Pairing and shell effects in the transport coefficients of collective motion

The linear response approach to nuclear transport has been extended to pair correlations. The latter are treated within a mean field approximation to a pairing interaction with constant matrix elements $G$. The constraint of particle number conservation has been accounted for on a time dependent average, which leads to modified response functions, both in the pairing degree of freedom as well as in the shape variable. The former is expressed by the gap parameter $\Delta$ and the latter by a $Q$ which specifies the elongation of a fissioning nucleus. The tensors for friction and inertia corresponding to these two collective coordinates are computed along the fission path of $^{224}Th$ for temperatures around T=1 \mev and less. It is seen that dissipation decreases with decreasing temperature and increasing pairing gap and falls well below the values of common "macroscopic models". Both friction and inertia show a sensible dependence on the configurations of the mean field caused both by shell effects as well as by avoided crossings of single-particle levels.Comment: 39 pages, LaTex, 10 postscript figures; submitted to Nuclear Physics A; e-mail: [email protected]; [email protected]

A Particle number conserving shell-correction method

The shell correction method is revisited. Contrary to the traditional Strutinsky method, the shell energy is evaluated by an averaging over the number of particles and not over the single-particle energies, which is more consistent with the definition of the macroscopic energy. In addition, the smooth background is subtracted before averaging the sum of single-particle energies, which significantly improves the plateau condition and allows to apply the method also for nuclei close to the proton or neutron drip lines. A significant difference between the shell correction energy obtained with the traditional and the new method is found in particular for highly degenerated single-particle spectra (as i.e. in magic nuclei) while for deformed nuclei (where the degeneracy is lifted to a large extent) both estimates are close, except in the region of super or hyper-deformed states.Comment: 11 pages in LaTeX, 7 figure

Time scales for fission at finite temperature

The concept of the "transient effect" is examined in respect of a "mean first passage time". It is demonstrated that the time the fissioning system stays inside the barrier is much larger than suggested by the transient time, and that no enhancement of emission of neutrons over that given by Kramers' rate formula ought to be considered.Comment: 6 pages, LaTex, 3 postscript figures; Keywords: Decay rate, transient effect, mean first passage time; "Symposium on Nuclear Clusters", Rauischholzhausen, Germany, 5-9 August 200

Variation of transport coefficients for average fission dynamics with temperature and shape

We study slow collective motion at finite thermal excitations on the basis of linear response theory applied to the locally harmonic approximation. The transport coefficients for average motion, friction \gamma, inertia M and the local stiffness C are computed along a fission path of Th-224 within a quasi-static picture. The inverse relaxation time \beta=\gamma/M and the effective damping rate \eta=\gamma/(2\sqrt{M|C|}) are found to increase with temperature, but do not change much with the collective variable. The values found for \eta and \beta as well as their behavior with temperature are in accord with experimental findings.Comment: 23 pages, including figures, Latex plus 11 postscript files for figures; home page: http://www.physik.tu-muenchen.de/tumphy/e/T36/hofmann.htm

Nuclear fission: The "onset of dissipation" from a microscopic point of view

Semi-analytical expressions are suggested for the temperature dependence of those combinations of transport coefficients which govern the fission process. This is based on experience with numerical calculations within the linear response approach and the locally harmonic approximation. A reduced version of the latter is seen to comply with Kramers' simplified picture of fission. It is argued that for variable inertia his formula has to be generalized, as already required by the need that for overdamped motion the inertia must not appear at all. This situation may already occur above T=2 MeV, where the rate is determined by the Smoluchowski equation. Consequently, comparison with experimental results do not give information on the effective damping rate, as often claimed, but on a special combination of local stiffnesses and the friction coefficient calculated at the barrier.Comment: 31 pages, LaTex, 9 postscript figures; final, more concise version, accepted for publication in PRC, with new arguments about the T-dependence of the inertia; e-mail: [email protected]

Dynamical restriction for a growing neck due to mass parameters in a dinuclear system

Mass parameters for collective variables of a dinuclear system and strongly deformed mononucleus are microscopically formulated with the linear response theory making use of the width of single particle states and the fluctuation-dissipation theorem. For the relative motion of the nuclei and for the degree of freedom describing the neck between the nuclei, we calculate mass parameters with basis states of the adiabatic and diabatic two-center shell model. Microscopical mass parameters are found larger than the ones obtained with the hydrodynamical model and give a strong hindrance for a melting of the dinuclear system along the internuclear distance into a compound system. Therefore, the dinuclear system lives a long time enough comparable to the reaction time for fusion by nucleon transfer. Consequences of this effect for the complete fusion process are discussed.Comment: 22 pages, 7 figures, submitted to Nucl.Phys.