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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]

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]

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 ≥ 172. For the synthesis of superheavy nuclei, we used a combination of neutron-rich reaction partners, with a [sup 244]Pu target and a [sup 48]Ca projectile. The sensitivity of the present experiment exceeded by more than two orders of magnitude previous attempts at synthesizing superheavy nuclides in reactions of [sup 48]Ca 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 [sup 288, 289]114, 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 modem theoretical approaches. [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]