Competition of fusion and quasi-fission in the reactions leading to production of the superheavy elements

The mechanism of fusion hindrance, an effect observed in the reactions of cold, warm and hot fusion leading to production of the superheavy elements, is investigated. A systematics of transfermium production cross sections is used to determine fusion probabilities. Mechanism of fusion hindrance is described as a competition of fusion and quasi-fission. Available evaporation residue cross sections in the superheavy region are reproduced satisfactorily. Analysis of the measured capture cross sections is performed and a sudden disappearance of the capture cross sections is observed at low fusion probabilities. A dependence of the fusion hindrance on the asymmetry of the projectile-target system is investigated using the available data. The most promising pathways for further experiments are suggested.Comment: 8 pages, 7 figures, talk presented at 7th International School-Seminar on Heavy-Ion Physics, May 27 - June 1, 2002, Dubna, Russi

Charged pions from Ni on Ni collisions between 1 and 2 AGeV

Charged pions from Ni + Ni reactions at 1.05, 1.45 and 1.93 AGeV are measured with the FOPI detector. The mean $\pi^{\pm}$ multiplicities per mean number of participants increase with beam energy, in accordance with earlier studies of the Ar + KCl and La + La systems. The pion kinetic energy spectra have concave shape and are fitted by the superposition of two Boltzmann distributions with different temperatures. These apparent temperatures depend only weakly on bombarding energy. The pion angular distributions show a forward/backward enhancement at all energies, but not the $\Theta = 90^0$ enhancement which was observed in case of the Au + Au system. These features also determine the rapidity distributions which are therefore in disagreement with the hypothesis of one thermal source. The importance of the Coulomb interaction and of the pion rescattering by spectator matter in producing these phenomena is discussed.Comment: 22 pages, Latex using documentstyle[12pt,a4,epsfig], to appear in Z. Phys.

Event Anisotropy in High Energy Nucleus-Nucleus Collisions

The predictions of event anisotropy parameters from transport model RQMD are compared with the recent experimental measurements for 158$A$ GeV Pb+Pb collisions. Using the same model, we study the time evolution of event anisotropy at 2$A$ GeV and 158$A$ GeV for several colliding systems. For the first time, both momentum and configuration space information are studied using the Fourier analysis of the azimuthal angular distribution. We find that, in the model, the initial geometry of the collision plays a dominant role in determining the anisotropy parameters.Comment: 18 pages, 7 figures, 2 table

A proposed reaction channel for the synthesis of the superheavy nucleus Z = 109

We apply a statistical-evaporation model (HIVAP) to calculate the cross sections of superheavy elements, mainly about actinide targets and compare with some available experimental data. A reaction channel $^{30}$Si + $^{243}$Am is proposed for the synthesis of the element Z = 109 and the cross section is estimated.Comment: 4 pages, 2 figures, 2 tables; two typos are corrected in Ref. [12] and [19

Emission times and opacities from interferometry in non-central Relativistic Nuclear Collisions

The nuclear overlap zone in non-central relativistic heavy ion collisions is azimuthally very asymmetric. By varying the angle between the axes of deformation and the transverse direction of the pair momenta, the transverse HBT radii oscillate in a characteristic way. It is shown that these oscillations allow determination of source sizes, deformations as well as the opacity and duration of emission of the source created in any non-central high energy nuclear collisions. The behavior of the physical quantities with centrality of the collisions is discussed --- in particular changes caused by a possible phase transition to a quark-gluon plasma.Comment: Revised version, to appear in Phys. Rev. Letter

Antisymmetrized molecular dynamics with quantum branching processes for collisions of heavy nuclei

Antisymmetrized molecular dynamics (AMD) with quantum branching processes is reformulated so that it can be applicable to the collisions of heavy nuclei such as Au + Au multifragmentation reactions. The quantum branching process due to the wave packet diffusion effect is treated as a random term in a Langevin-type equation of motion, whose numerical treatment is much easier than the method of the previous papers. Furthermore a new approximation formula, called the triple-loop approximation, is introduced in order to evaluate the Hamiltonian in the equation of motion with much less computation time than the exact calculation. A calculation is performed for the Au + Au central collisions at 150 MeV/nucleon. The result shows that AMD almost reproduces the copious fragment formation in this reaction.Comment: 24 pages, 5 figures embedde

Origin of subthreshold K^+ production in heavy ion collisions

We investigate the origin of subthreshold $K^+$ production in heavy ion collisions at intermediate energies. In particular we study the influence of the pion induced $K^+$ creation processes. We find that this channel shows a strong dependence on the size of the system, i.e., the number of participating nucleons as well as on the incident energy of the reaction. In an energy region between 1--2 GeV/nucleon the pion induced processes essentially contribute to the total yield and can even become dominant in reactions with a large number of participating nucleons. Thus we are able to reproduce recent measurements of the KaoS Collaboration for 1 GeV/nucleon Au on Au reactions adopting a realistic momentum dependent nuclear mean field.Comment: 6 pages Latex using RevTex, revised version accepted for publication in Phys. Rev.

Transverse Energy per Charged Particle and Freeze-Out Criteria in Heavy-Ion Collisions

In relativistic nucleus-nucleus collisions the transverse energy per charged particle, E_T/N_ch, increases rapidly with beam energy and remains approximately constant at about 800 MeV for beam energies from SPS to RHIC. It is shown that the hadron resonance gas model describes the energy dependence, as well as the lack of centrality dependence, qualitatively. The values of E_T/N_ch are related to the chemical freeze-out criterium E/N about 1 GeV valid for primordial hadrons.Comment: 8 pages, 5 figure

Anisotropic flow in 4.2A GeV/c C+Ta collisions

Anisotropic flow of protons and negative pions in 4.2A GeV/c C+Ta collisions is studied using the Fourier analysis of azimuthal distributions. The protons exhibit pronounced directed flow. Directed flow of pions is positive in the entire rapidity interval and indicates that the pions are preferentially emitted in the reaction plane from the target to the projectile. The elliptic flow of protons and negative pions is close to zero. Comparison with the quark-gluon-string model (QGSM) and relativistic transport model (ART 1.0) show that they both yield a flow signature similar to the experimental data.Comment: 4 pages, 3 figures, Accepted for publication in Phys. Rev.

Differential flow in heavy-ion collisions at balance energies

A strong differential transverse collective flow is predicted for the first time to occur in heavy-ion collisions at balance energies. We also give a novel explanation for the disappearance of the total transverse collective flow at the balance energies. It is further shown that the differential flow especially at high transverse momenta is a useful microscope capable of resolving the balance energy's dual sensitivity to both the nuclear equation of state and in-medium nucleon-nucleon cross sections in the reaction dynamics.Comment: Phys. Rev. Lett. (1999) in pres