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Studies of superheavy nuclei produced in the 249Bk + 48Ca reaction were performed using the Dubna Gas Filled Recoil Separator. The cross section for the production of 293117 and 294117 isotopes was measured at five excitation energies of the 297117 compound nucleus ranging from 30 to 48 MeV and yielding maximum values of 1.1-0.6+1.2 pb for the 3n and 2.4-1.4+3.3pb for the 4n reaction channels. Alpha emission from 281Rg competing with spontaneous fission (ɑ/SF decay probability 1:9) was observed for the first time leading to the identification of the new isotope 277Mt (TSF ~ 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 ɑ energies and half-lives of odd-Z isotopes observed in the 293117 and 294117 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. [ABSTRACT FROM AUTHOR]

Results from the production and decay properties of element 115 nuclei observed using the reaction 243 Am + 48Caat 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 × 1019 and carried out in the excitation energy range E* = 31-47 MeV of the 291115 compound nucleus resulted in observation of three isotopes of element 115 with masses 287, 288, and 289. The 28 detected decay chains of 288115 show that this isotope is produced with the maximum probability at E* = 34.0-38.3 MeV with a corresponding cross section of σ3n = 8.5-3.7+6.4Pb- The four events attributed to the isotope 289115 that decays via a short ɑ --> ɑ -» SF chain could be detected only at the lowest excitation energy E* = 31-36 MeV, in accordance with what could be expected for the 2n-evaporation channel of the reaction. The decay characteristics of this nuclide were established earlier (2010) and more recently (2012) in the reaction 249Bk(48Ca,4n)239117 and following a decay to 289115. At the energy E* = 44.8 ± 2.3 MeV we observed only a single long chain of the isotope 287115. The decay properties of nuclei starting at 288115 and 287115 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 289115 in cross-bombardment reactions with the targets of 243Am and 249Bk provide additional evidence of the identification of the nuclei of elements 115 and 113. The experiments were carried out using the 48Ca beam of the U400 cyclotron of the Flerov Laboratory of Nuclear Reactions, JINR. [ABSTRACT FROM AUTHOR]

Two years after the discovery of element 117, we undertook a second campaign using the 249Bk + 48Ca reaction for further investigations of the production and decay properties of the isotopes of element 117 on a larger number of events. The experiments were started in the end of April 2012 and are still under way. This Letter presents the results obtained in 1200 hours of an experimental run with the beam dose of 48Ca of about 1.5 X 1019 particles. The 249Bk target was irradiated at two energies of 48Ca that correspond to the maximum probability of the reaction channels with evaporation of three and four neutrons from the excited 297117. In this experiment, two decay chains of 294117 (3M) and five decay chains of 293117 (4M) were detected. In the course of the long-term work, 249Cf--the product of decay of 249Bk (330 d)--is being accumulated in the target. Consequently, in the present experiment, we also detected a single decay of the known isotope 294118 that was produced during 2002-2005 in the reaction 249Cf(48Ca, 3M)294118. The obtained results are compared with the data from previous experiments. The experiments are carried out in the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, using the heavy-ion cyclotron U400. [ABSTRACT FROM AUTHOR]

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 243Am + 48Ca are presented. Twenty-one new decay chains originating from 288115 were established as the product of the 3n-evaporation channel by measuring the excitation function at three excitation energies of the compound nucleus 291115. 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 MeV, for the first time we registered the product of the 2n-evaporation channel, 289115, which was also observed previously in the reaction 249Bk + 48Ca as the daughter nucleus of the decay of 293117. The maximum cross section for the production of 288115 is found to be 8.5 pb at E* ≈ MeV. [ABSTRACT FROM AUTHOR]

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 [ Oganessian et al. Phys. Rev. Lett. 104 142502 (2010)]. [ABSTRACT FROM AUTHOR]

Production and decay of the isotopes of Hs were studied in the 226Ra+48Ca reaction at beam energies Elab=229, 234, and 241 MeV. At the Elab=234 MeV energy, the maximum of the 4n-evaporation channel of the reaction, six identical α-SF decay chains of the nucleus 270Hs were detected corresponding to a cross section of Σ4n=16-7+13 pb. At the other 48Ca energies, no Hs isotopes were observed. Nuclei of 270Hs undergo α decay with a Qα=9.15±0.08 MeV and the half-life of the daughter spontaneous fission (SF) isotope 266Sg is 0.28-0.08+0.19 s, in good agreement with the data previously observed in the 248Cm(26Mg,4n)270Hs reaction. The partial α-decay half-life of 270Hs was measured for the first time: Tα=7.6-2.2+4.9 s. For the spontaneous fission, we determined a lower limit TSF⩾10 s. Decay properties of 270Hs corroborate theoretical predictions of its relatively high stability caused by the effect of the deformed shells at Z=108 and N=162. [ABSTRACT FROM AUTHOR]

Irradiations of 239Pu and 240Pu targets with 48Ca 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 284Fl was produced for the first time in the 240Pu+48Ca (250 MeV) and 239Pu+48Ca (245 MeV) reactions. The cross section of the 239Pu(48Ca,3n)284Fl 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 244Pu+48Ca reaction. In the 240Pu+48Ca experiment, performed at 245 MeV in order to maximize the 3n-evaporation channel, three decay chains of 285Fl were detected. The α-decay energy of 285Fl was measured for the first time and decay properties of its descendants 281Cn, 277Ds, 273Hs, 269Sg, and 265Rf were determined with higher accuracy. The assignment of SF events observed during the irradiation of the 240Pu target with a 250 MeV 48Ca beam to 284Fl decay is presented and discussed. The cross sections at both 48Ca energies are similar and exceed that observed in the reaction with the lighter isotope 239Pu 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. [ABSTRACT FROM AUTHOR]

The discovery of a new chemical element with atomic number Z=117 is reported. The isotopes (293)117 and (294)117 were produced in fusion reactions between (48)Ca and (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 > or = 111, validating the concept of the long sought island of enhanced stability for superheavy nuclei.