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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.

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 $N=184$.

neutron-rich isotopes 242,244 Pu, 243 Am, 245,248 Cm and 249 Cf with 48 Ca projectiles. The decay properties of the synthesized nuclei are consistent with the consecutive α-decays originating in the decays of parent nuclides 286,287,288,289 114, 287,288 115, 290,291,293 116 and 294 118 produced in the 2n- to 5n-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 of 249 Bk with 48 Ca have been reinvestigated to provide new evidence for the discovery of element 117 on a larger number of events. The experiments were performed at five projectile energies and with a total beam dose of 48 Ca of about 4.6×10 19 . Two isotopes 293,294 117 were synthesized in the 249 Bk+ 48 Ca reaction, providing excitation functions and α-decay spectra of the produced isotopes that establishes these nuclei to be the products of the 4n- and 3n-evaporation channels, respectively. Decay properties of 293,294 117 and of all the daughter products agree with the data of the experiment in which these nuclei were synthesized for the first time in 2010. The new 289 115 events, populated by α decay of 293 117, demonstrate the same decay properties as those observed for 289 115 produced in the 243 Am( 48 Ca,2n) reaction thus providing cross- bombardment evidence. In addition, a single decay of 294 118 was observed from the reaction with 249 Cf - a result of the in-growth of 249 Cf in the 249 Bk target.

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 $N=184$.

Production and decay of the isotopes of Hs were studied in the ${}^{226}$Ra+${}^{48}$Ca reaction at beam energies ${E}_{\mathrm{lab}}=229$, 234, and 241 MeV. At the ${E}_{\mathrm{lab}}=234$ MeV energy, the maximum of the 4$n$-evaporation channel of the reaction, six identical $\ensuremath{\alpha}$-SF decay chains of the nucleus ${}^{270}$Hs were detected corresponding to a cross section of ${\ensuremath{\sigma}}_{4n}={16}_{\ensuremath{-}7}^{+13}$ pb. At the other ${}^{48}$Ca energies, no Hs isotopes were observed. Nuclei of ${}^{270}$Hs undergo $\ensuremath{\alpha}$ decay with a ${Q}_{\ensuremath{\alpha}}=9.15\ifmmode\pm\else\textpm\fi{}0.08$ MeV and the half-life of the daughter spontaneous fission (SF) isotope ${}^{266}$Sg is 0.28${}_{\ensuremath{-}0.08}^{+0.19}$ s, in good agreement with the data previously observed in the ${}^{248}$Cm(${}^{26}$Mg,4$n$)${}^{270}$Hs reaction. The partial $\ensuremath{\alpha}$-decay half-life of ${}^{270}$Hs was measured for the first time: ${T}_{\ensuremath{\alpha}}=7.{6}_{\ensuremath{-}2.2}^{+4.9}$ s. For the spontaneous fission, we determined a lower limit ${T}_{\mathrm{SF}}\ensuremath{\geqslant}10$ s. Decay properties of ${}^{270}$Hs corroborate theoretical predictions of its relatively high stability caused by the effect of the deformed shells at $Z=108$ and $N=162$.

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 ${E}^{*}\ensuremath{\approx}36\text{ }\text{ }\mathrm{MeV}$.

In bombardments of 248Cm with 22Ne we discovered two new isotopes, 265106 and 266106, by establishing genetic links between α decays of the 106 nuclides and spontaneous fission (SF) or α decays of the daughter (or granddaughter) nuclides. For 266106 we measured Eα = 8.63 ± 0.05 MeV followed by the SF decay of 262104 for which we measured a half-life value of 1.2+1.00.5 s. For 265106 we measured Eα = 8.71–8.91 MeV. We estimated α half-lives of 10–30 s for 266106 and 2–30 s for 265106 with SF branches of approximately 50% or less. The decay properties of 266106 indicate a large enhancement in the SF stability of this N = 160 nuclide and confirm the existence of the predicted deformed shells N = 162 and Z = 108.

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.

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.

The decay properties of {sup 290}116 and {sup 291}116, and the dependence of their production cross sections on the excitation energies of the compound nucleus, {sup 293}116, have been measured in the {sup 245}Cm({sup 48}Ca,xn){sup 293-x}116 reaction. These isotopes of element 116 are the decay daughters of element 118 isotopes, which are produced via the {sup 249}Cf+{sup 48}Ca reaction. They performed the element 118 experiment at two projectile energies, corresponding to {sup 297}118 compound nucleus excitation energies of E* = 29.2 {+-} 2.5 and 34.4 {+-} 2.3 MeV. During an irradiation with a total beam dose of 4.1 x 10{sup 19} {sup 48}Ca projectiles, three similar decay chains consisting of two or three consecutive {alpha} decays and terminated by a spontaneous fission (SF) with high total kinetic energy of about 230 MeV were observed. The three decay chains originated from the even-even isotope {sup 294}118 (E{sub {alpha}} = 11.65 {+-} 0.06 MeV, T{sub {alpha}} = 0.89{sub -0.31}{sup +1.07} ms) produced in the 3n-evaporation channel of the {sup 249}Cf+{sup 48}Ca reaction with a maximum cross section of 0.5{sub -0.3}{sup +1.6} pb.

We have studied the dependence of the production cross sections of the isotopes {sup 282,283}112 and {sup 286,287}114 on the excitation energy of the compound nuclei {sup 286}112 and {sup 290}114. The maximum cross section values of the xn-evaporation channels for the reaction {sup 238}U({sup 48}Ca,xn){sup 286-x}112 were measured to be {sigma}{sub 3n}=2.5{sub -1.1}{sup +1.8} pb and {sigma}{sub 4n}=0.6{sub -0.5}{sup +1.6} pb; for the reaction {sup 242}Pu({sup 48}Ca,xn){sup 290-x}114: {sigma}{sub 2n}{approx}0.5 pb, {sigma}{sub 3n}=3.6{sub -1.7}{sup +3.4} pb, and {sigma}{sub 4n}=4.5{sub -1.9}{sup +3.6} pb. In the reaction {sup 233}U({sup 48}Ca,2-4n){sup 277-279}112 at E*=34.9=2.2 MeV we measured an upper cross section limit of {sigma}{sub xn}{

We have studied the excitation functions of the reactions $^{244}\mathrm{Pu}$($^{48}\mathrm{Ca}$,$xn$). Maximum cross sections for the evaporation of 3--5 neutrons in the complete-fusion reaction $^{244}\mathrm{Pu+}^{48}\mathrm{Ca}$ were measured to be ${\ensuremath{\sigma}}_{3n}=2\phantom{\rule{0.3em}{0ex}}\text{pb}$, ${\ensuremath{\sigma}}_{4n}=5\phantom{\rule{0.3em}{0ex}}\text{pb}$, and ${\ensuremath{\sigma}}_{5n}=1\phantom{\rule{0.3em}{0ex}}\text{pb}$. The decay properties of $3n$-evaporation product $^{289}114$, in the decay chains observed at low $^{48}\mathrm{Ca}$ energy coincide well with those previously observed in the $^{244}\mathrm{Pu+}^{48}\mathrm{Ca}$ and $^{248}\mathrm{Cm+}^{48}\mathrm{Ca}$ reactions and assigned to $^{288}114$. Two isotopes of element 114 and their descendant nuclei were identified for the first time at higher bombarding energies: $^{288}114$ (${E}_{\ensuremath{\alpha}}=9.95\phantom{\rule{0.3em}{0ex}}\text{MeV}$, ${T}_{1∕2}=0.6\phantom{\rule{0.3em}{0ex}}\mathrm{s}$) and $^{287}114$ (${E}_{\ensuremath{\alpha}}=10.04\phantom{\rule{0.3em}{0ex}}\text{MeV}$, ${T}_{1∕2}=1\phantom{\rule{0.3em}{0ex}}\mathrm{s}$). We also report on the observation of new isotopes of element 116, $^{290,291}116$, produced in the $^{245}\mathrm{Cm}+^{48}\mathrm{Ca}$ reaction with cross sections of about $1\phantom{\rule{0.3em}{0ex}}\text{pb}$. A discussion of self-consistent interpretations of all observed decay chains originating at $Z=118$, 116, and 114 is presented.

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.