Journal of Nuclear and Radiochemical Sciences, Vol. 8, No.1, pp. A1-A7, 2007

Accounts

Electron-Capture Decay Rate of 7Be Encapsulated in C60 Cages
T. Ohtsuki,*,a K. Hirose,a and K. OhnobaLaboratory of Nuclear Science, Tohoku University, Mikamine, Taihaku, Sendai 982-0826, Japan
Department of Physics, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501,
Japanb

Received: March 30, 2007
The decay rate of 7Be electron capture (EC) was measured in C60 and Be metal with a reference method. The halflives of 7Be endohedral C60 (7Be@C60 ) and 7Be in Be metal (Be metal(7Be)) were found to be 52.65±0.04 and53.25±0.04 days, respectively. This amounts to a 1.13% difference in the EC-decay half-life between 7Be@C60
and Be metal(7Be). The result is a reflection of the different electron wave-functions for 7Be inside C60 compared
to when 7Be is in a Be metal. Here, the magnitude of the average charge-transfer from the L(2s) electrons of the7
Be atom plays an important role in such variation in the decay constant in the environments.

1. Introduction
In nuclear β-decay and in the closely related process of electron-capture(EC), the decay curve is an exponential function
versus time with a constant decay rate. The decay rate fromany parent state, usually the nuclear ground state, to any final
daughter state is proportional to the product of a nuclear matrix
element and factors related to the phase-space available to theneutrino and electron and to the overlap between the initial
electron and final nuclear wave functions. As first suggested
by Segrē et al,1-3 environmental factors such as the chemical
form and pressure, among others, may alter the electron contact
densities in the nucleus and thus affect the EC decay rates. In
this regard, the nucleus of 7Be is a good candidate to look forsuch variations in environmental factors because of its simple
electronic structure, 1s22s2, in the EC decay nucleus.4-9
A long-standing challenge has been to establish the degree
to which manipulation of these environmental factors can inpractice change nuclear decay rates in experiments to determine the decay rate of 7Be compounds. In addition, there havebeen several observations of variations in the half-life with hostmetals,10-19 chemical forms,20-23 and pressure.24-26 For example,
we introduce, in Figures 1 and 2, the variations of the half-lifein chemical form and pressure were presented by Jolidge et al21
in 1970 and Hensley et al 24 in 1973. From the early data, thehalf-life variations of 7Be as a function of different chemical

Figure 1. Differences of electron densities at the Be nucleus in various compounds (BeZ) of Be. The difference of horizontal value inunits of 10 –4 ×|ϕ0|2Be.
*Corresponding author. E-mail: ohtsuki@LNS.tohoku.ac.jp[(λ−λ0)/λ0]×10-3

10.0
8.0
6.0
4.0
2.0
0
0

100

200

Pressure / kbar

300

Figure 2. Pressure dependence of the half-life in the chemical formof 7BeO. The half-life of 7Be in 7BeO at 1 kbar (normal pressure)corresponds to 53.41 days. Open circles are the data presented by
Hensley et al.24 and a closed square is by Gogarty.26

forms and host materials have been limited almost to within
0.2%, (and even under high pressure up to 270 kbar, to within
0.59% 24 ). Other summaries of the early studies in the EC
decay are also given by Emery.27 In recent studies, however,large variations have been observed as a function of different
chemical forms and pressures.16, 20, 25 Therefore, a precise measurement is still needed to obtain the absolute decay rate in different circumstances.
After the discovery of C60 and the subsequent successful
production of several kinds of fullerenes,28, 29 endohedral fullerenes which have one or more atoms inside the C60 , C 70 , C82
cages etc. currently have attracted great interest.30,31 If theirmass production becomes possible, they would have many
interesting applications such as stable molecular devices in
nano-meter scales and/or as functionalized materials in many
fields. Although endohedral complexes can be created simultaneously as well as ordinary fullerenes by using arc-discharge
vaporization of composite rods made of graphite and metalcompounds, the production rate of endohedral C60 is quite low
compared to ordinary C60. An alternative way of producing
endohedral C60 is to insert foreign atoms into the cage of thepreexisting C60 afterwards.32-34 We have examined the forma-© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/30/2007

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OhtsukiJ. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

tion of endohedral fullerenes by a nuclear recoil implantationof several foreign atoms following nuclear reactions.35-39 We
found that the 7Be atom can be endohedrally doped to create
the 7Be endohedral C60 (7Be@C60).36
Because of the unique chemical form of C60 and/or several
factors contributing to this environment; many π electrons of
C60, and special dynamic motions inside C60 etc,40-42 the electron contact density on the 7Be nuclei can be affected significantly by the electron environment of C60 . Therefore, it is
intriguing to measure the half-life of the 7Be inside C60 ; "howdoes the EC decay rate of 7Be change inside the C60 cage relative to other situations?" We compared the half-life of 7 Be
when it is encapsulated in a C60 cage to that in Be metal as areference. In order to accurately calibrate our time measurements we relied on a standard clock time radio signal. We
found a surprisingly shorter half-life of 7Be inside C60.43 This
fact implied that the 7Be atoms are located in a unique environment inside C60.

Radioactivities of 7B e
UV Chromatogram (arb. unit)

2. Experimental procedures and simulation
2.1. Preparation of 7Be@C60. A nuclear recoil implantation technique was used to produce 7Be@C60. Purified fullerene (C60 ) and Li 2 CO3 were used in powder form. The grain
size of the material was smaller than 100 mesh(20µm) for
Li2CO3. Fullerene C60 was carefully mixed with Li2CO3(weightratio=1:1) in an agate mortar, and a few mL of carbon disulfide(CS2) was added. After the mixture dried, about 100 mg of themixed sample was pressed into a tablet and wrapped in a purealuminum foil of 10 µm in thickness for irradiation. Proton
irradiation with a beam energy of 16 or 18 MeV was performedat the Cyclotron and Radioisotope Center, Tohoku University.
The beam current was typically 3 µA and the irradiation time
was about 24 hours. The sample was cooled with a He-gas
during irradiation. The 7Li(p,n)7Be reaction leads to the production of 7Be. After the kinetic energy of 7Be has decreasedin the sample (target) to an appropriate value, it then penetrates
into the C60 cage to produce the endohedral fullerene (a nuclear
recoil implantation, the detailed mechanism of which is shownin Figure 3).
After irradiation, the sample was left for a few days to allow
several kinds of short-lived radioactive by-products to decay.
The sample was dissolved in o-dichrolobenzene and filteredthrough a millipore filter(pore size=0.2 µm) to remove insoluble materials. The soluble portion was injected into a high performance liquid chromatography(HPLC) device equipped witha 5PBB (Cosmosil) (silica-bonded with a pentabromobenzyl
group) column of 10 mm in inner diameter and 250 mm inlength at a flow rate of 2 or 3 mL/min. To confirm the pres-Figure 3. Schematic view of production mechanism for radioactive
fullerenes. 7Be nuclide is produced by (p,n) reaction, and its kinetic
energy of 7Be is reduced in the sample to a magnitude which is appropriate for the fusion. Finally, 7Be hits the C60 cages and stops in thecages.

Figure 4. HPLC elution curves of the soluble portion of the crude
extracted in the proton-irradiated sample of 7Li and C60. The verticalaxis indicates the radioactivities of 7Be in each fraction (solid circles)
and absorbance of the UV chromatogram of C60.

ence of fullerenes and their derivatives, a UV detector was
installed with a wavelength of 290 nm. In order to prepare thesample of 7 Be@C60, eluent fractions were collected for 30 sintervals (0–30, 30–60, 60–90, ... s), and the 478 keV γ-ray
activities emanating from the 7Be of each fraction were measured with a γ-ray detector for confirmation of the 7Be inside
the C60 fraction. The eluent behavior is shown in Figure 4 andthe solid circles in the figure show the results of the γ-ray measurements for each fraction. The horizontal axis represents theretention time (in min), which corresponds to each fraction collected at 30 s intervals, after injection into the HPLC. The vertical axis represents the counting rates of the 478 keV γ-ray
from 7Be. One strong peak of 7Be-radioactivity was observedin the retention of 8.5–10 min. Radioactivities of 7Be on thefraction were estimated to be of the magnitude of 20–30 Bq/
mg. The solid curve in Figure 4 indicates the absorbance monitored by a UV detector for the dissolved sample. A strong
peak was also observed in the elution curve, though the relatively smaller peaks were also seen in 10–16 min. This peak
position corresponds to the retention time of C60 which was
confirmed by the calibration run using the C60 sample before
the irradiation. Furthermore the second and third peaks werealso attributed to C60 dimers and trimers, respectively, produced by coalescence reactions.
It was noted that the peak position of 7 Be (solid circles)
exactly coincided with the UV chromatograph of C60. Fromthe correlation of the elution behavior between the UV chromatogram and the radioactivities of the 7Be atoms, we foundthat the atom-doped fullerene, 7Be@C60, was indeed producedby nuclear recoil implantation. In order to obtain a measurement sample, the solution which corresponded to a C60 fraction
was evaporated, and finally the C60 sample containing 7Be@C60
was converted to a solid tablet by press.
2.2. Simulation of 7Be@C60. In order to check the implantation procedure, an ab initio molecular dynamics (MD) simulation, based on all-electron mixed-basis approach with theframework of the local density approximation, was carried out
for collision between the 7Be atom and C60 cage.44,45 We put one
C60 molecule and one Be atom in a supercell with zero and 5
eV initial kinetic energies for C60 and Be, respectively. The
wave functions were expanded by 300 Slater-type atomic orbit-

Electron-capture decay rate of 7Be encapsulated in C60 cages

J. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

Core monitor
e-

e-

A3

9

Be( 2n)7Be

SamplePt

Beam line

Sweep magnet(7Be)
Be metale-Figure 6. Setup for the irradiation of a Be metal using the electronlinear accelerator at Tohoku University.

36

Figure 5. Time-ordered snapshots of a simulation, where Be atom
hits with 5 eV kinetic energy the center of a six-membered ring of
C60. In this case, 7Be@C60 is created.

multi-channel analyzer

Be metal(7Be)
7BCe@C60

Ge-detector

standard clock

sample holder

sample holder

Pb sheild

als and 2969 plane waves. The supercell was divided into
64× 64× 64 meshes with 2.7 meshes corresponding to 1
a.u.= 0.5918 Å. We performed six steepest-descent iterations
between two adjacent updates of atomic positions in order toconverge the electronic states, and choose δt=4 a.u.~0.1 fs as
the basic time step. For rescaling velocity and assuming a
Fermi-Dirac distribution of electronic states, the simulation
temperature of the whole system was set to be T=1000 K. In
the case where a Be atom with 5 eV kinetic energy hits the center of a six-membered ring of C60 perpendicularly, we foundthat the Be atom easily penetrated into the cage through thecenter of the six-membered ring. It was trapped at 1.0 Å depth
from the center of the same six-membered ring toward the cage
center; see Figure 5. The reason why the insertion proceededvery easily is that the ionic radius of the Be atom is 0.45 Å,much smaller than the radius of the hole of a six-memberedring. If a Be atom with the same kinetic energy hits a C-C
bond of a C60 atom, the bombardment gives a considerable
shock and deformation to the C 60 cage. If a Be atom withhigher kinetic energy, e.g. 100 eV, hits a C60 molecule at any
location, we found that the C60 cage was damaged considerably.
These results suggest the following scenario of the present
experiment: Many C60 cages are destroyed before the 7Be atoms
have lost their kinetic energies during the recoil process in thesample, and once sufficiently low kinetic energies are reached,
the insertion process 7Be + C60 → 7Be@C60 occurs. The snapshots of the simulation are shown in Figure 5.
2.3. Preparation of Be metal (7Be). The EC decay rate of
the 7Be nucleus inside the Be metal (Be metal (7Be)) have neverbeen surveyed to date. Therefore, it would be interesting tomeasure the decay rate in the Be metal (uniform lattice structure, (hcp). In the present study, we measured the half-life of
7
Be inside Be metal (7Be) for reference use compared to that of
the 7 Be@C60 sample. Be metal (10 mm in diameter and 0.3
mm in thickness) was utilized to produce 7Be uniformly in a
Be metal. After being washed with a diluted HCl solution, theBe metal was sealed in a quartz tube (vacuum packed) 12 mm
in diameter to be used as a target. Irradiation with a bremsstrahlung (50 MeV electrons) was carried out by the electronlinear accelerator, Laboratory of Nuclear Science, Tohoku
University. The sample in a quartz tube was set in the middle
path of a sweep magnet placed on the axis of the electron beam.
A platinum plate converter with a thickness of 2 mm was set infront of the sweep magnet in order to generate the bremsstrahlung. Then the sample was irradiated only by the bremsstrahlung in such a way that all the electrons were removed by therail

driverrail

Figure 7. The measurement system: a computer-controlled sample
changer. Two samples, 7Be@C60 and Be metal (7Be), were placed on
top of two arms of an automated sample changer, which moved thesamples horizontally in front of a γ-ray detector. A computer clock
was always calibrated by a standard clock time radio signal.

magnetic field. Therefore, damage to the lattice of the Be
metal was confined to a minimum. The experimental setup forthe irradiation procedure is shown in Figure 6. The 7Be can beproduced uniformly by the 9Be(γ, 2n) 7 Be reaction in the Be
metal. After the irradiation, the sample (Be metal (7Be)) was
baked in an electric oven with vacuum packing at 1150 °C (themelting point of Be metal is 1278 °C) for over one hour torecover a lattice defect caused by the (γ, 2n) reaction. Finally,
the Be metal (7Be) was washed again with a diluted HCl solution to clean the surface.
2.4. γ-Ray measurement by the reference method. We
developed a reference method to measure the half-life of 7Be
inside C60 and that in the Be metal. The system is shown in
Figure 7. This allowed the decay rates of the two samples to bemeasured in a comparable way. Two samples of the 7Be@C60
and the Be metal (7Be) were placed on top of two arms of an
automated sample changer, which moved the samples horizontally in front of a γ-ray detector.
The 7Be decays directly to a 3/2 – ground state of 7Li with abranching of 89.6%; it goes with a branching of 10.4% to a firstexcited state of Li(1/2 at 478 keV), which decays by γ-ray
emission to the ground state.46 Therefore, the activities associated with 7Be, in particular the 478 keV γ-rays emanating fromthe EC-decay daughter of 7 Be, were measured with a highpurity germanium (HPGe) detector (∆EFWHM is 1.8 keV at 1332
keV of 60 Co and 50% relative efficiency) coupled to a 2048and/or 4096-channel pulse-height analyzer. The excellent
energy resolution of the HPGe detector resulted in a good signal-to-noise ratio. The background was reduced by a leadshield. Therefore, the background peaks do not impair thedetermination of the half-life of 7Be in the present experiment.
The radioactivities associated with the decay of 7Be could be
uniquely detected through the identification of its characteristic
γ-rays, and all other sources could be ruled out.
We exactly set the measurement duration for one data pointto 21600 s (6 hours), (exactly 21480 s for the live measurement
time and 120 s for the dead time of the measurement system
plus the sample exchange time and the waiting time to the nextstep). This procedure was repeated more than 300 times (over150 days). Total measuring time was more than 150 days,
which is almost three half-lives of 7Be. The internal clock timeof a computer for data acquisition was always calibrated by atime-standard signal distributed via a long-wave radio center inJapan (i.e. the starting time for each run was correlated to atime distributed publicly). Therefore, the uncertainty in thetime measurements is negligibly small. The system was completely controlled by a computer to obtain precise measurement
positions.
The reproducibility of each position was determined to bewithin 0.05 mm. Systematic errors were reduced by using the
Be metal(7Be) as a reference. To keep the constancy of a geometrical factor between the sources and the γ-ray detector, thetemperature was kept at 20 °C by air conditioner during themeasurement.
7–

3. Results and Discussion
A typical γ-ray spectrum obtained in a measurement of
Be@C60 decay as a function in keV is shown in Figure 8. The
expected γ line at E γ =478 keV and a natural background line at
E γ =1461 keV can be seen as two giant peaks. No peak wasseen at around E γ =478 keV when the 7Be sources were absent.
A direct summation method, which is normally used for a neutron activation analysis etc, was applied to obtain the peak area
(E γ =478 keV). The background was subtracted by a straightline between the average counting number/channel on bothsides of the peak. In Figure 9, the two measured exponential
decay curves for the 7Be@C60 and Be metal(7Be) activities areplotted versus time. Closed circles and cross symbols, respectively, indicate the radioactivities (decay rate) of the 7Be@C60
and Be metal(7Be) samples. The initial radioactivities of the7
Be in each sample were almost identical, i.e. at time zero we
obtained around 2.7 counts per second (cps) for 7Be@C60 and
2.5 cps for Be metal(7 Be) in one run. Thus, the two decay
curves can be compared under almost the equal degree in countrate (cps). The data for Be metal(7Be) shown in Figure 9 was
normalized to those for 7Be@C60 by use of an adjustment procedure on initial points of the two decay curves. The decay
constant for the two samples were obtained by fitting straightlines in logarithm for the vertical line to the measured data
points by use of the MINUIT program distributed from the
CERN Program Library. This program takes into account thestatistical errors associated with each data point in Figure 9.
This statistical error is by far the dominating uncertainty. The
uncertainty of our measurement is given by the uncertainty ofthe slope of the straight line fitted to the logarithm of thecounts. We have measured the decay rates and deduced thecorresponding half-lives of 7Be in the 7Be@C60 and in the Be
metal(7 Be) samples in two separate measurements for each
7

OhtsukiJ. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

105
104

104
102

3

10

102
10.4%

1/2- 478keV
γ-ray

101

3/2- 0
7
3Li

0

10

0

3/27Be4

EC decay
89.6%

500

1000

2000

1500

Figure 8. Typical γ-ray spectrum of 7Be in the 7Be@C60 fraction.
Expanded scale around the E γ = 478 keV is shown in the figure.
Decay scheme of 7Be is inserted in the figure.
0.6

2
0.5

Decay rate (cps)

A4

1
0.9
0.8
0.7

Be metal ( 7Be)

0.4

7Be@C

60

0.6
0.5

0.3
120

0.3

0

20

40

160

140

0.4

60

80

100

120

140

160

Time (days)
Figure 9. Exponential decay curves of 7 Be in the samples of7
Be@C60 and Be metal(7Be). Inset figure corresponding to the decay
intervals of 120~162 days are displayed with an expanded scale.

sample in duration more than 150 days. The averaged results
for the samples of the 7 Be@C 6 0 and Be metal( 7 Be) are
T1/2 =52.65±0.04, and T1/2 =53.25±0.04 days, respectively. The
reduced χ-square values of the exponential fits are between
0.90 and 1.20. The dead time in the data acquisition system
has been found to be about 8~15 s for each 21480 s run.
Therefore, the uncertainty due to the dead time was estimatedto be less than 0.04%, and the systematic error in the measurements was estimated to be less than half of the statistical error
quoted above.43 (In order to reduce the dead time of the measurement system, the amount of the activities and the distance
between a γ-ray detector and the source were prepared suitably
in the measurement.)
The counting rates of the natural background, which consistof the 1461 keV γ-rays emanating from 40K, were also analyzed.
The data for 40K obtained were fitted by the same procedures

Electron-capture decay rate of 7Be encapsulated in C60 cages

Be metal ( 7B e )(reference sample)
T1/2=53.25 0.04 days

7B e@C60

T =293K
T1/2=52.65 0.04 days

52.5

53.0

53.5

54.0

Half-life (days)
Figure 10. Half-lives (T1/2) measured in this time period are plotted
as solid circles for the 7Be@C60. The cross symbols represent thehalf-lives used as reference samples of Be metal(7Be) (e.g. shown byarrows). The dashed area are the half-lives of the Be metal(7 Be)
samples compared to other samples.47 Half-lives previously presented
mostly range inside the thick-bar.

Be metal crystal

Boron nitride

Host materialusing the NIMUIT program. It was found that the fitted line
closely corresponded to a horizontal line. Therefore, the present decay rates for the samples of the 7 Be@C 6 0 and Be
metal(7Be) seem to be fair.
In Figure 10, solid circles indicate the half-lives obtained forthe 7Be@C60 sample compared with that for the Be metal(7Be)
given by cross symbols. It was surprising to observe that theh a l f-l i fe obt a i n e d for 7 Be i n t h e sa mple 7 Be @ C 6 0 ,T1/2 =52.65±0.04 days, was as much as 1.13% shorter than thatfor the Be metal(7Be) sample, where we define the percentage
difference by [100 (λ(C60 )−λ(Be metal)]/λ(Be metal). Thisdifference in half-lives is sufficiently large that it is clearly discernable when the data is displayed on the extended scale of
the right inserted figure in Figure 9. The half-life of 7Be inside
C60 is shorter than that in any previously reported in environments for any materials and/or pressure. This implies that the7
Be atoms are located in a unique environment inside the C60
cages.
On the other hand, the half-life of 7Be in the Be metal(7Be)averaged over many measurements was 53.25±0.04 days.47 The
half-lives obtained for 7Be in several other host materials andchemical forms such as graphite, gold, oxide, etc. have beenpresented by many researchers.10-18,20-26 In Figure 10, the halflives previously measured are also shown by the range of a
thick bar as a comparison. Here, only the half-lives obtainedusing a standard time distributed publicly (stated in the text)
are considered.10,13-15,17,18 The half-lives inside the values aremostly between 53.10 and 53.40 days. For example, the halflife values obtained for 7Be in several other host materials suchas graphite and boron nitride that have been presented by
Jaeger et al10 and Norman et al,14 respectively, are also shownin Figure 11. The values are almost all in the range of 53.1 to53.3 days.14 Therefore, we found that our reference measurement of the half-life for 7Be in the Be metal(7Be) was in satisfactory agreement with the other available data.
Several factors have contributed to creating this unique environment, for example, the many π electrons of C60 and the special dynamic conditions of the electrons inside the C60 cage,
which include the ratchet and/or tumbling motion,41,42 all ofwhich may affect the contact electron density at the 7 Be
nucleus. Here, a magnitude of the average charge-transfer from2s electrons of the 7Be atom, e.g. the K and L capture,12,48,49 canplay an important role for such a large variation in the EC
decay rate of 7Be between the samples of the 7Be@C60 and Be
metal(7Be). The L/K capture ratio (i.e. the ratio of the electron
density of L(2s) and K(1s) orbitals at the Be nucleus position)is estimated to be almost 10% on an isolated Be atom.49,50 In
theoretical calculations, we also found that the 7Be atom stays
at the center (potential minimum) of the C60 cage and the 2selectrons can be fully restricted to the Be nucleus at T=0 K as
if they were an isolated Be atom (1s22s2).48 A schematic view of
the closed electron shells (1s2 2s2 ) when 7 Be is inside C60 isshown in Figure 12. On the other hand, chemically and/or
metallically bonded Be atoms have always lost the 2s electrons
to some degree due to the alkaline-earth metal nature (relatively smaller electronegativity). Therefore, the L/K capture
ratio in the 7 Be atom is reduced when the 7 Be is chemicallybonded and/or inside the host metal materials. Here, the magnitude of the average charge-transfer from the L(2s) electronsof the 7Be atom may play an important role in such variation in
the decay constant in the environments, though the K(1s) electrons may also play a large influence on the factor of the environments around the 7Be nuclear position. It is our hope that
our results will stimulate further experimental and theoreticalstudies leading to a more quantitative understanding of thislarge change in the decay constant.
Finally, a dynamic motion inside C60 at room temperature
may contribute to the environment around the 7 Be nucleus.
Therefore, it is intriguing to study the temperature dependence

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J. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

Graphite
TantalumGold
Aluminum
Lithium fluoride

52.5

53.0

53.5

54.0

Half-life (days)
Figure 11. Half-life of 7 Be in Be metal and half-lives previously
measured in several host materials.10,14

Figure 12. Schematic view of electron shell structure of 7Be inside
C60.

of the half-life of 7Be inside C60. In our study, in order to suppress the dynamic motion of 7Be inside C60, we are planning tomeasure the half-life of 7Be inside C60 that is cooled to a temperature close to liquid helium (T=5 K). Now we are also carrying out calculations of the electron density at the 7Be nucleus

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J. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

position at the site inside C60 by different temperatures (at T=0
K and 293 K). The theoretical estimates will reveal the stable
position of the 7 Be nucleus inside C60 and the temperaturedependence of the electron density at the 7Be nuclear position.
4. Conclusion
In summary, we have measured the half-life of (1) 7 Be
encapsulated in C60 and (2) 7Be incorporated in Be metal using
an HPGe detector with a time reference from a standard-time
radio signal. We found that the half-life of 7Be inside C60 andin Be metal were T1/2 =52.65±0.04 and T1/2 =53.25±0.04 days,
respectively. This 1.13% difference between the EC decay rate
inside C60 and in Be metal represents a strong environmental
effect on the 7Be EC decay rate, caused by the different electronic wave-functions near the 7 Be nucleus inside a C60 cageand inside Be metal. These findings may give a fundamental
understanding of the EC decay problem and/or hyperfine interactions. Extreme pressures and/or plasma states in stellar environments are supposed to change the EC decay rate by ordersof magnitude. However, our more limited observed effects
have still significantly raised the magnitude of what is achievable in laboratories on earth.
Acknowledgement. This research work was carried out by
the collaborators, M. Muto, J. Kasagi, and H. Yuki inLaboratory of Nuclear Science of Tohoku University, T.
Morisato in Accelrys K.K. Tokyo, Y. Kawazoe, T. Mitsugashiraat the Institute of Materials Research of Tohoku University,
The authors are grateful to the 229mTh collaboration for the continuous support (the list includes T. Nakanishi and T. Kikunagaof Kanazawa University, S. Shibata and K. Takamiya of Kyoto
University, A. Shinohara and Y. Kasamatsu of Osaka
University, H. Haba and T.T. Inamura of RIKEN, M. Hara of
Tohoku University, and other all colleagues). The authors arealso grateful to K. Endo for his kindness and K. Sakamoto and
I. Fujiwara for the special attention to this work. We also
acknowledge the valuable discussions and support by P. Möller
(LANL), K. Masumoto (KEK), and K. Shikano (Hakodate).
We convey our special thanks to the staff in the division ofaccelerators of the Laboratory of Nuclear Science and the
Cyclotron and Radioisotope Center, Tohoku University.
Finally, the authors appreciate the kindness of H. Nakahara ofTokyo Metropolitan University, who gave thoughtful and continuous advice on this work. This work was supported by
Grant-in-Aids for Co-operative Research No.10640535,
No.12640532, No.17350024 from the Ministry of Education ofJapan, by the REIMEI Research Resources of Japan Atomic
Energy Research Institute, and by MITSUBISHI Fund.
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