Giant Nuclear Systems Research Articles

Binary Fission fragmentation of 184 466,476X

Based on the statistical theory of fission, we discuss here the binary fission fragmentation of these giant nuclear systems formed in low energy U+U collisions. Here, the mass and charge distribution of fragments from the binary fission of thesesystems are studied at T= 1 and 2 MeV. From our results at T= 1 MeV, fragments in the near-asymmetric and near-symmetric regions pronounce higher yield values. However, at T= 2 MeV, our results are grossly different. Furthermore, the binary fragmentation with the largest yield consists of at least one closed shell nucleus. Different possible binary fission modes are presented to look for U+Ugiant nuclear systems.

True ternary fission and quasifission of superheavy nuclear systems

We found that a true ternary fission with formation of a heavy third fragment (a new kind of radioac- tivity) is quite possible for superheavy nuclei due to the strong shell effects leading to a three-body clusterization with the two doubly magic tin-like cores. The three-body quasifission 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 detection of two coincident lead-like fragments in low-energy U+U collisions.

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.

Binding energy of giant nuclear systems in an empirical density functional model

Calculations of the radial shape and the binding energy of giant nuclei are made, using an empirical density functional. The obtained binding energies are smaller than those obtained with extrapolations of the liquid drop model. The density distributions show a growing central depression (due to Coulomb and symmetry energy) which finally (forA>700) leads to bubble nuclei.

Hartree-Fock and shell model calculations for giant nuclei

The properties of giant nuclear systems like Uranium/Uranium etc. (A−476) have been studied in statical, spherical Hartree-Fock approximation. Various effective forces of Skyrme type were used to calculate binding energies, density distributions and single particle spectra. A shell model of Woods-Saxon type is shown to be more or less equivalent with respect to the shell structure derived from Skyrme models and easy generizable to slightly deformed giant nuclei. Strong magic shells could be found even in this region of the periodic table, but it seems that they cannot contribute significantly to a stabilization of these systems. The total binding energy is in good agreement with liquid drop model extrapolations. Only very briefly we discuss possibilities and problems for calculating deformation energies.

The decay of the vacuum in supercritical fields of giant nuclear systems

The structure of the vacuum has become of general importance in modern physics. In this context the vacuum of quantum electrodynamics plays a special role because it changes drastically in strong external electric fields and these changes can be followed in a controllable way in the laboratory. It is reviewed how in supercritical electric fields the electron-positron-vacuum, i.e., the ground state of QED is changing from a neutral to a charged vacuum. The theoretical concepts, developed during the last 15 years, can be tested in collisions of very heavy ions, like U+U, U+Cm, etc. The important theoretical and experimental stages are described; in particular it is shown that the vacuum decay might have recently been experimentally verified: Sharp line structures in the positron spectra with pronounced threshold effects are the signals of this fundamental phase transition. Furthermore, the observed γ-ray and δ-electron spectra, the non-appearance of a massive line structure in undercritical systems like U+Ta and the quantitative understanding of the innershell ionization phenomena in giant quasimolecules (their agreement with theoretical predictions) give confidence into this interpretation. At the same time the positron line structures possibly indicate the existence of rather long living giant nuclear systems with Z ∼ 180–190. It is shown that most likely giant nuclear molecules have been formed as doorway states, but that other new not yet completely understood nuclear structure of giant complexes is indicated. The structure of the giant complexes can eventually be investigated spectroocopically by the positrons from the vacuum: The positron spectrum should not only show the vacuum decay line, but also Raman and Stokes satellites revealing the nuclear collective modes of the giant system formed. Various minima in the collective potential of giant systems should reveal the vacuum decay and their own local nuclear structure through corresponding classes of positron lines. The seemingly constancy with (Z T + Z P) of the principal positron line structures observed so far could indicate the formation of always the same giant object. Other possibilities to explain this effect (vacuum vibrations, excitation of the same nuclear mode in various giant systems and its supercritical conversion) are also discussed. In particular, the possibility of the creation and decay of a yet unknown particle causing constant positron line structure is shown to be in contradiction with atomic precision experiments. Finally the overcritical phenomenon in other areas of physics (e.g., overcritical gluo-electric fields, overcritical gravitational fields) is shortly discussed.

Atomic excitations in supercritical fields of giant nuclear systems

Theoretical results for the δ-electron distribution and positron emission in superheavy systems are compared with recent experimental data. Special emphasis is laid on atomic excitations in deep inelastic nuclear collisions. It is demonstrated that the study of heavy-ion collisions with nuclear time delay promises a clear signature for the spontaneous positron emission. Corresponding experimental results show evidence for this transition from the neutral vacuum to a charged ground state and for the existence of long living nuclear molecules.

The decay of the vacuum in supercritical fields of giant nuclear systems

The structure of the vacuum has become of general importance in modern physics. In this context the vacuum of quantum electrodynamics plays a special role because it changes drastically in strong external electric fields and these changes can be followed in a controllable way in the laboratory. It is reviewed how in supercritical electric fields the electron-positron-vacuum, i.e., the ground state of QED is changing from a neutral to a charged vacuum. The theoretical concepts, developed during the last 15 years, can be tested in collisions of very heavy ions, like U+U, U+Cm, etc. The important theoretical and experimental stages are described; in particular it is shown that the vacuum decay might have recently been experimentally verified: Sharp line structures in the positron spectra with pronounced threshold effects are the signals of this fundamental phase transition. Furthermore, the observed γ-ray and δ-electron spectra, the non-appearance of a massive line structure in undercritical systems like U+Ta and the quantitative understanding of the innershell ionization phenomena in giant quasimolecules (their agreement with theoretical predictions) give confidence into this interpretation. At the same time the position line structures possibly indicate the existence of rather long living giant nuclear system with Z ∼ 180–190. It is shown that most likely giant nuclear molecules have been formed as doorway states, but that other new not yet completely understood nuclear structure of giant complexes is indicated. The structure of the giant complexes can eventually be investigated spectroscopically by the positrons from the vacuum: The positron spectrum should not only show the vacuum decay line, but also Raman and Stokes satellites revealing the nuclear collective modes of the giant system formed. Various minima in the collective potential of giant systems should reveal the vacuum decay and their own local nuclear structure through corresponding classes of positron lines. The seemingly constancy with (Z T + Z P) of the principal positron line structure observed so far could indicate the formation of always the same giant object. Other possibilities to explain this effect (vacuum vibrations, excitation of the same nuclear mode in various giant systems and its supercritical conversion) are also discussed. In particular, the possibility of the creation and decay of a yet unknown particle causing constant positron line structure is shown to be in contradiction with atomic precision experiments. Finally the overcritical phenomenon in other areas of physics (e.g., overcritical gluo-electric fields, overcritical gravitational fields) is shortly discussed.

Positrons from supercritical fields of giant nuclear systems

During collisions of heavy nuclei with a combined chargeZ≳160 the electronic 1s-state is deeply bound due to the strong Coulomb field, forZ≧173 it even enters as a resonance the lower continuum of the Dirac-Hamiltonian. In pure Rutherford scattering no qualitative indication for the filling of a dynamically createdK-hole by the spontaneous positron creation process is predicted, but the study of heavy-ion collisions with nuclear time delay due to the attractive nuclear force promises clear signatures for the decay of the vacuum. Emphasis is laid also on the quantitative influence of the electron-electron interaction and ofE0-transitions in the giant nuclear system on positron emission, the latter treated in a classical approximation. We compare our results with recent experimental data of two different groups at GSI, Darmstadt.