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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 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.

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.

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.

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.

In recent experiments at the velocity filter Separator for Heavy Ion reaction Products (SHIP) (GSI, Darmstadt), an extended and improved set of $\ensuremath{\alpha}$-decay data for more than 20 of the most neutron-deficient isotopes in the region from lead to thorium was obtained. The combined analysis of this newly available $\ensuremath{\alpha}$-decay data, of which the $^{186}\mathrm{Po}$ decay is reported here, allowed us for the first time to clearly show that crossing the $Z=82$ shell to higher proton numbers strongly accelerates the $\ensuremath{\alpha}$ decay. From the experimental data, the $\ensuremath{\alpha}$-particle formation probabilities are deduced following the Universal Decay Law approach. The formation probabilities are discussed in the framework of the pairing force acting among the protons and the neutrons forming the $\ensuremath{\alpha}$ particle. A striking resemblance between the phenomenological pairing gap deduced from experimental binding energies and the formation probabilities is noted. These findings support the conjecture that both the $N=126$ and $Z=82$ shell closures strongly influence the $\ensuremath{\alpha}$-formation probability.

Decay properties of the very neutron-deficient isotopes ${}^{197\char21{}199}$Fr were studied at the velocity filter Separator for Heavy Ion reaction Products (SHIP) at GSI in Darmstadt. The isotopes were produced in the $2n\char21{}4n$ evaporation channels of the fusion-evaporation reaction ${}^{60}$Ni+${}^{141}$Pr $\ensuremath{\rightarrow}$ ${}^{201}$Fr*. Improved $\ensuremath{\alpha}$-decay properties of ${}^{199}$Fr were determined and the possible existence of two $\ensuremath{\alpha}$-decaying states in this nucleus is discussed. For the isotope ${}^{198}$Fr a broad $\ensuremath{\alpha}$-decay energy distribution was detected in the range of $(7470\char21{}7930)$ keV and two $\ensuremath{\alpha}$-decaying states were observed with half-lives of 1.1(7) and 15(3) ms. The new isotope ${}^{197}$Fr was identified based on the observation of one $\ensuremath{\alpha}$-decay chain yielding ${E}_{\ensuremath{\alpha}}=7728(15)$ keV and ${T}_{1/2}=0.{6}_{\ensuremath{-}0.3}^{+3.0}$ ms. The systematics of reduced $\ensuremath{\alpha}$-decay widths are presented for neutron-deficient francium, radon, and astatine isotopes.

The reduced probabilities of the delayed fission channels P s df for 188 Bi and 196 At are determined, from experimental data, to be 3.4 × 10 −4 and 8.8 × 10 −4 , respectively. The comparison of the obtained and calculated value of P β df allows to estimate the fission barriers for 188 Pb and 196 Po.

A detailed $\ensuremath{\alpha}$-decay study of the neutron-deficient isotope $^{181}\mathrm{Pb}$ has been performed in the complete fusion reaction $^{40}\mathrm{Ca}$$+$$^{144}\mathrm{Sm}$$\ensuremath{\rightarrow}$$^{184}\mathrm{Pb}$${}^{*}$ at the velocity filter SHIP (GSI, Darmstadt). In comparison with the literature, more precise data have been deduced for the ${I}^{\ensuremath{\pi}}=(9/{2}^{\ensuremath{-}})$ ground state in this nucleus, which is presumably based on the neutron $\ensuremath{\nu}{h}_{9/2}$ spherical orbital. Improved $\ensuremath{\alpha}$-decay data were also measured for $^{180}\mathrm{Pb}$.

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 .

An isomeric state in {sup 255}Lr with a half-life of t{sub 1/2} = 1.4(1) ms and E{sub x}>720-keV has been observed for the first time using the GABRIELA setup at the focal plane of the VASSILISSA separator. Based on its K-forbiddeness, the configuration of the state is most probably formed by coupling the valence proton to a two quasiparticle neutron excitation. Possible three quasiparticle configurations are discussed.

An isomeric state in {sup 209}Ra has been observed for the first time, using the GABRIELA setup at the focal plane of VASSILISSA, to decay to the ground state of {sup 209}Ra via a cascade of 238-keV (M2) and 644-keV transitions. The half-life of the isomer has been measured to be 117(5) {mu}s and from systematics is assigned as a neutron i{sub 13/2}{sup -1} excitation.

The reaction 30Si + 238U → 268Sg* was studied at beam energies close to the Coulomb barrier. At the above barrier energy of Ec.m. = 144.0 MeV (center‐of‐mass energy at half thickness of the target), we measured three decay chains of 263Sg produced by evaporation of five neutrons. The cross‐section was (67−37+67) pb. At subbarrier energy of Ec.m. = 133.0 MeV we measured three spontaneously fissioning nuclei which we assigned to the isotope 264Sg. The half‐life of (120−44+126) ms was determined. The production cross‐section was (10−6+10) pb, which is about four orders magnitude larger than the calculation based on the one‐dimensional barrier penetration model in the fusion process, showing fusion enhancement caused by the prolate deformation of 238U. At Ec.m. = 128.0 MeV an upper cross‐section limit of 15 pb was measured. 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.

An overview of present experimental investigation of superheavy elements is given. The data are compared with theoretical descriptions. Results are reported from an experiment to confirm production of element 112 isotopes in irradiation of 238UF4 with 48Ca. One spontaneous fission event was measured, which agrees with three events of previously measured data which had been assigned to the decay of 283112. However, more experimental work is needed in order to obtain an independent and unambiguous confirmation of previous results.

The new neutron-deficient isotope $^{187}\mathrm{Po}$ has been identified in the complete fusion reaction $^{46}\mathrm{Ti}$+$^{144}\mathrm{Sm}$\ensuremath{\rightarrow}$^{187}\mathrm{Po}$+$3n$ at the velocity filter SHIP. Striking features of the $^{187}\mathrm{Po}$ \ensuremath{\alpha} decay are the strongly-hindered decay to the spherical ground state and unhindered decay to a surprisingly low-lying deformed excited state at 286 keV in the daughter nucleus $^{183}\mathrm{Pb}$. Based on the potential energy surface calculations, the $^{187}\mathrm{Po}$ ground state and the 286 keV excited state in $^{183}\mathrm{Pb}$ were interpreted as being of prolate origin. The systematic deviation of the \ensuremath{\alpha}-decay properties in the lightest odd-A Po isotopes relative to the smooth behavior in the even-A neighbors is discussed. Improved data for the decay of ${}^{187}{\mathrm{Bi}}^{m,g}$ were also obtained.

Decay properties of the new neutron-deficient nuclide $^{192}\mathrm{At}$ have been studied in the complete fusion reaction $^{144}\mathrm{Sm}$($^{51}\mathrm{V}$,3n)$^{192}\mathrm{At}$ at the velocity filter SHIP. Two isomeric states with half-lives of 88(6) ms and 11.5(6) ms, respectively, and with complex $\ensuremath{\alpha}$-decay schemes were identified in $^{192}\mathrm{At}$. The decay pattern of one of the isomers suggests that it is based on the oblate-deformed $\ensuremath{\pi}2{f}_{7/2}\ensuremath{\bigotimes}\ensuremath{\nu}1{i}_{13/2}$ configuration, which confirms the expected onset of deformation in the At isotopes by approaching the neutron midshell at $N=104$.

The production of the very neutron-deficient nuclides $^{184\ensuremath{-}192}\mathrm{Bi}$ and $^{186\ensuremath{-}192}\mathrm{Po}$ in the vicinity of the neutron midshell at N = 104 has been studied by using heavy-ion-induced complete fusion reactions in a series of experiments at the velocity filter SHIP. The cross sections for the xn and pxn evaporation channels of the $^{46}\mathrm{Ti}$+$^{144}\mathrm{Sm}$$\ensuremath{\rightarrow}^{190}\mathrm{Po}{}^{*},$$^{98}\mathrm{Mo}$+$^{92}\mathrm{Mo}$$\ensuremath{\rightarrow}^{190}\mathrm{Po}{}^{*},$$^{50,52}\mathrm{Cr}$+$^{142}\mathrm{Nd}$$\ensuremath{\rightarrow}^{192,194}\mathrm{Po}{}^{*}$, and $^{94,95}\mathrm{Mo}$+$^{93}\mathrm{Nb}$$\ensuremath{\rightarrow}^{187,188}\mathrm{Bi}{}^{*}$ reactions were measured. The results obtained, together with the previously known cross section data for the heavier Bi and Po nuclides, are compared with the results of statistical model calculations carried out with the HIVAP code. It is shown that a satisfactory description of the experimental data requires a significant (up to 35%) reduction of the theoretical fission barriers. The optimal reactions for production of the lightest Bi and Po isotopes are discussed.

Isotopes of dubnium (element 105) with mass numbers A = 256, 257, and 258 were produced by the reaction 209Bi(50Ti,xn) 259-xDb (x = 1, 2, 3) at projectile energies of (4.59-5.08) AMeV. Excitation functions were measured for the 1n, 2n and 3n evaporation channels. The same position of the excitation function was observed for the 1n channel as for the previously measured 1n channel of the reaction 208Pb(50Ti,1n)257Rf. The measured α-decay data of 257Db and its daughter products resulted in the identification of α-decaying isomeric states in 257Db and 253Lr. Two new isotopes, 256Db and 252Lr, were produced at the highest bombarding energies of 4.97 AMeV and 5.08 AMeV. They were identified by delayed α-α coincidences. The measured half-lives are (1.6+0.5 -0.3) s for 256Db and (0.36+0.11 -0.07) s for 252Lr. Besides α-decay, a spontaneous fission activity of T 1/2 = (2.3+1.1 -0.6) s was observed and attributed to an electron-capture branch of 256Db, which feeds the fissioning nucleus 256Rf. A branching ratio of 0.36±0.12 was obtained. The isotope 255Rf was produced by the reaction 207Pb(50Ti,2n)255Rf. Improved decay data have been obtained by means of α- and α-γ spectroscopy.

Understanding the fundamental excitations of many-fermion systems is of significant current interest. In atomic nuclei with even numbers of neutrons and protons, the low-lying excitation spectrum is generally formed by nucleon pair breaking and nuclear vibrations or rotations. However, for certain numbers of protons and neutrons, a subtle rearrangement of only a few nucleons among the orbitals at the Fermi surface can result in a different elementary mode: a macroscopic shape change. The first experimental evidence for this phenomenon came from the observation of shape coexistence in 16O (ref. 4). Other unexpected examples came with the discovery of fission isomers and super-deformed nuclei. Here we find experimentally that the lowest three states in the energy spectrum of the neutron deficient nucleus 186Pb are spherical, oblate and prolate. The states are populated by the alpha-decay of a parent nucleus; to identify them, we combine knowledge of the particular features of this decay with sensitive measurement techniques (a highly efficient velocity filters with strong background reduction, and an extremely selective recoil-alpha-electron coincidence tagging methods). The existence of this apparently unique shape triplet is permitted only by the specific conditions that are met around this particular nucleus.

The isotopes 257,255Rf, 257,256Db, 253,252Lr have been produced in bombardments of 207,208Pb and 209Bi target nuclei with 50Ti and identified by their α-decay. New or improved decay data could be obtained. Analysis of the fine structure of the α-decay pattern of 257Rf allowed the construction of a first tentative level scheme for the daughter nucleus 253No and also the identification of a low lying high spin isomeric state, while from α-γ- coincidence measurements for 255Rf a first tentative level scheme of the daughter nucleus 251No was derived. For 257Db we found that two nuclear levels decay by α-emission and populate also different levels in the daughter nucleus 253Lr. The levels are produced by the reaction process. In bombardments of 209Bi with 50Ti at ECN*=26.4 MeV and 30.8 MeV the previously unknown isotopes 256Db and 252Lr were identified.

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.

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.