Journal of Nuclear and Radiochemical Sciences, Vol. 4, No. 1, pp. R1-R13, 2003

Reviews
Synthesis and Properties of Superheavy Elements
S. Hofmann*Gesellschaft für Schwerionenforschung (GSI), D-64291 Darmstadt, Germany
Received: April 3, 2003
The nuclear shell model predicts that the next doubly magic shell-closure beyond 208Pb is at a proton number Z =114, 120, or 126 and at a neutron number N = 172 or 184. The outstanding aim of experimental investigations is
the exploration of this region of spherical ‘SuperHeavy Elements’ (SHEs). Experimental methods are describedwhich allowed for the identification of elements 107 to 112 in studies of cold fusion reactions based on lead andbismuth targets. Also presented are data which were obtained on the synthesis of elements 112, 114, and 116 in
investigation of hot fusion reactions using actinide targets. The decay data reveal that for the heaviest elements,
the dominant decay mode is alpha emission, not fission. Decay properties as well as reaction cross-sections arecompared with the results of theoretical studies. Finally, plans are presented for the further development of theexperimental set-up and the application of new techniques. At a higher sensitivity, the exploration of the region of
spherical SHEs now seems to become feasible, more than thirty years after its prediction.

1. Introduction
Searching for new chemical elements is an attempt toanswer questions of partly fundamental character: How many
elements may exist? How long is their lifetime? Which properties determine their stability? How can they be synthesized?
What are their chemical properties? How are the electrons
arranged in the strong electric field of the nucleus?
Searching for new elements beyond uranium by the processof neutron capture and succeeding β − decay, Hahn andStraßmann1 discovered the possibility that a heavy nucleus
might “divide itself into two nuclei.” This was the correctinterpretation given by Meitner and Frisch,2 and the term
“fission” was coined for this process. By applying the existing
charged liquid-drop model of the nucleus, 3, 4 nuclear fission
was explained quite naturally, and it was shown that fission
will most likely limit the number of chemical elements. At
that time, the maximum number of elements was expected to
be about 100. This number results from the balance of two
fundamental nuclear parameters, the strength of the attractive
nuclear force which binds neutrons and protons together andcreates a surface tension and the repulsive electric force.
The properties of nuclei are not smooth uniform functions of
the proton and neutron numbers, but show non-uniformities asevidenced by variations in the measured atomic masses. Likethe electrons in an atom, also the nucleons in a nucleus described by quantum mechanical laws - form closed shells
called “magic” numbers. At the magic proton or neutron
numbers 2, 8, 20, 28, 50, and 82, the nuclei have an increased
binding energy relative to the average trend. For neutrons, N =126 is also identified as a magic number. However, thehighest stability is observed in the case of the “doubly magic”
nuclei with a closed shell for both protons and neutrons.
Amongst other special properties, the doubly magic nuclei arespherical and resist deformation.
The magic numbers were successfully explained by the
nuclear shell model, 5, 6 and an extrapolation into unknown
regions was reasonable. The numbers 126 for the protons and
184 for the neutrons were predicted to be the next shell*Corresponding author. E-mail: s.hofmann@gsi.de. FAX: +496159-71-2902.

closures. Instead of 126 for the protons also 114 or 120 were
calculated as closed shells. The term superheavy elements(SHEs) was coined for these elements.
The prediction of magic numbers, although not unambiguous, was less problematic than the calculation of thestability of those doubly closed shell nuclei against fission. As
a consequence, predicted half-lives based on various calculations differed by many orders of magnitude.7−12 Some of thehalf-lives approached the age of the universe, and attempts
have been made to discover naturally occurring SHEs.13, 14
Although discoveries were announced from time to time, none
could be substantiated after more detailed inspection.
There was also great uncertainty of the production yields for
SHEs. Closely related to the fission probability of SHEs in the
ground-state, the survival of the compound nuclei formed after
complete fusion was difficult to predict. Even the best choiceof the reaction mechanism, fusion or transfer of nucleons, wascritically debated. However, as soon as experiments could be
performed without technical limitations, it turned out that themost successful methods for the laboratory synthesis of heavy
elements are fusion-evaporation reactions using heavy-elementtargets, recoil-separation techniques, and the identification of
the nuclei by generic ties to known daughter decays after
implantation into position-sensitive detectors. 1 5 − 1 7 An
overview of various methods developed for the synthesis oftransuranium elements and a review of their discoveries up to
the most recent attempts to produce spherical SHEs is given in
Reference 18.
In the following sections a detailed description is given of
the set-ups of the physical experiments used for the investigation of SHEs. (The instrumentation based on chemical methods
for the study of heavy elements is presented in Reference 19.)
Experiments are presented, in which cold and hot fusion reactions were used for the synthesis of SHEs. These experiments
resulted in the identification of elements 107 to 112 at theGesellschaft für Schwerionenforschung (GSI) in Darmstadt,
and in the recent synthesis of elements 114 and 116 at the Joint
Institute for Nuclear Research (JINR) in Dubna. We also
report on a search for element 118, which started in 1999 at
Lawrence Berkeley National Laboratory (LBNL) in Berkeley.
In subsequent sections a theoretical description follows
discussing properties of nuclei in the region of SHEs and© 2003 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 05/27/2003

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

phenomena, which influence the yield for the synthesis of
SHEs. Empirical descriptions of hot and cold fusion nuclear
reaction systematic are outlined. Finally, a summary andoutlook is given.
2. Experimental Techniques
2.1. Targets and Accelerators. First attempts to synthesize
transuranium elements based on the idea to produce β −decaying nuclei by neutron capture, which decay into the next
heavier so far unknown element. Up to fermium, this method
made it possible to climb up the Periodic Table element by
element. While from neptunium to californium, some isotopes
can be produced in amounts of kilograms or at least grams inhigh neutron flux reactors, the two heaviest species, 254Es and257
Fm, are available only in quantities of micrograms andpicograms, respectively. At fermium, however, the method
ends due to the lack of β− decay and too short α and fission
half-lives of the heavier elements. Sufficiently thick enoughtargets cannot be manufactured from these elements.
The region beyond fermium is best accessible using heavyion fusion reactions, the bombardment of heavy-elementtargets with heavy ions from an accelerator. The cross-sectionis less than in the case of neutron capture and values are
considerably below the geometrical size of the nuclei.
Moreover, only thin targets of the order of 1 mg/cm2 can beused. This limitation arises from the energy loss of the ion
beam in the target, which results (using thicker targets) in anenergy distribution that is too wide for both the production offusion products and their in-flight separation. On the other
hand, the use of thin targets in combination with well definedbeam energies from accelerators results in unique information
about the reaction mechanism. The data are obtained bymeasuring excitation functions, the yield as a function of thebeam energy.
Various combinations of projectiles and targets are in principle possible for the synthesis of heavy elements: actinide
targets irradiated by light projectiles of elements in the range
from neon to calcium, targets of lead and bismuth irradiated byprojectiles from calcium to krypton, and symmetric combinations like tin plus tin up to samarium plus samarium. Also
inverse reactions using e.g. lead or uranium as projectile arepossible and may have technical advantages in specific cases.
Historically, the first accelerators used for the production ofheavy elements were the cyclotrons in Berkeley, California,
and later in Dubna, Russia. They were only able to accelerate
light ions up to about neon with sufficient intensity and up to
an energy high enough for fusion reactions. Larger and more
powerful cyclotrons were built in Dubna for the investigationof reactions using projectiles near calcium. These were theU300 and U400, 300 and 400 cm diameter cyclotrons. In
Berkeley a linear accelerator HILAC (Heavy Ion Linear
ACcelerator), later upgraded to the SuperHILAC, was built.
The shutdown of this accelerator in 1992 led to a revival ofheavy element experiments at the 88-Inch Cyclotron. Aiming
at the acceleration of ions as heavy as uranium, the UNILAC(UNIversal Linear ACcelerator) was constructed in Darmstadt,
Germany, during the years 1969–74.
In order to compensate for the decreasing cross-sections of
the synthesis of heavy elements, increasing beam currents areneeded from the accelerators. This demands a continuous
development of ion sources in order to deliver high currents athigh ionic charge states. Beam currents up to a few particle
microamperes (1 µApart = 6.24 × 1012 particles/s) are presently
reached. Such high currents, in turn, demand a higher resistance of the targets. An efficient target cooling and chemicalcompounds with higher melting points are presently tested.
The developments in the laboratories in Berkeley, Dubna, andalso in Finland, France, and Italy are similar and usually madeHofmannin close collaboration and exchange of know-how. Especiallygreat progress towards the synthesis of new elements wasreported recently from experiments performed at the Instituteof Physical and Chemical Research (RIKEN), in Wako,
Saitama, Japan (see sect. 3.1).
2.2 Recoil-Separation Techniques and Detectors. The
identification of the first transuranium elements was by chemical means. In the early 1960s physical techniques were developed which allowed for detection of nuclei with lifetimes ofless than one second at high sensitivity. A further improvement of the physical methods was obtained with the development of recoil separators and large area position sensitive
detectors. As a prime example for such instruments, we will
describe the velocity filter SHIP (Separator for Heavy-Ion
reaction Products) and its detector system, which were developed at the UNILAC. The principle of separation and detection techniques used in the other laboratories is comparable.
In contrast to the recoil-stopping methods, as used in He-jet
systems or mass separators, where ion sources are utilized,
recoil-separation techniques use the ionic charge andmomentum of the recoiling fusion product obtained in thereaction process. Spatial separation from the projectiles andother reaction products is achieved by combined electric andmagnetic fields. The separation times are determined by therecoil velocities and the lengths of the separators. They are
typically in the range of 1-2 µs. Two types of recoil separators
have been developed: (1) The gas-filled separators use thedifferent magnetic rigidities of the recoils and projectiles travelling through a low pressure (about 1 mbar) gas-filled volume
in a magnetic dipole field.20 In general, helium is used in orderto obtain a maximum difference in the rigidities of slow reaction products and fast projectiles. A mean charge state of theions is achieved by frequent collisions with the atoms of thegas. (2) Wien-filter or energy separators use the specific kinematic properties of the fusion products. The latter are createdwith velocities and energies different from the projectiles andother reaction products. Their ionic charge state is determinedwhen they escape from a thin solid-state target into vacuum.
Ionic-charge achromaticity is essential for high transmission.
It is achieved by additional magnetic fields or symmetricarrangements of electric fields. An example of such a separator used in experiments for the investigation of heavy
elements is the velocity filter SHIP in Darmstadt15 shown in
Figure 1.
γDetectors

SHIP 94

SiDetectors
TOF
Detectorso

7.5 Magnet

Beamstop

Targetwheel

Magnets
ElectricField
Lenses

Projectiles

Figure 1. The velocity filter SHIP (Separator for Heavy Ion reaction
Products) and its detection system.15−17 The drawing is approximately
to scale, however, the target wheel and the detectors are enlarged by afactor of two. The length of SHIP from the target to the detector is 11
m. The target wheel has a radius up to the center of the targets of 155
mm. It rotates synchronously with beam macrostructure at 1125
rpm. 21 The target thickness is usually 450 µ g/cm 2 . The detector
system consists of three large area secondary-electron time-of-flightdetectors22 and a position-sensitive silicon-detector array (see text).
The flight time of the reaction products through SHIP is 2 µs. The
filter, consisting of two electric and four magnetic dipole fields plus
two quadrupole triplets, was extended by a fifth deflection magnet,
allowing for positioning of the detectors away from the straight beamline and further reduction of the background.

Synthesis and Properties of Superheavy Elements

Recoil separators are designed to filter out those nuclei witha high transmission, which are produced in fusion reactions.
Since high overall yields result in increased background levels,
the transmitted particles have to be identified by detector
systems. The detector type to be selected depends on theparticle rate, energy, decay mode, and half-life. Experimental
as well as theoretical data on the stability of heavy nuclei showthat they decay by α emission, electron capture or fission, withhalf-lives ranging from microseconds to days. Therefore
silicon semiconductor detectors are well suited for the identification of nuclei and for the measurement of their decay properties. If the total rate of ions striking the focal plane of theseparator is low, then the particles can be implanted directly
into the silicon detectors. Using position-sensitive detectors,
one can measure the local distribution of the implanted particles. In this case, the detectors act as diagnostic elements tooptimize and control the ion optical properties of the separator.
Given that the implanted nuclei are radioactive, the positions measured for the implantation and all subsequent decay
processes are the same. This is the case because the recoil
effects are small compared with the range of implanted nuclei,
emitted α particles or fission products, and detector resolution.
Recording the data event by event allows for the analysis ofdelayed coincidences with variable position and time windows
for the identification of the decay chains.16
The presently used detector system is composed of three
time-of-flight detectors, seven identical 16-strip silicon wafers,
and germanium detectors.17 A schematic view of the detector
arrangement is shown in the focal plane of SHIP in Figure 1.
Three secondary-electron foil detectors in front of the silicondetectors are used to measure the velocity of the particles.22
They are mounted 150 mm apart from each other. The
detector signals are also used to distinguish implantation fromradioactive decays of previously implanted nuclei. Threedetectors are used to increase the detection efficiency.
A time-of-flight signal and an energy signal from the silicondetector provide the information for switching off the beam
after detection of an implanted residue.23 After a response timeof 20 µs a subsequent time window of preset duration opens
for counting a preset number of α particles of the decay chain.
If the desired conditions are fulfilled, the beam-off period is
prolonged up to the expected measurable end of the decay
chain by opening a third time window. This improvement
considerably reduces the background during the measuring
period of the decay chain and allows for the safe detection of
signals from long lived decays. The sequence of three time
windows is needed because time-of-flight and energy signals
alone would trigger the switching off process for the beam too
often due to background events in the corresponding windows.

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from the α decay of Mt, and the isotope Bh was identified as granddaughter in the decay chain of 272111.23, 29 The
isotopes 266Bh and 267Bh were produced using the hot fusion
reaction 22Ne + 249Bk → 271Bh*.39, 40 These nuclei were used
for a study of the chemical properties of bohrium.
Hassium, element 108, was first synthesized in 1984 using
the reaction 58Fe + 208Pb. The identification was based on theobservation of three atoms. 45 Only one α-decay chain wasmeasured in the irradiation of 207Pb with 58Fe. The measured
event was assigned to the even-even isotope 264 Hs. 46 The
results were confirmed in a later work,25, 27 and for the decay of
264
Hs, a fission branching of 50% was also measured. The
isotope 269Hs was discovered as a link in the decay chain of
277
112, 23, 28 and 270 Hs was identified in a recent chemistry
experiment.41
Meitnerium, element 109, was first observed in the year
1982 in the irradiation of 209Bi with 58Fe by a single α-decay
chain. 47, 48 This result was confirmed later. 49 In the mostrecent experiment44 twelve atoms of 266Mt have been measured,
revealing a complicated decay pattern, as could be concluded
from the wide range of α energies from 10.5 to 11.8 MeV.
This property seems to be common to many odd and odd-odd
nuclides in the region of the heavy elements. The moreneutron-rich isotope 268 Mt was measured after α decay of272
111.23, 29
Element 110 was discovered in 1994 using the reaction 62Ni
208
+ Pb → 269110 + 1n.27 The main experiment was precededby a thorough study of the excitation functions for thesynthesis of 257Rf and 265Hs using beams of 50Ti and 58Fe inorder to determine the optimum beam energy for the production of element 110. New information on the decay pattern ofthese nuclei was also obtained. The data revealed that the
maximum cross-section for the synthesis of element 108 wasshifted to a lower excitation energy, different from the predictions of reaction theories. An extra-push energy was notmeasured.
The heavier isotope 271110 was synthesized with a beam ofthe more neutron-rich isotope 64Ni.25 The important result for
the further production of elements beyond meitnerium was thatthe cross-section was enhanced from 2.6 pb to 15 pb byincreasing the neutron number of the projectile by two, whichgave hope that the cross-sections for the synthesis of heavier
266

44

264

3. Experimental Results
3.1. Elements Produced in Cold-Fusion Reactions. In
this section, we present results dealing with the discovery ofelements 107 to 112 using cold fusion reactions based on leadand bismuth targets. A detailed presentation and discussion of
the decay properties of elements 107 to 109 and of elements
110 to 112 was given in previous reviews.17, 24, 25 Known elements and their position in the Periodic Table of the Elements
are shown in Figure 2. An overview of nuclei and decay
chains in the region of SHEs, which are presently known or
under investigation, is given in the partial chart of nuclides(Figure 3).
Bohrium, element 107, was the first new element synthesized at SHIP using the method of in-flight recoil separationand generic correlation of parent-daughter nuclei. The reactionused was 54Cr + 209Bi → 263Bh*. Five decay chains of 262Bh
were observed.42 The next lighter isotope, 261Bh, was synthesized at a higher beam energy.43 Additional data were obtained

Figure 2. Periodic Table of the Elements. The known transactinide
elements 104 to 116 take the positions from below Hf in group IVA tobelow Po in group VIB. Element 108, hassium (Hs), the heaviest
element chemically investigated, is placed in group VIIIA. The
arrangement of the actinides reflects the fact that the first actinide
elements still resemble, to a decreasing extent, the chemistry of theother groups: Th (group IVA below Hf), Pa (group VA below Ta), andU (group VIA below W).26 The name ‘darmstadtium’, symbol ‘Ds’
was proposed for element 110 and recommended for acceptance by
the Inorganic Chemistry Division of IUPAC in February 2003.

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Hofmann

Figure 3. Upper end of the chart of nuclei showing the known nuclei and those which are presently (2003) under investigation. For each known
isotope the element name, mass number, and half-life are given. The dots mark compound nuclei of reactions which were used to study the heaviest elements. In cold fusion reactions using 208Pb target and beams of 64Ni, 70Zn for the study of elements 110 and 112 in Darmstadt.27, 28 Element
111 was produced with a 64Ni beam on a 209Bi target.23, 29 The isotopes of the new elements were identified by decay chains (nuclei connected witharrows) which ended in known, previously studied nuclei. The parent nucleus was created after evaporation of one neutron from the compound
nucleus. The compound nuclei of elements 112, 114 and 116 studied in Dubna using a 48Ca beam and targets of 238U,30 242Pu,31 244Pu,32, 33 and248
Cm.34, 35 The observed decay chains are shown together with their assignment to superheavy nuclei created after evaporation of three and four
neutrons, respectively. Finally, the compound nuclei used by nuclear chemists for the study of the chemical properties of elements seaborgium(Sg),36−38 bohrium (Bh)39, 40 and hassium (Hs).41 The elements were synthesized in reactions with beams of 22Ne and 26Mg and targets of 248Cm and249
Bk. Relatively long lived isotopes were produced after evaporation of four and five neutrons. The magic numbers for the protons at element 114
and 120 are emphasized. The bold dashed lines mark proton number 108 and neutron numbers 152 and 162. Nuclei with that number of protons or
neutrons have increased stability, however, they are deformed contrary to the spherical superheavy nuclei. The crossing at Z = 114 and N = 162reflects the uncertainty, whether nuclei in that region are deformed or spherical.

elements could decrease less steeply with available stable,
more neutron-rich projectiles. However, this expectation was
not proven in the case of element 112.
An overview of all data measured at SHIP from the decay
chains observed in the reaction 64Ni + 208Pb → 272110* is given
in Figure 4. The energies and lifetimes of α decays directly
succeeding the implantations are shown. (Describing single
event chains it is preferable to use the lifetime τ instead of thehalf-life, because τ is directly measured as time difference
between two succeeding signals). On top of the upper abscissa
the α spectra deduced from literature are plotted for the decaysof 255Md, 255No, 259Rf, and 263Sg, in order to compare the energy
and intensity pattern with the measured data assigned to the
decay of 271110. In case of 267Hs the three α decays observedby Lazarev et al.50 are plotted.
The event chains are ordered chronologically from number
1 to 13. For each chain, the time sequence of the α decays isfrom right to left following the decreasing α energies. The
lifetimes of the first α decays at 10.7 MeV are the time differences between implantation and α decay.
A total number of 57-α decays were measured and assigned
to the 13 decay chains. Thirty-eight α’s were emitted in beam
direction and stopped in the 300-µm thick detector. These α
events are marked by the little filled squares. The width of thesquares shows the energy resolution of 13 keV (2σ) obtained
for these full energy α signals. An exception are the 271110 α
decays of chain 9 and 11 for which the energy had to becorrected by +60 keV and −40 keV, respectively, due to short
lifetime and tails of the signals from the preceding implanta-

Figure 4. Energies and lifetimes of the thirteen α -decay chains
resulting from the reaction 64Ni + 208Pb → 271110 + 1n. The isotope
271
110 was identified by comparison of the decay properties of thedaughter products with literature data marked by an asterisk ( 267Hs,50
263
Sg,51 259Rf,52 255No,52 255Md.52). New decay data of the isotope 263Sg
could be deduced. The decay chains are arranged chronologicallywith the date of production given at the righthand ordinate. For
description of the symbols see text.

tion. A larger error bar, as indicated by the rectangles, wasused. The larger, open squares mark escape α’s for which thefull energy could be summed from ∆E signals between 0.6 and1.4 MeV in the stop detector and coincident residual energies
in the backward crystals. A total of 12 of these α events were
measured, for which an energy resolution of 34 keV (2σ), asSynthesis and Properties of Superheavy Elements

marked by the width of the squares, was obtained in test reactions at higher statistical significance. In one of these cases,
chain number 12, the first α decay was corrected by +60 keV
due to the short lifetime after the implantation, and a larger
error bar was used. Seven α’s escaped missing the backward
crystals, but still leaving a ∆E signal and thus the time andposition information. The amplitudes of the signals corresponded to energies between 1.0 and 5.2 MeV, which is characteristic for escaping α particles. They are marked in Figure
4 by the arrows pointing left. The ratio of the α’s measured
with full energy/escape plus residual energy/escape is 38/12/7.
It agrees within statistical error bars with the ratio 60/30/10
determined by test reactions.
The lower discriminator level for signals in the stop detector
was at 260 keV, which is below the lowest ∆E signals of
escaping α’s emitted from heavy nuclei implanted 4–6 µm into
the active detector material. An estimate of the implantation
depths was obtained by the measured implantation energy
signals of ≈25 MeV. As an important result a detection efficiency for α decay of 100% was obtained, the same as for the
much higher energetic fission events. In case of a missing α
decay or fission signal the reason must be that the nucleus,
which has been identified as an α-decay product, undergoes βdecay or electron capture. In Figure 4 these missing α or
fission decays are marked by “ ε”. If the time windows can be
opened wide enough to cover the full lifetime range, thedetector system allows an unambiguous measurement of βbranching ratios.
In agreement with the known branching ratios of 255No (bε =39%52) and 255Md (bα = 8%52) is the observation of the α decayof 255Md in one of 13 cases (see chain 12 in Figure 4). This
example reveals firstly that lifetimes up to 19 min are measurable at low background rate (in our case the 255Md α decay
occurred during a beam pause of 14.5 ms) and secondly thatagreement of α energies with literature data is much moresignificant for the identification in the case of monoenergetic
decays or simple decay patterns. The widely spread α energiesof the 255No decay represents an opposite example.
The decay data of the isotope 271110 and its daughter products were confirmed in a recent experiment at RIKEN, where
the same reaction, 64Ni + 208Pb → 272110*, was studied and a
total of 14 decay chains were measured.53, 54 The α energy
agrees well with our value of 10.74 MeV and also for the lifetime a long lived (3 events) and a short lived (11 events)
components were measured. Mean values obtained from all 5
long lived and 22 short lived decays are τ = 100 ms and 2.35
ms, respectively, from which half-lives of (69 +56−21 ) ms and)msfollow.
(1.63 +0.44−0.28
Obviously isomeric states are responsible for the twodifferent half-lives. Because the measured α energies arealmost identical, an explanation suggested by Morita et al. 53
seems very likely; the isomeric state has the longer half-lifeand decays dominantly by γ emission or internal conversion
into the shorter lived ground-state. Both states are populated
in the reaction, but because the lifetime is measured as interval
between implantation and α decay, we observe two different
values for one and the same α transition. Theoretically low
spin and high spin states (1/2+ to 13/2−) which could result inisomeric states close to the ground-state were predicted byĆ wiok et al.55 The isomeric ratio between population of the 69
ms and 1.6 ms states is 5/22. Therefore we may further
conclude that the relatively long lived isomeric state has a low
spin value and is less populated. However, this spin dependence known from lighter nuclei may be changed for heavy
systems due to reduced fission probability of the compound
nucleus at high spin. Also the possibility that both levels
decay by α emission with almost the same α energy cannot becompletely excluded. An indication could be the slightly
lower α energy in the case of chain number 5 for the longer

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lived decay, which was measured with high precision.
Further confirmation of the production of 271110 in the reaction 64 Ni + 208 Pb → 272 110* was reported recently from anexperiment performed at the Berkeley Gas-filled Separator(BGS).56 At a beam energy of 309 MeV at the center of thetarget two decay chains were measured from which a crosssection of 8.3 pb was deduced. This value is in agreementwith our result.25 We conclude that the maximum deviation ofbeam energy measured at the LBNL 88-Inch Cyclotron and the
UNILAC is ±2 MeV. Similar deviation is deduced from acomparison of the data measured at RIKEN 54 and at the
UNILAC. Possibilities to improve the accuracy of beamenergy measurements are presently discussed.
Two more isotopes of element 110 have been reported in theliterature. The first is 267110, produced at LBNL in the irradiation of 209Bi with 59Co.57 The second isotope is 273110, reported
to be observed at JINR in the irradiation of 244Pu with 34S after
the evaporation of five neutrons.58 Both observations needfurther experimental clarification.
The even-even nucleus 270110 was synthesized using thereaction 64Ni + 207Pb.59 A total of eight α-decay chains were
measured during an irradiation time of seven days. Decay data
were obtained for the ground-state and a high spin K isomer,
for which calculations predict spin and parity 9−, 10− or 8+.60
The relevant single particle Nilsson levels are ν [613] 7/2+ andν [615] 9/2+ below the Fermi level and ν [725] 11/2− above theFermi level. Configuration and calculated energy of theexcited states are { ν [613] 7/2+ ν [725] 11/2− } 9− at 1.31 MeV,{ ν [615] 9/2+ ν [725] 11/2− } 10− at 1.34 MeV, and { ν [613] 7/2+
ν [615] 9/2+ }8+ at 1.58 MeV.
The new nuclei 266Hs and 262Sg were identified as daughter
products after α decay. Spontaneous fission of 262Sg terminates the decay chain. A proposed partial decay scheme of270
110 is shown in Figure 5.

Figure 5. Tentative assignment of measured α and γ decay and spontaneous fission data observed in the reaction 64Ni + 207Pb → 271110*.
The data were assigned to the ground-state decays of the new isotopes
270
110, 266Hs, and 262Sg and to a high spin K isomer in 270110. Arrows
in bold represent measured α and γ rays and spontaneous fission. The
proposed partial level schemes are taken from theoretical studies of
Muntian et al.61 for the rotational levels, of Ć wiok et al.60 for the K
isomers and of Smolanczuk62 and Smolanczuk et al.63 for the α energies and spontaneous fission half-life of 262Sg, respectively. For adetailed discussion see Reference 59.

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64

Element 111 was synthesized in 1994 using the reaction Ni+ 209Bi → 273111*. A total of three α chains of the isotope
272
111 were observed.29 Three additional decay chains were
measured in a confirmation experiment in October 2000.23
Element 112 was investigated at SHIP using the reaction70
Zn + 208Pb → 278112*.28 The irradiation was performed in
January-February 1996. Over a period of 24 days, a total of
3.4 × 10 18 projectiles were collected. One α -decay chain,shown in the left side of Figure 6, was observed resulting in across-section of 0.5 pb. The chain was assigned to the one
neutron-emission channel. The experiment was repeated in
May 2000 aiming to confirm the synthesis of 277112.23 During
a similar measuring time, but using slightly higher beamenergy, one more decay chain was observed, also shown in
Figure 6. The measured decay pattern of the first four αdecays is in agreement with the one observed in the first experiment.
A new result was the occurrence of fission which ended thesecond decay chain at 261Rf. A spontaneous-fission branch ofthis nucleus was not yet known, however, it was expected fromtheoretical calculations. The new results on 261Rf were provenin a recent chemistry experiment,41 in which this isotope wasmeasured as granddaughter in the decay chain of 269Hs (see thefollowing sect. 3.2).
A reanalysis of all decay chains measured at SHIP since
1994, a total of 34 decay chains were analyzed, revealed thatthe previously published 28 first decay chain of 277 112 (notshown in Figure 6) and the second of the originally published27
four chains of 269110 were spuriously created. Details of theresults of the reanalysis are given in Reference 23.
Results from an experiment at the 88-Inch Cyclotron in
Berkeley aiming to synthesize element 118 were published in
1999. 64 Using the new BGS the reaction 86 Kr + 208 Pb →294
118* was investigated. From three decay chains consistingof six subsequent α decays a surprisingly high cross-section of
2 pb was deduced for the one neutron emission channel.
In order to confirm the data obtained in Berkeley, the same
reaction was investigated at SHIP in the summer of 1999. The
experiment is described in detail in Reference 17. During ameasuring time of 24 days a beam dose of 2.9 × 1018 projectiles was collected which was comparable to the Berkeley
value of 2.3 × 10 18 . No event chain was detected, and thecross-section limit resulting from the SHIP experiment for thesynthesis of element 118 in cold fusion reactions was 1.0 pb.
Also from the laboratories GANIL in Caen, France, 65 and
RIKEN66 negative results were reported. The Berkeley data
were retracted in the summer of 200167 after negative results ofa repetition experiment performed in the year 2001 in Berkeley
itself and after a reanalysis of the data of the first experiment,

Figure 6. Two decay chains measured in experiments at SHIP in thecold fusion reaction 70Zn + 208Pb → 278112*. The chains were assigned
to the isotope 277112 produced by evaporation of one neutron from thecompound nucleus. The lifetimes given in brackets were calculatedusing the measured α energies. In the case of escaped α particles thealpha energies were determined using the measured lifetimes.

which showed that the three reported chains were not in the1999 data. A comparison of the measured cross-section limit
for this reaction with predictions from theoretical models is
given in sect. 5.
3.2. Elements Produced in Hot-Fusion Reactions. Hot
fusion reactions are based on targets made from actinide
elements. A number of differences exist compared with reactions using lead or bismuth targets. Probably the most significant is the excitation energy of the compound nucleus at thelowest beam energies necessary to initiate a fusion reaction.
Values are at 10 – 20 MeV in reactions with lead and bismuth
targets and at 35 – 45 MeV in reactions with actinide targets,
which led to the widely used terminology of ’cold’ and ’hot’
fusion reactions. Due to the lack of targets between bismuth
and thorium, a gradual change from cold to hot fusion cannot
be studied experimentally. A second important difference ofactinide target based reactions is the synthesis of moreneutron-rich isotopes compared with a cold fusion reaction
leading to the same element, e.g. 269Hs from a 248Cm target (seebelow) and 264Hs from a 208Pb target using beams of 26Mg and
58
Fe, respectively.
Actinides served already as targets, when neutron captureand subsequent β− decay were used for the first synthesis oftransuranium elements. Later, up to the synthesis ofseaborgium,51 actinides were irradiated with light-ion beams
from accelerators. Then, cold fusion reactions were used withlead and bismuth targets, which resulted in higher yield for thesynthesis of heavy nuclei.
The argumentation changed again when elements 110 to
112 had been discovered in cold fusion reactions and continuously decreasing cross-sections were measured. The combination of actinide targets with beams as heavy as 48Ca became
promising to study more neutron rich isotopes which are closer
to the region of spherical SHEs and for which also longer halflives were expected. In addition the lowest excitation energies
of compound nuclei from fusion with actinide targets are
obtained with beams of 48Ca.
The experimental difficulty with using a 48Ca beam is the
low natural abundance of only 0.19% of this isotope, whichmakes enrichment very expensive. Therefore, the development of an intense 48Ca beam at low consumption of material
in the ion source and high transmission through the accelerator
was the aim of the work accomplished in Dubna during aperiod of about two years until 1998.68
The experiments at the U400 cyclotron were performed at
two different recoil separators, which had been built during the
1980s. The separators had been upgraded in order to improvethe background suppression and detector efficiency. Theenergy-dispersive electrostatic separator VASSILISSA wasequipped with an additional deflection magnet.69, 70 The gasfilled separator GNS was tuned for the use of very asymmetric
reactions with emphasis on the irradiation of highly radioactive
targets.71 Both separators were equipped with time-of-flightdetectors and with an array of position-sensitive Si detectors in
an arrangement similar to the one shown in Figure 1.
At the separator VASSILISSA attempts were undertaken to
search for new isotopes of element 112 by irradiation of 238U
with 48Ca ions.30 The irradiation started in March, 1998. Twofission events were measured resulting in a cross-section of 5.0
pb. The two events were tentatively assigned to the residue
283
112 after 3n evaporation.
The experiments were continued in March 1999. The reaction 48Ca + 242Pu → 290114* was investigated.31 It was expected
that, after evaporation of three neutrons, the nuclide 287114
would be produced and would decay by α emission into thepreviously investigated 283112. Over a period of 21 days, a
total of four fission events were detected. Two of them could
be assigned to fission isomers. The other two fission signals
were preceded by signals from α particles (one was an escapeSynthesis and Properties of Superheavy Elements

α of 2.31 MeV) and implantations. A cross-section of 2.5 pb
was obtained for the two events. They were assigned to thenuclide 287114. The four events, two from 112 and two from
114 of the 238U and 242Pu irradiation with 48Ca, are consistent.
The fission lifetimes are within the limits given by statisticalfluctuations. Fission was measured again after α decay, when
the target was changed from 238 U to 242 Pu. The low background rate in the focal plane of VASSILISSA makes mimicking by chance coincidences unlikely. However, further
investigation is needed for an unambiguous assignment.
At GNS a search for element 114 was started in NovemberDecember, 1998. The experiments were performed in collaboration between the Flerov Laboratory of Nuclear Reactions(FLNR) of JINR and the Lawrence Livermore National
Laboratory (LLNL), Livermore, California. A 244Pu target wasirradiated for a period of 34 days with a 48Ca beam. One decay
chain was extracted from the data. The chain was claimed to
be a candidate for the decay of 289114. The measured crosssection was 1 pb.32
The 48Ca + 244Pu experiment was repeated in June-October,
1999. During a period of 3.5 months, two more α -decay
sequences, terminating in spontaneous fission, wereobserved.33 The two chains were identical within the statisticalfluctuations and detector-energy resolution, but differed fromthe first chain measured in 1998. The two new events wereassigned to the decay of 288114, the 4n evaporation channel.
The cross-section was 0.5 pb.
An investigation of element 116 was started in June 2000.
Using a 248Cm target, the previously detected isotopes 289114 or
288
114 were expected to be observed as daughter products fromthe decay of the corresponding element 116 parent nuclei
produced after evaporation of 3 or 4 neutrons. The first decay
chain which was assigned to 292116 was measured after 35 dayson July 19, 2000.34 The irradiation was continued later, andtwo more decay chains were measured on May 2 and 8, 2001.35
All three decay chains are plotted in Figure 7. The crosssection is about 0.6 pb deduced from a total beam dose of 2.25
× 1019.
The newly measured chains are of high significance. The
data reveal internal redundancy, and the lifetimes are relatively
short, making an origin by chance events extremely unlikely.
In particular, because all further decays in the chain, after theparent decay was observed, were measured during a beam free
period. The beam was switched off using as a trigger the timeof-flight and energy signals from the implantation and the αdecay from the parent. The assignment to the 4n channel islikely, but remains subject to further investigation until an
unambiguous identification will become possible. As the

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

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280

chains end at 110 by spontaneous fission, generic relations
to known nuclei cannot be used. Other possible procedures
which could help to establish a unique assignment, could bemeasurements of excitation functions, further cross bombardments, direct mass measurements, and chemical analysis ofparent or daughter elements. Also a systematic investigationof the nuclei in the gap between those studied mainly at GSI
and that measured in Dubna would be useful.
How well chemical properties can be used for the separationand identification of even single atoms was recently demonstrated in an experiment to study hassium. 41 Using the hot
fusion reaction 26Mg + 248Cm → 274Hs*, the isotope 269Hs wasproduced after evaporation of five neutrons. The hassium
atoms, a total of three decay chains were measured, reactedwith oxygen to form the volatile compound HsO4. This way it
was proven independently by chemical means that the
produced atom belongs, like osmium which also forms avolatile tetroxide, to group VIIIA and thus to element 108 in
the Periodic Table of the Elements (Figure 2). The measured
decay properties of the separated atoms fully confirmed thedata obtained from the decay 277112.23
Hot fusion reactions applied to synthesize long-lived
nuclides of elements 104 through 108 for chemical studies are
summarized in Table 1.26 Cross-sections vary from about 10
nb to a few pb.39−41, 72−74 With typical beam intensities of 3 ×1012 ions per second on targets of about 0.8 mg/cm2 thickness
production yields range from a few atoms per minute forrutherfordium and dubnium isotopes to five atoms per hour for265
Sg and even less for 267Bh and heavier nuclides. Therefore,
all chemical separations are performed with single atoms on an“atom-at-a-time” scale. Similar to the experiments with recoil
separators characteristic α decays and time correlated α-αdecay chains are used to identify these nuclides in specific fractions or at characteristic positions after chemical separation.
TABLE 1: Nuclides From Hot Fusion Reactions26 Used in
Chemical Investigations
Nuclide T1/2
(s)
261m

Rf

Beam
18

78

O

Target Channel Cross-section Yield
(pb)
248

Cm

5n

≈10,000

2 min−1

Ne

244

Pu

5n

4,000

1 min−1

34

18

249

Bk

5n

6,000

2 min−1

19

F

248

Cm

5n

1,000

0.5 min−1

263

27

18

O

249

Bk

4n

10,0003 min−1

265

Cm

22
262

Db
Db

O

7.4

22

Ne

248

5n

≈240

5 h−1

266

21

22

Ne

248

4n

≈25

0.5 h−1

267

Sg
Sg

Cm

17

22

Ne

249

Bk

5n

≈70

1.5 h−1

269

14

26

Mg

248

Cm

5n

≈6

3 d−1

270

2-7

26

Mg

248

4n

4

2 d−1

Bh
Hs
Hs

Cm

4. Nuclear Structure and Decay Properties

Figure 7. Three decay chains measured in the reaction 48Ca + 248Cm→ 296116* at JINR in Dubna.35 After implantation of the evaporation
residue and detection of the first α decay, the beam was switched off
and the succeeding decays were measured under almost background
free conditions. The decays assigned to the daughter nucleus 288114
were in agreement with the data measured previously from two decay
chains in the reaction 48Ca + 244Pu → 292114*.33 The time values given
are the measured intervals between successive signals and represent
the lifetimes of the decays. Also given are the measured positions in
mm, where in the vertically arranged detector strips the eventsoccurred. The figure was prepared using data given in Reference 35.

The calculation of the ground-state binding energy provides
the basic step to determine the stability of SHEs. In macroscopic-microscopic models the binding energy is calculated as
the sum of a predominating macroscopic part (derived from theliquid-drop model of the atomic nucleus) and a microscopic
part (derived from the nuclear shell model). By this way moreaccurate values for the binding energy are obtained than in thecases of using only the liquid drop model or the shell model.
The shell correction energies of the ground-state of nuclei nearclosed shells are negative which results in further decreased
values of the negative binding energy from the liquid drop
model - and thus increased stability. An experimental signature for the shell-correction energy is obtained by subtracting a

R8

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

calculated smooth macroscopic part from the measured totalbinding energy.
The shell-correction energy is plotted in Figure 8a using
data from Reference 75. Two equally deep minima are
obtained, one at Z = 108 and N = 162 for deformed nuclei with
deformation parameters β2 ≈ 0.22, β4 ≈ –0.07 and the other one
at Z = 114 and N = 184 for spherical SHEs. Different results
are obtained from self-consistent Hartree-Fock-Bogoliubov
calculations and relativistic mean-field models.76, 77 They
predict for spherical nuclei shells at Z = 114, 120 or 126 (indicated as dashed lines in Figure 8a) and N = 184 or 172.
The knowledge of ground-state binding energies, however,is not sufficient for the calculation of partial spontaneous
fission half-lives. Here it is necessary to determine the size of

Figure 8. Shell-correction energy (a) and partial half-lives for spontaneous fission, α and β decay (b-d). The calculated values in (a) (c) aretaken from References 63 and 75 and in (d) from Reference 78. The
squares in (a) mark the nuclei presently known or under investigation,
the filled squares in (d) mark the β stable nuclei.

Hofmann
the fission barrier over a wide range of deformation. The mostaccurate data were obtained for even-even nuclei using amacroscopic-microscopic model.63 Partial spontaneous fission
half-lives are plotted in Figure 8b. The landscape of fission
half-lives reflects the landscape of shell-correction energies,
because in the region of SHEs the height of the fission barrier
is mainly determined by the ground-state shell correctionenergy, while the contribution from the macroscopic liquiddrop part approaches zero for Z = 104 and above.
Nevertheless we see a significant increase of spontaneous
fission half-life from 103 s for deformed nuclei to 1012 s forspherical SHEs. This difference originates from an increasing
width of the fission barrier which becomes wider in the case of
spherical nuclei.
Partial α half-lives decrease almost monotonically from 1012
s down to 10−9 s near Z = 126 (Figure 8c). The valley of βstable nuclei passes through Z = 114, N = 184. At a distance ofabout 20 neutrons away from the bottom of this valley, β halflives of isotopes have dropped down to values of one second78(Figure 8d).
Combining results from the individual decay modes one
obtains the dominating partial half-life as shown in Figure 9
for even-even nuclei. The two regions of deformed heavy
nuclei near N = 162 and spherical SHEs merge and form a
region of α emitters surrounded by spontaneously fissioning
nuclei. The longest half-lives are 1000 s for deformed heavy
nuclei and 30 y for spherical SHEs. It is interesting to notethat the longest half-lives are not reached for the doubly magic
nucleus 114, but for Z = 110 and N = 182. This is a result ofcontinuously increasing Q α values with increasing atomic
number. Therefore, α decay becomes the dominant decay
mode beyond element 110 with continuously decreasing halflives. For nuclei at N = 184 and Z < 110 half-lives are determined by β− decay.
The four member α -decay chain of 292 116, the heaviest
even-even nucleus, observed in the recent experiment in
Dubna,35 is also drawn in Figure 9. The arrows follow approximately the 1-s contour line down to 280110. This is in agreement with the experimental observation. The nucleus 280110 ispredicted to decay by spontaneous fission. The average valuesof the measured half-lives of the nuclei along the decay chain,see sect. 3.2, are 53 ms – 2.6 s – 45 s – 7.6 s, respectively,
which is about a factor of 10 longer than the calculated values.
However, this deviation is well within the accuracy limits ofthe calculation. E.g., a change of the α energy of 288114 by
350 keV only changes the half-life by a factor of 10. The
decay chains of two other recently synthesized even-evennuclei, 270110 59 and 270Hs,41 are also drawn in the figure. In
these cases the decay chains end by spontaneous fission at
262
Sg and 262Rf, respectively.
For odd nuclei, partial α and spontaneous fission half-lives
calculated by Smolanczuk and Sobiczewski75 have to be multi-

Figure 9. Dominant half-lives for α, β + decay/electron capture, β −decay, and spontaneous fission. The data are valid for even-evennuclei only.

Synthesis and Properties of Superheavy Elements

plied by a factor of 10 and 1000, respectively, thus making
provisions for the odd particle hindrance factors. However, wehave to keep in mind that fission hindrance factors show awide distribution from 101 to 105, which is mainly a result of
the specific levels occupied by the odd nucleon. For odd-odd
nuclei, the fission hindrance factors from both the odd proton
and the odd neutron are multiplied. For odd and odd-odd
nuclei, the island character of α emitters disappears and fornuclei with neutron numbers 150 to 160 α -decay prevails
down to rutherfordium and beyond. In the allegorical representation where the stability of SHEs is seen as an island in asea of instability, even-even nuclei portray the situation at
high-tide and odd nuclei at low-tide, when the island isconnected to the mainland.
The interesting question arises, if and to which extent uncertainties related to the location of proton and neutron shellclosures will change the half-lives of SHEs. Partial α and β
half-lives are only insignificantly modified by shell effects,
because their decay process occurs between neighboring
nuclei. This is different for fission half-lives which areprimarily determined by shell effects. However, the uncertainty related to the location of nuclei with the strongest shelleffects, and thus longest partial fission half-life at Z = 114, 120
or 126 and N = 172 or 184, is irrelevant concerning the longest‘total’ half-life of SHEs. All regions for these SHEs are dominated by α decay. Alpha-decay half-lives will only be modified by a factor of up to approximately 100, if the double shellclosure is not located at Z = 114 and N = 184. Only in the caseof shell effects similar strong as in the double magic 208Pb, thehalf-lives could become significantly shorter for nuclei above
the shell closure and longer for the nuclei below.
The line of reasoning is, however, different concerning the
production cross-section. The survival probability of thecompound nucleus is determined among other factors significantly by the fission-barrier. Therefore, with respect to an efficient production yield, the knowledge of the location ofminimal negative shell-correction energy is highly important.
However, it may also turn out that shell effects in the region of
SHEs are distributed across a number of subshell closures. In
that case a wider region of less deep shell-correction energy
would exist with corresponding modification of stability andproduction yield of SHEs.

J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003
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Data obtained in reactions with Pb and Bi for the 1n-evaporation channel at low excitation energies of about 10–15 MeV(therefore named cold fusion) and in reactions with actinide
targets at excitation energies of 35–45 MeV (hot fusion) for
the 4n channel are plotted in Figure 10a and b, respectively.
Some features which the data reveal are pointed out in thefollowing:(1) So far no data were measured below cross-section valuesof about 0.5 pb. This is the limit presently set by experimental
constraints. Considering the already long irradiation time of≈1 month to reach a cross-section of 0.5 pb, it seems impractical to perform systematic studies on this cross-section levelor even below. Further improvement of the experimental
conditions is mandatory. Note in this context that the experimental sensitivity increased by three orders of magnitude sincethe 1982–83 search experiment for element 116 using a hot
fusion reaction.79(2) The cross-sections for elements lighter than 113
decrease by factors of 4 and 10 per element in the case of coldand hot fusion, respectively. The decrease is explained as a
combined effect of increasing probability for re-separation ofprojectile and target nucleus and fission of the compound
nucleus. Theoretical consideration and empirical descriptions,see e.g. References 17, 25, 80, and 81, suggest that the steep
fall of cross-sections for cold fusion reactions may be strongly
linked to increasing re-separation probability at high values of
Z1Z2 while hot fusion cross-sections mainly drop because ofstrong fission losses at high excitation energies. Extremely
small values result from an extrapolating these data into the

5. Nuclear Reactions
The main features which determine the fusion process ofheavy ions are (1) the fusion barrier and the related beamenergy and excitation energy, (2) the ratio of surface tension
versus Coulomb repulsion which determines the fusion probability and which strongly depends on the asymmetry of thereaction partners (the product Z1Z2 at fixed Z1 + Z2), (3) the
impact parameter (centrality of collision) and related angularmomentum, and (4) the ratio of neutron evaporation and of γ
emission versus the fission of the compound nucleus.
In fusion reactions towards SHEs the product Z1Z2 reaches
extremely large and the fission barrier extremely small values.
In addition, the fission barrier itself is fragile, because it issolely built up from shell effects. For these reasons the fusionof SHEs is hampered twofold: (1) in the entrance channel by ahigh probability for re-separation and (2) in the exit channel bya high probability for fission. In contrast, the fusion of lighter
elements proceeds unhindered through the contracting effect of
the surface tension and the evaporation of neutrons instead offission.
The effect of Coulomb repulsion on the cross-section starts
to act severely for fusion reactions to produce elements beyond
fermium. From there on a continuous decrease of crosssection was measured from microbarns for the synthesis ofnobelium down to picobarns for the synthesis of element 112.

Figure 10. Measured cross-sections for reactions with 208Pb and 209Bi
targets and 1n evaporation (a) and for reactions with actinide targets
and 4n evaporation (b).

R10 J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003
region of element 114 and beyond. However, strong shelleffects for SHEs could lead to an increase of the fission barrier
and thus to an increase of the survival probability of thecompound nucleus, see also discussion in sect. 4. The relatively high values measured in Dubna for the synthesis ofelements 114 and 116 would be in agreement with this argumentation. In the case of cold fusion only cross-section limits
are known for the synthesis of elements 116 and 118.
(3) Locally an increase of the cross-section by a factor of
5.8 was measured for element 110 in cold fusion reactions
when the beam was changed from 62Ni to 64Ni. It was speculated that this increase could be due to the increased value ofthe projectile neutron number. However, the assumption could
not be confirmed in the case of element 112 which was synthesized using the most neutron rich stable zinc isotope with mass
number 70.
A number of excitation functions were measured for thesynthesis of elements from No to 110 using Pb and Bi
targets.17 For the even elements these data are shown together
with the two data points measured for 278112 in Figure 11. The
maximum evaporation residue cross-section (1n channel) was
measured at beam energies well below a one dimensional
fusion barrier.82 At the optimum beam energy projectile andtarget are just reaching the contact configuration in a central
collision. The relatively simple fusion barrier based on theBass model82 is too high and a tunnelling process through thisbarrier cannot explain the measured cross-section. Various
processes are possible, and are discussed in the literature,
which result in a lowering of the fusion barrier. Among theseprocesses transfer of nucleons and an excitation of vibrational
degrees of freedom are the most important.83, 84
Target nuclei of actinide targets are strongly deformed andthe height of the Coulomb barrier depends on the orientation of
the deformation axes. The reaction 48Ca + 248Cm, studied in
Dubna, was performed at a beam energy resulting in an excitation energy of approximately 34 MeV.35 The observed decay
chains were assigned to the 4n-evaporation channel. An excitation function which could provide experimental evidence foran orientation effect on the fusion cross-section is not yetmeasured.
It was pointed out in the literature85 that closed shell projectiles and target nuclei are favorable synthesizing SHEs. The
reason is not only a low (negative) reaction Q-value and thus alow excitation energy, but also that fusion of such systems isconnected 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. In this view the difference between cold and hot

Figure 11. Measured even element excitation functions. On the left,
the cross-sections are plotted as function of the dissipated energy E*,
on the right, the neutron binding energies are subtracted.

Hofmannfusion is not only a result from gradually different values of
excitation energy, but there exists a qualitative difference,
which is on the one hand (cold fusion) based on a well orderedfusion process along paths of minimum dissipation of energy,
and on the other hand (hot fusion) based on a process governedby the formation of a more or less energy equilibrated
compound nucleus. The use of the double magic 48Ca andactinide targets seems to proceed via an intermediate fusion
process, possibly along a fusion valley less pronounced than in
the case of cold fusion. Triggered by the recent experimental
success of heavy element synthesis, a number of theoreticalstudies are in progress aiming to obtain a detailed understanding of the processes involved.84, 86−91
Due to the great uncertainty concerning the influence of thevarious steps in the fusion of heavy elements, more and more
precise experimental data are needed. It is especially important that various combinations of projectile and target be investigated, from very asymmetric systems to symmetric ones, andthat excitation functions are measured. This provides information on how fast the cross-section decreases with increasingenergy due to fission of the compound nucleus, and how fastcross-sections decrease on the low energy side due to thefusion barrier. From both slopes, information about the‘shape’ of the fission and the fusion barriers can be obtained.
At a high enough cross-section, these measurements can becomplemented by in-beam γ-ray spectroscopy using the recoildecay tagging method in order to study the influence ofangular momentum on the fusion and survival probability.92−94
6. Summary and Outlook
The experimental work of the last two decades has shownthat cross-sections for the synthesis of the heaviest elements
decrease almost continuously. However, recent data on thesynthesis of elements 114 and 116 in Dubna using hot fusion
seem to break this trend when the region of spherical superheavy elements is reached. Therefore a confirmation isurgently needed that the region of spherical SHEs has finallybeen reached and that the exploration of the ‘island’ has started
and can be performed even on a relatively high cross-sectionlevel.
The progress towards the exploration of the island of spherical SHEs is difficult to predict. However, despite the exciting
new results, many questions of more general character are still
awaiting an answer. New developments will not only make itpossible to perform experiments aimed at synthesizing newelements in reasonable measuring times, but will also allow fora number of various other investigations covering reaction
physics and spectroscopy.
One can hope that, during the coming years, more data will
be measured in order to promote a better understanding of thestability of the heaviest elements and the processes that lead tofusion. A microscopic description of the fusion process will beneeded for an effective explanation of all measured
phenomena in the case of low dissipative energies. Then, therelationships between fusion probability and stability of thefusion products may also become apparent.
An opportunity for the continuation of experiments in theregion of SHEs at decreasing cross-sections affords, among
others, further accelerator developments. High current beams
and radioactive beams are options for the future. At increased
beam currents, values of tens of particle µA’s may become
accessible, the cross-section level for the performance ofexperiments can be shifted down into the region of tens offemtobarns, and excitation functions can be measured on thelevel of tenths of picobarns. High currents, in turn, call for thedevelopment of new targets and separator improvements.
Radioactive ion beams, not as intense as the ones with stable
isotopes, will allow for approaching the closed neutron shell N

Synthesis and Properties of Superheavy Elements= 184 already at lighter elements. The study of the fusion
process using radioactive neutron rich beams will be highlyinteresting.
The half-lives of spherical SHEs are expected to be relatively long. Based on nuclear models, which are effective
predictors of half-lives in the region of the heaviest elements,
values from microseconds to years have been calculated forvarious isotopes. This wide range of half-lives encourages theapplication of a wide variety of experimental methods in theinvestigation of SHEs, from the safe identification of short
lived isotopes by recoil-separation techniques to atomic
physics experiments on trapped ions, and to the investigationof chemical properties of SHEs using long-lived isotopes.
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