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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 $N=184$.

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 $N=162$.

In bombardments of $^{244}\mathrm{Pu}$ with $^{34}\mathrm{S}$ we discovered the \ensuremath{\alpha}-decaying nuclide $^{273}110$. We conducted an extensive off-line search of the raw data for event sequences which fit the expected pattern of implantation in a position-sensitive detector and subsequent decay of $^{273}110$ and its descendants. We observed one three-member sequence of genetically linked \ensuremath{\alpha} decays, resulting in ${\mathit{E}}_{\mathrm{\ensuremath{\alpha}}}$=11.35 MeV, a half-life of 0.${3}_{\mathrm{\ensuremath{-}}0.2}^{+1.3}$ ms, and a production cross section of about 0.4 pb for $^{273}110$. Other possible $^{273}110$ event chains were also observed. The measured \ensuremath{\alpha}-particle energy for the N=163 nuclide $^{273}110$ provides direct evidence for a neutron shell closure at N=162. \textcopyright{} 1996 The American Physical Society.

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.

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.

The decay properties of the new isotope ${}^{282}113$ and its daughter nuclei have been measured in the $^{237}\mathrm{Np}$($^{48}\mathrm{Ca}$, $3n$)${}^{282}113$ reaction. During an irradiation with a beam dose of $1.1\ifmmode\times\else\texttimes\fi{}{10}^{19}244$-MeV $^{48}\mathrm{Ca}$ projectiles, two decay chains originating from the odd-odd isotope ${}^{282}113$ $({E}_{\ensuremath{\alpha}}=10.63\ifmmode\pm\else\textpm\fi{}0.08\phantom{\rule{0.3em}{0ex}}\mathrm{MeV},{T}_{\ensuremath{\alpha}}={73}_{\ensuremath{-}29}^{+134}\phantom{\rule{0.3em}{0ex}}\mathrm{ms})$ were produced in the complete fusion reaction with a cross section of $0.{9}_{\ensuremath{-}0.6}^{+1.6}$ pb; these properties are all in agreement with expectations based on the results of previous experiments.

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.

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.