Main restrictions in the synthesis of new superheavy elements: quasifission or/and fusion-fission

The synthesis of superheavy elements stimulates the effort to study the peculiarities of the complete fusion with massive nuclei and to improve theoretical models in order to extract knowledge about reaction mechanism in heavy ion collisions at low energies. We compare the theoretical results of the compound nucleus (CN) formation and evaporation residue (ER) cross sections obtained for the $^{48}$Ca+$^{248}$Cm and $^{58}$Fe+$^{232}$Th reactions leading to the formation of the isotopes A=296 and A=290, respectively, of the new superheavy element Lv (Z=116). The ER cross sections, which can be measured directly, are determined by the complete fusion and survival probabilities of the heated and rotating compound nucleus. That probabilities can not be measured unambiguously but the knowledge about them is important to study the formation mechanism of the observed products. For this aim, the $^{48}$Ca+$^{249}$Cf and $^{64}$Ni+$^{232}$Th reactions have been considered too. The use of the mass values of superheavy nuclei calculated in the framework of the macroscopic-microscopic model by Warsaw group leads to smaller ER cross section for all of the reactions (excluding the $^{64}$Ni+$^{232}$Th reaction) in comparison with the case of using the masses calculated by Peter M\"oller {\it et al}.Comment: 26 pages and 15 figures. arXiv admin note: text overlap with arXiv:1109.201

Entrance channel effects in superheavy element production

The difference between evaporation residue cross sections measured in the cold (X+208Pb, 209Bi) and hot (48Ca+actinides) fusion reactions can be related to the stage of compound nucleus (CN) formation and/or to the stage of its survival against fission. The cold fusion reactions are favorable in synthesis of the superheavy elements (SHE) with charge numbers Z < 112 in comparison with the hot fusion reactions due to small excitation energy and large fission barrier of the CN formed in these reactions. The strong decrease of the cross sections of the synthesis of the SHE Z = 113 in the cold fusion reactions in comparison with the ones in the hot fusion reactions is the result of the increase of hindrance to the CN formation in the cold fusion reactions. The origin of the intrinsic fusion barrier, B*fus, causing the strong decrease of the probability PCN in the cold fusion is discussed

Capture and fusion dynamics in heavy-ion collisions

The dependence of the difference between the processes of capture and complete fusion is studied as a function of the entrance channel in the framework of the combined dynamical and statistical model. The excitation functions for the 126Ba compound nucleus formed in the 19F + 107Ag and 28Si + 98Mo reactions were calculated to compare the Emin and Emax values of the beam energy window for the fusion cross section. The importance of the competition between quasi-fission and complete fusion in interpreting the experimental data obtained for the 40Ar + natAg reaction is discussed. The increase in the difference between the experimental evaporation residue production and the extracted fusion cross section at increasing 40Ar beam energy is explained as an increasing effect of the quasi-fission process. © 2000 Elsevier Science B.V. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

The influence of the entrance channel dynamics on the evaporation residue formation

The dynamical effects of the entrance channel on the competition between quasifission and fusion processes, and on the evaporation residue formation are investigated. We have analyzed the results and compared our calculations with the experimental data obtained in the 16O + 204Pb and 96Zr + 124Sn reactions having very different mass asymmetries and leading to the 220Th* compound nucleus. We have found that different partial capture cross-sections σlcap(E) for these reactions lead to different fusion-quasifission competitions and, consequently, to different partial fusion cross-sections σlfus(E) of the compound nucleus formed in the two reactions. The dynamical conditions also affect the fission-evaporation competition of the excited intermediate nuclei along the CN de-excitation cascade and, consequently, the evaporation residue formation. © Società Italiana di Fisica/Springer-Verlag 2004.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Comparison of the fusion-fission and quasifission mechanisms in heavy-ion collisions

The decrease of the evaporation residue yields in reactions with massive nuclei is explained by an increase of the competition between quasifission and complete fusion processes and by the decrease of the survival probability of the heated and rotating nuclei against fission along the de-excitation cascade of the compound nucleus. The experimental data on the yields of evaporation residue, fusion-fission and quasifission fragments in the 48Ca + 154Sm reaction 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 of the 48Ca + 154Sm reaction have been well reproduced and yields of fission fragments were analyzed using the partial fusion and quasifission cross sections calculated in the dinuclear system model. Such a way of calculation is used to find optimal conditions for the synthesis of the new element Z = 120 (A = 302) by studying the excitation functions of evaporation residues of the 54Cr + 248Cm, 58Fe + 244Pu, and 64Ni + 238U reactions. Our estimations show that the 54Cr + 248Cm reaction is preferable in comparison with the two others. © 2009 World Scientific Publishing Company.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Appearance of fast-fission and quasi-fission in reactions with massive nuclei

The experimental data on the capture and evaporation residue cross-sections obtained in the 48Ca+208Pb reaction were analyzed in the framework of the dynamical model based on the dinuclear system concept and advanced statistical method to clarify the reaction mechanism. The experimental excitation function of the capture reactions was decomposed into contributions of the fusion-fission quasifission and fast-fission processes. Total evaporation residues and ones after neutron emission were only calculated and compared with the available experimental data. © World Scientific Publishing Company.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Investigation of the role of the projectile-target orientation angles on the evaporation residue production

The measured yield of evaporation residues in reactions with massive nuclei have been well reproduced by using the partial fusion and quasifission cross sections obtained in the dinuclear-system model. The influence of the orientation angles of the projectile- and target-nucleus symmetry axes relative to the beam direction on the production of the evaporation residues is investigated for the 48Ca + 154Sm reaction as a function of the beam energy. At the low beam energies only the orientation angles close to αP = 30° (projectile) and αP = 0°-15° (target) can contribute to the formation of evaporation residues. At large beam energies (about Ec.m. = 140-180 MeV) the collisions at all values of orientation angles αP and αT of reactants can contribute to the evaporation residue cross section which ranges between 10-100 mb, while at Ec.m. > 185 MeV the evaporation residue cross section ranges between 0.1-1 mb because the fission barrier for the compound nucleus decreases by increasing its excitation energy and angular momentum. © Pleiades Publishing, Ltd. 2009.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Effect of the entrance channel on the fission of the compound nucleus

The effects of entrance channel with different projectile and target nuclei combinations on the fission of the 90220Th*, 102254No* and 104256Rf* compound nuclei are investigated. The dependence of the de-excitation of the compound nucleus on the dynamics of the entrance channel is analyzed for the first time. We find that the fission branching ratio Γf= Γtot of the compound nucleus formed in the two different reactions with massive nuclei is different due to the dynamical effects in the entrance channel even at the same excitation energy E*. It is caused by the different partial fusion cross section σlfus (E) for those reactions. Consequently, the excitation function of evaporation residues measured in such reactions is strongly related to the effect of the entrance channel. This phenomenon is explained by the dependence of the capture and fusion cross sections on the orbital angular momentum of collision. ©2005 The Physical Society of Japan.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Effect of the entrance channel on the fission of the compound nucleus

The effects of entrance channel with different projectile and target nuclei combinations on the fission of the 90220Th*, 102254No* and 104256Rf* compound nuclei are investigated. The dependence of the de-excitation of the compound nucleus on the dynamics of the entrance channel is analyzed for the first time. We find that the fission branching ratio Γf= Γtot of the compound nucleus formed in the two different reactions with massive nuclei is different due to the dynamical effects in the entrance channel even at the same excitation energy E*. It is caused by the different partial fusion cross section σlfus (E) for those reactions. Consequently, the excitation function of evaporation residues measured in such reactions is strongly related to the effect of the entrance channel. This phenomenon is explained by the dependence of the capture and fusion cross sections on the orbital angular momentum of collision. ©2005 The Physical Society of Japan.SCOPUS: ar.jinfo:eu-repo/semantics/publishe