Radiochim. Acta 99, 405–428 (2011) / DOI 10.1524/ract.2011.1854 © by Oldenbourg Wissenschaftsverlag, München

Synthesis of superheavy elements by

By S. Hofmann1,2,∗

1 GSI Helmholtzzentrum für Schwerionenforschung, Planckstraße 1, 64291 , 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 V. M. Strutinsky [3]. Using this method, a number of the and . 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 . 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 made to discover naturally occurring SHEs [11, 12]. Al- targets and beams of rich isotopes of elements from though the corresponding discoveries were announced from 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 [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 and 184 for the increasing excitation energy of the compound nucleus led to 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 , 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 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 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 radioactivity, α 406 S. Hofmann

Fig. 1. Upper end of the chart of nuclei showing the presently (2011) known nuclei. For each known the element name, , 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 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 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). The figure was taken from [17]. Historically, the first accelerators used for the production of heavy elements were the cyclotrons at LBNL and later at γ rays have to be emitted to cool the nucleus down to the FLNR. They were only able to accelerate light up to ground-state. In the second case, the reaction is cold and about with sufficient intensity and up to energy high the emission of only one neutron plus γ rays is sufficient to enough for fusion reactions. Larger and more powerful cy- reach the ground-state. clotrons were built in Dubna for the investigation of reac- Until the beginning of the 1960’s, only hot fusion had tions using projectiles near . These were the U-300 been examined experimentally because only light projectiles and U-400, 300 and 400 centimetre diameter cyclotrons. In were available from accelerators. It was Y. T. Oganessian at Berkeley the linear accelerator HILAC was built. The shut- Dubna who first considered cold fusion as an alternative path down of this accelerator in 1992 led to a revival of heavy for the synthesis of the heavy elements [17]. element experiments at the 88-inch cyclotron. Aiming at the A number of experiments were carried out at FLNR using acceleration of ions as heavy as , the UNILAC was the 3- diameter cyclotron U-300 operating since 1960. constructed in Darmstadt. Beams from 40Ar to 76Ge were used and targets of lead and In order to compensate for the decreasing cross-sections bismuth, but until 1973 the experiments were not success- of the synthesis of heavy elements, increasing beam currents ful. The negative results were explained by the high fissility, are needed from the accelerators. This demands a contin- i.e. the tendency of the reacting system to re-separate on uous development of ion sources in order to deliver high the way to fusion due to high repulsive Coulomb forces. currents at high ionic charge states. Beam currents of several μ = . × 12 / Quasifission or fast fission are alternative terminologies for particle microamperes (1 p A 6 24 10 particles s) are the re-separation before fusion occurs. The fissility increases presently reached. Such high currents, in turn, demand with the product of the proton numbers of the projectile and a higher resistance of the targets. An efficient target cool- target. In the case of Ne + U the product is 10 × 92 = 920; ing and chemical compounds with higher melting points with Ca + Pb it is 20 × 82 = 1640; in both cases, element are presently tested. The developments in the laboratories 102 is the fusion product. We see a tremendous increase of in Berkeley, Dubna, and also in Finland, France, Italy and the fissility for the reactions with lead, which led people to Japan are similar. believe that cold fusion would never work. This assumption 3.2 Rotating cylinders and track detectors was supported, of course, by the negative results of the early experiments but, as it soon turned out, these were not suffi- A typical early experimental arrangement which was used at ciently sensitive. FLNRisshowninFig.3. The ion beam was incident tangen- In 1973 the detection methods were improved and the re- tially to the surface of a thin walled cylinder cov- action 40Ar + 208Pb for synthesising (element 100) ered by the target material, thin layers of , lead, or was investigated. Instead of the usual several tens of , bismuth. The diameter of the cylinder was 10 cm. The whole the yield amounted to several hundreds and even thousands device was installed inside the cyclotron, near the outer cir- of atoms in a one-day experiment [18]. The largest cross- cumference but still between the poles of the magnet. Only section characterised the reactions involving the evaporation there was the beam intensity high enough to make a search of 2 and 3 neutrons from the compound nucleus rather than for new elements successful. The cylinder was mounted so the reactions accompanied by the evaporation of 4 to 5 neu- that the axis of rotation was vertical along the magnetic field trons. The latter are typical for hot fusion. This fact was lines inside the cyclotron. The cylinder could be rotated at 408 S. Hofmann

Fig. 3. Schematic view of the rotating target cylinder used at FLNR for irradiation inside the cyclotron. The mica fission-track detectors were fixed around the cir- cumference. The figure was taken from [19].

rotation speed of the cylinder is known, the location can be easily converted into a time scale.

3.3 Recoil-separation techniques and detectors In contrast to the recoil-stopping methods, as used in the mechanical device described in Sect. 3.2, in He-jet systems or mass separators where ion sources are utilized, recoil- separation techniques use the ionic charge and of the recoiling fusion product obtained in the reaction pro- cess. Spatial separation from the projectiles and other reac- tion products is achieved by combined electric and magnetic fields. The separation times are determined by the recoil vel- ocities and the lengths of the separators. They are typically in the range of 1–2 μs. Two types of recoil separator have Fig. 4. Time distribution of spontaneous fission tracks in the reaction been developed: 50Ti + 208 Pb → 258Rf∗ (the star symbol is used to indicate an excited compound nucleus). The tracks became visible under a microscope (1) The gas-filled separators use the different magnetic after etching the mica fission-track detectors which were fixed around rigidities of the recoils and projectiles travelling through the rotating cylinder (see Fig. 3). In the lower part of the figure the de- a low pressure (about 1 mbar) gas-filled volume in cay curve is plotted after subtraction of the contribution from a long a magnetic dipole field [22]. In general, is used 256 lived activity with a half-life of several . For the nucleus Rf in order to obtain a maximum difference in the rigidities (the resulting nucleus after evaporation of 2 neutrons) a half-life of 5 ms was deduced from the decay curve. The most recent value is of slow reaction products and fast projectiles. A mean 6.2ms[20].Thefigurewastakenfrom[21]. charge state of the ions is achieved by frequent collisions with the atoms of the gas. (2) Velocity-filters or energy separators use the specific kinematic properties of the fusion products. The latter various speeds (with a maximum of 5600 rpm) to measure ones are created with velocities and energies different lifetimes over a wide range, with the shortest one only a few from the projectiles and other reaction products. Their milliseconds. ionic charge state is determined when they escape from The fusion products were found close to the target’s sur- a thin solid-state target into vacuum. A whole charge face; they were stopped almost immediately after the reac- state distribution is created with a width of about ±10% tion had taken place. If the heavy nucleus produced decayed around the mean value. Therefore, ionic-charge achro- by fission, one of the two fission fragments could always maticity is essential for high transmission. It is achieved easily escape from the target layer. by combining electric and magnetic fields or by means The escaping fission fragments were detected using thin of symmetric arrangements of electric fields. The solu- foils of a mineral (muscovite mica). The foils were mounted tion which was implemented in the design of SHIP is in a fixed position around the rotating cylinder enabling shown in Fig. 5. some 90% of all fission events to be detected. An example of how the tracks are distributed on the foils is shown in Fig. 4. Recoil separators are designed to filter out those nuclei with The half-life was determined from the decreasing of a high transmission, which are produced in fusion reactions. the tracks with distance from the irradiation area: when the Since higher overall yields result in increased background Cold fusion 409

Fig. 5. The velocity filter SHIP (Separator for Heavy Ion reaction Products) and its detection system [13–15]. The drawing is approximately to scale; however, the target wheel and the detectors are enlarged by a factor of two. The length of SHIP from the target to the detector is 11 m. The target wheel has a radius up to the centre of the targets of 155 mm. It rotates synchronously with beam macrostructure at 1125 rpm [23]. The target thickness is usually 450 μg/cm2. The detector system consists of three large area secondary- time-of-flight detectors [24] and a position- sensitive silicon-detector array (see text). The flight time of the reaction products through SHIP is 1–2 μs depending on the reaction. 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 beam line and further reduction of the background. levels, the transmitted particles have to be identified by de- beam after detection of an implanted residue [25]. After a re- tector systems. The detector type to be selected depends on sponse time of 20 μs a subsequent time window of preset the particle rate, energy, decay mode, and half-life. Experi- duration opens for counting a preset number of α particles mental as well as theoretical data on the stability of heavy of the decay chain. If the desired conditions are fulfilled, the nuclei show that they decay by α emission, , beam-off is prolonged up to the expected measurable or SF, with half-lives ranging from microseconds to days. end of the decay chain by opening a third time window. This Therefore silicon detectors are well suited improvement considerably reduces the background during for the identification of nuclei and for the measurement of the measuring period of the decay chain and allows for the their decay properties. If the total rate of ions striking the fo- safe detection of signals from long lived decays. The se- cal plane of the separator is low, then the particles can be quence of three time windows is needed because time-of- implanted directly into the silicon detectors. Using position- flight and energy signals alone would trigger the switching sensitive detectors, one can measure the local distribution off process for the beam too often due to background events of the implanted particles. In this case, the detectors act as in the corresponding window. diagnostic elements to optimize and control the ion optical A further extension of the measuring possibilities was properties of the separator. achieved with γ-ray, X-ray, or particle detectors mounted Given that the implanted nuclei are radioactive, the pos- around the target. If these detectors are operated in delayed itions measured for the implantation and all subsequent de- coincidence with signals from the implantation of reaction cay processes are the same. This is the case because the re- products and their in the focal plane of the coil effects are small compared with the range of implanted separator, the sensitivity of in-beam spectroscopy is signifi- nuclei, emitted α particles or fission products, and detector cantly improved. This so called recoil-decay tagging method resolution. Recording the data event by event allows for the was first applied in a study of the heavy ion radiative capture analysis of delayed coincidences with variable position and mechanism, a cold fusion process without any evaporation time windows for the identification of the decay chains [14]. of nucleons, using the reaction 90Zr + 90Zr → 180Hg [26]. The presently used detector system is composed of Meanwhile the method has become a standard tool in nu- three time-of-flight detectors, seven identical 16-strip silicon clear in-beam spectroscopy and it has become known as wafers, and germanium detectors [15]. A schematic view RDT method. of the detector arrangement is shown in the focal plane of SHIP in Fig. 5. Three secondary-electron foil detectors [24] 3.4 Ion traps behind recoil separators in front of the silicon detectors are used for measuring the particle velocity and for tagging ions which are im- The low background conditions behind recoil separators al- planted into the stop detector. These detectors are mounted low for installation of additional devices which would be de- at distances of 780, 425, and 245 mm in front of the stop stroyed by high beam intensities if mounted directly after the detector. Three detectors are used to increase the detection target. An example of such devices is the SHIPTRAP set- efficiency. up consisting of a gas-filled stopping cell, a radio-frequency A time-of-flight signal and an energy signal from the sil- quadrupole (RFQ) cooler and buncher, and a double Penning icon detector provide the information for switching off the trap system [27–30]. The set-up is shown in Fig. 6. 410 S. Hofmann

Fig. 6. Schematic overview of the SHIP- TRAP set-up. It consists of a gas-filled stop- ping cell for stopping of the high energetic reaction products separated by SHIP, an RFQ cooler and buncher for cooling and accu- mulating the ions, and a double Penning trap system for high-precision mass meas- urements. The length of the apparatus from the gas cell to the channeltron detector is 4.5 m. The figure was taken from [30].

The ions, e.g., ions produced in the reac- tion 48Ca + 208Pb, coming from SHIP with a kinetic energy of around 22 MeV are stopped in the gas cell which is filled with ultrahigh purity (1 ppm) helium at a pressure of 50–60 mbar [31]. The gas-filled volume is separated from the 10−6 mbar vacuum of SHIP by a entrance window with a thickness of about 2 mg/cm2 andanopen diameter of 60 mm. An additional set of Mylar degraders is installed in front of the entrance window. Their thickness is variable from 0.5 μm in steps of 0.5 μm. The total thickness is adjusted so that most of the ions are stopped in the centre of the gas cell. Inside the gas cell, an system consisting of a DC voltage cage and an RF funnel is mounted to gener- ate an electric field that drags the ions toward the exit nozzle within a collection time of a few milliseconds [32]. Upon ar- riving at the nozzle the ions are ejected from the gas cell by the helium gas flow. Subsequently, the ions are transported to the RFQ cooler and buncher made of four segmented rods. Here, the ions are cooled in collisions with helium gas atoms at a pressure of about 5 × 10−3 mbar and are accumulated in a potential minimum. The cooling time is about 3 ms. Fig. 7. Systematics of spontaneous fission half-lives of even-even nu- clei at neutron number N = 152. Experimental data are shown by open After a typical storage time of 1–2 s in the buncher the circles and squares connected by full lines. The dashed line shows the ions are ejected in pulses with a width of about 500 ns extrapolation by A. Ghiorso [34] for the element 104 isotopes. The and transferred to the double Penning trap system which figurewastakenfrom[18]. is placed in a superconducting solenoid magnet providing a field strength of 7 T. In the first trap isobaric separation with a mass resolving power of up to 100 000 is achieved by from the extrapolation of the data established in Berkeley by applying buffer-gas cooling. In the second trap the cyclotron the late A. Ghiorso [34]. frequency of the ions is determined with the time-of-flight Fig. 7 shows the trend of the half-lives for the elements ion cyclotron resonance (TOF-ICR) detection method [33]. from to 104; a sharp half-life maximum at neutron While one sample of ions is cooled and investigated in the number 152 is clearly visible. Extrapolating the shape of the Penning traps, another sample is accumulated and cooled in curves beyond element 102, one would expect a half-life of the RFQ cooler and buncher. minutes for element 104 but instead it was no more than milliseconds. On the other hand, the isotope with neutron number 156, 260Rf, had a half-life a million times longer than 4. Early results obtained in Dubna expected. The result was very important for the future of heavy 4.1 The double humped fission barrier element research, because it indicated a drastic change in An important milestone in heavy element research was the systematics of fission half-lives. If confirmed, it could achieved at Dubna in 1974–75. Using the rotating cylinder mean that all elements beyond element 102 could fission and fission fragment detectors, isotopes of element 104 were and, even more significant, that the fission half-lives could investigated by cold fusion. A beam of 50Ti ions and lead become shorter and shorter as predicted by theoretical calcu- isotopes of mass numbers 206, 207, and 208 as targets were lations. If the half-lives dropped below a microsecond, there used. For the newly produced nucleus 256Rf a half-life of would be no chance at all of identifying those short lived about 5 ms was measured, 10 000 times less than expected elements. Cold fusion 411

4.2 Radiochemical separation Some α-decay chains of heavy elements end in well-known isotopes and some of these isotopes have half-lives from a few hours to several days, long enough to perform a clean off-line chemical separation of the daughter elements from the bulk of other reaction products. After chemical sepa- ration, the probe is put in front of α or fission detectors. These measure the activity while the measured decay en- ergy allows determination of the isotopic composition of the sample. Applying this procedure for heavy elements, information about the parent nucleus could be obtained even if that nucleus could not be measured directly, per- haps because the experimental conditions did not enable the detection of α decay or short half-lives. The method can, of course, be applied only if the parent nucleus de- cays by α emission but, on the other hand, a positive re- sult is also an indication of α decay of the parent nucleus. This was the basic idea used at FLNR to obtain informa- tion on the synthesis of elements 107 and 109 in cold fu- sion reactions and on the decay properties of the produced nuclei [37]. Technically, an intensive internal beam of the cyclotron U-400 was available since the 1970s, and the reliable tar- get technology of the cooled rotating cylinder could be used. The hope was that the data would give information about the Fig. 8. Illustrative representation of the various fission barriers for production cross-section for the heaviest, and at the time still even-even nuclei with neutron number near N = 152. The wide and unknown, elements and whether they decayed by α emission double-humped fission barrier in the case of 252 Fm and 254No is respon- or fission. sible for the long partial fission half-life of those nuclei. The figure was For the production of the elements 107 and 109, one more taken from [35]. advantage was the variety of natural projectile isotopes. Starting with the most neutron-rich isotope of titanium, 50Ti, there exists a series of projectiles which differ by additional A qualitative physical explanation for the short half-life α particles: 54Cr and 58Fe. In the irradiation of 209Bi targets of 256Rf was given in [35]. Fig. 8 shows the fission barriers with these projectiles, the following compound nuclei are of 252Fm (Z = 100) and some neighbouring nuclei. 252Fm is produced: 259Db (, Z = 105), 263Bh (, 107) the isotope with the longest fission half-life located at neu- and 267Mt (, 109). After emission of one neutron tron number N = 152 (see Fig. 7). The ground-state of 252Fm the evaporation residues are: 258Db, 262Bh, and 266Mt. Each is the valley at the left of the hatched peak area. This energy differs from the next by one α particle. Assuming that these determines at which altitude the fission tunnel through the isotopes are α emitters, the 35 h half-life isotope 246Cf (98) mountain is located (dashed line). In the case of 252Fm the would eventually be produced after α decay of the nucleus mountain above the tunnel is high and also wide because of 254Lr (103) (which was already known), electron capture de- the second smaller peak at the right side. The isotope 252Fm cay of 250Md and another α decay of 250Fm (100). The decay has a so called double humped fission barrier. This makes chain is shown in Fig. 9. the probability for tunnelling small or, correspondingly, the In a series of experiments the production of element 105 fission half-life long. was studied, then 107, and 109 using projectile beams of A double humped fission barrier still exists for 254No, al- 50Ti, 54Cr and 58Fe, respectively [37]. The daughter isotope though it is less pronounced. However, in the case of 256Rf, 246Cf was readily separated radiochemically after an irradi- the minimum for the ground-state is less deep and the level ation period of 1–2 d and identified by its α-decay spectrum. for the tunnel at a higher energy. The result in this case is The line intensities decreased by a factor of 10 from 258105 that the tunnel lies above the second hump and the fission to 262107, and by a factor of almost 100 from 262107 to barrier for 256Rf is much narrower. That is why the fission 266109. These results were the first indications that elem- half-life of 256Rf is 10 000 times less than expected from the ents 107 and 109 could be produced by cold fusion reactions data measured in Berkeley. and that the isotopes 262107 and 266109 decay by α emis- The Dubna result was later confirmed, the most recent sion. However, a direct identification was not possible and value for the half-life of 256Rf being (6.2±0.2) ms [20]. The one important value, the half-life, could not be measured. qualitative explanation was also substantiated theoretically, see review article [36]. It could be shown that nuclei with = neutron number N 152 are more stable than the neigh- 5. New elements made by cold-fusion reactions bouring isotopes and that there does indeed exist a second hump in the fission barrier which significantly influences the In this section, we present results dealing with the syn- fission half-life. thesis of elements 107 to 113 using cold fusion reactions 412 S. Hofmann

A summary of recently measured spectroscopic data on elements from to is given in [64], see also contribution by R. Herzberg.

5.1 Element 107, bohrium Bohrium, element 107, was the first new element synthe- sized at SHIP using the method of in-flight recoil separation and generic correlation of parent-daughter nuclei. The reac- tion used was 54Cr + 209Bi → 263Bh∗ . Five decay chains of 262Bh were observed [65]. The next lighter isotope, 261Bh, was synthesized at higher beam energy [66]. The decay data of these two isotopes were confirmed in [48]. Addi- tional data were obtained from the α decay of 266Mt [67], and the isotope 264Bh was identified as granddaughter in the decay chain of 272Rg [25, 68]. The isotopes 266Bh and 267Bh were produced using the hot fusion reaction 22Ne + 249Bk → 271Bh∗ [69, 70]. These nuclei were used for a study of the chemical properties of bohrium. A new isotope of bohrium, 265Bh, was synthesized in 2003 at HIRFL (heavy ion research facility) in Lanzhou, China [71]. It was produced in a 4n evaporation channel using the hot fusion reaction 26Mg + 243Am → 269Bh∗ .For completeness we add here that the identification of the new isotope 259Db was published by the same already in 2001. In this case the reaction 22Ne + 241Am → 263Db∗ was Fig. 9. Sequential α and electron capture (EC or ε) decay chain for doubly-odd nuclei 258105, 262 107 and 266109, leading to long lived 246Cf used [72]. 260 production. The reactions for the synthesis of these are also The so far lightest isotope of bohrium, Bh, was synthe- given. The figure was taken from [37]. sized in the reaction 52Cr + 209Bi → 260Bh +1n [49], see also Sect. 5.9. based on lead and bismuth targets. A detailed presenta- 5.2 Element 108, hassium tion and discussion of the GSI-SHIP results on the decay properties of elements 107 to 112 was given in previous re- Hassium, element 108, was first synthesized in 1984 using views [15, 38, 39]. The results of experiments at RIKEN on the reaction 58Fe + 208Pb. The identification was based on the the confirmation of elements 110 to 112 and the first produc- observation of three atoms [73]. Only one α decay chain was tion of an isotope of element 113 using a cold fusion reaction measured in the irradiation of 207Pb with 58Fe. The measured were published in [40–44]. A systematic work on cold fu- event was assigned to the even–even isotope 264Hs [74]. The sion reactions aiming to study the influence of neutron num- results were confirmed in later works [39, 75], and for the ber and of target-projectile asymmetry on the cross-section decay of 264Hs an SF branching of 50% was measured. The was performed at LBNL. Data were measured for elem- presently known most neutron deficient isotope of hassium ent in [45], dubnium [46], [47], is 263Hs produced with a cross-section of 21 pb in the cold bohrium [48–50], hassium [51], meitnerium [52], and darm- fusion reaction 56Fe + 208Pb [51]. stadtium and [53, 54]. The results will be pre- The isotope 269Hs was discovered as a link in the de- sented in Sect. 5.9. Also in these works, previous data were cay chain of 277112 [25, 76]. It was also directly produced confirmed. in the hot fusion reaction 26Mg + 248Cm (5n channel) and An overview of nuclei in the region of SHEs, which are separated by chemical means [77, 78]. In the same reaction, presently known, is given in the partial chart of nuclides, dis- the new isotopes 270Hs and 271Hs were measured in the 4n played in Fig. 1. A number of new neutron rich isotopes of channel [78]and3n channel [79], respectively. The inter- elements above was produced as daughter prod- esting result of these studies of the hot fusion reaction was ucts in decay chains starting at isotopes of elements from that the cross-section for the 3n channel at E∗ = 35 MeV 113 to 118. These new elements were synthesized in recent has a value of 2.5 pb and is almost as high as the cross- experiments at FLNR using hot fusion reactions [55, 56], see sections for the 4n channel (3 pb) and the 5n channel (7 pb) also contribution by Yu. Ts. Oganessian. The results of these at excitation energies of 40 and 49 MeV, respectively (see experiments will not be described here but the produced nu- also Fig. 16). This observation opens new prospects for the clei are shown also in Fig. 1. search for more neutron rich isotopes in reactions between Also not discussed here are the results of experiments in actinide targets and neutron rich projectiles of elements be- which part of the data measured at FLNR were confirmed. tween and sulphur. It could be even more important These experiments were conducted by chemical means at in reactions towards the synthesis of superheavy elements as FLNR [57, 58] and using recoil separator techniques at in these reactions the cross-sections at high excitation en- GSI [59–61]andLBNL[62, 63]. ergy are low due to an increasing fission probability. Cold fusion 413

Attempts to produce 269Hs and 268Hs in the hot fusion results of reaction theories. A so called extra-push energy reaction 25Mg + 248Cm failed [80]. The cross-section limits predicted in [84, 85] was not measured. obtained by chemical means were 0.4pbforthe4n channel The heavier isotope 271Ds was synthesized with a beam (269Hs) and 1.3pbforthe5n channel (268Hs). of the more neutron-rich isotope 64Ni [39]. The important result for the further production of elements beyond meit- nerium was that the cross-section was enhanced from 2.6 to 5.3 Element 109, meitnerium 15 pb by increasing the neutron number of the projectile by Meitnerium, element 109, was first observed in 1982 in two. This observation gave hope that the cross-sections for the irradiation of 209Bi with 58Fe by a single α-decay the synthesis of heavier elements could decrease less steeply chain [81, 82]. This result was confirmed in [83]. In a more with available stable, more neutron-rich projectiles. How- recent experiment conducted at GSI twelve atoms of 266Mt ever, this expectation was not proven in the case of element were measured [67]. Five atoms of 266Mt were observed at 112, see below. LBNL using the reaction 59Co + 208Pb [52]. The decay data An overview of all data measured at SHIP from the de- reveal a complicated decay pattern, as concluded from the cay chains observed in the reaction 64Ni + 208Pb → 272Ds∗ is wide range of α energies from 10.5 to 11.8 MeV. This prop- giveninFig.10. We will describe these data in more detail, erty seems to be common to many odd and odd-odd nuclides because they are representative for the detection and assign- in the region of the heavy elements. The more neutron-rich ment of new decay data by correlation to the decay of known isotope 268Mt was measured after α decay of 272Rg [25, 68]. nuclei. In the figure the energies and lifetimes of α decays directly succeeding the implantations are shown. (Describ- ing single event chains it is preferable to use the lifetime 5.4 Element 110, τ instead of the half-life, because τ is directly measured as Darmstadtium, element 110, was discovered in 1994 using time difference between two subsequent signals). On top of the reaction 62Ni + 208Pb → 269Ds + 1n [75]. The main ex- the upper abscissa of Fig. 10 the α spectra deduced from lit- periment was preceded by an accurate study of the excitation erature are plotted for the decays of 255Md, 255No, 259Rf, and functions for the synthesis of 257Rf and 265Hs using beams of 263Sg, in order to compare the energy and intensity pattern 50Ti and 58Fe in order to determine the optimum beam en- with the measured data assigned to the decay of 271Ds. In ergy for the production of element 110. New information on case of 267Hs the three α decays observed in [86] are plotted. the decay pattern of these nuclei was also obtained. The data A total number of 57 α decays was measured and as- revealed that the maximum cross-section for the synthesis of signed to the 13 decay chains shown in Fig. 10. Thirty-eight element 108 was shifted to a lower excitation energy relative α particles were emitted in beam direction and stopped in to the peak for production of element 104, different from the the 300 μm thick detector. These α events are marked by

Fig. 10. Energies and lifetimes of the thirteen α decay chains resulting from the reaction 64Ni + 208Pb → 271Ds +1n. Plotted are the data measured in experiments at SHIP. The isotope 271 Ds was identified by comparison of the decay properties of the daughter products with literature data marked by an asterisk (267Hs [86], 263Sg [87], 259Rf [88], 255No [88], 255Md [88]). New decay data of the isotope 263Sg could be deduced. The thirteen decay chains are arranged chronologically with the date of production given at the right hand ordinate. For each chain, the time sequence of the α decays is from 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. For description of the symbols see text. Obviously, the α serves as a finger print for the identification of nuclei. The higher the energy resolution is and the less transitions of different energy for one nucleus occur, the higher is the significance of the assignment. See text for details. 414 S. Hofmann the little filled squares. The width of the squares shows action, 64Ni + 208Pb → 272Ds∗ , was studied and a total of 14 the energy resolution of 13 keV (2σ) obtained for these full decay chains was measured [40]. The α energy agrees well energy α signals. An exception are the 271Ds α decays of with the SHIP value of 10.74 MeV and also for the lifetime, chain 9 and 11 for which the energy had to be corrected a long-lived (3 events) and a short-lived (11 events) compon- by +60 keV and −40 keV, respectively, due to short life- ent was measured. Mean values obtained from all 5 long- time and tails of the signals from the preceding implanta- lived and 22 short-lived decays are τ = 100 ms and 2.35 ms, +56 . +0.44 tion. The exact shape of these tails as a function of energy corresponding to half-lives of (69−21)msand(163−0.28 )ms, was measured with energy degraded fission fragments of respectively. Note that the uncertainties decrease consider- a 252Cf source. A larger uncertainty, as indicated by the ably with increasing number of events. A proper application rectangles, was assigned to these data. The larger, open of the Poisson statistics for the determining of uncertainties squares mark escape α particles for which the full energy at low statistics is given in [89]. was summed from ΔE signals between 0.6 and 1.4MeV Obviously, isomeric states are responsible for the two in the stop detector and coincident residual energies in the different half-lives. Because the measured α energies are al- backward crystals. A total of 12 of these α events was ob- most identical, an explanation suggested in [40] seems very served, for which an energy resolution of 34 keV (2σ), as likely: The isomeric state has the longer half-life and de- marked by the width of the squares, was obtained in test cays dominantly by γ-ray or conversion-electron emission reactions at higher statistical significance. In one of these into the shorter-lived ground-state. Both states are populated cases, chain number 12, the first α decay was corrected in the reaction, but because the lifetime is measured as in- by +60 keV due to the short lifetime after the implanta- terval between implantation and α decay, we observe two tion, and a larger error bar was used. Seven α particles different values for one and the same α transition. Theoret- escaped from being recorded in the backward crystals but ically, low-spin and high-spin levels (1/2+ → 13/2−)which still yielded a ΔE signal and thus the time and position in- could result in isomeric states close to the ground-state were formation. The amplitudes of the signals corresponded to predicted in [90]. The isomeric ratio between population of energies between 1.0 and 5.2 MeV, which is characteristic the 69 ms and 1.6 ms states is 5/22. Therefore, we may fur- for escaping α particles. They are marked in Fig. 10 by the ther conclude that the relatively long-lived isomeric state has arrows pointing left. The ratio of the α’s measured with full a low spin value and is less populated. This spin depen- energy/escape plus residual energy/escape is 38/12/7. It dence of the production cross-section known from lighter agrees within the respective experimental uncertainties with nuclei may be further enhances for heavy systems due to re- the ratio 34/17/6 determined by test reactions and normal- duced fission probability of the compound nucleus at high ized to the total number of 57 α particles as measured in the spin. However, also the possibility that both levels decay by 271Ds experiment. α emission with almost the same α energy cannot be com- The lower discriminator level for signals of the stop de- pletely excluded. An indication could be the slightly lower tector amounted to 260 keV, which is below the lowest ΔE α energy for the longer-lived decay in the case of chain num- signals of escaping α particles emitted from heavy nuclei ber 5 (see Fig. 10), which was measured with high precision. implanted 4–6 μm into the active detector material. An esti- Further confirmation of the production of 271Ds in the re- mate of the implantation depths was obtained by the meas- action 64Ni + 208Pb → 272Ds∗ was reported from experiments ured implantation energy signals of ≈ 25 MeV. As an im- performed at BGS (Berkeley Gas-filled Separator) [53, 54]. portant result a detection efficiency of 100% for α decay At beam energies of 309.2, 311.5, and 314.3MeVathalf was obtained, the same as for the much higher energetic SF target thickness, a total of 2, 5, and 2 decay chains were events. Therefore, the reason for non-observation of an α- measured, respectively. Cross-sections of 8.3, 20, and 7.7pb decay or fission signal is almost certainly that the nucleus, were deduced. The decay chains are in excellent agreement which has been identified as an α-, undergoes with the previously measured data [39, 40]. Position and β+ decay or EC. In Fig. 10 such missing α or fission decays shape of the excitation functions agree within experimental are marked by ‘ε’. If the time window between implantation uncertainties. and disintegration is wide enough to cover the full lifetime We conclude that the maximum deviation of beam energy range, the detector system allows an unambiguous measure- measured at the LBNL 88-Inch Cyclotron and the UNILAC ment of β branching ratios. is ±2 MeV. A similar deviation is deduced from a compari- In agreement with the known branching ratios of 255No son of the data measured at RIKEN [40] and at the UNILAC. 255 (bε = 39% [88]) and Md (bα = 8% [88]) is the observa- Possibilities to improve the accuracy of beam-energy meas- tion of the α decay of 255Md in one of 13 cases (see chain urements are under discussion [91]. 12 in Fig. 10). This result allows one to draw two conclu- Two more isotopes of darmstadtium have been reported sions. Firstly, lifetimes up to 19 min are measurable at low in the literature. The first one is 267Ds, produced at LBNL in background rate (in our case the 255Md α-decay occurred the irradiation of 209Bi with 59Co [92]. The second isotope during a beam pause of 14.5 ms). Secondly, the agreement is 273Ds, reported to be observed at FLNR in the irradiation of the measured α energies with literature data is much of 244Pu with 34S after the evaporation of five neutrons [93]. more significant for monoenergetic decays, simple decay Both observations need further experimental clarification. patterns, and high energy resolution of the detector. The The even–even nucleus 270Ds was synthesized using widely spread α energies of the 255No decay represent an the reaction 64Ni + 207Pb [94]. A total of eight α-decay opposite example. chains was measured during an irradiation time of seven The decay data of 271Ds and its daughter products were days. Decay data were obtained for the ground-state and confirmed in an experiment at RIKEN, where the same re- a high spin K , for which calculations predict Cold fusion 415

Fig. 11. Assignment of measured α and γ de- cay and SF data observed in the reaction 64Ni + 207Pb → 271 Ds∗ . The data were assigned to the the ground-state decays of the new isotopes 270Ds, 266Hs, and 262Sg and to a high-spin K isomer in 270Ds. Arrows in bold represent measured α and γ rays and SF. The data of the proposed partial level schemes are taken from theoretical studies of the rotational levels [96], of K [95], of α en- ergies [96] and of SF half-lives [98]. For a detailed discussion see [94]. New experimental data were measured in [99] (see text). spin and parity 9−,10−,or8+ [95]. The relevant sin- 5.5 Element 111, roentgenium gle particle Nilsson levels are ν[613]7/2+ and ν[615]9/2+ below the Fermi level and ν[725]11/2− above the Fermi Roentgenium, element 111, was synthesized in 1994 using level. Configuration and calculated energy of the excited the reaction 64Ni + 209Bi → 273Rg∗ . A total of three α chains 272 states are {ν[613]7/2+ν[725]11/2−}9− at 1.31 MeV, {ν[615]9/2+ of the isotope Rg were observed [68]. Another three de- ν[725]11/2−}10− at 1.34 MeV, and {ν[613]7/2+ν[615]9/2+}8+ at cay chains were measured in a confirmation experiment in 1.58 MeV. 2000 [25]. The new nuclei 266Hs and 262Sg were identified as daugh- The GSI data on 272Rg were confirmed in a 50 d irradi- ter products of the α decay of 270Ds. Spontaneous fission of ation at RIKEN performed in the period from February 12 to 262Sg terminates the decay chain. A proposed partial decay May 12, 2003 [41]. A total of 14 α decay chains were meas- scheme of 270Ds is shown in Fig. 11. ured. In eight cases, the α decays were followed down to the In a recent SHIP experiment the statistical accuracy of α decay of 256Lr and in three cases, down to the α decay of the data was improved considerably [99]. Twenty-five more 260Db. In one case, the chain terminated by SF after popu- decay chains of 270Ds were measured. The previous data lation of 260Db, and in two cases, by SF after population of were confirmed. A new result was the detection of an SF 264Bh. The resulting SF branching ratios are 9.6 and 15%, branching of 266Hs and of an α branching of 272Sg. A com- respectively. It remains open, whether the two nuclei them- plete α-decay chain exists now for the even–even nuclei selves decay by SF or whether the known spontaneously from 270Ds to 254No allowing for a precise mass determin- fissioning even–even nuclei 260Rf and 264Sg are populated by ation of these heavy nuclei by connecting the Qα values to EC (see Fig. 1). However, considering the SF half-lives of the precisely measured mass of 254No, see Sect. 6.2. about 10–100 ms of even–even nuclei in this region, a SF 416 S. Hofmann hindrance factor for odd-odd nuclei of about 106 [100], (not shown in Fig. 12) and the second one of the origi- and a calculated partial EC half-life of about 10–100 s (see nally published four chains of 269Ds [75] represented spuri- Sect. 4), it seems more likely that 260Db and 264Bh have an ous events. Details of the results of the reanalysis are given EC branching. From the total half-lives of 1.5 and 1.0sand in [25]. the measured branching ratios [41] follow reasonable partial In 2004, the 70Zn + 208Pb → 278112∗ irradiation was re- EC half-lives of 15 and 10 s, respectively. A purely experi- peated at RIKEN [43]. Using a beam energy comparable to mental determination of a possible EC or SF branch of 260Db that used at SHIP, two decay chains were measured, which and 264Bh decay is difficult because the SF half-lives of the fully confirmed the SHIP data. For comparison, all four de- daughters 260Rf and 264Sg are short, i.e. 20 and 68 ms, re- cay chains are shown in Fig. 12. The two RIKEN chains also spectively. This means that the measured time difference verified the SF branch of 261Rf. In both of these chains, the between α decay of 264Bh and SF of 260Rf is not prolonged α energy of 265Sg produced in the decay chain of 277112 was significantly by the SF lifetime of 260Rf in the case of an measured for the first time and is now available for com- (unobservable) EC decay of 260Db, compared to the time dif- parison with the value measured in [78]. Also in this case ference between α decay of 264Bh and SF of 260Db. The same agreement was observed giving further support to the assign- argument holds for the chain 268Mt → 264Bh → 264Sg. ments made in [25, 76]. Further confirmation of the decay pattern of 272Rg was achieved in an experiment at LBNL [54]. However, for the 5.7 The A = 278 isotope of element 113 synthesis, the reaction 65Cu + 208Pb → 273Rg∗ used. So far, element 113 is the heaviest one produced by a cold fusion reaction, namely in 70Zn + 209Bi → 279113∗ .Afirstat- 5.6 Element 112, tempt to synthesize element 113 was made at the GSI SHIP Element 112 was investigated at SHIP using the reaction in 1998, using a net irradiation time of 46 d and a beam dose 70Zn + 208Pb → 278112∗ [76]. The irradiation was performed of 7.5 × 1018 [15]. The experiment was continued in 2003, in 1996. Over a period of 24 d, a total of 3.4 × 1018 pro- reaching a net irradiation time of 36 d and a beam dose of jectiles were collected. One α-decay chain, shown as the 7.4 × 1018 [101]. This latter part of the experiment was run- first one from the left in Fig. 12, was observed resulting in ning in parallel to the search for element 113 at RIKEN. a cross-section of 0.5 pb. The chain was assigned to the 1n Combining both parts of the SHIP experiment, an upper one- channel. The experiment was repeated in 2000 with the aim event cross-section limit of 160 fb was deduced. The one- of confirming the synthesis of 277112 [25]. During a simi- event limit corresponds to a cross-section in the case that lar measuring time, but using slightly higher beam energy, one event would have been observed, statistical fluctuations one more decay chain was observed, which is also shown are not considered. For an estimate of observation limits in in Fig. 12. The two experiments yield agreement concern- the case of negative results including statistical fluctuations, ing the decay pattern of the first four α decays of the 277112 see [89]. decay chain. The search experiment for element 113 at RIKEN was A new result was the occurrence of fission which termi- performed in 2003 and 2004, the net irradiation time being nated the second decay chain at 261Rf. An SF branch of this 79 d and the beam dose 17 × 1018 [42]. During this experi- nucleus was not known, however, it was expected from the- ment, one decay chain was observed and assigned to the oretical calculations. The new results on 261Rf were proven isotope 278113, see Fig. 13, left side. The chain terminated in a recent chemistry experiment [77, 78] in which this iso- after four subsequent α decays by SF of the known isotope tope was measured as granddaughter in the decay chain of 262Db. Also known was the last α emitter 266Bh. New were 269Hs and SF of 261Rf was also observed. the chain members 270Mt and 274Rg. The measured cross- +150 A reanalysis of all relevant results obtained at SHIP since section of (55−45 ) fb is in agreement with the limit obtained 1994, a total of 34 decay chains was analyzed, revealed that at SHIP. Despite comparable beam doses, the three times the previously published first decay chain of 277112 [76] lower cross-section value reached at RIKEN is due to the

Fig. 12. Decay chains measured in the cold fusion reaction 70Zn + 208Pb → 278112∗ . In the left part the two chains are shown, which were measured in 1996 and 2000 at SHIP [25, 76], in the right part those measured in 2004 at RIKEN [43]. The chains were assigned to the isotope 277112 produced by evaporation of one neutron from the compound nucleus. The lifetimes given in brackets were calculated using the measured α energies. In the case of escaped α particles the α energies, given in brackets, were determined using the measured lifetimes. Cold fusion 417

Fig. 13. Decay chains measured in the cold fusion reaction 70Zn + 209Bi → 279113∗ at RIKEN [42, 44]. higher efficiency, 80% instead of 50% at SHIP, and the use that the same compound nucleus can be formed. In add- of thicker targets in the RIKEN experiment. ition, for all projectiles of even elements, a number of The 70Zn + 209Bi irradiation was continued at RIKEN different isotopes exists, which can be used for systematic between January 2005 and May 2006, with several inter- studies. missions. The net irradiation time of this second part of A guideline in search experiments for new elements was the experiment was 161 d and the beam dose amounted selection of reactions with lowest fissility, which means 19 to 4.45 × 10 . During this period, a second decay chain lowest product Z1 × Z2 of projectile and target and with was measured, which confirmed the previous decay chain lowest excitation energy at the fusion barrier. On the ba- and its assignment to 278113, Fig. 13, right side [44]. The sis of these arguments, 208Pb and 209Bi were chosen as tar- cross-section determined from the RIKEN experiments is gets and the most neutron rich isotope of even elements +40 (31−20) fb. This value is the lowest one ever measured for a heavy-ion fusion reaction. Table 1. Cross-sections and corresponding excitation energies E∗ of > 1n-evaporation channels measured in various cold fusion reactions. 5.8 Search for elements 113 using cold-fusion Excitation energies in bold mark data obtained from the maximum reactions of the excitation function. In the lower part three pairs of hot fusion Cold fusion was also applied to search for elements 116 reactions are given for comparison. 208 and 118. In both cases Pb targets were irradiated. The Reaction σ(1n)/pb E∗/MeV Ref. beams were 82Se and 86Kr, respectively. The 116 experiment was performed at SHIP in order to search for the radiative 50Ti + 208 Pb → 258Rf 40 000± 5000 16.6 [45] capture (0n) channel. At five different excitation energies 48Ti + 208 Pb → 256Rf 380 ± 70 16.8 [45] between 0 and 11 MeV, cross-section limits of about 5 pb 51 + 208 → 259 +1100 V Pb Db 2070−760 16.0 [46] 50 + 209 → 259 +1730 were reached [15]. Ti Bi Db 5480−1370 16.0 [46] Subsequent to reports on positive results of the syn- 55Mn + 208Pb → 263Bh 530 ± 100 14.1 [50] thesis of element 118 in 1999 [102], confirmation ex- 54Cr + 209 Bi → 263Bh 430 ± 110 15.7 [50] 52 + 209 → 261 +29 periments were performed. However, only cross-section Cr Bi Bh 59−20 15.0 [49] limits of about 1 pb were reached at various laborato- 58Fe + 208 Pb → 266 Hs 69 ± 12 13.9 [15] 56 208 264 +13 Fe + Pb → Hs 21− . 15.2 [51] ries [15, 103, 104]. Eventually, the first announcement was 8 4 59 + 208 → 267 . +5.2 retracted in 2001 [105, 106] after additional experiments and Co Pb Mt 7 7−3.3 14.9 [52] 58 209 267 +4.8 Fe + Bi → Mt 7.4− . 13.4 [67] after a reanalysis of the data of the first experiment. 3 3 64 + 208 → 272 +9 Ni Pb Ds 15−6 12.1 [15] 64Ni + 207 Pb → 271 Ds 13 ± 5 14.0 [94] 5.9 Comparative cross-section studies 62 + 208 → 270 . +2.6 Ni Pb Ds 2 6−1.4 12.3 [25] 65 + 208 → 273 . +3.9 A comparative study of cold fusion reactions was started Cu Pb Rg 1 7−1.4 13.2 [54] 64 + 209 → 273 . +1.9 at LBNL. Two aspects influencing the fusion cross-sections Ni Bi Rg 2 9−1.3 12.5 [25] 70 + 208 → 278 . +1.1 should be studied. Firstly, entrance channel effects were in- Zn Pb Cn 0 5−0.4 12.0 [25] 68 + 208 → 276 ≤ . ≈ vestigated in reactions leading to the same compound nu- Zn Pb Cn 1 2 12 [39] 48 + 238 → 286 . +1.8( ) cleus and, secondly, the response of the reaction when pro- Ca U 112 2 5−1.1 3n 35.0 [107] 48 + 233 → 281 ≤ . jectile and target are changed only slightly. Ca U 112 0 6 34.9 [107] 48 + 244 → 292 . +3.6( n) 41.0 Ideal conditions for these studies are given in cold fu- Ca Pu 114 5 3−2.1 4 [108] 48Ca + 242Pu → 290114 4.5+3.6(4n) 40.2 [107] sion reactions with projectiles of titanium and heavier. −1.9 48 + 248 → 296 . +2.5( ) Ca Cm 116 3 3−1.4 4n 38.9 [107] There, stable of even and odd Z elements exist, + . 48Ca + 245Cm → 296116 3.7 2 5(3n) 38.0 [55] which can be combined with targets of 208Pb and 209Bi so −1.6 418 S. Hofmann as projectile. However, these assumptions were not proven experimentally. The new data measured at LBNL in comparison with pre- vious data reveal the interesting result that odd elements can be made with similar cross-sections in reactions with lead targets as in those with bismuth targets. In Table 1 the avail- able data are listed. A significant reduction of cross-section was measured when the number of neutrons of the projectile was reduced by two. The reduction of cross-sections seems to be less when lighter lead isotopes are used as targets. For these sys- tems, however, more experimental data have to be measured. Despite important information on the reaction mechanism these studies would also result in synthesis of new neutron deficient isotopes.

6. Binding energies Nuclear structure and binding energies determine the prop- erties of each individual nucleus, dominantly its half-life, decay mode and shape. Via these properties, they also sig- nificantly influence the dynamical behaviour of the reaction mechanism. In the region of the heaviest elements, these properties are especially important, because these nuclei owe their existence mainly to nuclear shell effects. Because Fig. 14. First decay chain of 277112 measured in 1996 [76]. In the lower nuclear structure of heavy elements, both experimentally as part the chain is plotted in a landscape of shell-correction energies well as theoretically, is subject of two other articles of this according to the macroscopic-microscopic model [111]. The deep min- issue of Radiochimica Acta, we present here only two ex- imum at Z = 108 and N = 162 is due to increased stability if nuclei amples, where nuclei produced in cold fusion reactions con- in this region are deformed. In the upper part the measured α-decay tributed significantly to the development of heavy element half-lives are plotted. Graphics courtesy of P. Möller. research.

change of the half-lives from 0.2msto10sbyalmostfive 6.1 Increased stability for deformed nuclei orders of magnitude between 273Ds and 269Hs. This change Already in early theoretical studies of heavy and superheavy is due to a considerable change of the Qα values from high elements, a region of increased stability was obtained for de- values when the α decay proceeds into the direction of in- formed nuclei at Z = 106–108 and A = 260–270 [109]. In creased stability and low values when this region is left. the chart of nuclei, this region is located south-west from This behaviour is in agreement with the theoretical calcula- the region of spherical superheavy nuclei, see Fig. 1.Nuclei tions. Together with the decay data from 278113, it marks the in this region owe their stability to large energy gaps be- extension of the region of increased stability in north-east tween Nilsson levels at Z = 108 and N = 162, which result direction. in a compression of energy levels below these num- bers. In subsequent theoretical studies, decay properties of 6.2 Direct mass-measurements by Penning traps nuclei located in this region were calculated in great detail, see contribution by A. Sobiczewski. The very high mass resolving power of Penning traps can First experimental evidence was obtained from the study now be applied to rare isotopes of nearly all elements. With of α-decay chains from nuclei produced in cold fusion re- the Penning trap mass spectrometer SHIPTRAP at GSI, actions [110]. Deduced shell effects revealed that the values elements even above Z = 100 are accessible. decreased from about − 0.5 MeV for uranium continuously After SHIPTRAP has achieved mass measurements down to −5 MeV for element 107 and 109. These shell ef- of about 60 rare isotopes contributing to nuclear astro- fects turned out to be so strong that the produced nuclei physics [28], in 2008, high-precision mass measurements decayed by α emission and not by spontaneous fission. This of the nobelium isotopes 252−254No have been performed. was even the case for the even element isotopes, where fis- A cyclotron resonance of 253No is shown in Fig. 15.These sion hindrance did not prolong the partial fission half-life. experiments are the first direct mass measurements in the According to the calculations, the region of stability region above uranium. for deformed nuclei should extend up to about A = 280 Previously, the masses of 252,254No were determined from 253 in north-east direction, see Fig. 1. These predictions were Qα measurements, whereas the No mass was only known proven experimentally by the decay chain starting from from extrapolations of systematic trends [112]. The experi- the isotope 277Cn. This chain is plotted in Fig. 14 in the ments were very challenging due to the low production rates. landscape of shell-correction energies. In the upper part Since 253No has a production cross-section of about 1 μb the measured half-lives are drawn. Remarkable is the large in the reaction 48Ca + 207Pb → 253No + 2n at 4.55 × A MeV, Cold fusion 419

heavy element region will be measured, which then serve as anchor points for determining masses of heavier nuclei via known α-decay chains.

7. Properties of cold-fusion reactions The main features which determine the fusion process of heavy ions are (1) the fusion barrier, and the related beam energy, and excitation energy, (2) the ratio of surface tension vs. Coulomb repulsion which determines the fusion prob- ability and which strongly depends on the asymmetry of

the reaction partners (the product Z1 Z2 at fixed Z1 + Z2), (3) the impact parameter (centrality of collision) and related angular momentum, and (4) the ratio of neutron evapora- tion and of γ emission vs. the fission of the compound nucleus. Fig. 15. Cyclotron resonance of 253No2+ . The line represents a fit of the In fusion reactions towards SHEs, the product Z1 Z2 theoretical line shape to the experimental data points. The figure was reaches extremely large and the fission barrier extremely taken from [29]. small values. In addition, the fission barrier itself is fragile, because it is solely built up from shell effects (see contri- bution by A. Sobiczewski). For these reasons, the fusion of less than one ion per second was entering the SHIPTRAP SHEs is hampered twofold: (1) in the entrance channel by stopping cell. Mainly doubly charged nobelium ions were a high probability for re-separation, and (2) in the exit chan- extracted from the stopping cell due to its high cleanliness, nel by a high probability for fission. In contrast, the fusion the low ionization rate by the incident ion beam, and an ex- of lighter elements occurs due to the contracting effect of the traction time of a few milliseconds. The new, directly meas- surface tension and the evaporation of neutrons instead of ured nobelium masses provide reference points in this mass fission. region and can be used to accurately determine the masses of Cross-section data obtained in reactions for production of heavier nuclei up to darmstadtium (Z = 110) using α-decay heavy elements are plotted in Fig. 16.The1n,2n,and3n links. channels in cold fusion reactions with 208Pb and 209Bi targets It is planned to increase the efficiency of the stopping cell result from low excitation energies of about 10–15, 15–25, so that stepwise the masses of more and more nuclei in the and 25–35 MeV, respectively, whereas hot fusion reactions

Fig. 16. Measured cross-sections for fusion reactions with 208Pb and 209Bi targets and evaporation of one (a), two (b), and three (c) neutrons and for fusion reactions with actinide targets and evaporation of three (d), four (e), and five (f) neutrons. Values (N − Z)/2 characterize the neutron excess of the projectile. The straight lines are fits to data points drawn to guide the eye. 420 S. Hofmann with actinide targets yield excitation energies of 35–55 MeV with the 3n and 5n channels being particularly interesting. The lowest cross-section value was measured to be 31 fb for the production of element 113 in cold fusion [42, 44]. This is the extreme limit presently set by experimental con- straints. Considering the already long irradiation time of ≈ 2 weeks to reach a cross-section of 1 pb, it seems difficult to perform systematic studies at this cross-section level or even below. Further improvement of the experimental con- ditions is mandatory. Note in this context that the experi- mental sensitivity increased by three orders of magnitude since the 1982–83 LBNL–GSI search experiment for elem- ent 116 [113] using a hot fusion reaction, see Fig. 16e. The cross-sections for elements up to 113 decrease by a factor of 4 per element in the case of cold fusion (1n chan- nel) and those for elements lighter than 110 by a factor of 10 in the case of hot fusion (4n channel). Theoretical considerations and studies, see, e.g., Refs. [15, 39, 114–119] suggest that the steep decrease of cross-sections for cold fusion reactions with increasing Z may be strongly linked to increasing re-separation probabil- ity at high values of Z1 Z2 while hot fusion cross-sections mainly drop because of strong fission losses at high exci- tation energies and decreasing fission barrier already in the ground-state. The improved cross-sections presently available clearly reveal odd–even effects in the case of 1n cold fusion data, Fig. 16a and Table 1. The decrease of cross-sections for syn- thesis of the odd elements 105, 107, and 109 using 209Bi targets is about a factor of ten compared to the cross-sections of the next lighter even elements 104, 106, and 108 pro- duced with 208Pb targets and, in both cases, with beams of 50Ti, 54Cr, and 58Fe, respectively. Similar is the difference between the even elements from 104 to 110 when the pro- jectile is changed from 50Ti to 54Cr to 58Fe and to 62,64Ni. In addition, the combination of odd elements projectiles 51V, 55Mn, 59Co, and 65Cu with targets of 208Pb for synthesis of the odd elements 105, 107, 109, and 111 results in similar cross-sections as in reactions with even element projectiles and 209Bi targets (see Table 1). This comparison shows that the presence of an odd pro- ton, independent whether it belongs to the projectile or to the target nucleus, reduces the cross-section by an amount which is similar as the increase of the fissility (probabil- ity for re-separation) when the projectile is changed from Z = 22 to 24 etc. The interesting question arises if in reac- tions with odd element projectiles and odd elements targets Fig. 17. Measured excitation functions of even elements from ruther- fordium to element 112 produced in reactions with 208Pb targets and the two odd protons will couple in an early stage of the reac- beams from 50Ti to 70Zn. The data were measured in experiments at tion or if fusion will be reduced twofold. Experimental data SHIP. For comparison the excitation function for synthesis of element from reactions with odd element projectiles and targets are 114 in the reaction 48Ca + 244 Pu is plotted in the bottom panel [107]. not yet available. The arrows mark the energy at reaching a contact configuration using The low cross-section value for element 113 using a bis- themodelbyBass[120]. muth target seems to confirm the systematic trend. However, it also suggests that synthesis of element 114 in the cold opposite trend. The cross-sections increase again and reach fusion reaction 76Ge + 208Pb could have a comparable and values of about 1–5 pb at element 114 and 116 for both 3n possibly even higher cross-section than synthesis of elem- and 4n evaporation channels [107], Fig. 15d,e. Even higher 70 + 209 . +7.4 . +3.9 ent 113 in the reaction Zn Bi due to the higher neutron values of (8 0−4.5)pband(98−3.1)pbweremeasuredinthe excess of the projectile. reaction 48Ca + 244Pu → 292114∗ for the 3n and 4n channel, Extremely small cross-section values result from an ex- respectively, in [60]. Only cross-section limits at relatively trapolating of the hot fusion data into the region of element high level exist for element 116 and 118 in the case of cold 114 and beyond. However, the experimental data reveal an fusion, Fig. 16a. Cold fusion 421

A number of excitation functions were measured for the E∗ = 19.7 MeV for the reaction 58Fe + 208Pb → 266Hs is al- synthesis of elements from nobelium to darmstadtium using ready 4.5pb. cold fusion reactions based on lead and bismuth targets [15]. For comparison, we show a 4n excitation function meas- For the even elements these data are shown together with the ured in the hot fusion reaction 48Ca + 244Pu → 292114 [107] two data points measured for the compound nucleus 278112 in the bottom panel of Fig. 17. Obviously significant differ- in Fig. 17. The maximum evaporation residue cross-section ences exist. The element 114 excitation function is located (1n channel) was measured at beam energies well below completely above the Bass contact configuration which was a one dimensional fusion barrier [120]. calculated for a mean radius of the deformed target nucleus. It should be noted that for the 1n channel of the reac- In addition, the curve for the hot fusion reaction having tion 50Ti + 208Pb, a significantly higher cross-section was a width of 10.6 MeV (FWHM) is significantly broader than measured at LBNL than at GSI, whereas the cross-sections the width of 4.6 MeV as measured for 265Hs. And most im- of the 2n channel were in good agreement. The maximum portant, cross-section values of 5 pb are measured at an ex- ( +10) values of the excitation function are 43−8 nb [45]and citation energy where rather small values are expected in the (15.6 ± 1.9) nb for the 1n channel and (15.7 ± 0.2) nb [45] case of cold fusion. and (18.3 ± 0.9) nb for the 2n channel. The higher value It was pointed out in the literature [126–128]thatclosed for the 1n channel measured at LBNL is drawn in Fig. 16a, shell projectile and target nuclei are favorable synthesiz- whereas the excitation functions of cold fusion in Fig. 17 ing SHEs. The reason is not only a low (negative) reaction show the GSI SHIP data. Q-value and thus a low excitation energy, but also that fu- The lower value of the 1n channel measured at SHIP is sion of such systems is connected with a minimum of energy very likely due to losses of reaction products which are pro- dissipation. The fusion path proceeds along cold fusion val- duced in short living isomeric states. If isomers decay by leys, where the reaction partners maintain kinetic energy up conversion after the nuclei have passed the charge equili- to the closest possible distance. In the case of cold fusion bration foil, their ionic charge state could be significantly with spherical targets, the maximum fusion yield is obtained increased. If the isomers decay during the flight time of at projectile energies just high enough so that projectile and 1.7 μs up to the last dipole magnet of SHIP, the transmission target nucleus come to rest when just the outer orbits are in for these nuclei is considerably reduced. The time-of-flight contact. The configuration at this point is plotted in Fig. 18. from the target to the charge equilibration foil mounted at From there on, the fusion process occurs well ordered along a distance of 12.5 cm is 22 ns. A reasonable explanation of paths of minimum dissipation of energy. Empty orbits above the discrepancy of the cross-sections of 257Rf is that about the closed shell nucleus 208Pb favour a transfer of nucle- 60% of 257Rf is produced via isomeric states with lifetimes ons from the projectile to the target and thus initiate the in the range from 0.02 to 2 μs. An indication that a high-spin fusion process. An adequate theoretical description of this isomer is populated, is given by the fact that the whole 1n process is given by the application of the two-centre shell excitation function measured at LBNL is shifted to higher model [129–132]. energy values relative to the SHIP data shown in Fig. 17 (see At the optimum beam energy, projectile and target are Fig. 2 in [45]). This assumption is supported by results of re- just reaching the contact configuration in a central colli- cent spectroscopy studies, where a number of one and multi sion. The relatively simple fusion barrier based on the Bass quasiparticle states were calculated in the energy range from model [120] is too high and a tunnelling process through 1 to 2 MeV, which could form the required isomers [121]. this barrier cannot explain the measured cross-section. Var- Already known in 257Rf is an isomer having a half-life of ious processes may result in a lowering of the fusion barrier. +42 μ (160−31) s[121, 122]. In the right range of lifetimes are iso- Among these processes, transfer of nucleons and an exci- mers known in the isotones 251Cf (38 ns, 1.3 μs) [20]and tation of vibrational degrees of freedom are most import- 253Fm (0.5 μs) [123]. It is likely that similar isomers exist ant [124, 125, 133]. also in 257Rf. On a first glance, the situation seems to be different in We take the opportunity to report about another observa- the case of hot fusion. The maximum of the excitation func- tion regarding the 2n channel, the fissioning nucleus 256Rf. tion is located at the higher energy side of the value needed In an experiment at SHIP the distance between target and to reach the contact configuration according to the Bass charge equilibration foil was reduced gradually. Up to the model [120], see Fig. 17. However, taking into account the closest distance of 4 mm no change of the fission rate of deformation of the target nucleus and considering fusion at 256Rf was measured in the focal plane of SHIP. However, near equatorial orientation, it is evident that the projectile when the foil was removed, the rate of fission events after and target nuclei come to rest when just the outer orbits are SHIP was zero. We therefore conclude that the main produc- in contact. This distance corresponds to the energy, where tion path of 256Rf is via isomeric states with lifetime in the the maximum yield is measured. For an accurate calculation, range from 1 to 100 ps. the relatively large hexadecapole deformation of the target The data shown in Figs. 16 and 17 reveal that the 2n nucleus has to be considered, which results in a closest ap- channel in cold fusion decreases more rapidly up to the syn- proach slightly off a central equatorial collision. Also in this thesis of element hassium than the 1n channel. Although no case, the empty orbits in the near equatorial plane of the pro- data were measured for elements heavier than Z = 104 in the late deformed target nucleus favor the transfer of nucleons. domain of the 3n and 4n channels at E∗ = 35 to 50 MeV, it Theoretical investigations taking into account the deforma- seems likely that the cross-sections are considerably smaller tion of the reaction partners are presented in [134–137]. than 1 pb for elements above seaborgium at these excita- At low projectile energies, where a contact configura- tion energies. The value measured for the 2n channel at tion at zero kinetic energy in the centre of mass system is 422 S. Hofmann

Fig. 19. Excitation energy of compound nuclei produced in fusion reactions at beam energies just high enough for reaching a contact configuration. For explanation of the symbols see text.

Although a realistic fusion scenario is not yet available and not all details of the fusion process resulting in SHE’s are completely understood, we already can investigate vari- ous reactions concerning the Q-value, including radioactive projectiles. For the interesting cases, the projectiles are not extremely neutron rich and their binding energy is known. The binding energy of SHE’s from various models is in suf- ficient agreement. Therefore, the uncertainty of the Q-value is estimated to be a few MeV only. Fig. 18. Energy-against-distance diagram for the reaction of an almost spherical 64Ni projectile with a spherical 208Pb target nucleus resulting In a first step, we consider the excitation energy of vari- 208 238 in the deformed fusion product 271Ds after emission of one neutron. At ous reactions using targets of Pb and U and preferably the centre-of-mass energy of 236.2 MeV, the maximum cross-section stable projectiles. The results are plotted in Fig. 19.The was measured. In the top panel, the reaction partners are represented lines connect projectiles with the same isospin. Larger sym- by their nuclear potentials (Woods–Saxon) at the contact configuration bols denote stable, the smaller dots radioactive projectiles. where the initial kinetic energy is exhausted by the Coulomb potential. At this configuration projectile and target nuclei are 14 fm apart from The excitation energy was calculated for beam energies so each other. This distance is 2 fm larger than the Bass contact configura- that a contact configuration is reached according to the Bass tion [120], where the mean radii of projectile and target nucleus are in model [120]. Data for which maximum cross-sections were contact. In the bottom panel, the outermost proton orbitals are shown 64 measured (marked by ‘Exp.’) are even below that energy, at the contact point. For the projectile Ni, an occupied 1 f7/2 orbit 208 halfway to the limiting (for the 1n channel) one neutron is drawn, and for the target Pb an empty 1h9/2 orbit. The protons circulate in a plane perpendicular to the drawing. The Coulomb repul- binding energy. sion, and thus the probability for separation, is reduced by the transfer Striking is the decrease of the excitation energy below of protons. In this model, the fusion is initiated by transfer (see also the 1n-binding energy at about element 114. For that rea- Refs. [124, 125]). son the reaction 82Se + 208Pb → 290116 was studied in [39]at excitation energies from 0 to 10 MeV in order to search for reached only in polar collisions, the heavy nuclei do not a possible radiative capture channel. The result was nega- fuse. The reasons are, firstly, that at the elongated configura- tive, cross-section limits of 5 pb were reached at four differ- tion re-separation is enhanced due to the unfavourable ratio ent beam energies. of Coulomb repulsion and surface attraction and, secondly, In the case of element 118 synthesis the reaction 86Kr + due to the occupied Nilsson levels originating from orbits 208Pb even gets endothermic. A surplus of kinetic energy is of high spin in the target nucleus, which hinders a transfer needed beyond the minimum necessary for reaching a con- of nucleons from the projectile to the target. Recent experi- tact configuration in order to evaporate one neutron. This mental work is aiming to study the transition from fusion feature was interpreted as unshielded fusion reaction [153]. with deformed actinide target nuclei and light projectiles, The extraordinary low excitation energy for the reaction e.g. 12Cor16O, where polar collisions at low beam energy using a 48Ca beam and a 238U target is obvious from the result in enhanced sub-barrier fusion, to heavier projectiles upper bunch of curves. Also with the uranium target the like 48Ca just described, as function of the projectile mass reaction gets colder with increasing element number. The and charge [138]. Triggered by the recent experimental suc- excitation energy reaches values close to the 1n-binding en- cess of heavy element synthesis, a number of theoretical ergy near element 124 using a 76Ge beam. The hot fusion studies were performed or are in progress, aiming to obtain reactions using actinide targets have excitation energies of a detailed quantitative understanding of the reaction pro- about 40 MeV for synthesis of elements up to about 110. cesses involved in heavy element synthesis [117, 118, 133, They change gradually into cold fusion for the heavier elem- 137, 139–152]. ents. This could be a promising aspect for the synthesis of Cold fusion 423

SHE’s near Z = 126 (which is one of the predicted closed region of the predicted spherical SHEs has finally been proton shells) using actinide targets. reached and the exploration of the island has started and In the case of 208Pb as a target, a surplus of 30 MeV of ki- can be performed even on a relatively high cross-section netic energy is necessary at these high element numbers in level. order to obtain an excitation energy high enough for emis- The progress towards the exploration of the island of sion of one neutron. However, such high kinetic energies spherical SHEs is difficult to predict. Hot fusion based on will probably lead to dissipation of energy already in early actinide targets and 48Ca beams terminate at element 118, stages of the fusion process. An increase of deep inelastic because targets beyond can be produced only processes will result. with tremendous costs and efforts. How heavier beams like Using more neutron rich radioactive projectiles (higher 50Ti, 54Cr, etc. will affect the fusion cross-section is subject values of isospin as shown in Fig. 19) does not, in general, of experiments planned for the near future. However, these result in lower excitation energies at the fusion barrier. In heavier beams are mandatory for exploration of the island the contrary, the excitation energy rises again (see Fig. 7 of SHEs into the north-east direction. Strong shell effects, if in [154]). An open question is how the stronger binding en- they exist at Z = 120 or 126, could positively influence the ergy of the protons and, vice versa, the low binding energy reaction cross-sections. of the neutrons will influence the fusion process, especially Also worth to study is the transition from hot to cold fu- the first stages which are governed by transfer of nucle- sion expected to occur with actinide targets and the most ons. A quantitative estimate of fusion cross-sections with strongly bound and for synthesis of radioactive beams was performed in [155]. Both cold and elements at Z = 126. Losses by re-separation in the entrance hot fusion reactions were considered. It was shown that for channel due to the higher value of Z1 Z2 could be compen- many of the cold fusion reactions, the cross-sections with ra- sated by a smaller probability of fission of the compound dioactive beams of elements between calcium and rubidium nucleus. and of isotopes having 1 to 9 neutrons more than the heav- No technical limitations exist for exploration of the is- iest stable isotope could be by about a factor of 20 higher land of SHEs in west and north-west direction. Sufficient than the cross-sections with stable beams. If these estimates neutron-deficient projectile isotopes are available. However, would turn out to be correct, new neutron rich isotopes of due to Q-value effects the excitation energy of the com- elements between nobelium and element 120 could be pro- pound nucleus at the barrier will increase. duced. These isotopes are expected to have longer half-lives Excitation functions have to be measured, which provide than the presently known isotopes, which make them es- information on how fast the cross-section decreases with in- pecially interesting for chemistry, atomic physics, and trap creasing energy due to fission of the compound nucleus, and experiments. how fast cross-sections decrease on the low energy side due to the fusion barrier and re-separation of projectile and tar- get nuclei. From both slopes, information about the shape 8. Summary and outlook of the fission and the fusion barriers can be obtained. The study of transfer products may open a direct access to the In the middle of the 1970s heavy ion accelerators became first steps of the processes resulting in fusion. Due to the low available, which allowed to accelerate ions as heavy as iso- beam energy, the reactions occur in central collisions and the topes of uranium with energies high enough for fusion reac- reaction partners re-separate in and opposite to the beam di- tions. Without technical restrictions, the optimum combina- rection. Therefore velocity separators like SHIP are an ideal tion of projectile and target for synthesis of new heavy nu- tool to study these processes [156]. clei in fusion reactions could be chosen. Cold fusion based Most interesting but also most difficult will be the syn- on unproblematic targets of lead or bismuth and projectiles thesis of more neutron rich isotopes located in south-east of neutron rich isotopes of elements from calcium to direction of the island of SHEs. There, the longest half-lives were the choice at that time. New elements from bohrium are expected. Reactions using radioactive beams and transfer (Z = 107) up to copernicium (112) were produced in cold reactions are options to be studied in the future. fusion reactions. Using transfer reactions as suggested in [157], low beam However, the experimental work of the last three decades energies and hence observation in zero degree direction are has shown that cross-sections in cold fusion reactions de- mandatory, in order to produce the fragments at the lowest crease continuously up to the synthesis of element 113, possible excitation energy and thus reducing fission of the where a present technical limit is reached at a production fragments. Systems as heavy as 238U + 248Cm are technically cross-section of 31 fb. possible and should be investigated with modern separation During the last decades, considerable technical im- and detection methods. These methods will also allow for provements were made. Available are now beam inten- measuring contact times of dinuclear systems as function of sities of about 1 p μA, separators with efficiencies near the fissility parameter Z1 Z2 by making use of the kinematics 50%, detectors having efficiencies of 100%, and electronic of the reaction products at the moment of re-separation after devices for measuring correlated events of decay chains rotation of the system. within a range of lifetimes from microseconds to days. At a high enough cross-section, the measurements can be This progress allowed for a re-investigation of hot fu- complemented by in-beam γ-ray spectroscopy using recoil- sion reactions based on actinide targets, which resulted in decay tagging methods in order to study the influence of the discovery of new elements from 113 to 118. Decay angular momentum on the fusion and survival probabil- data and cross-sections reveal that in these reactions the ity [158] (see also contribution by R. Herzberg). 424 S. Hofmann

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