Table of contents

The search for new elements is based on the irradiation of heavy element targets with heavy ions followed by identification through physical or chemical methods. Success seems assured for the production and identification of short-lived elements immediately beyond the heaviest presently known. Predictions indicate that still heavier elements, the superheavy elements as part of an "island of stability", would have sufficient stability to allow their identification; however, there is no equal assurance that nuclear reactions suitable for their synthesis can be found.

The chemical properties of a wide range of such prospective new elements have been predicted, in many cases in rather extensive detail. Special considerations related to their high atomic numbers suggest interesting differences between the chemical properties of these heavy elements and their lighter homologs.

Chemical separation schemes for the identification of superheavy elements have been devised and applied to heavy targets irradiated with heavy ions. While there may be some positive indications of such identification in experiments performed at Dubna, experiments at Berkeley have given negative results.

In the Berkeley experiments the yields of a broad range of isotopes of the known elements in the separated chemical fractions have been determined. The resulting distribution of products formed in the irradiation of uranium with krypton ions suggest some interesting new reaction mechanisms.

The topics covered include: (a) Elementary modes of excitation with special emphasis on the isoscalar highfrequency quadrupole vibrations. (b) Conditions for shell structure in both spherical and deformed potentials. (c) Modifications in nuclear structure produced by large angular momentum, including a discussion of stability conditions as well as of the spectrum of quantum states in the yrast region for very large angular momentum.

The experimental evidence supporting the double humped character of the fission barrier in actinide nuclei is reviewed and compared with theoretical predictions. The discussion covers the properties of spontaneously fissioning isomers, intermediate structure in fission cross sections, the systematics of fission barrier heights, and the implications of a possible octupole deformation at the second barrier for fragment mass distributions.

The Bevalac is briefly described and its schedule for completion is given. The Physics Research Program to be carried out with heavy ion beams with energy in the range 0.250 to 2.1 GeV/nucleon is described.

Heavy ion beams of ⁴⁰Ar (288 MeV) and ⁸⁴Kr (∼ 600 MeV) from Hilac acceleration and ¹⁴N (100, 160, 250, MeV) and ²⁰Ne (250 MeV) from the 88" cyclotron have been used to bombard natural silver targets. Formation cross sections, angular distributions and kinetic energy distributions for reaction products in the range Z = 6-20 have been measured using gas ΔE-E telescopes. Kinetic energy distributions generally agree well with coulomb repulsion energies for binary division. Angular distributions are discussed in terms of relaxation time for the mass asymmetry mode compared to rotational periods.

Experiments made at Orsay to synthesize superheavy elements by heavy ion induced reactions are described, and the results are presented. An attempt to explain the negative results obtained is presented, based on the mechanisms of the reaction involved: interaction barrier, critical distance of approach, deep inelastic transfers.

Recent predictions for lifetimes of superheavy (A ≈ 300) and super-superheavy (A300) nuclei are reviewed. Various factors affecting the predicted lifetimes and the uncertainties in these predictions are discussed.

The relation between nuclear shell structure and nuclear shapes is brought out in diverse mass regions. Thus certain nuclear orbitals, shown to be responsible for the development of mass-asymmetric distortions, are here followed in more detail for large and realistic shape changes. The same holds for axial asymmetries. The study is applied to actinide and superheavy nuclei as as well as very light ones, among the latter ²⁴Mg, which is shown to exhibit a deep octupole-shape second minimum.

The stability problem for the superheavy elements has been reconsidered based on the use of the shape-dependent droplet model and the modified-oscillator potential. The effect of axial asymmetry on the barrier heights is included. A semi-empirical inertial-mass function is employed. Alternatively the inertial-mass parameters are calculated microscopically. The spontaneous-fission half-lives are calculated with these inertials. Both of the latter were renormalized to give optimal agreement with experimental half-lives in the actinide region. The resulting spontaneous-fission half-lives in the SHE region are somewhat shorter than those of most previous estimates, predicting, for example, around 10⁵ years as the total half-live for the most stable nucleus ²⁹⁴110.

The effects of the simultaneous presence of monopole and quadrupole pairing interactions on the fission barriers for heavy and superheavy elements are considered. The first barrier for light Th isotopes is found to be raised by 1 MeV, while heavier actinide nuclei seem to be less affected by the inclusion of quadrupole pairing. For the superheavy elements, the inclusion of quadrupole pairing tends to increase the barriers considerably. Moreover, the Coulomb energy, associated with the non-homogeneous charge distribution of the modified oscillator model, is compared with the liquid-drop value and with value emerging from a self-consistent calculation based on the Skyrme interaction. The shell effect, which may be several MeV in magnitude, appears to be highly sensitive to model assumptions concerning the forces between neutrons and protons.

A review is given of the methods used in the search for superheavy elements in nature and of the results obtained. It is concluded that there is no convincing evidence for the presence of such elements in nature.

In searching for superheavy elements in nature it is usually assumed that the SHE (1) follows the chemistry of its lighter homolog and (2) decays by spontaneous fission. The sensitivity of SHE detection can be substantially improved by replacing these assumptions by a consideration of the geo-chemical fractionation processes, and by using a mass spectrometer. A good candidate for a search would be element 112, eka-mercury. It is expected to be more volatile and more noble than Hg so that it may behave like a heavy rare gas and be concentrated in the earth's atmosphere.

It is emphasized that the single-particle levels calculated from a shell model potential are physically meaningful only if the shell model potential is calculated in a selfconsistent Hartree-Fock manner. Rearrangement corrections to the single-particle levels calculated in this fashion are small for atoms. For nuclei there is experimental evidence that these corrections are quite large. One of these corrections is the 1h-2h1p coupling that shifts single-particle levels close to the Fermi surface up and gives a decay (Auger decay) width to the deeper states.

Self-consistent calculations of super-heavy elements are presented. The Skyrme effective interaction SIII has been used. This force gives an overall good fit of a wide range of nuclear properties, in particular for closed shell nuclei. The quality of these results puts reasonable limits on the reliability of extrapolation to unknown nuclei. Q_α and Q_β values are calculated and corresponding lifetimes are estimated. For the nucleus ²⁹⁸114, the fission barrier has been obtained by a constrained self-consistent calculation.

A theory of nuclear collisions where a frictional force is introduced explicitly from the beginning is worked out based on a macroscopic and leptodermous idealization. A solution of the problem is made by freezing all degrees of freedom but four: the distance between centres and the two angles measuring the spins of the nuclei. The conditions for capture as a function of energy and angular momentum is discussed in detail. Results on the kinetic energy and angular distributions of collision products are also obtained.

We calculate the nuclear potential energy of deformation for the collision of two heavy nuclei by means of a macroscopic-microscopic method. The nuclear macroscopic energy is calculated in terms of a double volume integral of a Yukawa function, and the microscopic shell and pairing corrections are calculated by use of Strutinsky's method from the single-particle levels of a realistic diffuse-surface single-particle potential. The time evolution of the system after the point of first contact is determined by solving the classical equations of motion for incompressible, irrotational hydrodynamical flow. The effect of nuclear viscosity on the fusion path is to slow down the formation of the neck and to inhibit the excitation of collective shape vibrations. For nuclear systems in which the fission saddle point lies well outside the contact point it is possible to interpret experimental fusion cross sections at relatively low bombarding energies in terms of a one-dimensional interaction barrier, as is customarily done. For heavier nuclear systems and higher bombarding energies, where the larger Coulomb and centrifugal forces tend to deform the fusing nuclei and lead to immediate fission, only those dynamical paths that pass inside the fission saddle point contribute significantly to fusion.

This is a survey of the work done at Orsay on the accelerator ALICE, on reactions induced by very heavy ions. (Ar to Kr). It deals mainly with the questions of complete fusion and very inelastic exchanges of nucleons which are relevant to the problem of nuclear viscosity. New results are presented on the very inelastic processes which produce light fragments in Ar-induced reactions on Ni and Th targets. The angular and mass correlation has been verified between the light and the heavy partner, and it is now entirely proven that a large amount of energy is dissipated into intrinsic excitations. Measured cross sections for complete fusion are compiled and a number of comments are given on the limitation for fusion in relation to critical angular momenta and critical distances of approach. For reactions induced by krypton and copper ions, the fusion barrier is higher than the interaction barrier. Below the fusion barrier, peculiar phenomena occur, named "quasi-fission". The fusion barrier is explained in terms of a cut-off in the low lℏ population for partial waves of the incoming ion, due to friction forces.

The spectrum of mass yields from spontaneous fission and thermal neutron induced fission of even nucleides sometimes shows a fine structure, indicating a preference for doubly even mass divisions. This occurs only when the fissility parameter is relatively small. For a given nuclide, the fine structure is more pronounced when the total kinetic energy carried away by the fragments is larger than average. The preference for doubly even mass divisions is interpreted as the preservation of superfluidity, i.e. of the paired configuration, during the descent from saddle to scission. Conversely, loss of fine structure means excitation of quasiparticles during the descent. Alternative models for the saddle-to-scission transition are discussed in relation to the observed variance in the number of neutrons evaporated from a single fragment, under the constraint of a fixed total kinetic energy and a fixed mass ratio. It is concluded that fine structure in the mass yields may also occur, when the total kinetic energy is much lower than average.

Heavy ion reactions high above the Coulomb barrier are analysed by using a classical model with dissipative forces. It is shown, in agreement with experiment, that the reaction can be divided into three parts: (1) quasielastic scattering, associated with high-angular momenta, (2) deep inelastic scattering, with intermediate angular momenta, and (3) complete fusion associated with the lowest angular momenta. Good agreement between theory and experiment is obtained for the cross section for complete fusion and for the cross section for deep inelastic scattering.

We present a theory of energy dissipation in heavy ion and fission process. Beginning with the time-independent coupled channels generator coordinate equations, a statistical treatment leads to a quantal equation in the collective coordinates and excitation energy. Assumptions of adiabaticity lead to a momentum coupled Schroedinger-like equation for the statistical wave-function. This equation describes, in a statistical manner, quantum mechanical collective motion, including dissipation. Therefore, average inelastic cross sections or fragment excitation energies may be obtained. Of course, phenomenologically known functions such as the nuclear mass parameter or potential energy surface can be simply utilized. The new dissipation function (like all the others) is determined by averages of microscopically calculable quantities. Numerical result for a model calculation exhibiting the structure of the equation are presented.

The properties of the nuclear tensor of inertia are discussed in connection with the fission process. In particular, the relations between the nuclear inertial parameters and the characteristics of the nuclear intrinsic motion are considered.

In the first part, calculated lifetimes for spontaneous fission from the ground and from the isomer state are presented. The agreement reached with experiments correponds to the crude approximations done, but is better than expected. The calculations are based on the shell correction approach for the potential energy and on the cranking model for the collective mass. In the second part we discuss the behaviour of the so-called least action trajectories: there are two of them, one for symmetric and one for asymmetric shapes. The systematic behaviour of the dynamic barriers and of the peak-to-peak ratio of the fission fragment mass distribution and their correlation with the experimental in formation support this point of view.

A synopis of the transmutations of matter in a supernova to the final stages of neutron-star matter, r-nuclei, p-nuclei and cosmic rays if given. In particular discussed are (1) the freezing-out of nuclear reactions during cooling, (2) the consequences of an improved β-decay and β-delayed neutron emission systematics for the calculations of the r-process element abundances and of its application to the estimation of the nucleosynthesis-age of the galaxy, (3) the production mechanism of the high-energy light-element cosmic rays, which we propose to originate from the neutron star's surface.

We discuss the possibility of producing superheavy elements in the astrophysical r-process. Thus, we consider a detailed calculation of fission-barrier heights and neutron separation energies for heavy neutron-rich nuclei. The dependence of the neutron-induced fission cutoff of the r-process on the uncertainty in the nuclear models is discussed in some detail.

The fission barriers of the nuclei in the r-process region have been calculated with inclusion of axial asymmetry and reflection asymmetry. Neutron separation energies and spontaneous-fission half-lives have been calculated and the possibilities of producing superheavy elements by means of neutron-capture processes are discussed.

Thermonuclear neutron-capture approaches to heavy transuranium element synthesis are investigated. Certain feature of capture experiments using thermonuclear macro-explosives are re-analyzed in view of recent results of nuclear shell-structure theory and theoretical developments in laser-energized fusion micro-explosions. The micro-explosion conditions for prompt multiple neutron capture are estimated for the simplest moderation techniques. We speculate on some practical neutor-capture paths to the superheavy island, especially a hybrid approach, using both macro-and micro-explosion events.

Recent advances in the calculation of fission barriers and in the experimental determination of beta strength functions make it possible to derive reliable estimates of the effect of beta-delayed fission in the decay of very heavy nuclides far from the line of beta-stability. The model presented here is used to calculate the amount of delayed fission during the decay back of nuclides produced in the astrophysical decay r-process. An interesting feature of these calculations is that they reproduce the reversal of the odd-even effect found for the yield of beta-stable nuclides obtained in the thermonuclear explosions. The preliminary results from the r-process calculations indicate that the effect of delayed fission has to be considered when the yield of very heavy beta stable nuclides from this process is estimated.

Self-consistent calculations of highly excited nuclei are presented. The changes in the average nuclear field as a function of temperature are discussed; they are found to be negligible in the calculation of the entropy versus excitation energy at a fixed deformation. The disappearance of shell effects at temperatures T ≃ 2-3 MeV in heavy nuclei is demonstrated both by calculations of deformation energy curves at different temperatures and by considering the asymptotic behaviour of the entropy. Finally, the validity of some simplifying approximations is discussed.

In collisions of very heavy ions (Z₁ + Z₂ > 172) molecular electronic orbitals are formed. Generalized Hartree-Fock equations for this case are derived from field theoretic principles. It is shown that the lowest ground state (i.e. the vacuum) becomes charged for Z > Z_cr and R < R_cr. If the K-shell is vacant, positron production sets in. The energy distribution of the positrons and the cross-section for this process are calculated.

The concept of a "Superheavy Quasiatom" is discussed. Radiative transition times are compared with the lifetime of the intermediate system, cross sections are calculated within a two-collision model and induced transitions and their anisotropic emission are discussed. Recent experimental and theoretical results are presented from collision systems obtained with I-beams bombarding various heavy targets, giving combined Z-values between 120 and 145. Results include the energy dependence of the peak structure interpreted as M X-rays from superheavy quasiatoms and the anisotropy of X-ray emission referred to the beam direction. The data are discussed within the models available. These cannot explain the strong emission of anisotropic radiation in the X-ray energy range of quasiatomic M X-ray at small bombarding energies.

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