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Register a new userEffects of the entrance channel and fission barrier in synthesis of superheavy element $Z$=120
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Authors
A. K. Nasirov
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The fusion and evaporation residue cross sections for the $^{50}$Ti+$^{249}$Cf and $^{54}$Cr+$^{248}$Cm reactions calculated by the combined dinuclear system and advanced statistical models are compared. These reactions are considered to be used to synthesize the heaviest superheavy element. The $^{50}$Ti+$^{249}$Cf reaction is more mass asymmetric than $^{54}$Cr+$^{248}$Cm and the fusion excitation function for the former reaction is higher than the one for the latter reaction. The evaporation residue excitation functions for the mass asymmetric reaction is higher in comparison with the one of the $^{54}$Cr+$^{248}$Cm reaction. The use of the mass values of superheavy nuclei calculated in the framework of the macroscopic-microscopic model by the Warsaw group leads to smaller evaporation residue cross section for both the reactions in comparison with the case of using the masses calculated by Peter Moller {it et al}. The $^{50}$Ti+$^{249}$Cf reaction is more favorable in comparison with the $^{54}$Cr+$^{248}$Cm reaction: the maximum values of the excitation function of the 3n-channel of the evaporation residue formation for the $^{50}$Ti+$^{249}$Cf and $^{54}$Cr+$^{248}$Cm reactions are about 0.1 and 0.07 pb, respectively, but the yield of the 4n-channel for the former reaction is lower (0.004 pb) in comparison with the one (0.01 pb) for the latter reaction.
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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.
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 Moller {it et al}.
Structural properties and the decay modes of the superheavy elements Z $=$ 122, 120, 118 are studied in a microscopic framework. We evaluate the binding energy, one- and two- proton and neutron separation energy, shell correction and density profile of even and odd isotopes of Z $=$ 122, 120, 118 (284 $leq$ A $leq$ 352) which show a reasonable match with FRDM results and the available experimental data. Equillibrium shape and deformation of the superheavy region are predicted. We investigate the possible decay modes of this region specifically $alpha$-decay, spontaneous fission (SF) and the $beta$-decay and evaluate the probable $alpha$-decay chains. The phenomena of bubble like structure in the charge density is predicted in $^{330}$122, $^{292,328}$120 and $^{326}$118 with significant depletion fraction around 20-24$%$ which increases with increasing Coulomb energy and diminishes with increasing isospin (N$-$Z) values exhibiting the fact that the coloumb forces are the main driving force in the central depletion in superheavy systems.
Within the framework of the dinuclear system (DNS) model, the fusion reactions leading to the compound nuclei 274Hs and 286Cn are investigated. The fusion probability as a function of DNS excitation energy is studied. The calculated results are in good agreement with the available experimental data. The obtained results show that the fusion probabilities are obviously enhanced for the reactions located at high place in potential energy surface, although these reactions may have small values of mass asymmetry. It is found that the enhancement is due to the large potential energy of the initial DNS.
A complete set of existing data on hot fusion reactions leading to synthesis of superheavy nuclei of Z =114-118, obtained in a series of experiments in Dubna and later in GSI Darmstadt and LBNL Berkeley, was analyzed in terms of a new angular-momentum dependent version of the Fusion by Diffusion (FBD) model with fission barriers and ground-state masses taken from the Warsaw macroscopic-microscopic model (involving non-axial shapes) of Kowal et al. The only empirically adjustable parameter of the model, the injection-point distance (sinj), has been determined individually for all the reactions and very regular systematics of this parameter have been established. The regularity of the obtained sinj systematics indirectly points at the internal consistency of the whole set of fission barriers used in the calculations. Having fitted all the experimental excitation functions for elements Z = 114-118, the FBD model (with the new sinj systematics) was used to predict cross sections for synthesis of elements Z = 119 and 120. Regarding prospects to produce the new element Z = 119, our calculations prefer the 252Es(48Ca,xn)300-x119 reaction, for which the synthesis cross section of about 0.2 pb in 4n channel at Ec.m.= 220 MeV is expected. The most favorable reaction to synthesize the element Z = 120 turns out to be 249Cf(50Ti,xn)299-x120, but the predicted cross section for this reaction is only 6 fb (for 3n and 4n channels).
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