V. I. Zagrebaev

Synthesis of superheavy nuclei: A search for new production reactions

Two important pages in synthesis of superheavy (SH) nuclei have been overturned within last twenty years. In the “cold” fusion reactions based on the closed shell target nuclei, lead and bismuth, SH elements up to Z = 113 have been produced [1, 2]. The “world record” of 0.03 pb in production cross section of 113 element has been obtained here within more than half-year irradiation of Bi target with Zn beam [2]. Further advance in this direction (with Ga or Ge beams) seems to be very difficult. Note also that the SH elements obtained in the “cold” fusion reactions with Pb or Bi target are situated along the proton drip line being very neutron-deficient with a short half-life.

TWO-NEUTRON EXCHANGE OBSERVED IN THE 6HE + 4HE REACTION. SEARCH FOR THE DI-NEUTRON CONFIGURATION OF 6HE

Differential cross sections for the elastic scattering of 6 He exotic nuclei from a gaseous helium target has been measured in a wide angular range in the CM system at a 6 He beam energy of 151 MeV. The large cross-sections obtained at backward angles are discussed in terms of a two-neutron exchange process. The results of DWBA calculations show that this effect can account for the cross sections obtained between 1208 and 1608 assuming the spectroscopic factor to be about 1 for the di-neutron cluster as was predicted by theory for 6 He. q 1998 Published by Elsevier Science B.V. All rights reserved.

Unified consideration of deep inelastic, quasi-fission and fusion–fission phenomena

A new approach is proposed for a unified description of strongly coupled deep inelastic (DI) scattering, fusion, fission and quasi-fission (QF) processes of heavy-ion collisions. The standard (most important) degrees of freedom of the nuclear system, unified driving potential, and a unified set of dynamic equations of motion are used in this approach. This makes it possible to perform a full (continuous) time analysis of the evolution of heavy nuclear systems, starting from the approaching stage, moving up to the formation of the compound nucleus and eventually emerging into two final fission fragments. The calculated mass, charge, energy and angular distributions of the reaction products agree well with the corresponding experimental data. It gives us hope to obtain rather accurate predictions for the probabilities of superheavy element formation in near-barrier fusion reactions.

DECAY PROPERTIES AND STABILITY OF HEAVIEST ELEMENTS

Decay properties and stability of heaviest nuclei with Z≤132 are studied within the macro-microscopical approach for nuclear ground state masses and phenomenological relations for the half-lives with respect to α-decay, β-decay and spontaneous fission. We found that at existing experimental facilities the synthesis and detection of nuclei with Z>120 produced in fusion reactions may be difficult due to their short half-lives (shorter than 1 μs). The nearest (more neutron-rich) isotopes of superheavy elements with 111≤Z≤115 to those synthesized recently in Dubna in 48Ca-induced fusion reactions are found to be β+-decaying. This fact may significantly complicate their experimental identification. However it gives a chance to synthesize in fusion reactions the most stable superheavy nuclei located at the center of the island of stability. Our calculations yield that the β-stable isotopes 291Cn and 293Cn with a half-life of about 100 years are the longest-living superheavy nuclei located at the island of stability.

Low-energy collisions of heavy nuclei: dynamics of sticking, mass transfer and fusion

The dynamics of heavy-ion low-energy collisions is studied within the realistic model based on multi-dimensional Langevin equations. Interplay of strongly coupled deep inelastic scattering, quasi-fission and fusion-fission processes is discussed. Collisions of very heavy nuclei ( 238 U+ 238 U, 232 Th+ 250 Cf and 238 U+ 448 Cm) are investigated as an alternative way for the production of super-heavy elements with increasing neutron number. Large charge and mass transfer were found in these reactions due to the inverse (anti-symmetrizing) quasi-fission process leading to the formation of surviving super-heavy long-lived neutron-rich nuclei. In many events the lifetime of the composite giant system consisting of two touching nuclei turns out to be rather long (≥10 -20 s), sufficient for observing line structure in spontaneous positron emission from super-strong electric fields, a fundamental QED process.

Superheavies: Theoretical incitements and predictions

It is well known that in fusion reactions one may get only neutron deficient superheavy nuclei located far from the island of stability. The multi-nucleon transfer reactions allow one to produce more neutron enriched new heavy nuclei but the corresponding cross sections are rather low. Neutron capture process is considered here as alternative method for production of long-lived neutron rich superheavy nuclei. Strong neutron fluxes might be provided by nuclear reactors and nuclear explosions in laboratory frame and by supernova explosions in nature. All these cases are discussed in the paper. PACS numbers: 25.70.Jj, 25.70.Lm

Potential Energy of a Heavy Nuclear System in Fusion-Fission Processes

We discuss the problem of description of low-energy nuclear dynamics and the derivation of a multi-dimensional potential energy surface that depends on several collective degrees of freedom and allows a unified analysis of deep inelastic scattering, fusion, and fission processes. A unified description is required due to the strong coupling and significant overlapping of these reaction channels in heavy nuclear systems, which are used, in particular, for synthesis of superheavy elements. The multidimensional adiabatic potential is derived based on an extended versio of the two-center shell model. This model leads to a correct asymptotic value and height of the Coulomb barrier in the entrance channel (fusion), and appropriate behavior in the exit channel, giving the required mass and energy distributions of reaction products and fission fragments. The derived driving potential is proposed to be applied in a consistent dynamic analysis of low-energy interactions of heavy nuclei.

True ternary fission of superheavy nuclei

True ternary fission with formation of a heavy third fragment is quite possible for superheavy nuclei because of the strong shell effects leading to a three-body clusterization with the two doubly magic tinlike cores. The simplest way to discover this phenomenon in the decay of excited superheavy nuclei is a detection of two tinlike clusters with appropriate kinematics in low-energy collisions of medium-mass nuclei with actinide targets. The three-body quasi-fission process could be even more pronounced for giant nuclear systems formed in collisions of heavy actinide nuclei. In this case a three-body clusterization might be proved experimentally by the detection of two coincident leadlike fragments in low-energy U + U collisions.

Shell effects in damped collisions: a new way to superheavies

The dynamics of heavy-ion low-energy damped collisions is studied within the model based on the Langevin-type equations. Shell effects on the multidimensional potential energy surface play an important role in these reactions. An enhanced yield of nuclides far from the projectile and target masses was found in multi-nucleon transfer reactions due to the shell effects. Our theoretical predictions need experimental confirmation.

Fusion-fission dynamics and perspectives of future experiments

The paper is focused on reaction dynamics of superheavy-nucleus formation and decay at beam energies near the Coulomb barrier. The aim is to review the things we have learned from recent experiments on fusion-fission reactions leading to the formation of compound nuclei with Z≥102 and from their extensive theoretical analysis. Major attention is paid to the dynamics of formation of very heavy compound nuclei taking place in strong competition with the process of fast fission (quasifission). The choice of collective degrees of freedom playing a fundamental role and finding the multidimensional driving potential and the corresponding dynamic equation regulating the whole process are discussed. A possibility of deriving the fission barriers of superheavy nuclei directly from performed experiments is of particular interest here. In conclusion, the results of a detailed theoretical analysis of available experimental data on the “cold” and “hot” fusion-fission reactions are presented. Perspectives of future experiments are discussed along with additional theoretical studies in this field needed for deeper understanding of the fusion-fission processes of very heavy nuclear systems.