Flow Of Nucleons Research Articles

FLUID-DYNAMICAL APPROACH TO THE DAMPING OF NUCLEAR TRANSVERSE EXCITATIONS

Within the dissipative nuclear fluid-dynamics, the predictions are presented for the widths of Mλ, T=0 twist and E1, T=0 torus resonances associated with the longwavelength transverse vibrations of a nucleon flow. The macroscopic damping mechanism is presumed to be originated from the individual nucleon collisions with each other resulting in the shear viscosity. The collisional width of magnetic twist resonance as a function of mass number and multipole degree is found to be γ(Mλ)≈6(2λ+3)(λ−1) A−2/3 MeV. The width of the dipole torus mode is given by Γ tor (E1, T=0)≈130 A −2/3 MeV .

Is collective pion flow anticorrelated to nucleon flow?

Abstract The in-plane transverse momentum of pions in Ne(2A GeV)Pb and Au(2A GeV)Au collisions is analysed in the framework of the Vlasov-Uehling-Uhlenbeck model. The rapidity dependence of pion flow shows the opposite sign as the nucleon flow. The anticorrelation of pions and nucleons can be explained by multiple πN scattering. It turns out that the nonzero transverse momentum for pions at target rapidities is no proof for pion absorption in contrast to claims in previous works. Pion absorption does not influence the angular distribution of pions but plays an important role for the absolute numbers of produced pions.

Transverse baryon flow as possible evidence for a quark-gluon-plasma phase.

In order to investigate the coupling between collective flow of nucleons and pions in hot pion-dominated hadronic matter, we calculate the pion-nucleon drag coefficient in linearized transport theory. We find that the characteristic time for flow equalization is longer than the time scale of the expansion of a hardonic fireball created in high-energy collisions. The analysis of transverse-momentum data from p+p\ifmmode\bar\else\textasciimacron\fi{} collisions at \ensuremath{\surd}s =1.8 TeV reveals the same flow velocity for mesons and antinucleons. We argue that this may be evidence for the formation of a quark-gluon plasma in these collisions.

Enhanced emission of high-energy photons perpendicular to the reaction plane in alpha +Th reactions.

High-energy photon and neutron emission has been measured in coincidence with fission fragments in \ensuremath{\alpha}${+}^{232}$ Th reactions at 170 MeV. From measurements parallel and perpendicular to the fission plane, anisotropies relative to the reaction plane were determined. The in-plane/out-of-plane intensity ratio is 0.72(7) for photons with energies above 20 MeV and 11(3) for neutrons at 35 MeV. The result for high-energy photons can be explained by nucleon-nucleon bremsstrahlung if the initial flow of nucleons has a correlation to the reaction plane similar to the one observed for fast neutrons.

Nucleon exchange and heat partition in damped collisions.

Mass and charge distributions have been measured for damped projectile-like fragments in the reaction $^{74}$Ge${+}^{165}$Ho at 8.5 MeV per nucleon bombarding energy. Coincidences were measured between angle-correlated reaction partners in order to derive the primary mass distribution and the excitation energy distribution for projectile-like fragments. The evolution of the primary N and Z distributions as a function of energy loss is found to deviate significantly from predictions of the nucleon exchange transport model. The fraction of excitation energy residing in the projectile-like fragment is shown to depend both upon energy loss and the direction of the nucleon flow.

Partial-transparency effects in heavy-ion collisions at energies of the order of 1 GeV/nucleon

Abstract A model of heavy-ion collisions at energies E lab ∼ 1 GeV/nucleon taking into account the effects of incomplete statistical equilibrium in highly-excited nuclear matter is presented. The collision process is considered as the interaction of two flows of nucleons decelerating each other. This process is described within the framework of a relativistic kinetic approach employing the Fokker-Planck approximation. Assuming two-flow nonequilibrium the momentum distribution function is represented as a sum of two maxwellian distributions displaced by the average relative velocity of flows. Equations for time evolution of space-averaged velocities and internal energies of flows are derived. These equations contain a single model parameter, i.e. the effective deceleration length λ d . Using the firestreak model geometry inclusive cross sections of protons and composite particles (d, t) in the reactions Ne + NaF, Ar + KCl and Ar + Pb are calculated at various values of E lab . In contrast to the firestreak model, a complete stopping of colliding tubes in the centre-of-mass frame was not assumed in the present calculation. Composite particle spectra are calculated on the basis of the coalescence model. The theory is thoroughly compared with the experimental data. The approach suggested allows us to reproduce a two-humped structure in the rapidity distributions of the secondary particles. The experimental data analysis leads to the value λ d = 8 fm for E lab = 0.8 GeV/nucleon, which is in good agreement with the estimation based on experimental NN cross sections.

Fast nucleon emission: A probe of the heavy ion reactions at energies below 100 MeV/u

After a review of the various theoretical models which describe the fast nucleon emission in heavy-ion reactions at bombarding energies lower than 100 MeV/u, the Boltzmann equation is discussed as a tool for studying the reaction mechanisms in this energy range. The Boltzmann equation allows to analyze the balance between one — and two — body processes. A linearized version is derived to study the transport in nuclei of fast nucleon flows generated by the reaction dynamics. Results are presented showing that the high energy tails of the spectras can be interpreted without reference to statistical emissions from hot nuclear fragments.

Energetic interplanetary nucleon flux anisotropies: The effect of Earth's bow shock and magnetosheath on sunward flow

Recent statistical surveys of interplanetary MeV energy nucleon flux anisotropies observed between prompt solar particle events during solar activity minimum have included time blocks of data obtained when the interplanetary magnetic field (IMF) connects the near earth spacecraft to earth's bow shock. The ensemble average nucleon flux anisotropy in the solar wind reference frame <ξD> obtained on these ‘connected’ field lines does not accurately represent the unperturbed <ξD> obtained on field lines free of this magnetic connection. Hourly average observations of interplanetary 0.5‐ to 1.8‐MeV proton fluxes, obtained near earth from 1972 to 1977, are correlated herein with simultaneous measurements of the IMF and solar wind plasma. At moderate flux levels during solar activity minimum, <ξD> indicates that nucleon flow in the solar wind frame is toward the sun and primarily along the IMF, although <ξD> is directed more sunward than strict field‐aligned propagation requires. Cross‐field transport is statistically significant and in the direction expected from the large‐scale MeV energy nucleon flux distributions throughout the heliosphere. The unperturbed nucleon flow direction relative to the IMF is used to demonstrate and characterize the interaction of MeV energy nuclei with the earth's bow shock and magnetosheath. The result of this interaction is that the mean value of <ξD> perpendicular to connected field lines is consistent with zero and therefore is not statistically significant. Obstruction of sunward nucleon flow on connected field lines is indicated by the variation of <ξD> with spacecraft position.

Microscopic Dynamical Calculation of Nucleon Flow in Heavy-Ion Reactions

The nucleon exchange process in deep-inelastic collisions is investigated by studying the one-way current constructed using the Wigner transform of the time-dependent Hartree-Fock one-body density matrix. Detailed calculations for head-on $^{16}\mathrm{O}$+$^{16}\mathrm{O}$ collisions at three different bombarding energies have been performed. A combination of two simple models for the contributions from relative motion and tunneling is shown to predict one-way currents close to the time-dependent Hartree-Fock results.

Neutron rich nucleon flow in central heavy ion collisions

In reactions of 1.2 GeV 208Pb ions with 110Pd nuclei, mass, charge, and energy of the products were measured at various angles. Transfers up to 50 nucleons connected with very low excitation energies of the fragments were observed especially for reactions leading to large mass asymmetry. For central collisions the neutron to proton ratio of the nucleon flow has the unusually high value of 2.5.