Journal of Nuclear and Radiochemical Sciences, Vol. 7, No.1, pp. R25-R29, 2006

Study of Superheavy Elements at the GSI-SHIP
S. Hofmann,*, a D. Ackermann,a S. Antalic,b H.G. Burkhard,a R. Dressler,c F.P. Heßberger,aB. Kindler,a I. Kojouharov,a P. Kuusiniemi,d M. Leino,d B. Lommel,a R. Mann,a G. Münzenberg,aK. Nishio,e A.G. Popeko,f S. Saro,b H.J. Schött,a B. Streicher,b B. Sulignano,a J. Uusitalo,d and
A.V. YereminfaGesellschaft für Schwerionenforschung (GSI), D-64220 Darmstadt, Germany
Department of Nuclear Physics, Comenius University, SK-84248 Bratislava, SlovakiacPaul Scherrer Institut, 5232 Villigen, SwitzerlanddDepartment of Physics, University of Jyväskylä, FIN-40351 Jyväskylä, Finlande
JAEA, Tokai, Ibaraki 319-1195, Japanf
Flerov Laboratory of Nuclear Reactions, JINR, Ru-141 980 Dubna, Russiab

Received: December 26, 2005
An overview of present experimental investigation of superheavy elements is given. The data are compared with theoretical descriptions. Results are reported from an experiment to confirm production of element 112 isotopes in irradiation of 238UF4 with 48Ca. One spontaneous fission event was measured, which agrees with three events of previously
measured data which had been assigned to the decay of 283112. However, more experimental work is needed in orderto obtain an independent and unambiguous confirmation of previous results.

1. Introduction and status of experiments
For the synthesis of heavy and superheavy elements (SHE)
fusion-evaporation reactions are used. Two approaches havebeen successfully employed. Firstly, reactions with medium
mass ion beam impinging on targets of stable Pb and Bi isotopes (cold fusion). These reactions have been successfully
used to produce elements up to Z=112 at GSI1 and to confirm
these experiments at RIKEN2 and LBNL.3 Using a 209Bi target
the isotope 2 78 113 was recently synthesized at RIKEN. 2
Secondly, reactions between lighter ions, especially with beams
of 48Ca, and radioactive actinide targets (hot fusion) have beenused to produce more neutron rich isotopes of elements from
Z=112 to 116 and 118 at FLNR.4 Figure 1 summarizes the dataas they are presently known or under investigation.
Besides the discovery of the existence of these high-Z elements, two more important observations emerged. Firstly, theexpectation that half-lives of the new isotopes should lengthenwith increasing neutron number as one approaches the islandof stability seems to be fulfilled. Secondly, the measured
cross-sections for the relevant nuclear fusion processes reachvalues up to 5 pb, which is surprisingly high. Furthermore,they seem to be correlated with the variation of shell-correctionenergies as predicted by macroscopic-microscopic calculations.5,6
A number of excitation functions were measured for the synthesis of elements from nobelium to darmstadtium using cold
fusion reactions. Some of the curves are shown together withthe two data points for 277112 in Figure 2. Maximum evaporation residue cross-sections (1n channel) were measured at beamenergies well below a contact configuration, where projectile
and target nucleus come to rest according to the fusion modelby Bass.7
Excitation functions for hot fusion reactions were measured
recently at FLNR.4 The data for the reaction 48Ca + 244Pu →288
114 + 4n is shown in the lowest panel in Figure 2. Here thepeak is located well above the contact configuration calculated*Corresponding author. E-mail: S.Hofmann@gsi.de. “Josef
Buchmann Professor Laureatus”

from a mean value of the nuclear potential of the deformed target nucleus. This shift as well as the increased width of thecurve (10.6 MeV instead of 4.6 MeV FWHM for 265Hs) are inaccord with an orientation effect on fusion using a deformed
target nucleus. The shift to higher energy indicates collisions
in direction of the short deformation axis.
It was pointed out in the literature9 that closed shell projectile and target nuclei are favorable for the synthesis of SHEs.

Figure 1. Calculated ground-state shell correction energy (a) anddominating partial half-lives for α, β + or EC, β– decay, and spontaneous fission (SF) of even-even nuclei (b). The calculated data weretaken from References 5 and 6. The squares in (a) show the isotopesof heavy and superheavy elements, which are known or presently
under investigation, and the arrows in (b) mark the measured decay
chains. In all cases the decay chains end, in agreement with theoretical predictions, at nuclei decaying by SF.

© 2006 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/30/2006

R26 J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006

Hofmann
2. Study of the 48Ca + 238U Reaction

Figure 2. Excitation functions measured at SHIP for the synthesis ofelements Rf and Hs plus two data points measured for Z=112 using
cold fusion reactions1 (208Pb target) and at the DGFRS of FLNR for
the synthesis of element Z=1144 using hot fusion reaction (244Pu target). The arrows mark the excitation energy for reactions, when the
kinetic energy of the projectile is just sufficient high to reach a contact configuration according to the fusion model by Bass.7 The excitation energies were calculated using binding energies for thecompound nuclei from Reference 8.

The reason is not only a low (negative) reaction Q-value andthus a low excitation energy, but also that fusion of such systems is connected with a minimum of energy dissipation. The
fusion path proceeds along cold fusion valleys, where the reaction partners maintain kinetic energy up to the closest possible
distance. Recent theoretical studies are able to reproduce themeasured data. That work is continued by various groups (seee.g. References 10,11, and references therein) aiming to workout reliable predictions for the production cross-sections of
SHEs.

2.1. Results from previous investigations. Data from studies of the reaction 48Ca + 238U → 286112* are contradictory concerning half-lives and cross-sections. A summary of the
published data is shown in Table 1.
Most comprehensive studies of hot fusion reactions with 48Ca
beams were performed at the DGFRS of FLNR, see Reference
4. The results from this group are based not only on the reaction 48Ca + 238U, but also on data from reactions for the synthesis of nuclei of elements beyond Z=112. These data served as abasis for the preparation of our experiment to search for 283112
at SHIP using the reaction 48Ca + 238U.
2.2. Technical aspects and test reactions. The 48Ca beam
was prepared from the ECR ion source and accelerator
UNILAC at GSI. Metallic, isotopically enriched (89.5%) 48Ca
and the ECR oven technique were used. Ions with charge state
10+ were extracted and accelerated by the high charge state
injector (RFQ + IH structure) and the UNILAC to Coulomb
barrier energies. A mean current of 1.2 pµA was reached ontarget at a duty factor of 28% (5.5 ms wide pulses at 20 Hz repetition frequency). The consumption of 48Ca was 0.6 mg/h on
the average.
The targets were prepared from the chemical compoundUF4. Layers of 488, 408, and 451 µg/cm2 were evaporated onbacking foils of 42 µg/cm2 C and subsequently covered with C
layers of 10 or 20 µg/cm2. Details of the target preparation are
given in Reference 16. The target thickness was controlled online by registration of elastically scattered projectiles and scattering of 20 keV electrons.17 The data showed a continuous
decrease of the thickness during irradiation. The target wheel
was replaced at the latest, when the losses reached a value of
30%. We estimated a mean fading of 20% of target thickness
for the calculation of the cross-section. A total of 14 uranium
target wheels were used. The uranium content of new and irradiated targets including the performance of the whole experimental set-up was tested by measuring the yield of fusion
products from reactions with a 22Ne beam supplied for two days
at half time of the experiment. This test confirmed the results
obtained by the measurements using scattered projectiles orelectrons.
Properties of SHIP and of the detector system are describedin Reference 1. Since that time, however, the detector system
was modified. A Si veto detector was mounted behind the stop
detector and the Ge detectors were replaced by a four crystal
Ge clover detector. The new set-up is shown in Figure 3.
During the uranium irradiation the first of the TOF detectors
was removed after few days, which resulted in reduced scattering of the ions and deeper implantation into the Si stop detector.

Table 1: Published data from investigation of the reaction 48Ca + 238U → 286112*.
Duration / day
25
15
29
15
14
17

E* / MeV
33
39
33
35.5
31.4
35.0

dose / 1018
3.5
2.2
5.9
4.7
5.8
7.1

13
22.5
≈ 8.7
≈ 7.1

39.8
34.2
31.9
36.3

5.2
2.8
2.26
1.85

events
2
0
0
2
1
2
3
1
1
7
0
0

event type
ER–SF
–
–
ER–SF
ER–(α)–SF
ER–(α)–SF
ER–α–SF
ER–4α–SF
ER–SF
SF
–
–

T1/2 / s
81
–
–
568
(3.4)
(1.4)
2.7
6.1
0.00014
≥ 60
–
–

xn
3

3
3
3
3
3
4
3

σ / pb
5.0
< 7.3
< 2.2
3.0
0.5
2.5

Ref.
12
12
13
13
4
4

0.6
2
< 0.80< 0.964
14
15
15

E* at half thickness of the target layer; cross-section limits given are “one-event” limits which do not include errors from statistical fluctuations.

Study of SuperheavyElements at the GSI-SHIP

J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006 R27

Figure 3. Detector set-up at SHIP. For details see text and description in Reference 1.

Figure 4. Detector response to SF events of 252No produced in thereaction 48Ca + 206PbS for three different implantation depth obtainedby mylar degrader foils of 1, 4, and 8 µm thickness located in front of
the Si detector array. The measured energy (α calibration) of theimplanted nuclei is given on the right. The three columns show fromthe left the energy singles spectra obtained from the stop detector,
coincident signals detected in the box and the stop detector (herecombined in one spectrum) and the total kinetic energy (TKE) as
sum of the coincident signals. The energy values given in MeV are
the peaks centre of gravity. The energy calibration is based on α particles and does not include heavy ion deficit energies. Note that at14.2 MeV implantation energy both fission fragments are detected
simultaneously in the stop and the box detector with a probability of
42% relative to all SF signals in the stop detector. This probability isslightly increasing at less deep implantation, because then the probability decreases that both SF fragments are stopped in the stop detector. The arrows in the first row mark the energy values measured
from the SF event on May 8, 2005. The implantation energy was 19.5
MeV (see Figure 6), which corresponds roughly to the same implantation depth as for 252No shown in row one, assuming implantation ofa heavy isotope of element 112.
48

4.99 MeV/u

Ca+

238

22

UF4

4.92 MeV/u

Ne+

238

48

UF4

Ca+

238

UF4

4.92 MeV/u

5.20 MeV/u

5

Beam Current / pµA

Two different field settings for SHIP were calculated using aMonte Carlo method, in order to optimize efficiency for the
separation of fusion products from the 48Ca + 238UF4 reaction.
Values of 17% and 24% were obtained. Test reactions using a
208
Pb target confirmed the dependence and, because the background did not increase more than proportional, we used thesetting with higher efficiency.
The response of the detector system to SF events was investigated especially carefully, expecting that fission will terminate the decay chains of element 112 isotopes. These
measurements were performed using the 48Ca beam and targetsof 208Pb and 206Pb before and after the main irradiation, respectively. Half time the main irradiation the reaction 22Ne + 238UF4
was also used for testing purposes. The isotopes produced
were 254No, 252No and 255,256No, 256Md (p3n channel) and theirdaughter products, respectively. In order to give an impressionof the detector response, we plot in Figure 4 part of the data.
The response of the Ge clover detector to SF was obtained
from 10,000 SF events. SF−γ coincidences were measured
with fractions of 25.6, 31.0, 23.8, and 7.7% for signals in 1, 2, 3,
or all 4 of the 4 Ge crystals, respectively. Only in 11.0% of allcases no signal from the clover detector was in coincidencewith SF. Vice versa the response of the clover detector to background events was also measured. From 19,000 background
events in the Si stop detector with energies >150 MeV the corresponding fractions are 1.8, 0.4, 6×10−4, 5×10−5, and 97.7%.
2.3. The experiment at SHIP. The irradiations at SHIP,
including various test reactions, took place from April 6 to June
9, 2005. Duration of the 238UF4 irradiation, beam energies,
bea m doses, a nd results a re sum ma r ized in Table 2.
Chronologically, energies were chosen as given in the tablefrom top to bottom.
No SF events were measured at excitation energies of 37.0
and 32.0 MeV. At the beginning of the irradiation at E* = 34.5
MeV within 12 hours after the test reaction 22Ne + 238UF4, we
measured 3 SF events. The period of interest is shown in
Figure 5. The TKE (energy calibration based on α particles) ofthese SF events is 149.4, 141.3, and 154.8 MeV, respectively,
which agrees well with the energies of SF events from 255No or
256
Fm measured during the test reaction. We assign these
events to the decay of 256Fm on the basis of well known halflives and SF branching ratios.
A forth SF event was measured on May 8, 2005, also shownin Figure 5. The measured parameters clearly identify thisevent as SF. The energies, based on α calibration, are E stop =190.4 MeV and Ebox = 15.3 MeV. These data are shown together
with the energies from SF of 252No in Figure 4. The considerable high TKE is clearly visible. In order to determine the true
TKE we used the known TKE of the 252No decay, which is 195
MeV18 (see Figure 7). The difference to the peak position at
158.7 MeV in Figure 4 is 36 MeV. This energy difference wasadded to the TKE of the SF event from May 8, which resulted
in a TKE of 242 MeV. Assuming a width of the TKE distribution similar to that of 252No, we determine a one σ uncertainty
of ±15 MeV for this one event.
The following properties of the SF event were measured further: no signals from the TOF detectors, signals of 386 and2068 keV in two of the Ge detectors, appearance during thebeam pause at 12.508 ms after beginning of the macro-pulse
period which starts with the 5.5 ms wide beam pulse. Finally,
the event occurred in strip number 9 of the 16 strip stop detec-th

SF May 8 , 2005, 22:45

SF

4
3
2
1
0
2 8 29 3 0

1

2

3

4

5

6

7

8

9

10

11 12 13 14

May

April

Figure 5. Mean beam currents in particle µA and appearance of SF
events during irradiation of an 238UF4 target with 48Ca projectiles.
From May 3 to 6 a 22Ne beam was used for testing purposes.

Table 2: Parameters and results of the 48Ca + 238UF4 irradiation studied at SHIP.
Duration / day
20.9
16.8
14.8

Eproj / MeV
239.3
236.2
233.3

E* / MeV
37.0
34.5
32.0

For E* and cross-section limits see footnote to Table 1.

dose / 1018
12
10
7

event
0
1
0

event type
–
ER–SF
–

T1/2 / s
–
5.2
–

xn
–
?
–

σ / pb<0.6
0.7 +1.6–0.6<0.8

Energy / MeV

R28 J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006

Hofmann

30

25

25

20

20

15

15

10

10

5

5
1340

1360

1380

1400

1420

0
800

900

1000

1100

1200

1300

– Time-of-flight / channel
Figure 6. TOF (inverse time scale)–energy spectrum from the reaction 48Ca + 238UF4 with all signals (marked by crosses) appearing 1000 s
before the SF event from May 8, 2005 and within a position window of ±1 mm (left side) in comparison with a similar plot from the test reaction
48
Ca + 206PbS (right side). In that plot the branches show, with increasing energy, scattered projectiles, scattered target nuclei and 252No evaporation residues. It was taken from an 11.5 hours irradiation in which a beam dose of 3.4 ×1017 was reached. Contour lines begin at 201 counts and areplotted in steps of 201 counts. The structure on the projectile branch is an artifact from the graphics program. The contour lines on the left plot
begin with 49 counts and are plotted in steps of 49 counts. For this underlying contour plot all data of the 238UF4 irradiation at 236.2 MeV beamenergy were taken, which were measured after the test with the 22Ne beam during a period of 12.1 days and at a beam dose of 7.1×1018 projectiles.

260
mass-sym.
fission

250

271
258

TKE / MeV

240
230

294

Sg

No

262

No

267,268
258

118

Db

282,284

112
114
279,281
110

Fm

286

220

m a s s - a s ym .
fission210
200

2 56 - 2 62

190

Rf

252-258,262

NoFm

3. Conclusion and Outlook

244-268,254-258

180

2 4 8 -2 5 6242-2 50

170

Cf
Cm

160
1400

1600

1800

2000

time window of 1000 s, we found a total of 36 implantations,but no α particle was measured. Due to the low discriminator
level (200 keV) for detection of α particles escaping from thestop detector, we exclude non-registration of such events withhigh probability. TOF and energy values of the 36 implanted
nuclei are plotted in Figure 6, left side. Most of the events
coincide with background events from elastically scattered target nuclei. Only one appears at a position where we expect signals from implanted evaporation residues. Where this region is
located relative to the background distribution is clearly seen in
Figure 6, right side, which was taken from the test reaction 48Ca
+ 206PbS. The measured time difference between implantationand SF is 7.57 s, which corresponds to a half-life of (5.2 +25.1−2.4 ) s.
Finally, we show in Figure 7 the Viola-Seaborg systematicof SF nuclei.

2200

Z 2 / A1/3
Figure 7. Shown is the Viola-Seaborg systematic of SF nuclei. The
horizontal line at 242 MeV marks the TKE value of the SF event
measured on May 8 in our experiment. The ±1σ uncertainty of themean TKE for this SF decay is also given. Marked at 195 MeV is the
TKE of 252No,18 which was used for calibration. The TKE of our event
was added to a figure taken from Reference 4, which shows data
recently measured at FLNR (filled squares) and previously measured
data19 (open squares).

tor at a vertical position of 30 mm from the bottom. The signal
from the 28 segment box detector was derived from segment
number 13.
At the position of the SF event in the stop detector wesearched for preceding α particles and the implanted evaporation residue. Within a reasonable position window of ±1 mm(a more accurate position calibration is in preparation) and a

The data measured for the decay of nuclei in the region ofheavy and superheavy nuclei agree well with the predictions
from the macroscopic-microscopic model. Using fission barriers from these models, the production cross-sections were reasonably well described in recent theoretical work. Aiming at
confirmation of results on element 112 using hot fusion reaction, we measured one SF event which agrees with data measured in Dubna. However, if we tentatively assign the SF to theisotope 283112 in accordance with the interpretation given in
Reference 4, then we have to introduce an adequate SF branching of this nucleus, because no α particle was measured
between implantation and SF. More experimental work isneeded in order to obtain an independent and unambiguous
confirmation of previous results. As a next step we will explore
the production of metallic uranium targets which result inhigher efficiency of the SHIP separator than targets of chemical compounds, in order to study the reaction 50 Ti + 238U →
288
114* and to repeat the reaction 48Ca + 238U → 286112* underfurther improved conditions.
Acknowledgement. We thank the UNILAC staff for excellent performance of the 48Ca beam with respect to high stability, high current, and low consumption of material.

Study of SuperheavyElements at the GSI-SHIP

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