The Synthesis of Superheavy Elements
Dieter Ackermann
Gesellschaft für Schwerionenforschung GSI, Planckstr.1, D64291 Darmstadt, Germany
Institut für Physik, Johannes Gutenberg-Universität, Staudingerweg 7, D-55099 Mainz, Germany
Abstract. The elements with the atomic numbers 107-112 have been synthesized and unambiguously identified at the
velocity filter SHIP at GSI. The technique allowing for this successful experimental program is the combination of the
detection of correlations between evaporation residues and subsequent a-decays with a powerful separator. Systematic
investigations, the construction of decay chain networks and mass measurements are some of the possible approaches to
study the decay chains tentatively attributed to the isotopes of the elements 114 and 116 at Dubna, which are, in contrast
to those observed at GSI, not connected to α-decays of known isotopes. The sensitivity limit of the set-up at GSI has
reached the 1pb level. For systematic investigation in this region of extremely low cross section and to synthesize nuclei
of higher Z this limit has to be pushed to even lower values. An extensive development program is pursued at SHIP in
order to reach at least an order of magnitude lower cross sections. Apart from target cooling and separator development a
super conducting CW linear accelerator is studied to reach this goal. To design a successful experimental program for
the possible discovery of new elements the nuclear structure of the heaviest nuclei has to be understood as well as the
reaction mechanism which leads to their production in heavy ion reactions. We have initiated series of systematic studies
for both subjects.
INTRODUCTION
heavier elements step by step. Following the concept
of “cold” fusion of lead or bismuth targets with
medium heavy projectiles like 40Ar or 50Ti, first
applied successfully by Oganessian et al [7], the SHIP
group succeeded to produce and identify about 25 new
isotopes with atomic numbers from Z=98 up to Z=112.
Mutual interaction of experimental results and
theoretical calculations led to a better understanding of
their stability, while measured excitation functions
allowed for a reliable empirical extrapolation of
optimum bombarding energies and cross sections for
1n de-excitation channels. Continuous technical
development pushed the sensitivity of the set-up down
to a cross section value of about 1 pb. To proceed
towards higher Z an extensive development program is
being followed at present. Recently the synthesis of
isotopes of the elements 114, 116 and 118 has been
reported at Dubna and Berkeley. The unambiguous
assignment of those events, howeve r, is not yet
possible. An attempt to confirm the Berkeley results
for element 118 at SHIP did not yield a positive result.
A recent review on the discovery of the heaviest
elements [8] gives a complete overview over the recent
achievements in the field. There can also be found a
detailed description of the experimental set-up at GSI.
The search for superheavy elements, predicted
close to the double magic nucleus 298114 [1] - more
recent theoretical results are found in [2,3] - was a
substantial motivation for the construction of the
UNILAC and the velocity filter SHIP [4] at GSI in
Darmstadt. To reach the “island of superheavy
elements” in the beginning of the experimental work at
SHIP in 1976 only one method seemed possible: to
jump across the “sea of instability”. Although this
method was tempting, it contained severe
uncertainties. Decay properties of nuclei in the
intended region, such as decay modes, decay energies
and half-lives, were not known and could only be
estimated on the basis of predicted mass excesses,
shell effects, fission barriers etc., and were therefore
extremely uncertain. The same held for the prediction
of production cross sections using fusion-evaporation
codes optimized to reproduce data in the region of
known elements. Experiments, performed at SHIP, to
produce superheavy elements in bombardments of
170
Er with 136Xe or 238U with 65Cu [5], as well as by the
reaction 48Ca + 248 Cm [6] did not show positive results.
It turned out to be more successful to approach the
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213
1.1·1018 at E* = 11.6 MeV and 1.1·1018 at E* = 13.0
MeV were collected. While no decay chain that could
be attributed to 272111 was registered at E* = 10.0
MeV, one event was observed at E* = 11.6 MeV, and
two events at E* = 13.0 MeV [9], referring to a cross
RECENT RESULTS ON THE
SYNTHESIS OF HEAVY ELEMENTS
WITH Z=110-112
The elements with Z=107-112 have been
synthesized and unambiguously identified at SHIP.
The elements 107-109 have already been named and
have been entered as Bohrium (Bh, Z=107), Hassium
(Hs, Z=108) and Meitnerium (Mt, Z=109) in the
periodic table of elements. The properties found for
the elements 110, 111 and 112 are presented in this
section. A linear extrapolation of the optimum
excitation energies for the production of 257Rf and
265
Hs resulted in an 'optimum' value of E*=12.3MeV
for the production of 269110 via the reaction 62Ni +
208
Pb. In an experiment in November 1994, where a
total projectile dose of 2.2 ·1018 was collected, four αdecay chains were observed, which were attributed to
the isotope with the mass number 269 of the new
element 110 [9]. The assignment was based on the
observation that the α-decays directly preceded the
well established α-decay chain of 265Hs and, therefore,
they have to origin from the α-mother 269110. From the
measured decay data an average decay energy of
E=(11.112±0.020)MeV
and
a
half-life
of
+160
T1 2 = 170 − 60 µ s was obtained. The production cross
+4.6
section of σ = 3.5− 2.3 pb . In the series of
experiments performed in October 2000 we also
confirmed the synthesis of 272111 observing additional
three
decay
chains
of
this
isotope.
In early 1996 the search for element 112 was
undertaken using the projectile target combination
70
Zn + 208 Pb. A total projectile dose of 3.4·10 18 was
collected. Following the systematics on optimum
excitation energies a bombarding energy according to
E* = 10.1 MeV was chosen. Two decay chains which
had been attributed to 277112 were reported [12].
+ 2 .7
section was σ = 3.5−1.8 pb . Since it is well
established in the region of transfermium nuclei that
more neutron rich projectiles lead to higher formation
cross sections, one could expect for the combination
64
Ni + 208Pb a still higher ER cross section than for
62
Ni + 208Pb. In a directly following experiment in
November/December 1994 the ER production by the
reaction 64Ni + 208Pb was investigated at E*=(813)MeV. Nine α-decay chains observed in this
experiment could be attributed to 271110. A maximum
FIGURE 1. The two decay chains observed for 277112
in 1996 and 2000.
+9
cross section of σ = 15− 6 pb was measured at
E*=12.1MeV. In an experiment in October 2000 we
observed in the reaction 64Ni + 207Pb eight decay
chains of correlated ER-α-fission events which we
attribute to the decay of the new isotope 270110. Also
the daughter and grand daughter products 266 Hs and
262
Sg have not been observed before [10].
It has been found later that one of the decay chains
was spuriously generated [13]. In a recent experiment
in May 2000 a second decay chain of 277112 has been
recorded. It is shown together with the first chain from
1996 in fig.1. This latter chain has been observed at an
excitation energy of about 2 MeV higher at E* = 12
MeV. During an irradiation time of 19 days a total of
3.5·1018 projectiles were sent onto the target. The
resulting cross section at this energy is
On the basis of these encouraging results for the
synthesis of element 110 in the reactions 62,64 Ni +
208
Pb the production of an isotope of element 111 by
the reaction 64Ni + 209 Bi was undertaken in an
experimental run in December 1994. Three
bombarding energies at 10.0 MeV, 11.6 MeV, and
13.0 MeV for the compound nucleus 273111 excitation
energies were chosen using the predicted mass excess
of [11]. Projectile doses of 1.0·1018 at E* = 10.0 MeV,
σ = 0.5+−10..14 pb [13]. This value fits well into the
cross section systematics of maximum cross sections
for the synthesis of elements with Z$100. The chain
end with the fission of 261Rf in difference to the first
chain which ran all the way down to 253Fm which
cannot be correlated due to its half-life of 3.0d. Both
decay modes of 261Rf have been observed also in an
214
experiment on the chemistry of Hs where this isotope
was produced in the decay chain of 269 Hs [14]
confirming our findings.
the closed proton shell at Z=82 and N=126 using the
partial wave (spin) distribution σR [23]. It has been
shown that you can extract from the spin distribution
information on the barrier structure for heavy ion
collisions [24]. Moreover, it yields information on the
single partial wave cross sections and can reveal
effects at high angular momenta which might hint to a
stabilization due to shell effects as shown
schematically in fig. 2. The energy consumed in the
rotation of the system could lead to an extra
stabilization and to a population/survival at higher
partial waves which could be detected in a measured
spin distribution.
NUCLEAR STRUCTURE OF HEAVY
NUCLEI
The nuclear structure of heavy nuclei with Z$82 is
interesting in itself as many interesting features, like
e.g. isomers, shape transitions and shape co-existence,
are expected and found in this region. One recent
example is the hint for a k-isomer in the 270110 we
reported recently [10]. The detailed understanding of
nuclear structure and its development in the vicinity of
closed shells, and towards heavier masses and greater
Z is on the other hand a necessary ingredient for a
successful progress in the synthesis of new heavy
elements. We have applied the technique of α fine
structure and α-(α)-γ spectroscopy to study seve ral
radium isotopes (A=209-212) [24], neutron-deficient
nuclei with Z=86-92 [26] up to the isotopes 252,253 Lr,
255
Rf and 256,257 Db[27].
FIGURE 2. Cartoon illustrating the effect of a shell
correction energy Eshell on the critical angular
momentum for fusion l crit .
REACTION MECHNANISM STUDIES
The steep decrease with increasing Z of the
compound system in cross section for the production
of heavy elements observed for fusion reactions with
Pb and Bi-targets and for previous reactions on
actinide targets has been contrasted by recent
observations made in Dubna. There decay chains have
been seen in bombardments of 244Pu and 248 Cm with
48
Ca projectiles which were interpreted as the decay
of 288114 [15] and 292116 [16] as 4n evaporation
residues (ER) with a more less constant production
cross section of about 1pb. According to recent selfconsistent calculations [17] these nuclei are close to
the region where a stabilization due to shell effects can
be expected – those calculations propose the proton
numbers 120 and 126, and the neutron number 172 as
possible new candidates for the next closed shells.
However, it was up to now not possible to detect an
increasing production cross section for heavy ER’s
with magic proton or neutron numbers [18,19,20]. For
medium to heavy masses it has been shown that
nuclear structure and nuclear deformation are
important ingredients for the formation of the
compound system [21,22]. We have started a program
to examine the reaction mechanism in the vicinity of
FIGURE 3. Part of the chart of nuclei from Z=100105 with the studied nuclei indicated by shaded
squares [26].
The latter ones are indicated as shaded squares from
Z=100-105 in the part of the chart of nuclides shown
in fig. 3.
215
ACKNOWLEDGMENTS
The recent experiments were performed togethe r
with H.-G. Burkhard, F.P. Heßberger, S. Hofmann, B.
Kindler, B. Lommel, S. Reshitko, H.-J. Schött (GSI
Darmstadt), A.Yu. Lavrentev, A.G. Popeko, A.V.
Yeremin (FLNR-JINR Dubna), S. Antalic, P. Cagarda,
R. Janik, Š. Šaro (University of Bratislava), J. Uusitalo
and M. Leino (University of Jyväskylä).
TECHNICAL DEVELOPMENT AT SHIP
The three areas presently
investigation at SHIP are:
•
beam development
•
target development
•
background reduction.
under
technical
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