Fusion of very heavy nuclei

Details of the dynamical nature of the fusion process are beginning to emerge from experiments with heavy targets and projectiles at bombarding energies near and up to twice the barrier energy. Thus, subbarrier fusion is found to be enhanced in a way that cannot be understood as tunneling through a static, one-dimensional, radial potential barrier. Likewise, the extra push concept, which is dynamic in nature, appears more successful in describing fusion excitation functions than for example the critical distance model, which is based on a static, sudden potential in one variable. Finally, the occurrence of the fast fission process seems to be understandable in terms of the cliff hypothesis, which again is dynamic in origin, rather than as a result of energetic instability of the totally fused configuration.     

Pion and proton emission in interactions induced by relativistic heavy nuclei withZ≧12

The emission of pions and protons in interactions between heavy nuclei from the cosmic radiation (12≦Z≦26) and photoemulsion nuclei has been studied. The angular and energy distributions of target protons are similar to corresponding distributions in proton-nucleus interactions for emission angles >30°. In heavy ion interactions a forward peak of high energy target protons (E>200 MeV)is observed. The energy spectrum of target protons for angles <30° is remarkably flat for heavy ion interactions. For increasing disintegration of the target nucleus the number of produced pions per emitted recoil target proton is almost constant in heavy ion interactions, while it decreases rapidly in proton-nucleus interactions. The logtgΘ distributions of pions have small standard deviations even in interactions with a large target disintegration.     

Emission of helium nuclei in heavy ion interactions at energies ≧ 100 MeV/nucleon

The energy and angular distributions of helium particles emitted in interactions between nuclei in the cosmic radiation and nuclei in photoemulsions at energies ≧ 100 MeV/nucleon have been studied. The data obtained is impossible to interpret on the basis of a statistical decay of excited nuclei. For example, it is found that more than 28% of the helium nuclei are emitted in processes different from simple evaporation. The differential energy distribution of the helium nuclei in the energy interval (40–200) MeV can be represented by the relationN(E)dE=constE ^?a dE, wherea≈1.2. The large spread in angles and energies of the fast helium particles emitted in heavy ion interactions can to a certain degree be understood, if it is assumed that interactions between nucleons and clusters of nucleons occur.     

In-beam studies of very neutron-deficient heavy nuclei

The JYFL gas-filled recoil separator RITU, combined with Ge detector arrays and a SACRED magnetic solenoid spectrometer, has been successfully employed in recoil-decay-tagging (RDT) experiments in order to probe structures of very neutron-deficient heavy nuclei. The present contribution focuses on the light Pb region where the new data extend the systematics of shape-coexisting yrast states towards the proton dripline. Similarities between band structures and their relation to possible multi-particle multi-hole intruder excitations will be discussed. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: rauno.julin@phys.jyu.fi     

Two-nucleon transfer reactions in deformed nuclei using very heavy ions

Semiclassical methods found to be highly accurate for inelastic scattering are applied to the calculation of rotational population signatures in heavy-ion two-neutron transfer reactions involving highly deformed targets. Basic features to be expected for such reactions are predicted, and are shown to have straightforward semiclassical interpretations. The rotational population signatures for 2-neutron transfer are shown to be quite different from those expected for the analogous inelastic scattering case. Several calculations are shown for Xe projectiles on rare-earth targets, and it is demonstrated that such reactions can provide a unique probe of nuclear structure in high angular momentum states. The extension of the general ideas employed here to 1-nucleon transfer in deformed nuclei and to several other examples of transfer to highly collective states in deformed and vibrational nuclei using very heavy ions is briefly considered. Experimental possibilities are discussed, and it is concluded that relevant experiments in this virtually unexplored area are possible using sophisticated particle-γ spectroscopic techniques.     

Folding interactions in the scattering of soft heavy ions by light nuclei

Scattering of polarized ⁷Li by ¹²C and polarized ⁶Li by ¹⁶O at E_lab～20 MeV is studied by both cluster folding interactions and double folding ones, where projectile virtual excitation effects are included by the coupled-channel method. The results strongly depend on the choices of interactions and reaction mechanisms reflecting low Coulomb barrier effects.     

Proton-proton and deuteron-deuteron correlations in interactions of relativistic helium nuclei with protons

The reactions⁴Hep pp+X,³Hep pp+X and⁴Hep ddp have been investigated and the correlation function has been measured for protons and deuterons with small relative momenta. Strong positive correlation has been observed for protons related mainly to the final state interactions in¹S₀ state. The root mean square radius of the proton source calculated from the correlation function has been found to be equal to (1.7±0.3) fm and (2.1±0.3) fm for⁴He and³He respectively. It agrees with the known radii of these nuclei.We would like to thank Dr R. Lednicky for discussion, helpful suggestions and for making available the computer program that calculates the theoreticalpp correlation function. The authors also thank Dr D.H. Boal for providing the results of the two-deuteron correlation function calculations.     

Coherent photon-photon interactions in very peripheral relativistic heavy ion collisions

Heavy ions at high velocities provide very strong electromagnetic fields for a very short time. The main characteristics of ultraperipheral relativistic heavy ion collisions are reviewed, characteristic parameters are identified. The main interest in ultraperipheral heavy ion collisions at relativistic ion colliders like the LHC is the interactions of very high energy (equivalent) photons with the countermoving (equivalent) photons and hadrons (protons/ions). The physics of these interactions is quite different from and complementary to the physics of the strong fields achieved with current and future lasers.     

Emission of helium nuclei in relativistic proton interactions leading to total break-up of Ag and Br nuclei

In this paper the probability of total break up of Ag and Br emulsion nuclei under the bombardment with different relativistic hadrons and heavy nuclei is discussed. The differential energy spectra and angular distribution of the He-nuclei (up to 120 Mev) in inelastic interactions of 69 Gev/C protons leading to both low and high degree of disintegration of Ag and Br nuclei are investigated on the basis of the conventional evaporation and direct mechanisms.     

Prompt fission induced by antiproton annihilation at rest on heavy nuclei

Antiproton induced fission probabilities of U²³⁸, Bi²⁰⁹, Pb²⁰⁸ and Au¹⁷⁷ are reported together with the mass distribution of the fission fragments in the U²³⁸ and Bi²⁰⁹ cases. The charged particles multiplicities observed in co-incidence with fission have, also, been measured for U and Bi and are presented.Dedicated to Peter Armbruster on the occasion of his 60th birthday in recognition of his important contribution to modern Nuclear Physics     

Inelastic interactions of high energy nucleons with heavy nuclei

The intranuclear cascade model including the effect of changes in the properties of the target nucleus due to development of the shower of cascade particles is applied to calculations of the interactions of protons with fissioning nuclei in the energy region T ≈ 0.1–30 GeV. The fission cross sections, yields of different isotopes after the evaporation stage, and the characteristics of accompanying particles are calculated. The properties of the excited evaporating and fissioning nuclei are discussed. The calculated results are compared with experiment.     

Fast helium production in interactions of 3.7 A GeV <Superscript>24</Superscript>Mg with emulsion nuclei

We have studied the properties of the relativistic helium fragments emitted from the projectile in the interactions of ²⁴Mg ions accelerated at an energy of 3.7 A GeV with emulsion nuclei. The total, partial nuclear cross-sections and production rates of helium fragmentation channels in relativistic nucleus-nucleus collisions and their dependence on the mass and energy of the incident projectile nucleus are investigated. The yields of multiple helium projectile fragments disrupted from the interactions of ²⁴Mg projectile nuclei with hydrogen H, light CNO and heavy AgBr groups of target emulsion nuclei are discussed and they indicate that the breakup mechanism of the projectile seems to be independent of the target mass. Limiting fragmentation behavior of fast-moving helium fragments is observed in both the projectile and target nuclei. The multiplicity distributions of helium projectile fragments emitted in the interactions of ²⁴Mg projectile nuclei with the different target nuclei of the emulsion are well described by the KNO scaling presentation. The mean multiplicities of the different charged secondary particles, normally defined shower, grey and black ( n_s, n_g and n_b) emitted in the interactions of 3.7 A GeV ²⁴Mg with the different groups of emulsion nuclei at different ranges of projectile fragments are decreasing when the number of He fragments stripped from projectile increases. These values of n_i ( i = s, g, b and h particles) in the events where the emission of fast helium fragments were accompanied by heavy fragments having Z 3 seem to be constant as the He multiplicity increases, and exhibit a behavior independent of the He multiplicity.     

Dynamical effects on the way to fusion of very heavy nuclei

The central collision of ⁴⁰Ar and ²⁰⁸Pb is studied considering the ellipsoidal deformations and isovector dipole mode of motion in the approaching phase. The collective energy dissipation is suggested to originate from the Fermi surface deformation which is treated as a kinematically independent mode of motion within the canonical Lagrange-Rayleigh dynamics. The possible extensions of the approach are discussed. The potential energy surface, calculated using the generalized (folded) surface potential, is studied. The saddle point in the potential energy surface lying at the border of strongly deformed compact configurations is located. The potential energy at this point is about 10MeV smaller than that of the ions touching each other in the spherical shape. The examination of trajectories followed by the system in its evolution shows that the inertia forces strongly hinder the motion of ions along the potential energy valley. The collective energy dissipated during the approach is found to be smaller than the difference in the potential energies at saddle point and at the touching configuration of unpolarized ions.     

Alpha scattering by heavy nuclei

We develop an approximation scheme to calculate the Glauber amplitude for elastic and inelastic alpha-nucleus scattering. It is rapidly converging and works for any nuclear density. The α-particle density is assumed Gaussian.     

Complete destruction of heavy nuclei by hadrons and nuclei

The total disintegration is considered of nuclei with atomic weights ～100 and 200 by high energy hadrons and He⁴, C¹² nuclei with a momentum of 4.5 GeV/c per nucleon. It is shown that mainly nucleons are emitted, and there is no residual nucleus the mass of which is comparable to that of the primary nucleus. The probability of total disintegration is considered as a function of projectile energy and mass. The multiplicity, energy and emission angle of particles are considered as well. It is shown that the density of nuclear matter in the overlap zone of colliding nuclei exceeds the usual one by a factor of ～4. A comparison is made with interaction models. A conclusion is drawn of the collective interaction mechanism (perhaps, of the shock wave type) of particle ejection from the target nucleus at the first stage of interaction and of explosive decay of the residual nucleus at the next one.