Resonance Model for π\piN Scattering and η\eta-Meson Production in the s11_{11} Channel

A model for $\pi$N scattering and $\eta$-meson production in the S$_{11}$ channel is presented. The model includes $\pi$N-scattering Born terms as well as the N$^*${} resonances S$_{11}$(1535) and S$_{11}$(1610). The $T$-matrix is computed in the $K$-matrix approximation. The parameters of the model are determined by fitting the elastic $\pi$N-scattering $T$-matrix to empirical data. We find an excellent fit for all energies up to $\sqrt{s}$ = 1.75 GeV. Furthermore, a good description of the cross section for $\pi^- + p \rightarrow \eta + n$ is obtained without further adjustment of parameters.Comment: LaTex 10 pages, 4 figures (not included;the postscript-figures are available from [email protected] in subdirectory pub/sauerman, files: fig1.ps, fig2.ps,fig3.ps,fig4.ps- or contact the authors via e-mail), GSI-94-4

The path of hot nuclei towards multifragmentation

The initial production and dynamical expansion of hot spherical nuclei are examined as the first stage in the projectile-multifragment ation process. The initial temperatures, which are necessary for entering the adiabatic spinodal region, as well as the minimum temperatures and densities, which are reached in the expansion, significantly differ for hard and soft equations of state. Additional initial compression, occurring in central collisions leads most likely to a qualitatively different multifragmentation mechanism. Recent experimental data are discussed in relation to the results of the proposed model

Thermodynamics of Delta resonances

The thermodynamic potential of a system of pions and nucleons is computed including the piN interactions in the P33 channel. A consistent treatment of the width of the resonance in this channel, the Delta(1232) resonance, is explored in detail. In the low-density limit we recover the leading term of the virial expansion for the thermodynamic potential. An instructive diagrammatic interpretation of the contributions to the total baryon number is presented. Furthermore, we examine within a fireball model the consequences for the pion spectra in heavy-ion collisions at intermediate energies, including the effect of collective flow. A consistent treatment of the Delta width leads to a substantial enhancement of the pion yield at low momenta.Comment: 12 pages, 3 Postscript figures, LaTeX, elsart, epsfig, minor changes, references added, to be published in Physics Letters

Energy dependence of nucleus-nucleus potential close to the Coulomb barrier

The nucleus-nucleus interaction potentials in heavy-ion fusion reactions are extracted from the microscopic time-dependent Hartree-Fock theory for mass symmetric reactions $^{16}$O${}+^{16}$O, $^{40}$Ca${}+^{40}$Ca, $^{48}$Ca${}+^{48}$Ca and mass asymmetric reactions $^{16}$O$+^{40,48}$Ca, $^{40}$Ca${}+^{48}$Ca, $^{16}$O+$^{208}$Pb, $^{40}$Ca+$^{90}$Zr. When the center-of-mass energy is much higher than the Coulomb barrier energy, potentials deduced with the microscopic theory identify with the frozen density approximation. As the center-of-mass energy decreases and approaches the Coulomb barrier, potentials become energy dependent. This dependence signs dynamical reorganization of internal degrees of freedom and leads to a reduction of the "apparent" barrier felt by the two nuclei during fusion of the order of $2-3$ compared to the frozen density case. Several examples illustrate that the potential landscape changes rapidly when the center-of-mass energy is in the vicinity of the Coulomb barrier energy. The energy dependence is expected to have a significant role on fusion around the Coulomb barrier.Comment: 11 pages, 13 figures, 1 table, discussion of effects of coordinate-dependent mass added, accepted for publication in Phys. Rev.

Thermodynamics of nuclei in thermal contact

The behaviour of a di-nuclear system in the regime of strong pairing correlations is studied with the methods of statistical mechanics. It is shown that the thermal averaging is strong enough to assure the application of thermodynamical methods to the energy exchange between the two nuclei in contact. In particular, thermal averaging justifies the definition of a nuclear temperature.Comment: 9 pages, 1 figur

Effect of angular-momentum dissipation and fluctuation on energy coherence lengths and time evolution in the dissipative collision 28Si + 48Ti

Abstract We analyze the energy autocorrelation functions and the energy coherence lengths in the strongly dissipative collision 28 Si (E lab = 130 MeV ) + 48 Ti for Z = 11 and 12 reaction fragments. It is found that in order to obtain a good fit of both the energy-averaged angular distributions and the angular dependence of the energy coherence lengths one has to take into account (i) the dissipation and fluctuation of the relative angular momentum of the dinucleus and (ii) the contribution from direct (fast) reactions in addition to the statistical (relative slow) interaction processes. The established angular dependence is a direct consequence of the angular-momentum dissipation-fluctuation effects on the time-space evolution of the intermediate dinucleus

Final excitation energy of fission fragments

We study how the excitation energy of the fully accelerated fission fragments is built up. It is stressed that only the intrinsic excitation energy available before scission can be exchanged between the fission fragments to achieve thermal equilibrium. This is in contradiction with most models used to calculate prompt neutron emission where it is assumed that the total excitation energy of the final fragments is shared between the fragments by the condition of equal temperatures. We also study the intrinsic excitation-energy partition according to a level density description with a transition from a constant-temperature regime to a Fermi-gas regime. Complete or partial excitation-energy sorting is found at energies well above the transition energy.Comment: 8 pages, 3 figure

Melting or nucleon transfer in fusion of heavy nuclei?

The time-dependent transition between a diabatic interaction potential in the entrance channel and an adiabatic potential during the fusion process is investigated within the two-center shell model. A large hindrance is obtained for the motion to smaller elongations of near symmetric dinuclear systems. The comparison of the calculated energy thresholds for the complete fusion in different relevant collective variables shows that the dinuclear system prefers to evolve in the mass asymmetry coordinate by nucleon transfer to the compound nucleus.Comment: 14 pages, 3 figures, submitted to Phys.Lett.