Journal of Nuclear and Radiochemical Sciences,Vol. 3, No. 1, pp. 1–4, 2002

1SHE Research at GSI — Achievements and Plans
G. Münzenberg∗,a,ba
Gesellschaft für Schwerionenforschung, GSI mbH, Planckstr. 1, 64291 Darmstadt, Germanyb
Johannes Gutenberg-Universit ät Mainz, Germany
Received: January 24, 2002; In Final Form: January 24, 2002
Experimental methods for heavy element research will be presented. Recent experimental results are discussed
together with recent theoretical predictions. New instrumental developments and future plans will be discussed.

1. Introduction
Heavy-element research explores the borderline of the nuclear chart towards its upper end where the strong Coulomb
force acting between the many protons dominates the nuclear
stability and finally terminates the number of elements by instability against fission. The longstanding question is whether
nuclei can exist beyond that macroscopic limit only by microscopic stabilization through shell effects. These superheavy elements are predicted for the next double shell closure above leadlocated at Z = 114 and N = 184,1 according to the early predictions, and Z = 120, N = 184 for recent calculations.2, 3
A first exciting experimental result related to this question
was the discovery of the shell stabilized elements seaborgium
and hassium4, 5 which are part of a microscopic stabilized regionof deformed nuclei centered at Z = 108 and N = 162.
Element 112, the heaviest element unambiguously identified
at present6 by the parent-daughter α-α correlation method is already quite close to the superheavy region in proton number.
Recent results published by the Dubna group indicate the discovery of elements 114 and 116, already located in the predictedsuperheavy region.7 The presently known nuclides of the transfermium elements are shown in the portion of the chart of nuclei
presented in Figure 1.
The principal experimental problems of heavy-element research are the small production rates of less than one atom per
week achievable with the present technical possibilities and theshort half-lives ranging down to milliseconds.8 Therefore thekey technique used for the discovery of the heaviest known elements relies on the investigation of single atomic nuclei9 separated in-flight and identified by the parent-daughter correlationmethod.
To further explore the region of superheavy elements a
number of experimental developments are needed such as
more intense heavy-ion beams to produce a number of atoms
in few weeks of experiment and a direct mass and element
determination identification to identify the neutron rich nuclei
around the superheavy double shell closure, far away from the
known transactinide isotopes, and not reachable by α-decay
chains proceeding from these superheavy nuclides.
2. Production of Superheavy Elements
All artificial elements beyond fermium have been first created by complete fusion of heavy ions. Two types of reactions
have been used successfully so far: the cold heavy-ion fusion oflead or bismuth targets with appropriate projectiles of the mostneutron rich isotopes available, such as 64 Ni and 70 Zn for theproduction of elements 110, 111, or 112, and the hot fusion ofactinide targets7 such as 244 Pu or 248 Cm with 48 Ca projectiles toproduce elements 114 and 116.
Cold fusion is characterized by a low excitation energy of
the compound nucleus of 10 MeV to 15 MeV, while hot fu∗E-mail: g.muenzenberg@gsi.de.

sion leads to excitation energies of typically 30 MeV to 40 MeV.
As the fissility increases for the heaviest elements the survival
probability becomes of increasing importance towards the heaviest elements, cold compound systems are expected to have an
enhanced survival probability.10 As a consequence cold fusion
should be favorable to create the heaviest elements.
On the other hand fusion of massive systems is hinderedin the entrance channel by the large Coulomb force whichacts between target and projectile and the rearrangement of thelarge number of nucleons on the way from the dinuclear to themononuclear system. In the frame of this picture the synthesisof very heavy nuclei11 by cold fusion is less favored than theuse of the more asymmetric systems used in the actinide concept. The question whether the entrance channel effects or thesurvival probability dominate the production cross section of theheaviest systems is still debated and not solved theoretically ona quantitative basis.
The production cross sections for the elements 104 to 112 decrease by a factor of about 10 per two elements down to 1 picobarn for element 112 as shown in Figure 2. The corresponding
production rates are of the order of one atom per month.
With a usable target thickness of 1018 atoms cm−2 and beam
intensities of 1014 s−1 as expected for the new high-current accelerators, rates of one atom per ten hours are expected, necessary for a solid exploration of the superheavy element region
beyond element 112.
Radioactive beam facilities of the first generation will deliver
beams of intensities of the order of 108 s−1 to 109 s−1 and allow
to access the nanobarn region, appropriate for the light transactinides. Production cross sections may be enhanced by isospin
effects8 as observed for the production of e.g. nobelium and element 110. Radioactive beams and targets will give access toneutron rich species in the interesting transition region between
the deformed nuclei near N = 162 and the spherical nuclei in thevicinity of N = 184. The long half-lives of the neutron rich nuclei will allow for chemical studies and atomic physics investigations. These long-lived nuclei may undergo β decay or, if theshell stabilization in the transition region becomes too small,undergo spontaneous fission (see Reference 8).
Models to predict the production cross sections for superheavy elements are under development.12 Up to now they have
only limited predictive power. It is generally accepted that clusters, e.g. shells in target and projectile, as proven for the fusion with 208 Pb and 209 Bi targets, enhance the fusion probability.
Similar arguments7 hold for the use of the doubly magic 48 Ca.
The stabilization of shell effects against fission of the compoundsystem could not been convincingly proven up to now.
3. Recent Experimental Developments
In-flight separation is the method now established in heavyelement research, and used successfully in the discovery experiments of all trans-seaborgium elements. Gas filled magnetic
separators and kinematic separators including energy- and velocity filters are in use.13 The separation time is of the order ofc 2002 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 6/30/2002

2 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

Münzenberg

Figure 1. The upper end of the nuclear chart with the known isotopes and the experimental half-lives of the transactinides as it appears today.8

microseconds.
The advantage of in-flight separation is that the unretarded,energetic nuclei are implanted into silicon detectors where theirdecay is observed in situ. This opens the possibility for ultrasensitive single-atom physics. Identification is based on the nuclear decay characteristics. For the heaviest elements these arelong α-decay chains, in some cases they are terminated by spontaneous fission.
The method used presently for the unambiguous identification of new nuclides and even elements is based on the connection of these decay sequences to known nuclides. The identification of element 112 relies on the known isotopes of element
106 and below. Figure 3 shows the discovery chain of element
112. It should be noted here that the other chain presented inReference 6 could not be reproduced in a re-analysis of the data.
The synthesis of the elements 111 and 112 was successfully repeated, the data are published in Reference 14.
The recent chemical investigation of hassium yielded α-decay

data on the isotopes 269 Hs which are in full agreement to the datafrom Reference 6 and support the discovery of element 112 (seecontribution of A. Türler to this conference).
The synthesis of Z = 114 in the fusion of 244,242 Pu with 48 Ca
has been reported from Dubna. Very recently the discovery of adecay sequence in the reaction 248 Cm plus 48 Ca, leading to thepreviously known 114 chain has been announced (see contribution of Yu. Ts. Oganessian to this conference).
The assignment of the elements beyond Z = 112 is primarily based on general arguments such as excitation energy of
the compound nucleus, consistency checks of decay energy
and half-lives, and comparisons of the data to theoretical models. Their unambiguous identification according to the rules ofIUPAC remains a problem.15
New strategies for SHE identification need being developed.
Solutions are: the production of presently unknown neutron richFigure 2. Production cross sections for the production of the transfermium elements.8

Figure 3. The α chain observed for element 112.8

SHE Research at GSI — Achievements and Plans

J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

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daughter nuclei at the ends of the α chains with actinide targets
or with neutron rich beams of unstable nuclei, direct determination of the mass number by mass measurements in traps, timeof-flight spectrometers, or with calorimetric detectors. For thedetermination of Z chemical methods are being developed (seecontribution of M. Schädel to this conference).

predict Z = 126. The neutron shell is predicted for N = 184, andpartly also for N = 172. A detailed comparative investigation ofdifferent forces has been carried out by Bender et al. in selfconsistent calculations.2, 3

4. The Shell Region around Hassium

With the elements seaborgium and beyond the predicted newspecies of shell nuclei has been discovered. They close the gap
between the transuranium elements and the predicted sphericalsuperheavy shell closure. The unexpectedly long half-lives observed experimentally open up the possibilities for new typesof experiments such as the extension of heavy-element chemistry beyond the present limit of hassium as well as the application of ion traps for precision experiments including direct mass
determination or the investigation of atomic properties of theheaviest elements. SHIPTRAP, an ion trap attached to SHIP, iscompleted.
A key problem is the production of superheavy nuclei. Newtypes of reactions need being explored. Three paths to superheavy elements seem favorable18 : the cold fusion with lead andbismuth targets, the hot fusion with actinide targets, and as thethird, and only scarcely investigated path, the symmetric fusion
or reversed fission.7 Presently only the cold fusion has been explored systematically, for the hot, actinide based fusion, data areavailable. Neutron rich radioactive beams will help to accesshitherto not accessible regions of neutron rich isotopes of thetransfermium elements, important to understand structure andstability against fission and to possibly extract data for a better
understanding of the upper end of the r-process path. Reaction
studies with radioactive beams will contribute to the solution of
the synthesis problem.19
To proceed to new elements and to investigate their properties, high current accelerators are needed. The present situation clearly shows that identification of the elements beyond
Z = 112 needs new developments. The parent-daughter correlation method will not be applicable. Solutions under development are bolometric detectors and time-of-flight spectrometers
to identify the atomic mass and chemical methods to fix the nature of the chemical element. For short-lived species, chemicalinvestigations can be carried out on a long-lived member of thedecay chain as proven in the chemical investigation of hassium.

The experimental data show two general features:— in the transactinide region above rutherfordium α decay dominates,— the half-lives increase strongly towards the neutron richisotopes as can be observed nicely in the isotopic chains ofseaborgium and hassium where the half-lives increase by threeorders of magnitude up to about 20 s for the most neutron richspecies.
Both observations are in contrast to our expectation from amacroscopic description where fission, prevailing already forthe rutherfordium isotopes, would dominate in the tansactinide
region and half-lives would continuously decrease.
The enhancement of stability4 of the elements beyond rutherfordium (Z = 104) is the exciting discovery in heavy-element research. Theory explained this experimental result in terms of ashell stabilization created by a hexadecapole deformation.16, 17
The centre of this shell region is predicted for Z = 108 andN = 162 as shown in Figure 4 where the landscape of shell corrections for the transfermium elements16 is displayed. The decay
chains from element 112 give a clear proof for the existence of
the N = 162 shell in the decay-time intervals as well as in theQα values.8 The discovery of this new species of shell nuclei,interconnecting the transuranium- and the superheavy elements
create the basis of present transactinide research in physics andchemistry.
The location of the spherical superheavy nuclei is currently an
open question. Macroscopic-microscopic models tend to predictthe magic proton number Z = 114, relativistic mean field models prefer Z = 120, whereas Skyrme-Hartree Fock models also

5. Perspectives

The author wishes to thank the SHIP group with its leader S.
Hofmann. Fruitful discussions with A. Antonenko, W. Greiner,
W. Nazarewicz, R. Smolanczuk, and A. Sobiczewski are highlyappreciated.
References

Figure 4. The calculated microscopic corrections of the heaviest elements.16 The path of the decay chains from elements 112 are indicated.
The upper panel displays the measured correlation times for one of thechains.

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