Macroscopic formfactors for pair transfer in heavy ion reactions

A case is made for the use of macroscopic formfactors for nucleon pair transfer in heavy ion reactions. The formalism is based on the identification of a local pair transition density whose radial dependence can be converted in a macroscopic transfer formfactor related to the ion-ion potential in a similar way as it has traditionally been done for inelastic excitations.     

Inertia effects in the mass transport of heavy ion reactions

The general trend of the multi differential cross section plots for the products obtained from the dissipation collisions of the three systems, forming almost the same composite systems but with different degrees of mass asymmetry in the entrance channel,⁵²Cr (261 MeV)+⁵⁶Fe,⁴⁰Ca (182 MeV)+⁶⁴Ni and¹⁶O (187 MeV)+⁹²Mo, were analysed in the frame of the Diffusion Model, assuming that the intermediate system decays with a statistical life time. A fairly good agreement has been obtained between the calculated multi differential plots and the experimental ones. The half life time of the composite systems seem to depend on the mass asymmetry of the entrance channel.     

Bremsstrahlung pair production in relativistic heavy ion collision

We calculate production of electron- and muon-pairs by the bremsstrahlung process in hadron collisions and compare it with the dominant two-photon process. Results for the total cross section are given for proton-proton and heavy-ion collisions at energies of the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). Received: 4 March 1997 / Revised version: 8 July 1997 / Published online: 20 February 1998     

Observation of proximity- and non-equilibrium effects in ternary heavy ion reactions

Kinematically complete experiments have been performed on the two- and three-body exit channels in the reactions⁸⁴Kr+¹⁶⁶Er and¹²⁹Xe+¹²²Sn at 12.5 MeV/u. Three-body events occur with an unusually high probability. They arise from a fast two-step mechanism where a sequential fission-like process follows a deep inelastic collision with preferentially very large energy losses. Strong Coulomb proximity effects are observed in the three-body final state which, treated quantitatively in Coulomb trajectory calculations, establish a time-scale of 1·10^?21 s between the consecutive scission acts. The angular distribution of fission fragments is consistent with an orientation of the fission axis approximately collinear with the axis of the first scission, and the mass distribution of the fission is asymmetric with the heavier mass emitted preferentially opposite to the direction of the third particle. The high fission probability, the short time-scale, the near collinear orientation and the fission mass asymmetry together present consistent evidence for a new phenomenon of non-equilibrium fission.     

Critical scattering in heavy ion reactions

Heavy ion reactions at intermediate energies are mostly described by the VUU equations. Using methods of quantum statistics we derive a useful relation between the width of s.p. states and the density — density correlation function. Near the point of instability from this relation there follows the increase of the collision integral and enhanced equilibration.The author thanks Yu. Kalinovsky, R. Nazmitdinov and M. Di Toro for valuable discussions.     

On polarization in heavy ion reactions

Simple expressions are obtained for polarizations in heavy ion reactions by using the Strutinsky model and asymptotic forms for Clebsch-Gordan coefficients to give the weighting for the near side, far side and interference terms. The model is discussed and examples are given which show that it can reproduce many of the features of exact calculations.     

Core exchange symmetries in heavy ion reactions

The general symmetry properties of theT-matrix due to the exchange of identical cores in nuclear reactions between composite particles are given. For the case of direct transfer reactions an explicit expression for the cross section is derived in the distorted wave Born approximation. In this model the cross section is the coherent sum of two indistinguishable transfer processes. The relative phase between the corresponding transfer amplitudes is obtained from the Pauli principle and depends on the signs of the spectroscopic amplitudes which may, therefore, be determined experimentally.     

Energy dissipation in light heavy ion reactions

Analysis of the energy dissipation for light heavy ions (¹²C,¹⁴N,¹⁶O,²²Ne) reactions has been attempted. Classical models with or without a friction term and the diffusion model have been compared to the data. Fragmentation and prompt collective excitation have been considered also.     

Evidence for a molecular REC-effect in heavy ion collisions

Anisotropy spectra of the MOK X-ray transitions have been measured in F-Al, Al-Al and Cl-Al collisions as a function of the projectile energy. The measurements at higher energies indicate a new molecular effect due to transitions from quasicontinuum states to the MO 1s _σ-orbitals.     

Projectile-like fragment production in heavy ion reactions

A model for peripheral heavy ion reactions is proposed in which the reaction mechanism is governed by the number of nucleon-nucléon collisions taking place during the interaction. A general random walk process leads to simple expressions for the mass yields. Angular distributions are obtained as the convolution of distributions due to deflection by the ion-ion potential and recoil effects due to the change in mass.     

Gamma radiation following heavy ion reactions

The gamma radiation produced by bombarding ²⁷A1 and ²⁸Si targets with 25–50 MeV ¹⁶O ions was measured in order to study the formation and decay of the highly excited compound nuclei ⁴³Sc and ⁴⁴Ti. The relative population of the evaporation residues is fairly well reproduced by a statistical theory describing the successive emission of 1–3 light fragments. From the measured branching ratio between proton and α-particle emission, the moment of inertia parameter of the level density is found to be close to the rigid body value. The influence of heavy ion optical transmission factors, level density parameters and γ-ray decay widths of continuum states is investigated.     

Barrier-top resonances and heavy ion reactions

In reactions involving strong absorption, we show that the narrowest potential (two body) resonances may be of a novel type, barrier-top (or “orbiting”) resonances whose ReE are at the centrifugal-Coulomb barrier top and whose wave functions are localized at the barrier radius. These resonances provide the link between semiclassical orbiting and quantal Regge poles and establish the close relationship between these concepts. We exploit the dominance and simplicity of barrier-top scattering in a model in which direct reaction amplitudes for heavy ion particle transfers can be calculated analytically. The model assumes complete absorption at small r of inward propagating waves, and a parabolic barrier potential which approximates the optical potential in the barrier region; the wave functions of the model (Weber functions) correspondingly approximate the optical potential wave functions (distoreed waves) in the barrier region. The results of this model reproduce many of the features present in more detailed DWBA computations, and provide simple physical explanations for these features.     

Recent studies in heavy ion induced fission reactions

Nuclear fission process involves large scale shape changes of the nucleus, while it evolves from a nearly spherical configuration to two separated fission fragments. The dynamics of these shape changes in the nuclear many body system is governed by a strong interplay of the collective and single particle degrees of freedom. With the availability of heavy ion accelerators, there has been an impetus to study the nuclear dynamics through the investigations of nucleus-nucleus collisions involving fusion and fission process. From the various investigations carried out in the past years, it is now well recognized that there is large scale damping of collective modes in heavy ion induced fission reactions, which in other words implies that nuclear motion is highly viscous. In recent years, there have been many experimental observations in heavy ion induced fission reactions at medium bombarding energies, which suggest possible occurrence of various non-equilibrium modes of fission such as quasi-fission, fast fission and pre-equilibrium fission, where some of the internal degrees of freedom of the nucleus is not fully equilibrated. We have carried out extensive investigations on the fission fragment angular distributions at near barrier bombarding energies using heavy fissile targets. The measured fragment anisotropies when compared with the standard saddle point model (SSPM) calculations show that for projectile-target systems having zero or low ground state spins, the angular anisotropy exhibits a peak-like behaviour at the sub barrier energies, which cannot be explained by the SSPM calculations. For projectiles or targets with large ground state spins, the anomalous peaking gets washed out due to smearing of the K-distribution by the intrinsic entrance channel spins. Recently studies have been carried out on the spin distributions of fission fragments through the gamma ray multiplicity measurements. The fission fragments acquire spin mainly from two sources: (i) due to rigid rotation of the nascent fragments at scission and (ii) due to statistical excitation of the spin bearing collective modes in the fissioning nucleus. One of the collective modes — the tilting mode depends on the K quantum number and is responsible for the emission angle dependence of fragment spin. In our studies, we have shown conclusively that the collective statistical spin modes get strongly suppressed for high K values corresponding to large rotational frequencies along the fission axis. These results bring out the importance of the dynamical effects in the heavy ion induced fusion-fission reactions. The present article will review the work carried out on the above aspects in heavy ion fission reactions as well as on the fission time scales, and some of the recent studies on the mass-energy correlations of fission fragments at near-barrier bombarding energies.     

Multiple exchanges in lepton pair production in high-energy heavy ion collisions

As an archetype reaction for pQCD multigluon hard processes in collisions of ultrarelativistic nuclei, we analyze generic features of lepton pair production via multiphoton processes in peripheral heavy ion scattering. We report explicit results for collisions of two photons from one nucleus with two photons from the other nucleus, 2γ + 2γ → l⁺l^?. The results suggest that the familiar eikonalization of Coulomb distortions breaks down for oppositely moving Coulomb centers. The breaking of eikonalization in QED suggests that multigluon pQCD processes cannot be described in terms of collective nuclear gluon distributions. We discuss a logarithmic enhancement of the contribution from the 2γ + 2γ → l⁺l^? process to production of lepton pairs with large transverse momentum; similar enhancement is absent for the nγ + mγ → l⁺l^? processes with m, n > 2. We comment on the general structure of multiphoton collisions and properties of higher-order terms that cannot be eikonalized.     

Multiplicity distributions in hadronic and heavy ion reactions

It is shown that the study of multiplicity distributionsP(n) can provide valuable information about the reaction mechanism of strong interactions. The relation betweenP(n) and Bose-Einstein correlations is discussed and the recent Na³⁵ results are interpreted as possible evidence for the existence of a chaotic and a coherent component inP(n). An experiment to test the coherent nature of pion beams is proposed.