Inadequacy of the statistical model: Some evidence for compound nuclei in the A approximate to 150 and E-x approximate to 100-200 MeV region

Light charged-particle multiplicities in the evaporation residue (ER) and fission (FF) channels from the reaction 200 MeV (32)S + (100)Mo as well as ER and FF channel cross-sections have been measured and compared to the predictions of the statistical model (SM) to estimate the fission time scale. The statistical model fails in reproducing the whole set of data and no convincing estimate is possible. In particular, while pre-scission multiplicities can be reproduced, the model strongly overestimates proton and alpha particle multiplicities in the ER channel, irrespective of the SM input parameters and prescriptions used for the level density and the transmission coefficients. Same calculations performed on data from literature in the A approximate to 150 and excitation energy E(x) approximate to 100-200 MeV region, for the ER channel, provide similar conclusions. These findings repropose the problem of the reliability of the SM in describing the compound-nucleus decay and have a relevant impact on the extraction of the fission delay time through the use of the SM

Investigation of the reaction Ni-64+U-238 being an option of synthesizing element 120

This study is concerned with the search for entrance channels suitable to synthesize elements with Z > 118. Mass-energy distributions as well as capture cross-sections of fission-like fragments have been measured for the reactions (64)Ni+ (238)U -> (302)U ->(302)120 and (48)Ca + (238)U -> (286)112 at energies near the Coulomb barrier. Compound nucleus fission cross-sections were estimated from the analysis of mass and total kinetic energy distributions. The cross-section drops three orders of magnitude for the formation of the compound nucleus with Z = 120 obtained in the reaction (64)Ni + (238)U compared to the formation of the compound nucleus with Z = 112 obtained in the reaction (48)Ca + (238)U at an excitation energy of the compound nucleus of about 45 MeV. From our analysis it turns out that the reaction (64)Ni + (238)U is not suitable for the synthesis of element Z = 120

The peculiarities of the production and decay of superheavy nuclei

The interest in the study of the fission process of superheavy nuclei mainly deals with the opportunity to obtain information about the cross‐section of the compound nucleus (CN) formation at excitation energies E∗≈15–30 MeV. It allows one to estimate the survival probability of the superheavy composite system after evaporation of 1–3 neutrons, i.e. in “cold” or “warm” fusion reactions. However, in order to solve this problem deeper understanding of the coalescence processes between colliding nuclei, the competition between fusion‐fission and quasi‐fission processes is needed. The characteristics of both processes, their manifestation in the experimental observables and the relative contribution to the capture cross‐section in dependence on the excitation energies, reaction entrance channel etc were investigated for a wide range of target‐projectile combinations. Results of the experiments devoted to the study of the fusion‐fission and quasi‐fission processes in the reactions of the formation of the superheavy nuclei with Z = 102–122 are presented. The heavy ions 26Mg, 48Ca, 50Ti, 58Fe and 64Ni were used as projectiles. The choice of the reactions with 48Ca and actinide‐targets was inspired by the experiments on the production of the isotopes 283112, 289114 and 283116 in Dubna using the same reactions. The 50Ti, 58Fe and 64Ni projectiles were chosen since the corresponding projectile‐target combinations lead to the synthesis of even heavier elements. The experiments were carried out at the U‐400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR, Russia) and the XTU Tandem accelerator of the National Laboratory of Legnaro (LNL, Italy) using the time‐of‐flight spectrometer of fission fragments CORSET. The role of the shell effects, the influence of the entrance channel asymmetry and the deformations of colliding nuclei on the mechanism of the fusion‐fission and the competitive process of quasi‐fission are discussed. The recent results on synthesis of superheavy nuclei and the perspectives of the “hot” fusion reaction for the production of superheavy nuclei are considered