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The mass-energy distributions of fragments originating from the fission of the compound nucleus 226Th and their correlations with the multiplicity of gamma rays emitted from these fragments are measured and analyzed in 18O + 208Pb interaction induced by projectile oxygen ions of energy in the range E lab = 78–198.5 MeV. Manifestations of an asymmetric fission mode, which is damped exponentially with increasing E lab, are demonstrated. Theoretical calculations of fission valleys reveal that only two independent valleys, symmetric and asymmetric, exist in the vicinity of the scission point. The dependence of the multiplicity of gamma rays emitted from both fission fragments on their mass, Mγ(M), has a complicated structure and is highly sensitive to shell effects in both primary and final fragments. A two-component analysis of the dependence Mγ(M) shows that the asymmetric mode survives in fission only at low partial-wave orbital angular momenta of compound nuclei. It is found that, for all E lab, the gamma-ray multiplicity Mγ as a function of the total kinetic energy (TKE) of fragments, Mγ(TKE), decreases linearly with increasing TKE. An analysis of the energy balance in the fission process at the laboratory energy of E lab = 78 MeV revealed the region of cold fission of fragments whose total kinetic energy is TKE ∼ Q max. [ABSTRACT FROM AUTHOR]

Correlations between folding angular distributions of fission fragments and the gamma-ray multiplicity are studied for 18O + 208Pb interactions at energies of the beam of 18O ions in the range E lab = 78–198.5 MeV. The probabilities are determined for complete-and incomplete-fusion processes inevitably followed by the fission of nuclei formed in these processes. It is found that the probability of incomplete fusion followed by fission increases with increasing energy of bombarding ions. It is shown that, for the incomplete-fusion process, folding angular distributions of fission fragments have a two-component structure. The width of folding angular distributions (FWHM) for complete fusion grows linearly with increasing energy of 18O ions. The multiplicity of gamma rays from fission fragments as a function of the linear-momentum transfer behaves differently for different energies of projectile ions. This circumstance is explained here by the distinction between the average angular momenta of participant nuclei in the fusion and fission channels, which is due to the difference in the probabilities of fission in the cases where different numbers of nucleons are captured by the target nucleus. [ABSTRACT FROM AUTHOR]

The average multiplicity of gamma rays emitted by fragments originating from the fission of 226Th nuclei formed via a complete fusion of 18O and 208Pb nuclei at laboratory energies of 18O projectile ions in the range E lab = 78–198.5 MeV is measured and analyzed. The total spins of fission fragments are found and used in an empirical analysis of the energy dependence of the anisotropy of these fragments under the assumption that their angular distributions are formed in the vicinity of the scission point. The average temperature of compound nuclei at the scission point and their average angular momenta in the entrance channel are found for this analysis. Also, the moments of inertia are calculated for this purpose for the chain of fissile thorium nuclei at the scission point. All of these parameters are determined at the scission point by means of three-dimensional dynamical calculations based on Langevin equations. A strong alignment of fragment spins is assumed in analyzing the anisotropy in question. In that case, the energy dependence of the anisotropy of fission fragments is faithfully reproduced at energies in excess of the Coulomb barrier ( E c.m. − E B ≥ 30 MeV). It is assumed that, as the excitation energy and the angular momentum of a fissile nucleus are increased, the region where the angular distributions of fragments are formed is gradually shifted from the region of nuclear deformations in the vicinity of the saddle point to the region of nuclear deformations in the vicinity of the scission point, the total angular momentum of the nucleus undergoing fission being split into the orbital component, which is responsible for the anisotropy of fragments, and the spin component. This conclusion can be qualitatively explained on the basis of linear-response theory. [ABSTRACT FROM AUTHOR]

We have studied mass-energy distributions (MED) of fission fragments using two projectile-target combinations, [SUP12]C + [SUP204]Pb and [SUP48]Ca + [SUP168]Er, leading to the same compound nucleus [SUP216]Ra at the excitation energy E* ∼ 40 MeV. It has been found that the contribution of the asymmetric mode in the case of the former reaction is 1.5%, and it is 30% in the case of the latter one. We connect such a sharp increase in the yield of asymmetric products in the [SUP48]Ca + [SUP168]Er reaction with the quasifission process, the MED of which have a clearly expressed shell structure. The characteristics of the fission fragment MED are of such a kind that they can be interpreted by analogy with the low-energy fission of heavy nuclei as a manifestation of an independent mode of nuclear decay which competes with the classical fusion-fission process. [ABSTRACT FROM AUTHOR]

This article reports the results of experiments aimed at producing hypothetical long-lived superheavy elements located near the spherical-shell closures with Z ≥ 114 and N ≥ 172. For the synthesis of superheavy nuclei, we used a combination of neutron-rich reaction partners, with a [sup 244]Pu target and a [sup 48]Ca projectile. The sensitivity of the present experiment exceeded by more than two orders of magnitude previous attempts at synthesizing superheavy nuclides in reactions of [sup 48]Ca projectiles with actinide targets. We observed new decay sequences of genetically linked alpha decays terminated by spontaneous fission. The high measured alpha-particle energies, together with the long decay times and spontaneous fission terminating the chains, offer evidence for the decay of nuclei with high atomic numbers. The decay properties of the synthesized nuclei are consistent with the consecutive alpha decays originating from the parent nuclides [sup 288, 289]114, produced in the 3n- and 4n-evaporation channels with cross sections of about a picobarn. The present observations can be considered experimental evidence for the existence of the "island of stability" of superheavy elements and are discussed in terms of modem theoretical approaches. [ABSTRACT FROM AUTHOR]