T. I. Mikhailova

Collective dynamics of nuclear fusion: deformation changes and heating during the fusion

Abstract The formalism developed elsewhere for the theoretical description of the dynamics involved in the heavy nuclei fusion is applied in this paper to study the “history” of the fusion of two identical heavy nuclei experiencing central collision. The evolution of the shape and of the temperature of symmetrical fusing systems is studied. The role of the elastoplasticity of nuclear matter in the nonmonotonical changes of the shape is elucidated in this way. A tentative explanation of the “extra push” phenomenon is given in terms of the competition between elastic properties of fusing systems driving to the re-separation of colliding nuclei and the dissipative (plastic) properties of nuclear matter transforming the energy of collective motion into the energy of statistical excitation and thus leading to the fusion. The fingerprints of the heavy-nuclei fusion history as it is depicted by the model are traced in the anisotropy of the dipole and quadrupole γ-radiation emitted during the fusion. The parallels in the description of the fusion dynamics given by the simple model used in this paper and by the more fundamental approaches based on the kinetic equation are emphasised.

Asymmetry of velocity distributions in peripheral reactions with heavy ions at Fermi energies

The asymmetry of velocity distributions of projectile-like fragments produced in heavy ion collisions is considered. The calculations performed in the transport model approach (Vlasov kinetic equation with the collision term) are compared with the experimental data for the 22Ne (40MeV/nucleon) + 9Be and 18O (35 MeV/nucleon) + 9Be(181Ta) reactions. It is found that the velocity distributions contain two components: a direct component centered at the beam velocity and a dissipative component at lower energies, leading to asymmetry of velocity distributions. The direct component is interpreted empirically within the Goldhaber model, and the centroids and widths σ0 of the distributions for each fragment are extracted. It is found that value of σ0 derived from experimental data is smaller by a factor of 2 than the theoretical one. The dissipative (also called deep inelastic) component is described well by the transport calculations. It is shown that the ratio of yields of direct and dissipative components, which determines the asymmetry of velocity distributions, depends on shape of the deflection function.

Heating and fluctuations in elastoplastic systems

Abstract Starting from the equations of motion of a simple system possessing the properties of elastic and plastic bodies, we construct its Lagrangian and Hamiltonian functions and also the Rayleigh dissipation function. This allows us to find the rate of heating of the system and to analyze the fluctuations of basic observables. Introducing into the Hamilton-Rayleigh equation of motion a random force producing on average the same effects as a dissipation function, we arrive first at the Langevin equations describing the fluctuations and then at a kinetic equation for the distribution function defined in the space of the collective variables. In this way a rather general scheme is established for solving dynamical problems in different and more complex elastoplastic systems, in nuclear physics and maybe even in physics of molecules and atomic clusters. In a preliminary study, the model is applied to estimate the probability of the quasi-fission process coming from the thermal fluctuations of the nuclear shape.

Investigation of the fragmentation of 20Ne and 40Ar ions at the COMBAS setup

Properties of the COMBAS fragment separator are compared with respective properties of similar setups. Results of experiments aimed at obtaining products of one-proton-stripping reactions induced by a beam of 40Ar ions with an energy of 35 MeV/A and two-neutron-stripping reactions induced by a beam of 20Ne ions with an energy of 52 MeV/A are presented. A high resolution of the fragment separator in obtaining secondary neutron-rich 39Cl and neutron-deficient 18Ne ion beams is demonstrated.

Dissipative processes in peripheral heavy ion collisions at Fermi energies

Peripheral heavy ion reactions are of interest for the production of new isotopes. In this contribution we present an investigation of reactions of light projectiles O and Ne on Be and Ta targets at Fermi energies in the framework of transport theory. Transport theory describes dissipative (deep-inelastic) processes, where considerable amounts of mass and energy are exchanged. The data, on the other hand, also seem to include a more direct component with small energy loss. We separate the two components on the basis of the velocity distribution and compare the dissipative component to the transport calculations. The primary fragments of the transport calculation still have considerable excitation energies. For the comparison with experiment we take into account the secondary evaporation in a statistical model. This improves the qualitative agreement with the data.

Projectile fragmentation of 40,48Ca and isotopic scaling in a transport approach

We investigate theoretically projectile fragmentation in reactions of 40,48Ca on 9Be and 181Ta targets using a Boltzmann-type transport approach, which is supplemented by a statistical decay code to describe the de-excitation of the hot primary fragments. We determine the thermodynamical properties of the primary fragments and calculate the isotope distributions of the cold final fragments. These describe the data reasonably well. For the pairs of projectiles with different isotopic content we analyze the isotopic scaling (or isoscaling) of the final fragment distributions, which has been used to extract the symmetry energy of the primary source. The calculation exhibits isoscaling behavior for the total yields as do the experiments. We also perform an impact-parameter-dependent isoscaling analysis in view of the fact that the primary systems at different impact parameters have very different properties. Then the isoscaling behavior is less stringent, which we can attribute to specific structure effects of the 40,48Ca pair. The symmetry energy determined in this way depends on these structure effects.

Total reaction cross sections measurement for 6,8He and 8,9Li nuclei with energies of (25–45)/A MeV on natAl and natPb

Preliminary results of measurements of the total reaction cross sections σR for weakly-bound 6,8He and 8,9Li nuclei at energy range (25–45)/A MeV on 27Al and 208Pb targets are presented. The secondary beams of 6,8He and 8,9Li were produced by bombardment of the 11B (33 A MeV) primary beam on Be (89 mg cm–2) target and separated by COMBAS fragment-separator. In dispersive focal plane a horizontal slit defined the momentum acceptance as 1% and a wedge degrader of 200 μm Al was installed. The Bρ of the second section of the fragment-separator was adjusted for measurements in energy range (25–45)/A MeV. The secondary products were detected by a telescope consisting of two Si ΔE detectors 300, 1000 μm and E-detector, which consisted of nine CsI/Tl granules.

Projectile fragmentation at Fermi energies with transport simulations

Projectile fragmentation at Fermi energies is an important method to produce radioactive beams for the study of isospin asymmetric nuclear matter. Fragmentation is usually parametrized successfully by empirical phase space models. In this contribution we apply a microscopical method, semiclassical transport theory, to study in detail the reaction mechanism of the fragmentation process. We apply it to experimental data of 18O on 181Ta at E/A = 35 MeV measured in Dubna. We calculate consistently the excitation energy of the primary fragments and take into account their decay by a statistical model. It is found that the dissipative part of the fragment spectra is well described by transport theory. However, there are in addition important direct and collective contributions.

Total reaction cross sections and neutron-removal cross sections of neutron-rich light nuclei measured by the COMBAS fragment-separator

Preliminary results of measurements of the total reaction cross sections σR and neutron removal cross section σ-xn for weakly bound 6He, 8Li, 9Be and 10Be nuclei at energy range (20-35) A MeV with 28Si target is presented. The secondary beams of light nuclei were produced by bombardment of the 22Ne (35 A MeV) primary beam on Be target and separated by COMBAS fragment-separator. In dispersive focal plane a horizontal slit defined the momentum acceptance as 1% and a wedge degrader of 200 μm Al was installed. The Bρ of the second section of the fragment-separator was adjusted for measurements in energy range (20-35) A MeV. Two-neutron removal cross sections for 6He and 10Be and one –neutron removal cross sections 8Li and 9Be were measured.

Radioactive nuclear beams of COMBAS facility

The basic ion-optical characteristics of the luminosity and the high-resolution of kinematic separator COMBAS realized for the first time on the strong focusing principle are presented. The developed facility allows to separate the high-intensity secondary radioactive beams in a wide range of mass numbers A and atomic numbers Z which are produced in heavy ion reactions in the energy range of 20 ≤ E ≤ 100 MeV/A (Fermi energy domain). Two distinct detector systems such as realized Si strip detector telescope and the promising development of the three dimension time-projection chamber are discussed. Program of the investigations of nuclear reaction mechanisms at intermediate energies of 20–100 MeV/A, measurement of the radii of unstable nuclei, study of the cluster structure of light nuclei near the nuclear drip-line and search of 26,28O resonances in exchange reactions is proposed. The upgrading of experimental facility by the integration of COMBAS separator with the Ion Catcher is discussed.