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The search for new decay chains of oganesson isotopes is presented. The experiment utilized the Dubna Gas Filled Recoil Separator and a highly segmented recoil-decay detection system. The signals from all detectors were analyzed in parallel by digital and analog data acquisition systems. For the first time, a target of mixed californium (51% Cf249, 13% Cf250, and 36% Cf251) recovered from decayed Cf252 sources was produced and irradiated with an intense Ca48 beam. The observation of a new decay chain of Og294 is reported. The prospects for reaching new isotopes Og295,296 are discussed.

Currently, gas phase chemistry experiments with heaviest elements are usually performed with the gas-jet technique with the disadvantage that all reaction products are collected in a gas-filled thermalisation chamber adjacent to the target. The incorporation of a physical preseparation device between target and collection chamber opens up the perspective to perform new chemical studies. But this approach requires detailed knowledge of the stopping force (STF) of the heaviest elements in various materials. Measurements of the energy loss of mercury (Hg), radon (Rn), and nobelium (No) in Mylar and argon (Ar) were performed at low kinetic energies of around (40–270) keV per nucleon. The experimentally obtained values were compared with STF calculations of the commonly used program for calculating stopping and ranges of ions in matter (SRIM). Using the obtained data points an extrapolation of the STF up to element 114, eka-lead, in the same stopping media was carried out. These estimations were applied to design and to perform a first chemical experiment with a superheavy element behind a physical preseparator using the nuclear fusion reaction 244Pu(48Ca; 3n)289114. One decay chain assigned to an atom of 285112, the α-decay product of 289114, was observed.

Irradiations of $^{239}\mathrm{Pu}$ and $^{240}\mathrm{Pu}$ targets with $^{48}\mathrm{Ca}$ beams aimed at the synthesis of $Z=114$ flerovium isotopes were performed at the Dubna Gas Filled Recoil Separator. A new spontaneously fissioning (SF) isotope $^{284}\mathrm{Fl}$ was produced for the first time in the $^{240}\mathrm{Pu}+^{48}\mathrm{Ca}$ (250 MeV) and $^{239}\mathrm{Pu}+^{48}\mathrm{Ca}$ (245 MeV) reactions. The cross section of the $^{239}\mathrm{Pu}(^{48}\mathrm{Ca},3n)^{284}\mathrm{Fl}$ reaction channel was about 20 times lower than predicted by theoretical models and about 50 times lower than the maximum fusion-evaporation cross section for the $3n$ and $4n$ channels measured in the $^{244}\mathrm{Pu}+^{48}\mathrm{Ca}$ reaction. In the $^{240}\mathrm{Pu}+^{48}\mathrm{Ca}$ experiment, performed at 245 MeV in order to maximize the $3n$-evaporation channel, three decay chains of $^{285}\mathrm{Fl}$ were detected. The $\ensuremath{\alpha}$-decay energy of $^{285}\mathrm{Fl}$ was measured for the first time and decay properties of its descendants $^{281}\mathrm{Cn}, ^{277}\mathrm{Ds}, ^{273}\mathrm{Hs}, ^{269}\mathrm{Sg}$, and $^{265}\mathrm{Rf}$ were determined with higher accuracy. The assignment of SF events observed during the irradiation of the $^{240}\mathrm{Pu}$ target with a 250 MeV $^{48}\mathrm{Ca}$ beam to $^{284}\mathrm{Fl}$ decay is presented and discussed. The cross sections at both $^{48}\mathrm{Ca}$ energies are similar and exceed that observed in the reaction with the lighter isotope $^{239}\mathrm{Pu}$ by a factor of 10. The decay properties of the synthesized nuclei and their production cross sections indicate a rapid decrease of stability of superheavy nuclei as the neutron number decreases from the predicted magic neutron number $N=184$.

Studies of superheavy nuclei produced in the ${}^{249}$Bk + ${}^{48}$Ca reaction were performed using the Dubna Gas Filled Recoil Separator. The cross section for the production of ${}^{293}$117 and ${}^{294}$117 isotopes was measured at five excitation energies of the ${}^{297}$117 compound nucleus ranging from 30 to 48 MeV and yielding maximum values of 1.1${}_{\ensuremath{-}0.6}^{+1.2}$ pb for the $3n$ and 2.4${}_{\ensuremath{-}1.4}^{+3.3}$ pb for the $4n$ reaction channels. Alpha emission from ${}^{281}$Rg competing with spontaneous fission ($\ensuremath{\alpha}$/SF decay probability 1:9) was observed for the first time leading to the identification of the new isotope ${}^{277}$Mt (${T}_{\mathrm{SF}}\ensuremath{\approx}5$ ms). The measured decay properties are in good agreement with those expected based on the properties of neighboring even-$Z$ and odd-$Z$ nuclei. The $\ensuremath{\alpha}$ energies and half-lives of odd-$Z$ isotopes observed in the ${}^{293}$117 and ${}^{294}$117 decay chains together with results obtained for lower-$Z$ superheavy nuclei demonstrate enhanced stability with increasing neutron number toward the predicted new magic number $N=184$.

Results from the production and decay properties of element 115 nuclei observed using the reaction ${}^{243}\mathrm{Am}+{}^{48}\mathrm{Ca}$ at various beam energies between November 1, 2010, and February 26, 2012, at the Dubna Gas Filled Recoil Separator are presented. This long-running experiment with a total beam dose of $3.3\ifmmode\times\else\texttimes\fi{}{10}^{19}$ and carried out in the excitation energy range ${E}^{*}=31$--47 MeV of the ${}^{291}$115 compound nucleus resulted in observation of three isotopes of element 115 with masses 287, 288, and 289. The 28 detected decay chains of ${}^{288}$115 show that this isotope is produced with the maximum probability at ${E}^{*}=34.0$--38.3 MeV with a corresponding cross section of ${\ensuremath{\sigma}}_{3n}=8.{5}_{\ensuremath{-}3.7}^{+6.4}$ pb. The four events attributed to the isotope ${}^{289}$115 that decays via a short $\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\alpha}\ensuremath{\rightarrow}\text{SF}$ chain could be detected only at the lowest excitation energy ${E}^{*}=31$--36 MeV, in accordance with what could be expected for the 2$n$-evaporation channel of the reaction. The decay characteristics of this nuclide were established earlier (2010) and more recently (2012) in the reaction ${}^{249}$Bk(${}^{48}$Ca,4$n$)${}^{293}$117 and following $\ensuremath{\alpha}$ decay to ${}^{289}$115. At the energy ${E}^{*}=44.8\ifmmode\pm\else\textpm\fi{}2.3$ MeV we observed only a single long chain of the isotope ${}^{287}$115. The decay properties of nuclei starting at ${}^{288}$115 and ${}^{287}$115 isotopes obtained in the present work reproduce in full the results of the first experiment of 2003 that reported the discovery of elements 115 and 113. The excitation functions of the production of the isotopes of element 115 and observation of the isotope ${}^{289}$115 in cross-bombardment reactions with the targets of ${}^{243}$Am and ${}^{249}$Bk provide additional evidence of the identification of the nuclei of elements 115 and 113. The experiments were carried out using the ${}^{48}$Ca beam of the U400 cyclotron of the Flerov Laboratory of Nuclear Reactions, JINR.

Results of a new series of experiments on the study of production cross sections and decay properties of the isotopes of element 115 in the reaction $^{243}\mathrm{Am}+^{48}\mathrm{Ca}$ are presented. Twenty-one new decay chains originating from $^{288}115$ were established as the product of the $3n$-evaporation channel by measuring the excitation function at three excitation energies of the compound nucleus $^{291}115$. The decay properties of all newly observed nuclei are in full agreement with those we measured in 2003. At the lowest excitation energy ${E}^{*}=33\text{ }\text{ }\mathrm{MeV}$, for the first time we registered the product of the $2n$-evaporation channel, $^{289}115$, which was also observed previously in the reaction $^{249}\mathrm{Bk}+^{48}\mathrm{Ca}$ as the daughter nucleus of the decay of $^{293}117$. The maximum cross section for the production of $^{288}115$ is found to be 8.5 pb at ${E}^{*}\ensuremath{\approx}36\text{ }\text{ }\mathrm{MeV}$.

The heaviest isotopes of elements Z = 117 to Z = 105, 294117, 293117, 290115, 289115, 286113, 285113, 282Rg, 281Rg, 278Mt, 274Bh, and 270Db, were identified by means of the Dubna gas-filled recoil separator among the products of the 249Bk + 48Ca reaction. The details of the observed six decay chains, indicating the production and decay of isotopes 293117 and 294117, are presented and discussed. The decay energies and resulting half-lives of these new nuclei show a strong rise of stability with increasing neutron number, validating the concept of the island of enhanced stability for superheavy nuclei.

An experiment aimed at the synthesis of isotopes of element 120 has been performed using the $^{244}\mathrm{Pu}(^{58}\mathrm{Fe},\mathit{xn}){}^{302\ensuremath{-}x}120$ reaction. No decay chains consistent with fusion-evaporation reaction products were observed during an irradiation with a beam dose of $7.1\ifmmode\times\else\texttimes\fi{}{10}^{18} 330\text{\ensuremath{-}}\mathrm{MeV} {}^{58}\mathrm{Fe}$ projectiles. The sensitivity of the experiment corresponds to a cross section of 0.4 pb for the detection of one decay.

Thirty‐four new nuclides with Z = 104–116, 118 and N = 161–177 have been synthesized in the complete‐fusion reactions of 238U, 237Np, 242,244Pu, 243Am, 245,248Cm, and 249Cf targets with 48Ca beams. The masses of evaporation residues were identified through measurements of the excitation functions of the xn‐evaporation channels and from cross bombardments. The decay properties of the new nuclei agree with those of previously known heavy nuclei and with predictions from different theoretical models. A discussion of self‐consistent interpretations of all observed decay chains originating from the parent isotopes 282,283112, 282113, 286–289114, 287,288115, 290–293116, and 294118 is presented. Decay energies and lifetimes of the neutron‐rich superheavy nuclei as well as their production cross sections indicate a considerable increase in the stability of nuclei with the approach to the theoretically predicted nuclear shells with N = 184 and Z = 114.

Thirty‐four new nuclides with Z = 104–116, 118 and N = 161–177 have been synthesized in the complete‐fusion reactions of 238U, 237Np, 242,244Pu, 243Am, 245,248Cm, and 249Cf targets with 48Ca beams. The masses of evaporation residues were identified through measurements of the excitation functions of the xn‐evaporation channels and from cross bombardments. The decay properties of the new nuclei agree with those of previously known heavy nuclei and with predictions from different theoretical models. A discussion of self‐consistent interpretations of all observed decay chains originating from the parent isotopes 282,283112, 282113, 286–289114, 287,288115, 290–293116, and 294118 is presented. Decay energies and lifetimes of the neutron‐rich superheavy nuclei as well as their production cross sections indicate a considerable increase in the stability of nuclei with the approach to the theoretically predicted nuclear shells with N = 184 and Z = 114.

The reaction of 249Bk with 48Ca has been investigated with an aim of synthesizing and studying the decay properties of isotopes of the new element 117. The experiments were performed at five projectile energies (in two runs, in 2009-2010 and 2012) and with a total beam dose of 48Ca ions of about 9x1019 The experiments yielded data on a-decay characteristics and excitation functions of the produced nuclei that establish these to be 293117 and 294117 – the products of the 4n- and 3n-evaporation channels, respectively. In total, we have observed 20 decay chains of Z=117 nuclides. The cross sections were measured to be 1.1 pb for the 3n and 2.4 pb for the 4n-reaction channel. The new 289115 events, populated by α decay of 117, demonstrate the same decay properties as those observed for 115 produced in the 243Am(48Ca,2n) reaction thus providing cross-bombardment evidence. In addition, a single decay of 294118 was observed from the reaction with 249Cf – a result of the in-growth of 249Cf in the 249Bk target. The observed decay chain of 294118 is in good agreement with decay properties obtained in 2002-2005 in the experiments with the reaction 249Cf(48Ca,3n)294118. The energies and half-lives of the odd-Z isotopes observed in the 117 decay chains together with the results obtained for lower-Z superheavy nuclei demonstrate enhancement of nuclear stability with increasing neutron number towards the predicted new magic number N=184.

This paper presents results of experiments aimed at producing long-lived superheavy elements located near the spherical shell closures with Z ≥114 and N ≥172 in the reactions of neutron-rich isotopes 244Pu, 245,248Cm and 249Cf with 48Ca projectiles. The decay properties of the synthesized nuclei are consistent with the consecutive α-decays originating in the decays of parent nuclides 289114, 290,291,293116 and 294118 produced in the 2n and 3n-evaporation channels. The present observations can be considered to be experimental evidence of the existence of the “island of stability” of superheavy elements.

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.