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The reaction 54Cr$ + $248Cm was investigated at the velocity filter SHIP at GSI, Darmstadt, with the intention to study production and decay properties of isotopes of element 120. Three correlated signals were measured, which occurred within a period of 279ms. The heights of the signals correspond with the expectations for a decay sequence starting with an isotope of element 120. However, a complete decay chain cannot be established, since a signal from the implantation of the evaporation residue cannot be identified unambiguously. Measured properties of the event chain are discussed in detail. The result is compared with theoretical predictions. Previously measured decay properties of even element super-heavy nuclei were compiled in order to find arguments for an assignment from the systematics of experimental data. In the course of this review, a few tentatively assigned data could be corrected. New interpretations are given for results which could not be assigned definitely in previous studies. The discussion revealed that the cross-section for production of element 120 could be high enough so that a successful experiment seems possible with presently available techniques. However, a continuation of the experiment at SHIP for a necessary confirmation of the results obtained in a relatively short irradiation of five weeks is not possible at GSI presently. Therefore, we decided to publish the results of the measurement and of the review as they exist now. In the summary and outlook section we also present concepts for the continuation of research in the field of super-heavy nuclei.

The isotope 261Sg and its decay product 257Rf were produced in the nuclear reaction 208Pb(54Cr, n)261Sg . The radioactive decay of these isotopes was investigated by means of $ \alpha$ , $ \alpha$ - $ \gamma$ and/or $ \alpha$ -X-ray spectroscopy. Improved spectroscopic data concerning $ \alpha$ energies, $ \gamma$ transitions, tentative spin and parity assignments of low-lying Nilsson single-neutron orbitals are presented for these nuclei.

In a series of experiments performed at the velocity filter SHIP, new or improved decay data of neutron-deficient isotopes of elements from mendelevium (Z = 101) to hassium (Z = 108) were obtained. In particular, evidence for $ \alpha$ -decay or electron capture from isomeric states in 265Hs and 258Db was found.

The radioactive decay of the isotopes 254-256Lr and their daughter products was investigated by means of α , prompt α -γ and delayed conversion electron-γ coincidence spectroscopy. The isotopes were produced using the reaction 48Ca + 209Bi .

A K isomer with a half-life of (110±10) ms at an excitation energy of 1254keV has been identified in the nucleus 252No . The isomer is interpreted as a two-quasineutron state with K π = 8- . The isotope 252No was produced in the fusion-evaporation reaction 206Pb(48Ca, 2n)252No . The experiment has been performed at the velocity filter SHIP of GSI Darmstadt.

The decay of the isotopes 255Rf, 251No and 247Fm produced in the reactions $\mathrm{{}^{207}Pb\ensuremath({}^{50}Ti,2n){}^{255}Rf\ {}^{\alpha}_{\rightarrow}\ {}^{251}No\ \ensuremath{}^{\alpha}_{\rightarrow}\ {}^{247}Fm}$ , and $\mathrm{{}^{206}Pb\ensuremath({}^{48}Ca,3n){}^{251}No\ \ensuremath{}^{\alpha}_{\rightarrow}\ {}^{247}Fm}$ was investigated by means of α-γ spectroscopy. Previously observed γ transitions in coincidence with α decays of 255Rf were confirmed, their energies and line intensities were measured more precisely, and their multipolarities were determined as E1. In 251No a new isomeric state at E * > 1700keV with a half-life of ≈ 2μs was identified. The decay of 247Fm was measured more precisely. A partial level scheme of the daughter nucleus 243Cf could be established.

The reaction 30Si + 238U → 268Sg* was studied at beam energies close to the Coulomb barrier. At a center-of-mass energy of E c.m. = 144.0MeV for reactions at half thickness of the target we measured three decay chains of 263Sg produced by evaporation of five neutrons. The cross-section was ( 67+67 -37) pb. At E c.m. = 133.0MeV we measured three spontaneously fissioning nuclei which we assigned to the isotope 264Sg. The production cross-section was ( 10+10 -6) pb and a half-life of ( 120+126 -44) ms was determined. This half-life is a factor of twenty shorter than theoretical predictions. At E c.m. = 128.0MeV an upper cross-section limit of 15pb was measured. The cross-section data reveal a strong influence of the orientation of the deformed target nucleus on the production yield. Compared to excitation functions measured for the lighter system 16O + 238U → 254Fm*, a reduction of the fusion probability was observed at low beam energies indicating increasing competition from quasifission processes.

The decay of 255No was investigated by means of α-γ spectroscopy. The isotope was produced in the reactions 208Pb(48Ca, n)255No, \(\mathrm{{}^{209}Bi({}^{48}Ca,2n){}^{255}Lr \ {}^{\ensuremath \mathit{EC}}_{\rightarrow}\ {}^{255}No}\), and $238U(22Ne, 5n)255No$. Levels of the daughter nucleus 251Fm were assigned by α-γ coincidence measurements and on the basis of systematics. Level energies were determined precisely using measured γ-rays. The results are compared with the known level schemes of the lighter \(\ensuremath N=151\) isotones 247Cm and 249Cf as well as with data for 253No.

The 3/2- isomer in 185Pb and states above it have been populated in the α-decay of 189Po. The observed α-decay strengths to and the electromagnetic decay properties of the excited states in 185Pb have been combined with Potential Energy Surface and Particle-Plus-Rotor calculations to propose configuration assignments. It is suggested that the α-decaying isomer of 189Po is of prolate origin and that the prolate configuration becomes very low in energy in 185Pb.

The upgraded separator VASSILISSA was used to confirm results of previous experiments on the synthesis of the heavy isotopes of the element 112 obtained in complete-fusion reactions of 48Ca and 238U. An additional reason was the non-observation of any events in the same reaction studied with the BGS separator at Berkeley. Limits of $280 \leq A \leq 286$ for the atomic mass number of the observed spontaneously fissioning isotope were measured. The obtained results on the decay mode, half-life and production cross-section are in agreement with those obtained in our first experiment.

Detailed fine-structure $\alpha$ -decay studies of 188,190Bi were performed using the complete-fusion reactions of 50,52Cr ions with a 142Nd target at the velocity filter SHIP. The evaporation residues were separated in-flight and subsequently identified on the basis of recoil- $\alpha$ , recoil- $\alpha$ - $\gamma$ /X-ray and excitation function measurements. Improved data on the $\alpha$ -decay of 188,190Bi were obtained and a number of new low-lying excited states in the daughters 184,186Tl were observed. An intruder, presumably 10- state was identified in 184Tl, extending the systematics of such states in the odd-odd Tl nuclei beyond the neutron mid-shell at N = 104. Some technical aspects of experiment at recoil separators are also discussed.

Heavy-ion fusion reactions 48Ca + 204Pb and 44Ca + 208Pb leading to the same compound nucleus 252No* were run in attempts to produce new neutron-deficient spontaneous-fission isotopes of 249,250No using the electrostatic separator VASSILISSA. Production cross-sections for the spontaneous-fission activities with the half-lives 5.6 and 54 μs observed in these reactions are compared with the measured ones for the well-known isotopes of 251 - 255No formed in the heavy-ion fusion reactions 48Ca + 206Pb and 48Ca + 208Pb. The obtained excitation functions for the reaction products formed after the evaporation of 1-4 neutrons from the corresponding compound No nuclei have been compared with similar data obtained earlier and results of statistical model calculations.

The radioactive decay of 217Pa was investigated by means of α-γ-spectroscopy. Fine structure in the ground-state α-decay was established. Ambiguities in the fine structure of the α-decay of the previously known isomeric state could be clarified by α-γ-coincidence measurements. A previously unknown α-transition of Eα = (8306 ± 5) keV was detected and identified by means of delayed α-α- and α-γ-γ-coincidence measurements. A second isomeric state decaying by α-emission was not observed. The quality of the previously reported data of the α-decay fine structure of 217Th was improved.

New microsecond isomers in the neutron-deficient isotopes 189g, 190Bi have been identified after in-flight separation by the velocity filter SHIP. The evaporation residues were identified on the basis of delayed recoil-γ/X-ray, recoil-γ/X-ray-α and excitation function measurements. The systematics of the [ π1i 13/2]13/2+ excited states in the odd-mass Bi nuclei is discussed.

The even-even nucleus 270110 was synthesized using the reaction 64Ni + 207Pb. A total of eight α-decay chains was measured during an irradiation time of seven days. Decay data were obtained for the ground-state and a high-spin K isomer. The new nuclei 266Hs and 262Sg were identified as daughter products after α-decay. Spontaneous fission of 262Sg terminates the decay chain. The measured data are in agreement with calculations using the macroscopic-microscopic model and with self-consistent HFB calculations with Skyrme-Sly4 interaction.

Neutron deficient isotopes of elements Z = 86–92 have been produced by heavy-ion fusion reactions 12C + 208Pb, 209Bi, 22Ne + 208Pb, 51V + 170Er, and 50Ti + 170Er. The evaporation residues were investigated by means of α- and α-γ-spectroscopy after in-flight separation from the projectile beam by the velocity filter SHIP and implantation into a 16-strip position-sensitive Si-detector. New or improved decay data for 225,226U, 216,217m,218Pa, 215,216,217Th, 214,215,216,216mAc, 214Ra and 213Rn have been obtained.

New neutron-deficient isotopes 188,189Po have been produced in the complete fusion reaction of 52Cr ions with a 142Nd target at the velocity filter SHIP. The evaporation residues were separated in-flight and subsequently identified on the basis of α-γ and α-conversion electron coincidence measurements and of α-α position and time correlations. In 189Po a ground state to ground state α decay with Eα1= 7540(20) keV, T1/2= 5(1) ms and two fine structure α-decays at Eα2= 7264(15) keV and Eα3= 7316(15) keV have been observed. In 188Po (T1/2= 400+200 −150μs) a ground state to ground state α decay at Eα= 7915(25) keV and a fine structure α decay at Eα= 7350(40) keV have been found. Improved data on the α-decay of 189Bi were obtained.

The stability of heavy nuclides, which tend to decay by α-emission and spontaneous fission, is determined by the structural properties of nuclear matter. Nuclear binding energies and lifetimes increase markedly in the vicinity of closed shells of neutrons or protons (nucleons), corresponding to ‘magic’ numbers of nucleons; these give rise to the most stable (spherical) nuclear shapes in the ground state. For example, with a proton number of Z = 82 and a neutron number of N = 126, the nucleus 208Pb is ‘doubly-magic’ and also exceptionally stable. The next closed neutron shell is expected at N = 184, leading to the prediction of an ‘island of stability’ of superheavy nuclei, for a broad range of isotopes with Z = 104 to 120 (refs 1, 2). The heaviest known nuclei have lifetimes of less than a millisecond, but nuclei near the top of the island of stability are predicted to exist for many years. (In contrast, nuclear matter consisting of about 300 nucleons with no shell structure would undergo fission within about 10−20 seconds.) Calculations3,4,5 indicate that nuclei with N > 168 should already benefit from the stabilizing influence of the closed shell at N = 184. Here we report the synthesis of an isotope containing 114 protons and 173 neutrons, through fusion of intense beams of 48Ca ions with 242Pu targets. The isotope decays by α-emission with a half-life of about five seconds, providing experimental confirmation of the island of stability.

The reaction 48Ca+238U was investigated at the recoil separator VASSILISSA at the Flerov Laboratory of Nuclear Reactions, JINR Dubna, in attempts to synthesize new isotopes of element 112. The experiments were performed at two beam energies resulting in excitation energies of the compound nucleus of 33 and 39 MeV. The collected beam dose was 3.5 × 1018 and 2.2 × 1018, respectively. Two spontaneous fission events were observed at the lower beam energy, which tentatively were assigned to the new neutron rich isotope 283112 produced in the reaction 238U(48Ca,3n)283112. The measured cross-section is (5.0+6.3−3.2) pb and the half–life is (81+147−32) s. No event was observed at the higher beam energy resulting in the upper cross–section limit of 7.3 pb.

Spontaneous fission and α-decay of253−257104 and 258106 were investigated in irradiations of 204,206,208Pb with 50Ti and 209Bi with 51V, respectively. New spontaneous fission activities were identified and assigned to 253104, 254104, and 258106. The half-lives were measured as T1/2 = (48 −10 +14 ) μs for 253104, T1/2 = (23 ± 3) μs for 254104, and T1/2 = (2.9 −0.7 +1.3 ) ms for 258106. No indication for α-decay of any of these isotopes was found. For the α-decay branching ratios bα limits corresponding to bα ≤ 0.1 for 253104, bα ≤ 0.015 for 254104, and bα ≤0.2 for 258106 were obtained. These results prove a reduced fission probability for nuclei with neutron numbers N=152 up to Z=106 and a steep decrease of the fission half-lives for neutron numbers N < 152 up to element 104. α-decay data of 257104 and 255104 have been improved. An isomeric state decaying by α-emission was identified in 257104 and attributed to a low lying 11/2- [725] state. A small α-decay branch for the even — even nucleus 256104, indicated in an earlier experiment at SHIP, was confirmed, allowing a better founded extrapolation of experimental masses for even — even nuclei up to 264Hs (Z=108), the heaviest even — even nucleus identified so far.

Velocity distributions and production cross sections of evaporation residues have been measured in the reaction20Ne+208Pb at projectile energies of 8.6, 11.4, 14.9 A.MeV. Essential deviations from statistical model of deexcitation have been observed. Monte Carlo simulations involving emission of non-equilibrium particles have been used in order to reproduce experimental velocity, charge and mass distributions of evaporation residues and to estimate indirectly multiplicities of pre-equilibrium particles.

In an experiment carried out to identify element 110, we have observed anα-decay chain, that can be unambiguously assigned to269110. In a scries of preexperiments the excitation functions of the fusion reactions50Ti +208Pb→258104* and58Fe +208Pb→266108* were measured with high precision in order to get the optimum projectile energies for the production of these heavy elements. The cross-section maxima of the 1n evaporation channels were observed at excitation energies of 15.6 MeV and 13.4 MeV, respectively. These data result in an optimum excitation energy of 12.3 MeV of the compound nucleus for the production of269110 in the reaction62Ni +208Pb→269110 + 1n. In irradiations at the corresponding beam energy of 311 MeV we have observed a decay chain of 4 subsequent a decays. This can be assigned to the isotope with the mass number 269 of the element 110 on the basis of delayed α-α coincidences. The accurately measured decay data of the daughter isotopes of the elements 108 to 102, obtained in the previous experiments, were used. The isotope269110 decays with a hair-life of (270 -120 +1300 ) μs by emission of (11.132±0.020) MeV alpha particles. The production cross-section is (3.3 -2.7 +6.2 ) pb.

The synthesis of element 116 in fusion-evaporation reactions of a 48Ca beam with radioactive 248Cm targets was studied at the velocity filter SHIP of GSI in Darmstadt. At excitation energies of the compound nuclei of 40.9MeV, four decay chains were measured, which were assigned to the isotope 292116, and one chain, which was assigned to 293116. Measured cross-sections of (3.4−1.6+2.7) pb and (0.9−0.7+2.1), respectively, and decay data of the chains agree with data measured previously at the Flerov Laboratory of Nuclear Reactions in Dubna. As a new result, one α-decay chain was measured, which terminates after four α decays by spontaneous fission. The α energies of the second-to-fourth decay are considerably higher than those measured for the α decays of 289114, 285Cn, and 281Ds and the spontaneous fission half-life is significantly longer than that of 277Hs measured in previous experiments. A possible assignment is discussed in the frame of excited quasiparticle states of nuclei populated in the decay chain from 293116. Also other possible assignments were considered and are discussed. At an excitation energy of 45.0MeV no events were observed resulting in a one-event cross-section limit of 1.6 pb. The technical aspects related with the use of radioactive target material at SHIP are described in detail. The experience gained in this experiment will serve as a basis for future experiments aiming to study still heavier elements at the velocity filter SHIP.

Nuclear structure and decay of the isotope 253No and its decay products 249Fm and 253Md were investigated by means of $ \alpha$ - $ \gamma$ spectroscopy. Besides the established strong $ \gamma$ transitions from the $\ensuremath 9/2^{-}[734]$ Nilsson level in 249Fm , populated predominantly by the $ \alpha$ decay of 253No , into the ground-state (gs) rotational band, a couple of weaker $ \gamma$ lines (58.3, 129.2, 209.3 and 669.5keV) were observed and placed into the 249Fm level scheme. The transition from the $\ensuremath 7/2^{-}$ level in 249Es , populated by the $ \alpha$ decay of 253Md , into the $\ensuremath 9/2^{+}$ member of the gs rotational band, so far established for other odd-mass Es isotopes, was observed clearly. GEANT4 simulations were performed to investigate the influence of energy summing between $ \alpha$ particles and conversion electrons (CE) on the shape of the $ \alpha$ spectra at different implantation energies, leading to evidence for a weak $ \alpha$ decay branch of 253No into the gs of 249Fm or the ground-state rotational band, respectively.

Velocity distributions of heavy residuesARes>Atar,ZRes>Ztar identified by means ofα spectroscopy, have been investigated at the velocity filter SHIP in reactions20Ne+208Pb at projectile energies E/A=8.6, 11.4 and 15.0 MeV/u. Besides products from complete or nearly complete fusion, characterized by velocity distributions peaking atν/νCN≅0.8–1.0, heavy residues with mean velocities of about half of the compound nucleus velocity were observed. The Z-distribution of this component was found to peak atZ=87. It is interpreted as residues from fusion of target nuclei with projectile fragments produced by nearly symmetric break-up. The experimental results were compared with predictions of theoretical models: cross sections for incomplete fusion were calculated using the sum-rule model of Wilcynski et al., while residue cross sections were calculated using the evaporation code HIVAP. A fair agreement between experimental and calculated mass distributions of heavy residues and transferred projectile fragments is achieved if an energy dissipation of ≅23% (at E/A=8.6 MeV/u) and ≅8% (at E/A=11.4 MeV/u) of the incident projectile energy is introduced. The observed peak of theZ-distribution atZ=87 is predominantly effected due to a higher fission probability of products withZ>87 during the deexcitation process and experimental limitations in the identification of products withZ≦86 by means of a spectroscopy, which cause a decrease of the observed production rates towards lowerZ.

Nuclear structure and decay of isomeric states in 253No were investigated. The isotope was produced by the reaction 48Ca + 207Pb . The excitation energy of the known single-particle isomeric state ( 5/2+[622] was measured by delayed coincidences between γ-rays and implanted evaporation residues and was placed into the level scheme. In addition, decay of a high-lying multi-quasiparticle isomer in 253No was studied using e −-γ coincidence measurements. A rotational band populated by its de-excitation was identified in 253No . A new isomeric state in 253Fm was observed and a partial-level scheme for this isotope is suggested. In addition γ transitions from excited levels are reported for 253Md .

The decay chain of 246Fm has been investigated employing the SHIP separator at GSI Darmstadt. The 246Fm nuclei were produced via the 40Ar(208Pb, 2n)246Fm fusion-evaporation reaction. Improved values of the half-life, T 1/2 = 1.54(4) s, and of the spontaneous fission branching ratio, b SF = 0.068(6) , of 246Fm were obtained. The $ \beta^{+}_{}$ /electron capture branching ratio, b EC = 0.39(3) , of 242Cf was deduced. Possible structures of high-K states in 246Fm are discussed within the framework of a model calculation based on the Woods-Saxon potential.

A detailed $ \gamma$ spectroscopic decay study of two K isomers in 254No was performed. In addition to the previously reported $ \gamma$ lines two new transitions of E = 778 , 856keV could be attributed to the decay pattern of 254m1No ( T 1/2 = 275±7 ms). The population of an excited band built up on this isomer ( $\ensuremath K^{\pi} =8^{-}$ by the decay of 254m2No ( T 1/2 = 198±13 μs) could be proven by measuring delayed $ \gamma$ - $ \gamma$ coincidences between transitions stemming from the decay of both isomeric states. The energies of the band members could be established up to $\ensuremath I^{\pi} = 15^{-}$ . A spontaneous fission branch of (2.0±1.2)×10-4 was measured for 254m1No , an upper limit of $ \le$ 1.2×10-4 was estimated for 254m2No . These values demonstrate the high stability of multi-quasiparticle configurations against spontaneous fission. Evidence for an $ \alpha$ decay branch of 254m1No in the order of 1×10-4 was found.

The radioactive decay of the isotopes 247Md , 246Md and their daughter products was investigated by means of $ \alpha$ - $ \alpha$ and $ \alpha$ - $ \gamma$ coincidence spectroscopy. The isotopes were produced using the fusion reaction 40Ar$ + $209Bi. Decay schemes are suggested for 247Md and 243Es . A new isomeric state in 246Md with a half-life of ( 4.4±0.8 s was observed. Previous data of electron-capture delayed fission of 246Md and 242Es were confirmed. The probability for this decay branch in 246Md was measured to be P ECDF > 0.10 . The probability for electron-capture delayed fission in the case of 242Es was determined to be P ECDF = 0.013+0.012 -0.007 .

The new element 112 was produced and identified unambiguously in an experiment at SHIP, GSI Darmstadt. Two decay chains of the isotope277112 were observed in irradiations of208Pb targets with70Zn projectiles of 344 MeV kinetic energy. The isotope decays by emission of α particles with a half-life of (240 −90 +430 )µs. Two different α energies of (11,649±20) keV and (11,454±20) keV were measured for the two observed decays. The cross-section measured in three weeks of irradiations is (1.0 −0.4 +1.8 ) pb.

The four new neutron deficient isotopes246Md (T1/2=(1.0±0.4) s),245Md (T1/2=(0.35 −0.16 +0.23 ) s),242Es (T1/2=(16 −4 +6 ) s) and241Es(T1/2=(8 −4 +6 ) s) were produced in fusion reactions of40Ar ions with209Bi targets at bombarding energies of E p =4.78, 4.93 and 5.12 A*MeV. The assignment was based on evaporation residue-α-α time and position correlations. A spontaneous fission activity of T1/2=(0.90±0.25) ms was observed at a projectile energy of E p =5.12 A*MeV and assigned to245Md.

The new element 111 was produced and unambiguously identified in an experiment at SHIP, GSI Darmstadt. Three nuclei of the isotope272111 were observed in irradiations of209Bi targets with64Ni projectiles of 318 MeV and 320 MeV energy. The cross-sections are (1.7 −1.4 +3.3 ) pb and (3.5 −2.3 +4.6 ) pb, respectively. The nuclei decay by a emission into the new and so far the heaviest isotopes of the elements 109 and 107 with mass numbers A=268 and A=264. Theα-decay chains were followed down to the known nuclei260105 and256Lr.

Two new neutron-deficient isotopes,213Pa and214Pa were produced in complete fusion reactions of51V-ions with170Er targets at (5.2−5.6) AMeV. The assignment was based on delayed evaporation residue -α -α time and position coincidences. Theα- decay energies of213,214Pa were measured to be Eα=(8236±20) keV and Eα=(8116±20) keV, respectively. The half-lives of213,214Pa were determined to be T1/2=(5.3 −1.6 +4.0 ms and T1/2=(17 ±3) ms, respectively.

The neutron-deficient isotopes 257,258105 were produced in the reaction 27Al+236U in 6n and 5n evaporation channels, respectively. The evaporation residues emerging from the target were separated in-flight from the projectiles and from products of different nuclear reactions by the electrostatic separator VASSILISSA [1]. The isotopes were then implanted into position-sensitive silicon detectors and identified using the α-α-correlation method. The measured production cross-section is σ(5n)=(0.45±0.20)nb at E P =154 MeV and σ(6n)=(0.075±0.055) nb at E P =163 MeV. These cross-sections are compared with data measured for the same isotopes in the more symmetrical reaction 50Ti+209Bi.