Radiochim. Acta 99, 405–428 (2011) / DOI 10.1524/ract.2011.1854 © by Oldenbourg Wissenschaftsverlag, München Synthesis of superheavy elements by cold fusion By S. Hofmann1,2,∗ 1 GSI Helmholtzzentrum für Schwerionenforschung, Planckstraße 1, 64291 Darmstadt, Germany 2 Institut für Kernphysik, Goethe-Universität, Max von Laue-Straße 1, 60438 Frankfurt, Germany (Received February 3, 2011; accepted in revised form April 18, 2011) Superheavy elements / Synthesis / Cold fusion / in 1972, and the RILAC (variable-frequency linear acceler- Cross-sections / Decay properties / Recoil separators / ator) at the RIKEN Nishina Center in Saitama near Tokio in Detector systems 1980. In the middle of the 1960s the concept of the macrosco- pic-microscopic model for calculating binding energies Summary. The new elements from Z = 107 to 112 were of nuclei also at large deformations was invented by synthesized in cold fusion reactions based on targets of lead V. M. Strutinsky [3]. Using this method, a number of the and bismuth. The principle physical concepts are presented measured phenomena could be naturally explained by con- which led to the application of this reaction type in search sidering the change of binding energy as function of defor- experiments for new elements. Described are the technical mation. In particular, it became possible now to calculate the developments from early mechanical devices to experiments with recoil separators. An overview is given of present ex- binding energy of a heavy fissioning nucleus at each point of periments which use cold fusion for systematic studies and the fission path and thus to determine the fission barrier. synthesis of new isotopes. Perspectives are also presented Nevertheless, the calculation of the stability of SHEs for the application of cold fusion reactions in synthesis of against fission remained problematic. Predicted half-lives elements beyond element 113, the so far heaviest element based on calculations using various parameter sets differed produced in a cold fusion reaction. Further, the transition of by many orders of magnitude [4–10]. Some of the half-lives hot fusion to cold fusion is pointed out, which occurs in re- approached the age of the universe, and attempts have been = actions for synthesis of elements near Z 126 using actinide made to discover naturally occurring SHEs [11, 12]. Al- targets and beams of neutron rich isotopes of elements from though the corresponding discoveries were announced from iron to germanium. time to time, none of them could be substantiated after more detailed inspection. 1. Introduction There was also great uncertainty on the production yields for SHEs. Closely related to the fission probability of SHEs In 1948 the magic numbers were successfully explained in the ground-state, the survival of the compound nuclei by the nuclear shell model [1, 2], and an extrapolation into formed after complete fusion was difficult to predict. Argu- unknown regions of the nuclear landscape was thus under- ments considering the lowering of the fission barrier with taken. The numbers 126 for the protons and 184 for the increasing excitation energy of the compound nucleus led to neutrons were predicted to be the next shell closures. In- the conclusion that compound nuclei have to be produced as stead of 126 for the protons also 114 or 120 were calculated cold as possible. as closed shells. The term superheavy elements (SHEs) was Even the best choice of the reaction mechanism, fusion coined for these elements. or transfer of nucleons, was critically debated. However, as The perspectives offered by the nuclear shell model for soon as relevant experiments could be performed, it turned production of SHEs and the need for developing more pow- out that the most successful methods for the laboratory syn- erful accelerators for the synthesis of these elements in thesis of heavy elements are fusion-evaporation reactions heavy ion reactions was a main motivation for upgrading using heavy-element targets, recoil-separation techniques, existing facilities or for founding new laboratories. In ex- and the identification of the nuclei by generic ties to known pectation of broad research fields the HILAC (Heavy Ion daughter decays after implantation into position-sensitive Linear Accelerator), later upgraded to the SuperHILAC, was detectors [13–15]. built at LBNL (Lawrence Berkeley National Laboratory) in The newly developed detection methods extended the Berkeley in 1955, the U-300 and U-400 cyclotrons at FLNR range of measurable half-lives considerably. The lower half- (Flerov Laboratory of Nuclear Reactions) at JINR (Joint In- life limit of about 1 μs is determined by the flight time stitute for Nuclear Research) in Dubna in 1957 and 1970, through the separator. Long half-lives are measurable up to respectively, the UNILAC (Universal Linear Accelerator) at about one day. There, the limitation is given by the rate of GSI (Gesellschaft für Schwerionenforschung) in Darmstadt implanted reaction products and background considerations. The detectors are sensitive for measuring all radioactive de- *E-mail: [email protected]. cays based on particle emission like proton radioactivity, α 406 S. Hofmann Fig. 1. Upper end of the chart of nuclei showing the presently (2011) known nuclei. For each known isotope the element name, mass number, and half-life are given. The relatively neutron-deficient isotopes of the elements up to proton number 113 were created in cold fusion reactions based on 208Pb and 209Bi targets after evaporation of one neutron from the compound nucleus. The more neutron-rich isotopes from element 112 to 118 were produced in reactions using a 48Ca beam and targets of 238U, 237Np, 242Pu, 244Pu, 243Am, 245 Cm, 248 Cm, 249 Bk and 249Cf. The magic numbers for 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. At Z = 114 and N = 162 it is uncertain whether nuclei in that region are deformed or spherical. The background structure shows the calculated shell correction energy according to the macroscopic-microscopic model [16]. The lower the negative shell-correction energy the darker the background. and β decay, and spontaneous fission (SF). Nuclei which A reacting nuclear system behaves in a similar fashion al- are presently known in the region of heavy and superheavy though, unfortunately, it cannot be cooled from the outside. elements are summarized in Fig. 1. Cooling would be highly desirable because at the minimum In the following sections, the development of experimen- beam energy necessary for a reaction, the nuclear reaction is tal devices which were used in studies of cold fusion reac- still exothermic: heat is emitted and the reaction product, the tions is presented. Described is the progress from early me- compound nucleus, gets hot. Difficult though cooling may chanical devices to techniques using recoil separators. The be, further heating is easy: increasing the energy of the pro- main part of the article presents the discovery experiments jectile ion creates heat in the form of excitation energy of the of heavy elements produced in cold fusion reactions, and the nucleons within the nuclei. present application of this technique for the synthesis of new A remarkable variation of the excitation energy at mini- isotopes. In the subsequent section cross-sections measured mum beam energy occurs if a particular compound nucleus in cold fusion reactions are compared with those measured in is made using different projectile-target combinations. The hot fusion. Finally, a summary and an outlook are given. distribution of excitation energies in the case of 262Sg as compound nucleus is shown in Fig. 2. A maximum value of 45 MeV is reached for the combination 20Ne + 242Cm and 2. Implementation of cold fusion a minimum of 18 MeV for 54Cr + 208Pb. The reason for this variation lies in the different binding energies of the react- 2.1 Physical background ing nuclei: the binding energy for 54Cr + 208Pb is less than Chemical reactions can be exothermic or endothermic: en- that for 20Ne + 242Cm. Note that the binding energies of sta- ergy as heat is either liberated or required. How much energy ble nuclei are negative. Therefore less binding energy means – and in which direction – depends on the chemical bonds stronger binding. involved. Furthermore, a reacting chemical system can, of In the first case, the excitation energy of the compound course, easily be heated or cooled from outside. nucleus after fusion is relatively high and four neutrons plus Cold fusion 407 convincing evidence that the use of doubly magic lead nuclei (those with closed shells for the protons and neutrons) as tar- get material should lead to the formation of cold compound nuclei. 3. Experimental developments: from mechanical devices to recoil separators and ion traps 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 of less 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 sen- sitive detectors. As a prime example for such instruments, we will describe in Sect. 3.3 the velocity filter SHIP (separa- tor for heavy-ion reaction products) and its detector system, which were developed at GSI in Darmstadt. The principle of Fig. 2. Minimum excitation energies of various fusion reactions all re- separation and detection techniques used in the other labora- 262 sulting in the compound nucleus 106. Excitation energies not less tories is comparable. than about 45 MeV are obtained in reactions using actinide targets and projectiles with mass numbers A ≈ 20 (hot fusion). Minimum excita- tion energy of about 18 MeV is obtained with 208 Pb target and 54Cr 3.1 Accelerators and targets projectiles (cold fusion).