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We measured the average electronic charge $〈q〉$ of heavy ions with atomic numbers $Z=89$ through 116 traversing dilute hydrogen gas with velocities ranging from 1 to 2.6 times the Bohr velocity. We observe a strong linear dependence of the average charge on the velocity of the ions, and only a weak dependence on their atomic number Z. A more detailed examination of the experimental results shows that the $〈q〉$ values reflect the influence of the $5f$ electron shell on the properties of these elements.

This paper presents results of the experiments aimed at producing long-lived superheavy elements located near the spherical-shell closures with Z ⩾ 114 and N ⩾ 172 in the 244Pu + 48Ca and 248Cm + 48Ca reactions. The large measured α-particle energies of the newly observed nuclei, together with the long decay times and spontaneous fission terminating the chains, offer evidence of the decay of nuclei with high atomic numbers. The decay properties of the synthesized nuclei are consistent with the consecutive α-decays originating from the parent nuclides 288, 289114 and 292116, produced in the 3n and 4n evaporation channels with cross-sections of about a picobarn. The present observations can be considered as experimental evidence of the existence of the “island of stability” of superheavy elements.

In preparation for recent experiments on the synthesis of superheavy nuclei with $Z=114$ in the reaction ${}^{244}\mathrm{Pu}{+}^{48}\mathrm{Ca},$ we modified the Dubna gas-filled recoil separator and its detection system and carried out bombardments of lead targets with ${}^{48}\mathrm{Ca}$ projectiles. We studied excitation functions of the reactions ${}^{206}\mathrm{Pb}{(}^{48}\mathrm{Ca},1\ensuremath{-}4n)$ and ${}^{204,207,208}\mathrm{Pb}{(}^{48}\mathrm{Ca},2n).$ Maximum cross sections for the evaporation of 1--4 neutrons in the complete fusion reaction ${}^{206}\mathrm{Pb}{+}^{48}\mathrm{Ca}$ were measured to be ${\ensuremath{\sigma}}_{1n}=60 \mathrm{nb},$ ${\ensuremath{\sigma}}_{2n}=500 \mathrm{nb},$ ${\ensuremath{\sigma}}_{3n}=30 \mathrm{nb},$ and ${\ensuremath{\sigma}}_{4n}=0.3 \mathrm{nb}.$ In the bombardment of an enriched ${}^{204}\mathrm{Pb}$ target, we simultaneously obtained excitation functions of the ${}^{204,206,207,208}\mathrm{Pb}{(}^{48}\mathrm{Ca},2n)$ reactions induced on the isotopic admixtures present in the target material. The maximum cross sections for the evaporation of two neutrons from the compound nuclei ${}^{256}\mathrm{No},$ ${}^{255}\mathrm{No},$ and ${}^{252}\mathrm{No}$ were measured to be 2.1 $\ensuremath{\mu}\mathrm{b},$ 1.3 $\ensuremath{\mu}\mathrm{b},$ and 10 nb, respectively. The spontaneously fissioning even-even isotope ${}^{250}\mathrm{No},$ with a half-life ${T}_{1/2}=36 \ensuremath{\mu}\mathrm{s},$ was identified for the first time in this experiment. In the reaction ${}^{207}\mathrm{Pb}{+}^{34}\mathrm{S},$ we measured the excitation function for the production of the 21-ms spontaneously fissioning isotope ${}^{238}\mathrm{Cf},$ confirming our preliminary identification of this nuclide based on the results of cross bombardments.

We present the observation of the first decay event of the new nuclide {sup 292}116 in the running experiment on the synthesis of Z=116 nuclei in the reaction {sup 248}Cm+{sup 48}Ca. The experiment is in progress at FLNR, JINR, Dubna.

In bombardments of ${}^{244}\mathrm{Pu}$ targets with 114- and 120-MeV ${}^{22}\mathrm{Ne}$ projectiles we detected 69 $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\alpha}$ correlations linking $\ensuremath{\alpha}$ decays of ${}^{261}\mathrm{Rf}$ and ${}^{257}\mathrm{No}.$ We observed one $\ensuremath{\alpha}$ peak with ${E}_{\ensuremath{\alpha}}=8.30\ifmmode\pm\else\textpm\fi{}0.06$ MeV for ${}^{261}\mathrm{Rf}$ and peaks with \ensuremath{\alpha}-particle energies 8.07\char21{}8.40 MeV for ${}^{257}\mathrm{No}.$ The half-life of ${}^{257}\mathrm{No}$ was measured to be $25\ifmmode\pm\else\textpm\fi{}3$ s. No correlations were found between $\ensuremath{\alpha}$ decays and subsequent spontaneous fission events, from which we calculated an upper limit of 1.5% for the fission branch of ${}^{257}\mathrm{No}$ and estimated an upper limit of 3% for the \ensuremath{\alpha}-decay branch of ${}^{262}\mathrm{Rf}.$ The cross section of the ${}^{244}\mathrm{Pu}{(}^{22}\mathrm{Ne}{,5n)}^{261}\mathrm{Rf}$ reaction was measured to be about 4 nb at both ${}^{22}\mathrm{Ne}$ energies used. We also report on some results from ${}^{242}\mathrm{Pu}{+}^{22}\mathrm{Ne}$ and ${}^{238}\mathrm{U}{+}^{26}\mathrm{Mg}$ bombardments.

118 s); for the spontaneous fission (T1/257.522.9 114 s), the total energies deposited in the detector array were 213 62 and 221 62 MeV. The decay properties of the synthesized nuclei are consistent with the consecutive a decays originating from the parent even-even nucleus 288 114, produced in the 4n-evaporation channel with a cross section of about 1 pb. 288 114 and 284 112 are the heaviest known a-decaying even-even nuclides, following the production of 260 Sg and 266 Sg (Z5106) and the observation of a decay of 264 Hs (Z5108). The observed radioactive properties of 288 114 and the daughter nuclides match the decay scenario predicted by the macroscopic-microscopic theory.

The reaction 243Am + 48Ca has been reinvestigated to provide new evidence for the discovery of elements 113, 115. Twenty eight new 288115 decay chains were detected in this reaction to increase from three to 31 the number of 288115 atoms observed. In addition, four new decay chains were observed for the first time and assigned to the decay of 289115. These new 289115 events have the same properties for their decay chains as those observed for 289115 populated in the alpha decay of 293117 produced in the 249Bk + 48Ca reaction to provide cross-bombardment evidence. These new high statistics data sets and the cross-bombardment agreement provide definitive evidence for the discoveries of the new elements with Z = 113, 115, 117.

The synthesis of the new chemical element with atomic number Z=117 is presented. The isotopes 293117 and 294117were produced in fusion reactions between 48Ca and 249Bk. The 249Bk was produced in the High Flux Isotope Reactor and chemically separated at Oak Ridge. Decay chains involving eleven new nuclei were identified by means of the Dubna Gas Filled Recoil Separator. The measured decay properties show a strong rise of stability for super-heavy nuclei toward N=184.

We describe an on-line gas jet system in which the reaction products are transported during 1 s via a thin 6 m tube to a chamber where they are deposited on a jet filter. The fission fragments escaping the filter are registered by ionization chambers that determine their energy and the emission angle. By introducing a correction for energy losses in passive absorbers it is possible to measure the total kinetic energy of fission fragments with an accuracy of 1% for a geometric efficiency of 50%. The prompt neutron multiplicity is measured simultaneously with fission fragment detection.