The study of the $^{48}$Ca+$^{249,250,251,252}$Cf reactions in a wide energy interval around the external barrier has been achieved with the aim of investigating the dynamical effects of the entrance channel via the $^{48}$Ca induced reactions on the
$^{249-252}$Cf targets and to analyze the influence of odd and even neutron composition in target on the capture, quasifission and fusion cross sections. Moreover, we also present the results of the individual evaporation residue excitation functions obtained from the de-excitation cascade of the various even-odd and even-even $^{297-300}$118 superheavy compound nuclei reached in the studied reactions, and we compare our results of the $^{294}$118 evaporation residue yields obtained in the synthesis process of the $^{48}$Ca+$^{249,250}$Cf reactions with the experimental data obtained in the $^{48}$Ca+$^{249}$Cf experiment carried out at the Flerov Laboratory of Nuclear Reactions of Dubna.
The fusion excitation function is the important quantity in planning experiments for the synthesis of superheavy elements. Its values seem to be determined by the experimental study of the hindrance to complete fusion by the observation of mass, angu
lar and energy distributions of the fissionlike fragments. There is ambiguity in establishment of the reaction mechanism leading to the observed binary fissionlike fragments. The fissionlike fragments can be produced in the quasifission, fast fission, and fusion-fission processes which have overlapping in the mass (angular, kinetic energy) distributions of fragments. The branching ratio between quasifission and complete fusion strongly depends on the characteristics of the entrance channel. In this paper we consider a wide set of reactions (with different mass asymmetry and mass symmetry parameters) with the aim to explain the role played by many quantities on the reaction mechanisms. We also present the results of study of the $^{48}$Ca+$^{249}$Bk reaction used to synthesize superheavy nuclei with Z = 117 by the determination of the evaporation residue cross sections and the effective fission barriers $<B_{rm f}>$ of excited nuclei formed along the de-excitation cascade of the compound nucleus.
A variety of phenomena connected with the formation of a dinuclear complex is observed in the heavy ion collisions at low energies. The dinuclear system model allows us to analyze the experimental data and to interpret them by comparison of the parti
al capture, fusion and evaporation residue cross sections measured for the different reactions leading to the same compound nucleus. The comparison of theoretical and experimental values of the mass and angular distributions of the reaction products gives us a detailed information about reaction mechanism forming the observed yields. The observed very small cross sections of the evaporation residues may be explained by the strong fusion hindrance and/or instability of the heated and rotating compound nucleus and smallness of its survival probability. The fusion hindrance arises due to competition between complete fusion and quasifission while the smallness of survival probability is connected with the decrease of the fission barrier at large excitation energy and angular momentum of compound nucleus.
The yields of evaporation residues, fusion-fission and quasifission fragments in the $^{48}$Ca+$^{144,154}$Sm and $^{16}$O+$^{186}$W reactions are analyzed in the framework of the combined theoretical method based on the dinuclear system concept and
advanced statistical model. The measured yields of evaporation residues for the $^{48}$Ca+$^{154}$Sm reaction can be well reproduced. The measured yields of fission fragments are decomposed into contributions coming from fusion-fission, quasifission, and fast-fission. The decrease in the measured yield of quasifission fragments in $^{48}$Ca+$^{154}$Sm at the large collision energies and the lack of quasifission fragments in the $^{48}$Ca+$^{144}$Sm reaction are explained by the overlap in mass-angle distributions of the quasifission and fusion-fission fragments. The investigation of the optimal conditions for the synthesis of the new element $Z$=120 ($A$=302) show that the $^{54}$Cr+$^{248}$Cm reaction is preferable in comparison with the $^{58}$Fe+$^{244}$Pu and $^{64}$Ni+$^{238}$U reactions because the excitation function of the evaporation residues of the former reaction is some orders of magnitude larger than that for the last two reactions.
