient (Vzz) values determined from the TDPAC spectra at pH
6.0, 7.5, and 8.0 for a mutant-type mavicyanin, Thr15Ala-Mav,
as well as those for the wild-type. For both proteins there
seems to be an abrupt change of Vzz between pH 6.0 and 7.5.
What is interesting is that the Vzz values for the mutant-type
mavicyanin are smaller than the corresponding values for thewild-type. This observation indicates that Vzz is sensitive tosome structural change between the two types of mavicyanin,
related to redox potential change found for them, although their
UV-Vis spectra are identical with each other and thus do notprovide information on the structural change.
Acknowledgement. This work was carried out in part under
the Visiting Researcher’s Program of the Research Reactor
Institute, Kyoto University.

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