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Two gas phase adsorption chemistry experiments aimed at the chemical characterization of element 112 using its isotope 283 112 have been performed at the Flerov Laboratory for Nuclear Reactions (FLNR) Dubna, Russia. The applied Insitu-Volatilization and On-line Detection (IVO) technique is a thermochromatographic system combining the determination of the deposition temperature of volatile elements on a surface along a temperature gradient with an efficient detection of the deposited species by event-by-event alpha and SF-fragment spectroscopy. Two possibilities to produce the isotope 283 112 were used: 1.) the direct production reaction 238 U( 48 Ca,3n) 283 112; 2.) the reaction 242 Pu( 48 Ca,3n), where the primary product 287 114, decays via alpha emission to 283 112 with a half-life of 0.5 s. The chemistry experiments were aimed at a chemical identification of 283 112 and an independent confirmation of its decay properties. In the direct reaction no decays related to 283 112 were observed. However, two decay chains unambiguously attributed to the decay of 283 112 were observed using the second production path. Previously reported observation of 283 112 and 279 Ds and their decay properties were confirmed. From its thermochromatorgaphic deposition first thermochemical data were deduced for element 112, unveiling it as a typical group 12 element.

Aiming at an increase in the identification ability and higher sensitivity in the experiments on the synthesis and study of the decay properties of superheavy nuclei, the separator VASSILISSA has been upgraded. After preparatory experiments, the experiment aiming at the synthesis of the isotope 283 112 in the reaction 48 Ca + 238 U was performed. The obtained results on the decay mode, half-life and production cross-section are in agreement with those obtained in our first experiment.

The production cross sections were measured for the xn, pxn, and αxn de-excitation channels of the 212Rn compound nuclei formed at excitation energies of 80–160 MeV in the reaction 22Ne + 190Os. The maximum cross sections obtained for the xn (x = 8–12), pxn (x = 8–11) and αxn (x = 8–11) reaction channels were fitted satisfactorily with the computer code HIVAP using its standard parameter set except that the single scaling parameter C = 0.65 was implemented to adjust the values of the liquid-drop fission barriers predicted by the Cohen-Plasil-Swiatecki (CPS) model. Additionally, an analysis of a large set of data on the formation cross sections of neutron-deficient Z = 83–92 nuclei showed that the whole body of data obtained for the nuclear region notable for the large variations of both the liquid-drop barrier and the shell effects, can be described well with practically the same parameter C = 0.55–0.68. One may take this observation as an indication that, though the CPS model overestimates the liquid-drop fission barriers, it reproduces well the relative behaviour of these barriers in the whole range of neutron-deficient Z = 83–92 nuclei.

Excitation functions were obtained for the x n, p x n and αx n decay channels of 216,218,220 Ra compound nuclei produced in the reactions 22 Ne+ 194,196,198 Pt in the excitation energy range of 40–160 MeV. Due to the employment of three platinum isotope targets, the cross section ratios were measured with a high degree of accuracy for evaporation reaction channels resulting in the formation of fixed cold products after the termination of evaporation cascades of a different numbers of stages. Experimental values for the reduced neutron widths of highly excited compound nuclei were obtained from the measured cross section ratios. Evaporation reaction cross sections were well described in the framework of the statistical model taking into account shell effects according to Ignatyuk. Our present, and also earlier, investigations showed that calculations making use of one single set of the model parameters (the scaling factor to the liquid-drop fission barrier of Cohen, Plasil and Swiatecki, C = 0.6–0.7 and the ratio of the level density parameters, a f a v = 1.0 ) reproduces correctly the cross sections of the evaporation reactions in a wide range of compound nuclei extending from Bi to U. Moreover, the measured cross section ratios are exceptionally sensitive to the value of the parameter a f a v , i.e. are very suitable for a high accuracy estimation of this value. By fitting two sets of the experimental data — the absolute cross section values and cross section ratios — we were able to independently estimate the values of two principal parameters of the model, C and a f a y . Our calculations showed that a large part of the pre-fission neutrons is evaporated by hot pre-actinide compound nuclei before they reach the saddle point (50 to 100% of the total number of pre-fission neutrons are pre-saddle).

Excitation functions for xn-evaporation channels of the 22Ne+194,196,198Pt reactions have been measured in the compound nucleus excitation energy range of 45–110 MeV. The Γ n Γ tot = 0.88 ± 0.07 was obtained in the range of the excitation energy of 60–100 MeV from the analysis of the ratios of the formation cross sections for the same nuclides produced in reactions with different numbers of evaporated neutrons. From this value of Γ n Γ tot the fission time of (3–5)·10−20 s is derived for the compound nuclei.

Excitation functions are measured for the fusion reactions 27 Al+ 175 Lu and 31 P+ 169 Tm leading to the isotopes 192–198 Po. A comparison to the data for the reactions 100 Mo + 92–100 Mo shows that characteristics of the compound nucleus evaporation do not depend on the bombarding ion mass in the whole range of the entrance channel asymmetry up to the fully symmetrical systems. It is also shown that a satisfactory description of the experimental data is possible in the frame of the statistical deexcitation model only at an assumption involving, in comparison to theory predictions, a considerable reduction of macroscopic fission barriers for neutron-deficient isotopes of Bi.

The cross sections of the formation of Ac-, Ra- and Fr-evaporation residues in the 20 Ne + 197 Au reaction have been measured by means of the VASSILISSA kinematic separator in the range of the compound-nuclei excitation energies from 40 to 120 MeV. The experimental data have been analyzed on the basis of the statistical model of excited nuclei decay. The influence of different assumptions about the parameters of fission barriers on calculation results was considered.

The new neutron-deficient isotopes 250102, 242Fm and 254Ku have been synthesized by the reactions 233U(22Ne, 5n), 204Pb(40Ar, 2n) and 206Pb(50Ti, 2n), respectively. It is found that spontaneous fission is the main mode of decay. The spontaneous fission half-lives of these isotopes have been measured to be 0.25 msec, 0.8 msec and 0.5 msec, respectively. Some regularities in the spontaneous fission half-lives of transfermium isotopes are discussed.

Experiments have been performed to synthesize isotopes of elements 114–116 in fusion reactions between 48Ca ions and 246, 248Cm, 243Am and 242Pu target nuclei. The detection of reaction products by recording spontaneous fission and α-decay events allowed us to set upper limits of 5 × 10−35 cm2 and ≈ 2 × 10 −34 cm2 on the production cross sections for superheavy elements with 2 h ≦ Tsf ≦ 1 y and 2 h ≦ Tα ≦ 10 d, respectively.