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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.

For experiments aimed at the study of spontaneous fission of transfermium nuclei improvements in the focal plane detector system of recoil separator VASSILISSA have been made. The neutron detector consisting of 72 3He filled counters has been mounted around the focal plane detector chamber. In the first experiment the multiplicity of prompt neutrons emitted in spontaneous fission of 252No was measured.

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.

Within the past 15 years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in complete fusion reactions induced by heavy ions. The study of decay properties and formation of cross-sections of the isotopes of elements 110, 112 and 114 was performed using high-intensity 48 Ca beams and 232 Th , 238 U , 242 Pu targets. For further experiments aimed at the synthesis of the superheavy element isotopes ( Z ⩾110) with the use of intense 48 Ca extracted beams, improvements in the ion optical system of the separator and the focal plane detector system have been made. The results from the test reactions and new results for the isotope 283 112 are presented.

Since the past 10 years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in heavy-ion-induced complete fusion reactions. In the course of experimental work in the region of the elements with 92⩽Z⩽94 , 14 new isotopes have been identified by the parent–daughter correlations. For further experiments aimed at the synthesis of the superheavy element isotopes ( Z⩾110 ) with intensive 48 Ca extracted beams, improvements in the focal plane detector system have been made. As a result, energy and position resolutions of the detectors have been significantly improved, the detection efficiency for the α -particles, emitted from the implanted into the focal plane detector recoil nuclei, has been increased to 85%. The decay properties of the new isotope of element 112 and the isotope of the new element 114 with mass 287 have been measured in two-nuclei decays.

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).

Within the past seven years the electrostatic separator VASSILISSA [A.V. Yeremin et al., Nucl. Instr. and Meth. A 274 (1989) 528; A 350 (1994) 608] has been used for the investigations of evaporation residues (ER) produced in heavy-ion induced complete fusion reactions. Two types of such studies were of specific interest to us. These are the synthesis of heavy (Z ≥ 100) nuclides and investigations of heavy-ion fusion reactions leading to formation of neutron-deficient nuclei with Z ≥ 82 and high excitation energy ( 60 MeV ≤ E ∗ ≤ 200 MeV ). In the course of the experimental work the optimization of the ion-optical and other systems of the separator including the improvements in the focal plane detector system has been made. As a result, the separation efficiency values ranging from 3 to 30% were achieved for ER's produced in the reactions with heavy ions from 12C to 48Ti. The suppression factors > 1019 for the full energy beam particles and > 104 for multinucleon transfer reaction products were achieved. The results of the performed experiments illuminating the separator performance and future plans of the separator upgrade are briefly presented.

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.

Within the last five years the kinematic separator VASSILISSA [A.V. Yeremin et al., Nucl. Instr. and Meth. A 274 (1989) 528; and A.V. Yeremin et al., Preprint JINR E15-90-347, Dubna 1990] has been used for investigations of neutron deficient evaporation residues (ER) produced in heavy ion fusion reactions. In the course of this work optimization of the ion-optical and other systems of the separator including improvements of the focal plane detector system have been made. As a result, the separation efficiency values ranging from 3 to 30% were achieved for ERs produced in the reactions with heavy ions ranging from 16O to 40Ar. The suppression factors >1019 for the full energy beam particles and >104 for multinucleon transfer reaction products were achieved. The results of the performed experiments illuminating the separator performance are briefly presented.

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.

Construction and properties of a double ionization chamber (DIC) are described. Heavy ion fusion-evaporation reaction products obtained at the focal plane of the kinematic separator VASSILISSA enter the DIC volume through a thin plastic window. After their thermalization in the gas, they drift in the electric field to the DIC cathode. Their α-decay and/or spontaneous fission are detected in 4π-geometry in two ionization chambers, divided by the common cathode. Using DIC the α-decay of Bi and Pb isotopes produced in the fusion reactions 40 Ar + 159 Tb and 40 Ca + 153 Eu were detected. A new isomer, 189m Bi was observed.

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.

The Vassilissa facility is described. The main component is an electrostatic separator of complete-fusion reaction products from the ion beam. The facility is capable of transmitting recoil nuclei emitted by the target in the beam direction within a solid angle of 10 −2 sr and has a ± 10% spread in energy and ionic charge. The time required to transport the nuclei to the detectors is 3 × 10 −6 s The separation of scattered ions is performed using a system of three electrostatic condensers. Separation factors obtained for Ar, P and Ne are equal to 10 10 , 10 11 and 10 12 , respectively.

We describe an on-line gas jet system in which the reaction products are transported during 1 s via a thin 6 m tube to a chamber where they are deposited on a jet filter. The fission fragments escaping the filter are registered by ionization chambers that determine their energy and the emission angle. By introducing a correction for energy losses in passive absorbers it is possible to measure the total kinetic energy of fission fragments with an accuracy of 1% for a geometric efficiency of 50%. The prompt neutron multiplicity is measured simultaneously with fission fragment detection.