Journal of Nuclear and Radiochemical Sciences, Vol. 8, No. 2, pp. 73-78, 2007

Measurement of Evaporation Residue and Fission Cross Sections of the Reaction 30Si + 238U
at Subbarrier Energies
K. Nishio,*, a S. Hofmann,†, b, c H. Ikezoe,a F. P. Heßberger,b D. Ackermann,bS. Antalic,d V. F. Comas,e Z. Gan,f S. Heinz,b J. A. Heredia,e J. Khuyagbaatar,bB. Kindler,b I. Kojouharov,b P. Kuusiniemi,g B. Lommel,b M. Mazzocco,bS. Mitsuoka,a Y. Nagame,a T. Ohtsuki,h A. G. Popeko,i S. Saro,d H. J. Schött,b
B. Sulignano,b, j A. Svirikhin,i K. Tsukada,a K. Tsuruta,a and A. V. YereminiaJapan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
Gesellschaft für Schwerionenforschung mbH, D-64220 Darmstadt, GermanycInstitut für Kernphysik, Johann Wolfgang Goethe-Universität, D-60486 Frankfurt am Main,GermanydDepartment of Nuclear Physics, Comenius University, SK-84215 Bratislava, SlovakiaeHigher Institute of Technologies and Applied Sciences, Habana 10400, Cubaf
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, Chinag
CUPP, University of Oulu, FIN-86801 Pyhäjärvi, FinlandhLaboratory of Nuclear Science, Tohoku University, Sendai 982-0826, Japani
Flerov Laboratory of Nuclear Reactions, JINR, 141 980 Dubna, RussiajInstitut für Kernchemie, Johannes Gutenberg-Universität Mainz, D-55099 Mainz, GermanybReceived: October 30, 2006; In Final Form: July 13, 2007
Effects of the prolate deformation of 238U on fusion were studied in the reaction 30Si + 238U at bombarding energies
close to the Coulomb barrier. The ﬁssion (capture) cross sections were measured at the JAEA tandem accelerator
to see the enhancement of the cross sections in the subbarrier energy due to the lower Coulomb barrier in the collisions of projectile at the polar sides of 238U. In order to obtain the direct evidence for complete fusion, evaporation
residue cross sections were measured at UNILAC of GSI. At the subbarrier energy of E c.m. = 133.0 MeV, where
only polar collisions to 238U occur, we measured three spontaneously fissioning nuclei which we assigned to theisotope 264Sg. The obtained cross section (10 +10– 6 ) pb was smaller than the calculation that the system captured at
the polar collisions results in complete fusion, suggesting the competition between fusion and quasiﬁssion. At theabove barrier energy of E c.m. = 144.0 MeV, where collisions on the equatorial side of 238U start to contribute, we
observed α-decay chains starting from 263Sg and the cross section was determined to be (67 +67–37 ) pb. The data was
reproduced by the calculation that the system captured from the equatorial side of 238U results in complete fusion.

1. Introduction
Two types of fusion reactions were successfully used in synthesis of heavy and superheavy elements (SHE). These are thecold fusion reactions based on lead or bismuth targets and the
hot fusion reactions based on actinide targets. Using cold
fusion reactions isotopes of elements up to proton number 112(Reference 1) were produced at GSI in Darmstadt, Germany,
up to proton number 113 (Reference 2) at RIKEN in Wako,
Japan. Hot fusion reactions were used to produce isotopes of
elements up to 116 and one isotope of element 118 (References
3 and 4) were produced at FLNR in Dubna, Russia.
Another difference between two reactions is associated withthe static deformation of the target nuclei. Experimental data
for excitation functions producing SHEs reveal that in the caseof cold fusion the highest cross sections are obtained at beamenergies where a contact conﬁguration between projectile andspherical target nucleus is just reached.1 In the hot fusion the
cross-section maxima were measured at beam energies which
are high enough so that projectile (48 Ca) and prolate target
nuclei can come into contact at minimal distance (equatorial
collisions) and thus form a most compact starting conﬁgurationon the way to the compound nucleus. The cross sections drop
rapidly when the energy is decreased to values where the inter*Corresponding author. E-mail: nishio.katsuhisa@jaea.go.jp.
Fax: +81-29-282-5927.
†Josef Buchmann-Professor Laureatus.

action is limited to polar collisions. In this case the probability
for re-separation of the reaction partners is high.
However, the results are different for significantly lighter
projectiles than 48Ca. In the reaction 16O + 238U, the experimental data show a large enhancement of evaporation residue (ER)cross sections at subbarrier energies,5 compared to the calculation based on the one dimensional barrier penetration model inthe fusion process, and the system which overcomes the
Coulomb barrier results in complete fusion also for subbarrierenergies. The fusion probability is thus expected to be sensitive to the projectile mass (charge) for the polar collisions,whereas fusion would be less sensitive to projectile mass forequatorial collisions.
In order to study the anticipated phenomena, it is needed tomake a systematic measurement by changing projectiles. Here,
we report on the results for the reaction 30Si +238U,6 where theprojectile is right in the middle between 16O and 48Ca.
Firstly, we measured the ﬁssion cross sections for this reaction, which represents the probability of the system captured
inside the Coulomb barrier. The cross sections were largely
inﬂuenced by the properties of the colliding nuclei, especiallyprolate deformation of 238U. Secondly, we have produced theevaporation residues (ER) as the direct evidence of complete
fusion, and their cross sections were compared with calculations from fusion evaporation models taking into account theeffects of the prolate deformation of 238U on fusion. Similararguments were discussed and presented in earlier works onreactions using prolately deformed target nuclei of rare-earth© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/19/2007

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NishioJ. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007
7–10

elements.

2. Experimental Methods
2.1. Measurement of ﬁssion cross sections. Measurementof the ﬁssion cross section was performed by using 30Si beams
supplied by the JAEA tandem accelerator facility in Tokai.
The typical 30Si-beam current was 1.0–2.5 pnA, and the beamenergies were changed in the range from 140 MeV to 183 MeV
to measure the excitation functions of the cross sections. The
uranium target of 360 µg/cm2 was made by electro-deposition
on a Be-foil of 1.67 mg/cm 2 thickness, and the uranium layer
was coated by a 0.84 µm aluminum foil. The target plane wastilted at 30° to the beam axis with the Be-foil facing the downstream of the beam, and the aluminum foil was facing ﬁssion
fragment detectors described below.
Fission fragments were measured in singles mode by using∆E-E detectors consisting of a gridded ionization detector anda silicon surface barrier detectors. The detectors were all
mounted in single aluminum container. After passing throughthe 0.9 µm Mylar foil to shield the gas, ﬁssion fragments run
through the 20 Torr isobutane gas layer with the distance 45
mm to give the ∆E signal, then implanted into the 300 mm2 silicon detector to give the residual energy (E). In the container,
three Si detectors were mounted at angles of 90°, 120°, and150° relative to the beam direction to measure the angular distribution of the ﬁssion fragments. The grid and anode of the∆E detector covered three directions. The solid angle to eachdirection was determined by the entrance Mylar window andwas set at 2.13 msr. Another silicon surface barrier detector
was placed at angle 45° relative to the beam direction to determine the absolute value of the ﬁssion cross sections. The solid
angle was 1.08 msr.
Figure 1 shows examples of ∆E-E spectra obtained at two
different energies of E c.m. = 150.0 MeV and 130.0 MeV at the
middle layer of the target. The ﬁssion events can be distinguished from the elastic or quasi-elastic particles and otherlight particles.
For each beam energy, the differential ﬁssion cross section
for three center-of-mass angles θ, dσ /dΩ(θ), were ﬁtted with afunction describing the angular distribution of fission fragments W(θ) (Reference 11) as is explained in Reference 7. By
integrating the ﬁtted curve W( θ ) over the solid angle, we
obtained the ﬁssion cross section for each energy.
2.2. Measurement of evaporation residue cross sections.
Measurement of the evaporation residue cross sections was
performed using the velocity ﬁlter SHIP at the linear accelera500

120o

90o

400

150o

∆E / channel

300
200
100
0

0

500

100 200 300 400

0

90o

400

100 200 300 400

0

120o

100 200 300 400

150o

300
200
100
0

0

100 200 300 400

0100 200 300 400

0100 200 300 400

E / channel
Figure 1. Detected particles on the two dimensional map on E and∆E. The upper and lower panels show the data for the reaction energy
E c.m. = 150.0 MeV and 130.0 MeV at the middle of the target, respectively. Spectra measured at angles of 90°, 120°, and 150° in laboratory frame are shown. Region for the fission events are enclosed bysolid curves.

tor UNILAC of GSI in Darmstadt. The typical intensity of the
30 6+
Si beams was 0.7–1.0 pµA. The beam has time structure of
5.0 ms in width and 50 Hz in frequency. The detailed information for the experiment is published in Reference 6, so that thebrief description on the experimental method is given here.
The experimental setup consists of a rotating target wheel,
the velocity ﬁlter SHIP, and the detector system at the focal
plane. Detailed descriptions can be found in References 1 and
12–15.
The uranium targets were prepared by evaporating the 238UF4
material on a 45 µg/cm 2 carbon backing with a thickness of
375–404 µg/cm 2 . The uranium layer were coated with a 15
µg/cm2 thick carbon.
The transmission effciency of SHIP was determined byusing a Monte Carlo calculation, and we obtained 10% for the
probability of ERs being transmitted and focused at the focal
plane.
In the focal plane of the SHIP, ERs and their subsequent α
decay and/or spontaneous ﬁssion (sf) are detected by a position
sensitive 16-strip Si PIPS detector. Escaping α particles or
ﬁssion fragment are detected by a ‘box detector’ which covers
an area of 85% of the backward hemisphere. The energy resolution for fully stopped α’s was typically 25 keV (FWHM) for
the 8415 keV α particles from 252No. For escaped α particles
detected in coincidence with signals from the box detector, theresolution was determined to be 70 keV.
A timing detector is located in front of the silicon detector
array to perform the time-of-ﬂight measurement of the transmitted particles. Together with signals from the silicon detector, it was used for distinguishing implanted signals fromradioactive decay events. The timing detector was moved out
during some of the irradiations at the beginning of the experiment. In this case the radioactive decays were detected during
the 15.0 ms beam off periods.
The spontaneous ﬁssion (sf) events are measured by detecting high energy deposition in the silicon detectors. The calibration was performed with α particles from external sources.
The total kinetic energy (TKE) of sf events was obtained bysumming the energies from the stop and the box detectors.
Energy calibration of the TKE was performed by referring to
the known TKE of 195 MeV of 252No (Reference 16) producedin the reaction 48Ca + 206Pb. The pulse height deﬁcit and theenergy loss in the entrance windows of stop and box detectors
were determined from the difference between the literature
value and the measured energy based on the α -lines calibration. In this calibration run, we found a strong dependence ofthe measured fission fragment energies on the implantation
depth of ERs, which was regulated by changing the thickness
of Mylar degrader foils in front of the Si detectors. The results
are shown in Figure 2. The implantation depth was calculatedby the SRIM code.17 Taking into account the calculated
implantation depth of 1.6 µm for ERs produced in 30Si + 238U,
55 MeV should be added to the measured values to get the correct value of TKE.
Behind the focal-plane detector we mounted a clover detector consisting of four Ge crystals (50 mmφ × 70 mm length). It
was used to measure coincident γ - or X-rays accompanied byspontaneous ﬁssions, whose signals were served as strong evidence for the occurrence of sf.
The detection of correlated events is primarily based onagreement of the measured positions between implanted ERs
and subsequent α decays or sf.
3. Experimental Results and Discussions
3.1. Fission cross section for 30 Si + 238U. Experimental
results of the ﬁssion cross sections for 30Si + 238U are shown in
the upper part of Figure 3. The ﬁssion fragments should originate from quasiﬁssion and compound nucleus ﬁssion, so that

Measurement of Evaporation Residue and Fission Cross Sections

252

No spontaneous fission

55 MeV
150
100
50
0

1 2 3 4 5 6 7
Implantation depth
in silicon detector / µm

Figure 2. Sum of the energy of two fragments recorded by the stopand box detectors for the spontaneous fission of 252No, which shows
the dependence on the implanted depth in the stop detector. The literature value TKE = 195 MeV for 252No is indicated. Implantation depthof ERs for 30Si + 238U is also shown, and the correction energy of 55
MeV is obtained.

the cross section represents the sum of both origins. The data
are compared with the calculated capture cross section using
the coupled channel code CCDEGEN18 which is a modiﬁcationof the code CCFULL.19 In the calculation static deformation of
the target nucleus 238U with the deformation parameters of β2 =0.275 and β4 = 0.05 is taken into account. 5,20 The nuclear
potential was approximated with the same parameters as in thecase of the previously studied reaction 16O + 238U → 254Fm*.5
Also considered was the channel coupling to the 3 − state at
0.73 MeV in 238U and to the 2 + state at 2.235 MeV in 30Si.21 We
also show in Figure 3 the results from the CCDEGEN code byignoring deformation of 238U and couplings to the vibrational
st at es i n t he nuclei (one d i mensiona l mo del) . T hecorresponding Coulomb barrier height is 139.7 MeV, which isalmost equal to the Bass barrier energy 141.4 MeV.22
The coupled channel calculation reproduces well the experimental data points for the reaction 30 Si + 238U. On the other
hand the one dimensional model cannot describe the data
below the energy Ec.m. = 140 MeV.
3.2. Produced evaporation residues and their decay properties. For the production of evaporation residues in the 30Si +238
U, we used three different center-of-mass energies Ec.m. (excitation energies E*) of 144.4 (50.6), 133.0 (39.6), and 128.0(34.5) in MeV at the half thickness of the target layer. The
accumulated beam doses were 1.8 × 1018, 4.0 × 1018, and 1.7 ×
1018 particles, respectively.
At the highest energy of E c.m. = 144.5 MeV we expect thehighest cross section for the well known isotope 2 6 3 Sg
(References 23–28) produced in the 5n channel. In this energy
we observed decay events as shown in the upper row on Figure
4, chronologically ordered from 1 to 4.
Subsequent to the implantation we measured at the same
position of the detector within the position resolution two α
decays in the case of event number 1 and 2, three α decays in
the case of event number 3. Energies and half-lives agree wellwith the literature data for 263Sg (see references given before).

Fission cross-section / mb

195 MeV200

30

ER cross-section / mb

Total kinetic energy / MeV

250

75

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007
238

Si

103

30

U

40

50

E* / MeV

60

70

102
101
100
10-1
10-2

Coupled channel
One-dimensional

10-3
10-4

10-6
10

265

Sg(3n)264

263

Sg(5n)

Sg(4n)-7262

Sg(6n)

10-8
10-9
10-10
120

130

140

150

160

170

Ec.m. / MeV
Figure 3. (Upper part) Fission cross sections of the reaction 30Si +238
U as function of center-of-mass energy E c.m. and excitation energy
E*. The data are compared with coupled channels calculation (fullline) and the one dimensional fusion model (dashed line). (Lowerpart) Results of the evaporation residue cross sections. Calculation of
the statistical model by using the code HIVAP34 are shown, where thepartial cross sections from the CCDEGEN code18 was used as input.
At E c.m. = 133.0 and 144.0 MeV three events were measured in eachcase for the 4n (diamond) and 5n (circle) evaporation channel, respectively, and one event in each case was tentatively assigned to the 3n(open triangle) and 6n (open square) channel, respectively. The two
extreme touching configurations, polar and equatorial collisions areshown at the top. The arrows indicate the corresponding Coulomb
barrier heights.

The 9.06 and 9.25 MeV lines were reported for the α decayof 263Sg both in the direct production in a fusion reaction23 andin the α decay from 271Ds.25–28 Two different lines should originate from two different levels in 263Sg. In the case of the direct
production in 18O + 249Cf,23 the 9.05 MeV line is dominated. In
the decay chains of 271 Ds, the 9.25 MeV α particles areobserved almost exclusively. 2 5 –2 8 From the 22 events inReferences 25–28, the majority of 20 decays belongs to thehigher energy of 9.25 MeV and only 2 have the energy of 9.06
MeV. The mean half-lives for two groups are determined to be+1.02(0.56 +0.16–0.10 ) s and (0.56 –0.22 ) s, respectively.
In the present experiment we observed two α decays at 9.25
MeV and one at 9.05 MeV. The measured lifetimes given in
Figure 4 are in good agreement with the literature data. The
present intensity ratio of two to one for the energetically different α particles may indicate losses of the shorter-lived higherenergy α particles in the He-jet experiment.23 The cross section
for the production of three 263Sg nuclei in our experiment at
Ec.m. = 144.4 MeV is (67 +67–37 ) pb.

76

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

Ec.m.= 144.0 MeV

260
263Sg

α1

strip 16
Erec = 9.4 MeV
263Sg

CN

0.576 s9256 keV

259Rf

α1

strip 16
Erec = 5.1 MeV
263Sg

CN

α1

48.2 ms
9257 keV259Rf

α 2 8.65 s
8854 keV

strip 4
Erec = 7.9 MeV
262Sg

CN

0.764 s9052 keV

17.9 ms
Estop = 129 MeV
Ebox = 20 MeV
TKE = 204 MeV
E γ (1) = 964 keV

α 2 2.63 s
8839 keV
255No

α3

240

sf

259Rf

α 2 5.39 s
8741 keV (esc)
E stop = 4729 keV
E box = 4012 keV

259

CN

934 s
8130 keV (esc)
E stop = 251 keV
E box = 7879 keV

TKE / MeV

strip 8
Erec = 9.1 MeV

258

220

strip 10Erec = 5.0 MeV

259

264Sg

CN

265Sg

CN

sf

sf

sf

52.0 ms
E stop = 176 MeV
E γ (2) = 540 keV
E γ (3) = 638 keV

23.6 ms
E stop = 114 MeV
E box = 28 MeV
TKE = 197 MeV

443 ms
E stop = 144 MeV
E γ (2) = 94 keV
E γ (4) = 143 keV

α1

CN

160

15.2 s (esc)
E stop = 224 keV

Figure 4: Decay events obtained from the reaction 30Si + 238U →268
Sg* at E c.m. = 144.0 MeV (upper row) and at E c.m. = 133.0 MeV(lower row). In the case of α decay or spontaneous fission (sf) the
energies measured in the stop and box detectors are given. The total
kinetic energy (TKE) of sf is given, when both fission fragments weredetected. Also given are the energies of γ rays coincident with sf measured in one or more of the four crystals of the Ge clover detector.
The number in brackets gives the crystal number of the four fold Ge
clover detector.

At the highest beam energy we also observed one sf eventwith lifetime of 17.9 ms (T1/ 2 = (12.4 +56– 6 ) ms), number 4 inFigure 4. The sf event was in coincidence with a γ ray at 964
keV in one of the crystals of the clover detector. Two fragments
were measured in coincidence with the stop and box detector,
and we obtained 204 MeV as TKE after correcting for thepulse height deﬁcit of 55 MeV. The cross section for the sf
event was (22 +51–18 ) pb. We tentatively assign this sf event to theisotope 262 Sg produced by evaporation of six neutrons. The
measured TKE of 204 MeV agrees reasonably with the valueof 210 MeV based on the empirical Viola formula29 for 262Sg as
shown in Figure 5.
At the lower beam energy of Ec.m. = 133.0 MeV, we observed
four sf events as numbered from 5 to 8 in Figure 4. In the caseof event numbers 5, 7, and 8, coincident γ signals from the clover detector were observed. In the case of event number 6,both ﬁssion fragments were detected in the stop and box detector, and this event occurred during the beam-off period. Fromthe sum of the two fragment energies, we obtained a TKE of
197 MeV after correcting for the pulse height deﬁcit.
The lifetimes of the sf events number 5 to 7 are similar toeach other and relatively short. We determined a half-life of(120 +126– 44 ) ms. These events were produced at an excitationenergy of 40 MeV, 11 MeV less than the value for the production of 263Sg. We assign these three sf events to the isotope
264
Sg. The TKE of 197 MeV for event number 6 has reasonableagreement with 210 MeV calculated with the Viola formula for
264
Sg (Reference 29) as shown in Figure 5.
In Figure 6, the partial half-lives for the Sg isotopes areshown. The data for 258,260,262Sg are taken from References 31
and 30. Recent data for 266Sg are taken from Reference 32. We
also show in Figure 6 the theoretical calculation of the sf par-

Rf
262

Fm
252-256

Rf
No

Sg

264

Sg

U

1600
2

1/3

1800

Z /A

sf
11.4 s
E stop = 181 MeV
E γ (4) = 1172 keV

238

1400

261Rf

Md

Lr262

Es
252-248
Cf
244-250
Cm
236-242Pu

strip 13E rec = 7.1 MeV

Figure 5: Total kinetic energies of the spontaneously fissioning
nuclei are shown as a function of Z 2 /A1/3. The present data for 262Sg
and 264Sg fissions are shown. Dash-dotted line is the Viola formula.29

sf partial half-life / s

264Sg

CN

260

Fm

256-260

180
264Sg

No
258
No

253

strip 4E rec= 5.8 MeV

Viola

Md

262

244-257200

Ec.m.= 133.0 MeV
strip 4E rec= 3.7 MeV

259

Fm

105
Theory
104
103
Expt.
102
101
100
10-1
10-2
10-3
Present
10-4-5
10
148 150 152 154 156 158 160 162 164 166 168

Neutron Number N
Figure 6: Partial half-lives for spontaneous fission of the Sg isotopes(solid squares) are compared with the theoretical calculation33 (opensquares connected by the dash-dotted lines). The present data for264
Sg is indicated.

tial half-life.33 The calculated half-life for 264Sg is 2.3 s, which
is about factor ten longer than our data.
The production cross section for the three sf events assigned
to 264Sg is (10 +10– 6 ) pb.
Before the 11.4 s of the sf event of number 8, we observed an
escaped α particle with 224 keV signal. Then the ER candidate
prior to the sf event and escaped α particle was detected at 15.2
s before the α event. However, due to the appearance of one
ER-like event every 10 s on the average, the value of 15.2 s represents only a lower limit for the lifetime. We tentatively assign
this chain originating from the escaped α decay of 265Sg followed by spontaneous ﬁssion of 2 61 Rf as is discussed in
Reference 6. The production cross section of this decay is (3.5
+8.1
–2.9 ) pb.
At the lowest beam energy of Εc.m. = 128.0 MeV no decay
events were measured. We determined an upper cross-section
limit of 15 pb at 68% conﬁdence level (one event corresponds
to 8.2 pb).

Measurement of Evaporation Residue and Fission Cross Sections

3.3. Discussion of the production cross sections. Experimental results for the ER cross sections in the reaction of 30Si +238
U → 268Sg* are compared with model calculations as shownin the lower part of Figure 3. Using the two different models
for the capture cross section (entrance channel), i.e., the coupled channel calculation and one dimensional model, the ER
cross sections are calculated with the statistical model code
HIVAP.34
The measured ER cross section for the 5n channel, 263Sg, at
E c.m. = 144.0 MeV agrees well with both calculations, the coupled channel calculation and the one dimensional calculation.
At the lower energy E c.m. = 133.0 MeV the large enhancement of the 4n ER cross section compared to the one dimensional model conﬁrms the validity of taking into account theproperties of colliding nucleus. Especially the enhancement iscaused by the lowering of the Coulomb barrier height for collisions at the polar sides of 238U. However, the experimental 4n
cross section (10 pb) is about factor of 4 smaller than the calculated cross section (40 pb). This suggests the competition
between fusion and quasiﬁssion after system is captured inside
the Coulomb barrier. Also at the lowest energy E c.m. = 128.0
MeV the measured upper limit of the cross section of 15 pb (3n+ 5n channel) is about a factor 5 less than the calculation, suggesting the presence of quasiﬁssion.
Finally we compare the two extreme reactions mentioned
before, namely the very asymmetric system 16 O + 2 3 8 U(Reference 5) which has fusion probability independent of orientation and the heaviest systems studied so far using 48Ca projectiles, 3,4 with the reaction 30 Si + 238 U studied here. The
reaction 30 Si + 238 U shows an intermediate behavior. We
observed a measurable fusion cross section at subbarrier energies, where a contact is limited to polar collisions, but indicated
a competition between quasiﬁssion and complete fusion. On
the other hand, no suppression of the ER cross section wasmeasured at the energy where contact of the projectile on theequatorial side of 238U is possible, suggesting that the fusion
starting from the compact conﬁguration results in higher fusion
probability.
Acknowledgement. We thank the UNILAC staff for preparation of the stable and intense 30Si beam. We are also grateful
to W. Hartmann and J. Steiner of the GSI target laboratory for
manufacturing the target wheels and H. G. Burkhard for taking
care of the mechanical devices at SHIP. This work was partly
supported by a Grant-in-Aid for Scientiﬁc Research of theJapan Society for the Promotion of Science.
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