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Experimental works performed on the Dubna gas-filled magnetic recoil separator (GFS) to study the reactions of formation of 294Ts and 294Og nuclides and the properties of their radioactive decay are described. Future possibilities of synthesizing new elements with Z = 119 and Z = 120 and performing experiments to study the properties of heavier isotopes of known superheavy nuclei Lv (Z = 116), Ts (Z = 117), and Og (Z = 118) are discussed.

A new series of experiments aimed at the synthesis and study of decay properties of the most neutron-deficient isotopes of element Fl (Z = 114) and of the heaviest isotopes of 118 element is planned at the DGFRS (FLNR JINR). An appropriate registering system is to be implemented to transfer spectrometric data from double-sided silicon strip detector (DSSD). New analog modules were designed that allow to simplify existing multi-channel measurement system and to improve the real-time method of “active correlations” in search for the rare events of SHE formation and decay. The main features of the new modules the 16-channel charge-sensitive preamplifier, the 16-channel analog multiplexer and the 1.25 MSPS 12-bit Parallel ADC are presented.

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 $N=184$.

We have studied the dependence of the production cross-sections of the isotopes 282,283112 and 286–288114 on the excitation energy of the compound nuclei 286112 and 290114. The maximum cross-sections of the xn-evaporation channels for the reaction 238U(48Ca,xn)286-x 112 were measured to be: σ 3n = 2.5 -1.1 +1.8 pb and σ 4n = 0.6 -0.5 +1.6 pb; for the reaction 242Pu(48Ca,xn)290-x 114: σ 2n ∼ 0.5 pb, σ 3n = 3.6 -1.7 +3.4 pb and σ 4n = 4.5 -1.9 +3.6 pb. In the reaction 233U(48Ca, 2-4n)277–279 112 we measured an upper cross-section limit of σ xn ≤ 0.6 pb. An increase of σ ER in the reactions of actinide targets with 48Ca can be due to the expected increase of the survivability of the excited compound nucleus upon closer approach to the closed neutron shell N = 184. The observed nuclear decay properties of the nuclides with Z = 104–118 are compared with theoretical nuclear mass calculations and the systematic trends of α-decay properties. As a whole, they give a consistent pattern of decay of the 18 even-Z neutron-rich nuclides with Z = 104–118 and N = 163–177.

PC-based detection system for heavy element research at the Dubna Gas-filled Recoil Separator is presented. It allows to operate with the experimental data obtained from the detection system of GFRS which consists of 12-strip silicon position sensitive detector and two multiwire low-pressure proportional gaseous chambers to generate time-of-flight signal of incoming charge particles. Special attention is payed to the detection of genetically linked recoil-alpha sequences in a real-time mode. The results of the experiment aimed at the synthesis of new Z=113–115 elements with the mentioned technique are presented in brief.

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.

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.

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.

This article reports the results of experiments aimed at producing hypothetical long-lived superheavy elements located near the spherical-shell closures with Z≥114 and N≥72. For the synthesis of superheavy nuclei, we used a combination of neutron-rich reaction partners, with a 244Pu target and a 48Ca projectile. The sensitivity of the present experiment exceeded by more than two orders of magnitude previous attempts at synthesizing superheavy nuclides in reactions of 48Ca projectiles with actinide targets. We observed new decay sequences of genetically linked alpha decays terminated by spontaneous fission. The high measured alpha-particle energies, together with the long decay times and spontaneous fission terminating the chains, offer evidence for the decay of nuclei with high atomic numbers. The decay properties of the synthesized nuclei are consistent with the consecutive alpha decays originating from the parent nuclides 288,289114, produced in the 3n-and 4n-evaporation channels with cross sections of about a picobarn. The present observations can be considered experimental evidence for the existence of the “island of stability” of superheavy elements and are discussed in terms of modern theoretical approaches.

In bombardment of ${}^{244}\mathrm{Pu}$ with ${}^{48}\mathrm{Ca}$, we observed a decay sequence consisting of an implanted heavy atom, three subsequent $\ensuremath{\alpha}$ decays, and a spontaneous fission (SF), all correlated in time and position. The measured $\ensuremath{\alpha}$ energies and corresponding time intervals were ${E}_{\ensuremath{\alpha}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}9.71$ MeV ( $\ensuremath{\Delta}t\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}30.4$ s), 8.67 MeV ( $\ensuremath{\Delta}t\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}15.4$ min), and 8.83 MeV ( $\ensuremath{\Delta}t\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.6$ min); for the SF ( $\ensuremath{\Delta}t\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}16.5$ min), the total deposited energy was approximately 190 MeV. The $\ensuremath{\alpha}$-particle energies together with the decay times and SF terminating the chain offer evidence of the decay of nuclei with high atomic numbers. This decay chain is a good candidate for originating from the $\ensuremath{\alpha}$ decay of the parent nucleus ${}^{289}114$, produced with a cross section of about 1 pb.

The focal plane detector of the Dubna gass-filled recoil separator has been developed to study fusion reaction products from heavy-ion-induced nuclear reactions in the region of heavy nuclei with Z > 104. It consists of a 12 strip position-sensitive silicon detector to measure the energy/position of implanted nuclides and decay products and two low-pressure proportional chambers to generate a TOF signal. Additional information on ΔE from the STOP chamber is stored to provide an opportunity to discriminate alpha decays to the ground state of daughter product from the ones accompanied by conversion electron emission. The detector was successfully applied for the nuclear reactions which led to the discovery of new nuclides with Z = 108, 110 (A = 267, 273). An appropriate data acquisition system is described.

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.

In bombardments of ${}^{238}$U targets with 186-MeV ${}^{34}$S projectiles we discovered the $\ensuremath{\alpha}$-decaying nuclide ${}^{267}$108 with a half-life of ${19}_{\ensuremath{-}10}^{+29}$ ms, ${E}_{\ensuremath{\alpha}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}9.74$ to 9.87 MeV, and a production cross section of about 2.5 pb. The new nuclide was identified by measuring correlations in energy, time, and position to establish genetic links between its implantation in a position-sensitive silicon detector and subsequent $\ensuremath{\alpha}$ decay followed by $\ensuremath{\alpha}$ decays of known descendant nuclides.

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.

During the recent years, at the FLNR (JINR) a successful cycle of experiments has been accomplished on the synthesis of the superheavy elements with Z=112–118 with 48Ca beam. From the viewpoint of the detection of rare decays and background suppression, this success was achieved due to the application of a new radical technique - the method of active correlations. The method employs search in a real-time mode for a pointer to a probable correlation like recoil-alpha for switching the beam off. In the case of detection in the same detector strip an additional alpha-decay event, of “beam OFF” time interval is prolonged automatically. Reasonable scenarios of developing the method are considered. PC based data acquisition system as well as the monitoring and control system of the Dubna Gas Filled Recoil Separator is considered in brief too.

The discovery of a new chemical element with atomic number Z=117 is reported. The isotopes {sup 293}117 and {sup 294}117 were produced in fusion reactions between {sup 48}Ca and {sup 249}Bk. Decay chains involving 11 new nuclei were identified by means of the Dubna gas-filled recoil separator. The measured decay properties show a strong rise of stability for heavier isotopes with Z{>=}111, validating the concept of the long sought 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.

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.

The results of two experiments designed to synthesize element 115 isotopes in the {sup 243}Am+{sup 48}Ca reaction are presented. Two new elements with atomic numbers 113 and 115 were observed for the first time. With 248-MeV {sup 48}Ca projectiles, we observed three similar decay chains consisting of five consecutive {alpha} decays, all detected in a total time interval of 30 s. Each chain was terminated by a spontaneous fission (SF) with a high-energy release and a lifetime of about a day. With 253-MeV {sup 48}Ca projectiles, we registered a different decay chain of consecutive {alpha} decays detected in a time interval of 0.5 s, also terminated by spontaneous fission, but after 1.8 h. The decay properties of the eleven new {alpha}- and SF-decaying nuclei are consistent with expectations for consecutive {alpha} decays originating from the parent isotopes {sup 288}115 and {sup 287}115, produced in the 3n- and 4n-evaporation channels, respectively. Support for the assignment of the atomic numbers of all of the nuclei in the {sup 288}115 decay chain was obtained in an independent experiment in which a long-lived spontaneous fission activity, {sup 268}Db (15 events), was found to be chemically consistent with the fifth group of the periodic table.more » The odd-odd isotope {sup 288}115 was observed with largest cross section of about 4 pb. In the SF decay of {sup 268}Db, a total kinetic energy of 230 MeV and a neutron multiplicity per fission of 4.2 were measured. The decay properties of the 11 new isotopes with Z=105-115 and the production cross sections are in agreement with modern concepts of the role of nuclear shells in the stability of superheavy nuclei. The experiments were carried out at the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research.« less

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.

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.

A PC-based data acquisition system for heavy element research is described. Some special electronic modules were designed for the mentioned system. The system was tested for heavy-ion reactions at the main FLNR cyclotron U400. The detection module design is also described. This system was successfully applied in experiments to discover the shell closure at N=162 and Z=108 as well as for the discovery of new elements (two isotopes). A specific point of real-time detection of short recoil-alpha energy-time-position correlations is also presented.

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.

Formulation and solution of the problem of the U‐400 cyclotron arc ion source mode control optimization are considered. The optimization criterion is formulated as follows: to get the preset or maximum beam current in a limited range of the discharge and sputtering parameters. The ion source mode control is fulfilled by six input parameters variation. The algorithms of control are realized on the basis of CAMAC crate microprocessor controller and microcomputer compatible with IBM PC/XT/AT.