Production and decay of the heaviest nuclei (293,294)117 and (294)118.

Two years after the discovery of element 117, we undertook a second campaign using the (249)Bk+(48)Ca 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 (48)Ca of about 1.5×10(19) particles. The (249)Bk target was irradiated at two energies of (48)Ca that correspond to the maximum probability of the reaction channels with evaporation of three and four neutrons from the excited (297)117. In this experiment, two decay chains of (294)117 (3n) and five decay chains of (293)117 (4n) were detected. In the course of the long-term work, (249)Cf-the product of decay of (249)Bk (330 d)-is being accumulated in the target. Consequently, in the present experiment, we also detected a single decay of the known isotope (294)118 that was produced during 2002-2005 in the reaction (249)Cf((48)Ca,3n)(294)118. 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.

New insights into the 243Am + 48Ca reaction products previously observed in the experiments on elements 113, 115, and 117.

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)Am+(48)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 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)Bk+(48)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*≈36 MeV.

Synthesis of a new element with atomic number Z = 117.

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.

Chemical characterization of element 112.

The heaviest elements to have been chemically characterized are seaborgium (element 106), bohrium (element 107) and hassium (element 108). All three behave according to their respective positions in groups 6, 7 and 8 of the periodic table, which arranges elements according to their outermost electrons and hence their chemical properties. However, the chemical characterization results are not trivial: relativistic effects on the electronic structure of the heaviest elements can strongly influence chemical properties. The next heavy element targeted for chemical characterization is element 112; its closed-shell electronic structure with a filled outer s orbital suggests that it may be particularly susceptible to strong deviations from the chemical property trends expected within group 12. Indeed, first experiments concluded that element 112 does not behave like its lighter homologue mercury. However, the production and identification methods used cast doubt on the validity of this result. Here we report a more reliable chemical characterization of element 112, involving the production of two atoms of (283)112 through the alpha decay of the short-lived (287)114 (which itself forms in the nuclear fusion reaction of 48Ca with 242Pu) and the adsorption of the two atoms on a gold surface. By directly comparing the adsorption characteristics of (283)112 to that of mercury and the noble gas radon, we find that element 112 is very volatile and, unlike radon, reveals a metallic interaction with the gold surface. These adsorption characteristics establish element 112 as a typical element of group 12, and its successful production unambiguously establishes the approach to the island of stability of superheavy elements through 48Ca-induced nuclear fusion reactions with actinides.

Observation of excited states in 5H.

The 5H system was produced in the 3H(t,p)5H reaction studied with a 58 MeV tritium beam at small c.m. angles. High statistics data were used to reconstruct the energy and angular correlations between the 5H decay fragments. A broad structure in the 5H missing mass spectrum showing up above 2.5 MeV was identified as a mixture of the 3/2+ and 5/2+ states. The data also present evidence that the 1/2+ ground state of 5H is located at about 2 MeV.

Gamma-ray multiplicities and fission modes in (208)Pb((18)O,f).

Two components in the M(gamma)(M) distribution were established in detailed measurements of mean gamma-ray multiplicities from fission fragments of (226)Th. For the first time in the M(gamma)(M) dependencies we were able to distinguish two components associated with primary and the final (after the neutron evaporation) fission fragments, and show that at the scission point M(gamma) is extremely sensitive to symmetric and asymmetric modes of fission. Theoretical calculations of the pre-scission shapes of the fissioning nuclei confirm our conclusions.