Nuclear rotational population patterns in heavy-ion scattering and transfer reactions

A model of²³⁹Pu with decoupled neutron is used for theoretical calculations of rotational population patterns in heavy ion inelastic scattering and one-neutron transfer reactions. The system treated is⁹⁰Zr on²³⁹Pu at the near-barrier energy of 500 MeV and backscattering angles of 180° and 140°. The influence of the complex nuclear optical potential is seen to be very strong, and the Nilsson wave function of the odd neutron produces a distinctive pattern in the transfer reaction.     

Bremsstrahlung in heavy-ion reactions

We investigate bremsstrahlung processes induced by heavy ions: nuclear dipole and quadrupole radiation, radiation from bound target electrons into the continuum and secondary electron bremsstrahlung (SEB), which contribute to the background of the X-ray spectra. A comparison with experiment is presented.     

Polarization in heavy-ion reactions

A series of experiments is described in which beta-ray asymmetry has been used to determine polarization of heavy-ion reaction products¹²B and the present status of the studies of polarization phenomena in heavy-ion reactions is reviewed. A large amount of angular momentum sustained by the two colliding nuclei gives rise to polarization phenomena of reaction products. Coupling between the degrees of freedom accompanying the intrinsic and the relative motions is investigated from the systematic behaviour of polarization of reaction products disclosed by the experiments.     

Alpha-transfer processes in heavy-ion scattering

In the differential cross sections for the elastic scattering between an α-conjugate target and projectile, a rising oscillatory structure is often observed in the backward-angle region. An α-transfer mechanism is proposed to explain this anomalous phenomenon. A nuclear molecularorbit approximation theory for both 1α- and 2α-transfer processes has been formulated and applied to ¹⁶O + ²⁰Ne and ¹²C + ²⁰Ne scattering systems with different projectile energies. The experimental rising structures shown in these scatterings are well reproduced with parameters fairly consistent with spectroscopic data. An independent-α-particle model wave function has been used for the evaluation of the exchange potential, which gives better agreement with experiment than the Buttle-Goldfarb approximation can usually provide.     

Polarization potentials in heavy-ion scattering

A polarization potential is defined in terms of the Feshbach projection operator formalism to represent the effect upon the elastic channel of the coupling to non-elastic channels in heavy-ion scattering. The polarization potential represents coupling to specific surface degrees of freedom of the particular reaction considered and it is contrasted to the complementary global approaches for the volume potential such as the folding model and the proximity potential. The coupled channels method is used both as a source of exact model solutions for comparison with the various approximate potential forms and also as a numerical means of constructing trivially equivalent local potentials. The imaginary Coulomb polarization potential is due in lowest order to quadrupole coupling to the lowest collective 2⁺ state of a nucleus. It is considered in detail since it provides the insight of closed analytical forms in various approximations. Multistep coupling to higher states, energy loss and off-energy shell effects are also considered analytically. The real Coulomb polarization potential due to the virtual excitation of multipole giant resonances, and the polarization potential arising from relativistic corrections, are investigated in detail. Polarization potential components due to nuclear coupling are investigated numerically. Analytical cross section approaches are contrasted with the polarization potential approach and with coupled channels.     

Deformation effects in heavy-ion scattering

Many experiments on the scattering of heavy ions confirm that the elastic cross sections display characteristics of Fresnel diffraction. Recent experiments on the scattering of very heavy ions, however, show marked deviations from the expected Fresnel shapes and an analysis of these experiments using the “quarter-point-recipe” of the Fresnel cross section yields a larger radius for ²⁰⁸Pb than for ²³²Th. It is shown that the deviations may be described in terms of the ground state deformations of the nuclei involved. Taking the deformations into account removes the above anomaly in the radii. The actual fit to the experimental cross sections is not very good and suggests that the real part of the nuclear potential may have an appreciable effect on the orbits of nuclei which are elastically scattered below the Coulomb barrier.     

Parity dependence in heavy-ion scattering

A range is defined for the effects of parity dependence in heavy-ion scattering. This range is shown to be related to the terms of the antisymmetrizer which exchange the largest number of nucleons between both nuclei. A simple formula, derived in the two-center harmonic oscillator model, gives an upper bound for the parity range. A criterion is proposed to determine whether a parity-dependent real part should be used in heavy-ion optical potentials. The most important parity effects should be expected in scattering between nuclei with neighbouring masses.     

Resonance propagation in heavy-ion scattering

The formalism developed earlier by us for the propagation of a resonance in the nuclear medium in proton-nucleus collisions has been modified to the case of vector boson production in heavy-ion collisions. The formalism includes coherently the contribution to the observed di-lepton production from the decay of a vector boson inside as well as outside the nuclear medium. The medium modification of the boson is incorporated through an energy dependent optical potential. The calculated invariant ρ mass distributions are presented for the ρ-meson production using optical potentials estimated within the VDM and the resonance model. The shift in the invariant mass distribution is found to be small. To achieve the mass shift (of about 200 MeV towards lower mass) as indicated in the high energy heavy-ion collision experiments, an unusually strong optical potential of about −120 MeV is required. We also observe that, for not so heavy nuclear systems and/or for fast moving resonances, the shape, magnitude and peak position of the invariant mass distribution is substantially different if the contributions from the resonance decay inside and outside are summed-up at the amplitude level (coherently) or at the cross section level (incoherently).     

Factorization in relativistic heavy-ion scattering

We have calculated the total cross sections for relativistic nucleus-nucleus scattering in the Glauber theory and conclude that there will be no factorization, due to the short-range nature of nucleon-nucleon interaction as compared to the sizes of the colliding nuclei.     

Intermediate structure in heavy-ion reactions

The intermediate structure seen in some heavy-ion reactions is described by the fragmentation of a quasi-molecular shape resonance through coupling to at least two exit doorway states. We investigate particularly the correlated structures observed in the γ, p, n, d and α reaction channels for the system ¹²C-¹²C around E_c.m. = 19.5 MeV. Within the framework of the continuum shell model of Mahaux and Weidenmüller we are able to describe qualitatively the observed excitation functions in the elastic and reaction channels, assuming a spin assignment of 12⁺ for the gross structure.     

Catalytic reactions in heavy-ion collisions

We discuss a new type of reactions of a ?-meson production on hyperons, ??Y ?? ?Y and antikaons -KN ?? ?Y. These reactions are not suppressed according to Okubo-Zweig-Iizuka rule and can be a new efficient source of ? mesons in a nucleus-nucleus collision. We discuss how these reactions can affect the centrality dependence and the rapidity distributions of the ? yield.     

Coulomb distortions in heavy-ion reactions

In heavy-ion scattering the Coulomb potential deviates from the potential of two point charges since vibrations, rotations and giant resonances are excited in both projectile and target nuclei as a result of their mutual electrostatic forces. In this paper this effect is calculated using a classical model. The adiabatic and the dynamical solutions of the problem are compared and discussed.     

Neutron-to-proton ratios in heavy-ion reactions

An expression for determining then/p ratio in heavy-ion reactions has been tested using recently published data. The agreement between the calculated and measured ratios is satisfactory.     

Charge distribution in heavy-ion reactions

Marked oscillations are seen in the differential cross section of charge distributions in high-energy heavy-ion reactions with massive projectiles. It is suggested that this can be explained by a simple quantum mechanical potential model over an “enlarged” Hilbert space.     

Kinematical effects in heavy-ion reactions

Transfer reactions between heavy ions at energies well above the Coulomb barrier have a large cross-section only, if certain kinematical conditions are satisfied. These relate the Q-value of the reaction to the angular momentum of the transferred nucleons in the initial and final nuclei.     

Spin distributions in heavy-ion fusion reactions

Spin distributions are calculated for the ¹⁶O + ²³²Th fusion reaction, taking into account both inelastic and transfer channels. The static deformations of the target play an important role. The calculated cross sections for complete fusion are in good agreement with the measured fission yields, whereas the anisotropies are underpredicted. This indicates that there is a serious inconsistency in the interpretation of the data.