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Continued interest in learning about the giant collective dipole resonances in nuclei—both at the one- and two-phonon level—has brought about a gradual shift of the experimental bombarding energies into a higher range. As a consequence, the reactions of interest can no longer be investigated in terms of the traditional, classical formalisms but need be upgraded into the relativistic regime. This extension has been implemented correctly insofar as the Coulomb excitation mechanism is concerned. For nuclear excitation processes, on the other hand, the present stand is much less satisfactory. Very few calculations have been performed so far and these make use of boosting prescriptions of the nuclear fields that are not entirely justified. In this contribution we update the situation of this problem exploiting a recently developed approach that yields the nuclear couplings with much less uncertainty.

Radioactive beams expand considerably the range of reactions with a large charge-to-mass asymmetry between projectile and target that can be probed experimentally. We introduce a simple model to estimate the magnitude and energy distribution of the prompt dipole gamma-emission that takes place while the symmetry is restored during the short contact time. In addition to this pre-equilibrium component we also introduce a procedure to calculate the delayed gamma-emission of statistical character that occurs after thermal equilibration of the compound system —or the binary reaction ejectiles— is reached.

The existence of a neutron skin in neutron-rich nuclei is discussed in connection with the excitation of isovector dipole and quadrupole giant modes via isoscalar nuclear probes. In the case of large neutron excess, important contributions are obtained from the nuclear excitation, which may even become predominant according to proper kinematical conditions. At variance with the usual situation encountered in inelastic processes, constructive interference can be found between nuclear and Coulomb contributions.

The existence of a neutron skin in neutron-rich nuclei is discussed in connection with the excitation of isovector dipole and quadrupole giant modes via isoscalar nuclear probes. In the case of large neutron excess, important contributions are obtained from the nuclear excitation, which may even become predominant according to proper kinematical conditions. At variance with the usual situation encountered in inelastic processes, constructive interference can be found between nuclear and Coulomb contributions.

A new time-of-flight spectrometer with magnetic quadrupoles has been developed. Its high detection efficiency and resolution allows one to identify ions produced with low cross sections in a binary reaction. The study of multinucleon transfer channels in the system revealed a large drift of the experimental total cross sections with respect to calculations which include independent single nucleon transfer modes. Incorporating pair and -cluster channels into theory accounts, at least qualitatively, for the discrepancies. New data coming from the study of show the striking presence of , and channels, supporting once more the idea that cluster degrees of freedom must be considered in a correct description of the reaction mechanism.

Multinucleon transfer reactions in {sup 48}Ca+{sup 124}Sn have been studied at 174 MeV with a time-of-flight magnetic spectrometer. The pickup and stripping of several protons and neutrons have been observed. The experimental isotope yields for the {minus}2p and +2p channels cannot be reproduced with calculations that incorporate only independent single-nucleon transfer modes. The discrepancies can be accounted for by explicitly including nucleon pair and {alpha}-cluster degrees of freedom. {copyright} {ital 1997} {ital The American Physical Society}

Angular and {ital Q}-value distributions for a large variety of charge and mass partitions populated in the reaction {sup 40}Ca + {sup 124}Sn have been measured at 170 MeV with a new time-of-flight magnetic spectrometer. Reaction channels involving the net transfer of up to six protons and six neutrons have been detected. Population patterns, cross sections, and energy-loss distributions are compared with results of theoretical calculations based on independent single-nucleon transfer modes. The overall agreement is good, but more complex mechanisms may be needed to account for the larger drift towards neutron stripping revealed by the experimental data. {copyright} {ital 1996 The American Physical Society.}

The presence of regular and chaotic regimes in nuclear scattering processes has so far been established in the context of simple, schematic reaction models. It has been argued that traces of the underlying classical organization of phase space should also be present in complex quantal calculations performed with state-of-the-art coupled-channel codes. In this paper we show that this is indeed the case and develop concrete guidelines to infer - relying exclusively on features of the quantal results - the regular or chaotic character of the motion.

We study the multipole response of nuclei for situations in which the neutron content becomes progressively rich. It is observed that the occurrence of weakly bound states towards the neutron drip line gives rise to low-lying concentrations of strength in the continuum which are not of a resonant character. These do not result from a redistribution of the expected high-lying response, but rather amount to a significant source of additional strength. The effect can be accounted for in terms of sum rules that grow — beyond a straightforward increase in the number of particles involved — because of the large spatial extent of the weakly bound orbitals.

Extending the field of spectroscopy to very neutron-rich heavy nuclei has been impaired by reaction [ital Q] values that consistently favor the formation of the lighter isotopes. Radioactive beams offer a chance to overcome this difficulty. We carry out a systematic study of cross sections for multinucleon transfer processes in order to determine the actual neutron excess in the projectiles that is needed to revert the aforementioned tendency. This identification, supplemented by quantitative predictions for the production yields, provide concrete guidelines for the selection of reaction partners and bombarding conditions.

The depopulation of [sup 180]Ta[sup [ital m]] by Coulomb excitation was investigated in a thick-target experiment using beams of [sup 32]S and [sup 36]S with energies of 70--130 MeV on disks of natural tantalum. The resulting [sup 180]Ta ground-state population was measured by detecting its eight-hour decay. At the higher incident energies the observed [sup 180]Ta yield is dominated by sub-Coulomb neutron pickup transfer reactions on the 10[sup 4] times more abundant [sup 181]Ta. At the lower incident energies in [sup 180]Ta ground-state yield is found to be compatible with Coulomb excitation of the isomer to an intermediate level with energies [ital E][sub [ital x]][lt]1 MeV and a reduced transition probability [ital B]([ital E]2)[congruent]0.02--0.25 Weisskopf units. Consequences for the astrophysical production mechanism of [sup 180]Ta are discussed.

Elastic scattering and transfer reactions for the system [sup 32]S+[sup 208]Pb have been measured at the laboratory energy of 173.4 MeV. [ital Q]-value distributions and angle differential cross sections have been obtained for a variety of transfer channels. The experimental results for the different charge and mass partitions are analyzed in terms of multiple single-particle and pair-correlated transfer processes.

We study Coulomb excitation patterns for systems with static octupole deformations and the characteristic features that emerge from an interplay with quadrupole degrees of freedom. Q-value effects that may help discriminate between static and dynamical octupole deformations are also discussed.

The HeHeHeHe disassembly channel of the 16O on 27Al, 58Ni and 197Au targets has been studied at 94 MeV/nucleon. Charged particles have been detected by a multielement array of plastic scintillators covering the zenithal angular domain between 3° and 30° and the total azimuthal one. Standard relativistic kinematics has been used to reconstruct the excitation energy of the primary projectile-like nucleus. Mean values of the excitation energy are found independent of the target mass. The comparison with data taken at lower bombarding energies is compatible with a saturation of the mean excitation energy.

The construction of differential cross sections as a function of excitation energy for systems with a collection of low- and high-lying intrinsic vibrational modes has been attempted in the past. A prescription is proposed that simplifies the implementation of such calculation schemes with a remarkable reduction in computational time., Comment: 6 pages, 3 figures, to be published in Phys. Rev. C

We exploit a model describing the breakup of weakly bound nuclei that can be used as a laboratory for testing different prescriptions that have been advanced in the literature to take into account the nearby presence of continuum states. In the model, we follow the evolution of a single-particle wave function in one dimension, initially bound by a Woods-Saxon type potential and then perturbed by a time- and position-dependent external field. Proper choices of this potential can simulate the effect of the interaction between reaction partners in a nuclear collision. These processes generate inelastic excitation probabilities that-distributed over the bound and continuum states of the system-lead to either a partial or a total fragmentation of the final wave function. Ministerio de Educación y Ciencia (España) y FEDER FIS2008-04189 FPA2006-13807-C02-01 Programa Consolider Ingenio CSD2007-0042 INFN Junta de Andalucía P07-FQM-02894 FQM160

We study the problem of barrier penetration in the presence of excitation degrees of freedom that at the incident energies considered are strictly closed. To this end we solve coupled-channel equations with boundary conditions that are asymptotically free only in the entrance channel. Particular aspects of the enhancements of the transmission coefficients which result from these conditions are discussed.

We investigate the magnitude of the effective interactions for elastic and inelastic processes at bombarding energies in the vicinity of the nominal Coulomb barrier. The relevance of higher-order inelastic and transfer processes in defining the strength of the effective couplings is explored in a reaction with a superfluid target, 16 O on 120 Sn, at center-of-mass energies of 46, 50 and 54 MeV. Significant dynamical effects are found and these appear to be more pronounced in the off-diagonal matrix elements than in the diagonal ones. Theoretical arguments related to the data are presented.

We consider a simple model for a nuclear scattering problem involving the coupling of the relative motion between two heavy ions to an intrinsic (harmonic) degree of freedom. At the classical level, the reaction function — relating unobservable incoming quantities to outgoing observables — is the most sensitive probe of the chaotic nature of the scattering. A general procedure to construct the quantum counterpart to these reaction functions is developed and shown to be very sensitive to the presence of chaos even in the regime of very low quantum numbers. In the regular regime it is possible to reproduce quite accurately the quantal results within a semiclassical approximation, just by letting the different trajectories that lead to an identical outcome interfere. We explore ways to extend such a prescription into the chaotic regime.

In the discussion of the possible experimental identification of a diabolic point in deformed nuclei as a function of mass number and rotational angular momentum one should consider not only the pair transfer between yrast states but also the Coulomb excitation of the continuation of the ground-state bands and the pair transfer between these bands.

Calculations of the probabilities for two-neutron transfer in the reaction208Pb +162Dy at bombarding energies below the Coulomb barrier are presented. The reaction is dominated by the strong Coulomb excitation of the rotational bands in162Dy and164Dy, while the transfer process is a weak perturbation. The presence of a diabolic point in the interval [10+–12+] in the rotational sequence, above which the transfer matrix elements change sign, leads to a reduction in the total probability for transfer of up to 25%. The relative populations of the spin states above the diabolic point exhibit an even more pronounced variation.

In observations of multifragmentation processes with intermediate mass fragments, spherical events are apparently missing. This result turns out to be inferred only because of the small number of fragments which are typically involved. Numerical simulations show that for low multiplicity events the sphericity and coplanarity variables introduced for such analysis are not adequate for extracting signals of the shape of the emitting source. We recommend alternative global variables which are more sensitive to the fragmentation pattern.

Calculations for $^{154}\mathrm{Sm}$${+}^{154}$Sm collisions that account for the deformed surfaces indicate that the threshold for nuclear reactions is about 50 MeV below the nominal Coulomb barrier. Some specific features of such low-energy collisions are discussed.

We propose a simple model describing the break-up of weakly-bound nuclei that can be used as a laboratory for testing different prescriptions that have been advanced to take into account the presence of continuum states. The model describes the evolution of a single particle wave function in one dimension, initially bound by a Woods-Saxon type potential and then perturbed by a time- and position-dependent external field. Proper choices of this potential can simulate the effect of the interaction between reaction partners in a nuclear collision. These processes generate inelastic excitation probabilities that – distributed over the bound and continuum states of the system – lead to either a partial or a total fragmentation of the final wave function.

The semiclassical approach has proven to be a most valuable tool for the construction of the scattering matrix and accurate evaluation of cross sections in a large variety of heavy-ion collision problems. In its familiar implementation, however, its use is restricted to what is now known as the "regular regime", as it makes use of classical reaction functions that must be continuous and interpolable. In this paper we identify what version of the semiclassical formalisms may be especially suitable for extension into the chaotic regime that develops at energies close to the Coulomb barrier. We also show the crucial role of the absorptive part of the ion-ion potential to retain the usefulness of the semiclassical methods under conditions of irregularity. Ministerio de Educación y Ciencia FIS2005-01105 FPA2005-04460

The implementation of a number-conserving formalism for pair-addition and pair-removal modes has impaired the possibility of developing an intuitive and self-consistent picture for collective pairing modes. In this paper, we show how a relaxation of this constraint leads to a unified model with true predictive powe FEDER y Ministerio de Educación y Ciencia FIS2005-01105 FPA2005-04460

The semiclassical approach is one of the fundamental tools used to investigate and analyze heavy ion reaction processes. We show in this contribution that the evolution of the motion that emerges from the knowledge of the semiclassical amplitudes for the reaction channels as a function of time cannot be taken as a reliable representation for an actual wave function of the intrinsic states. Ministerio de Educación y Ciencia FIS2005-01105 FPA2005- 04460

The conclusions extracted from a recent study of the excitation of giant dipole resonances in nuclei at relativistic bombarding energies open the way for a further simplification of the problem. It consists in the elimination of the relativistic scalar and vector electromagnetic potentials and the familiar numerical difficulties associated with their presence in the calculation scheme. The inherent advantage of a reformulation of the problem of relativistic Coulomb excitation of giant dipole resonances along these lines is discussed. Ministerio de Educación y Ciencia IS2005-01105 FPA2005-04460

We investigate the consequences of anharmonic terms in the vibrational spectrum of giant dipole resonances for the double Coulomb excitation of such modes in relativistic heavy-ion collisions. It is found that apparent discrepancies between the results of two separate experiments can be put in harmony assuming minor departures from the harmonic limit because of the special features of the reaction mechanism. {copyright} {ital 1997} {ital The American Physical Society}

We investigate sub-barrier fusion phenomena in the presence of triaxial static deformations and under conditions of {gamma} instability. It is found that the distributions of effective barriers can reflect in a distinct way a general quadrupole profile specified in terms of both {beta}{sub 2} and {gamma}. Fusion cross sections as a function of energy are, on the other hand, rather insensitive to the {gamma} degree of freedom. {copyright} {ital 1997} {ital The American Physical Society}

We investigate alternatives to the standard formalism used for the study of relativistic Coulomb excitation of the giant dipole resonance in nuclei. The idea is to obtain reasonable results for the probabilities of excitation and cross sections to the one-phonon and two-phonon levels avoiding the substantial complexity of the treatments exploited so far. This is achieved for the relevant range of partial waves up to bombarding energies of at least $5\phantom{\rule{0.3em}{0ex}}\mathrm{GeV}$ per nucleon. The transfer of energy to the center of mass of the excited nuclei is also investigated.

Recent progress in experimental techniques have made possible accurate measurements of fusion cross sections far below the Coulomb barrier, revealing an unexpected behavior of these quantities as a function of the bombarding energy. Besides providing a plausible cause for the observed energy dependence we profit from the nature of this explanation and the high sensitivity of the experimental data to show how one can use these measurements to investigate the radial dependence of the nuclear ion-ion potential at extremely close distances.

The interplay of nuclear and Coulomb processes in the inelastic excitation of single- and double-phonon giant resonances in heavy ion collisions is studied within a simple reaction model. Predominance of the Coulomb excitation mechanism on the population of the single-phonon and, on the contrary, predominance of the nuclear excitation for the double-phonon is evidenced. The effect of the spreading of the strength distribution of the giant resonances on the excitation process is analyzed, showing sizeable modifications in the case of Coulomb dominated processes., Accepten in Nuclear Physics A. 10 eps figures and source file in an uncompressed tar package

Recent theoretical work has been devoted to investigating the way in which the breakup of neutron-rich nuclei such as [sup 11]Li should affect the probabilities for fusion at energies close to the Coulomb barrier. We critically analyze the role of this channel in the tunneling process and show how one can come to conclusions that differ radically from the ones presented so far in the literature.

We discuss different points that are fundamental to determine the actual magnitude of the cross sections measured in two-particle transfer reactions between heavy ions. These are important issues to consider when the aim is the experimental observation of the somewhat elusive Giant Pairing Vibrations.

Nuclear dissociation cross sections for halo nuclei have been assumed by various authors to have a A 1/3 dependence on the target mass. These cross sections have also been considered to be a sum of the nuclear and Coulomb components. Explicit calculations of the break-up of 19 C induced by various targets show that both of the above assumptions become invalid for halo nuclei.

Neutron-rich nuclei exhibit a soft dipole mode which we show may lead to an important dynamical enhancement of the fusion cross section at subbarrier energies. This effect could ease the formation of heavy systems through fusion reactions induced by radioactive beams.

We study the break-up of 8B in collisions with heavy-ions. The process is described in terms of inelastic excitations leading to states in the continuum. The effects of the nuclear and coulomb fields are included on the same footing in the microscopic construction of the transition matrix elements. The interplay between coulomb and nuclear excitation processes differs significantly from the situation encountered in reactions involving systems close to the stability line. In particular, nuclear excitation is found to remain predominant at energies well below the coulomb barrier, TEX file + 11 postscript figures

We investigate the effects of nuclear absorption on the population of intrinsic vibrational states in scattering processes involving heavy ions. It is found that at the classical level the removal of flux assumes a qualitatively different form in the regular or chaotic regimes. Specifically, it is shown how certain reaction channels turn out to be—in the latter case—relatively insensitive to the strength of the absorptive potential. This realization has a counterpart at the quantum level where, at the chaotic layer, conditions of selective transparency in the nuclear system are enforced. DFG Bo 1109/1 CONYCET Pid. 3233/92 EU Network Contract CHRX-CT92-0075 Comisión Interministerial de Ciencia y Tecnología PB95-0533-A

We exploit analytic properties of the optical potential for elastic scattering of \ensuremath{\alpha} particles on nuclei to extract information on the effective interaction that should be used to describe the motion of a cluster of two neutrons and two protons bound to a nuclear system. This prescription solves long-standing ambiguities in the formalisms used for the study of \ensuremath{\alpha} decay. \textcopyright{} 1996 The American Physical Society.

We study the breakup of weakly bound systems close to the neutron drip line in collisions with heavy-ions. The process is described in terms of inelastic excitations leading to states in the continuum. The nuclear and Coulomb multipole fields induced by the reaction partner are both included in the microscopic construction of the inelastic form factors. A fraction of the dissociation cross sections is triggered by the presence of resonances at high excitation energy. The largest probabilities for breakup result, however, from strongly coupled transitions to low-lying states in the continuum. Nuclear couplings associated with inelastic excitation to these states are found to extend to unusually large distances. As a consequence, the interplay of nuclear and Coulomb excitations differs from the ordinary situation encountered in heavy-ion inelastic processes involving systems close to the stability line.

We investigate the relative importance of the Coulomb and nuclear fields to induce the break-up of neutron-rich nuclei such as 11 Li at energies close to the Coulomb barrier. We assume that the mechanism that leads to the separation is the excitation of a low-lying dipole mode in which the weakly-bound neutron halo performs a collective oscillation against the residual nuclear core. To this end we exploit semiclassical prescriptions that are adequate to calculate not only the average break-up probabilities but also to estimate the size of fluctuations about the quantal expectation values. Possible outcomes are explored as a function of both bombarding energy and impact parameter. Consequences of the couplings for elastic scattering and fusion processes are also discussed.

We show that the neutron multiplicities observed in the reaction {sup 40}Ar on {sup 232}Th at incident energies between 27 and 77 MeV per nucleon are consistent with the predominantly binary character of the collision process. The most violent encounters lead to excitation energies of several MeV per nucleon, sufficiently high to crack the colliding nuclei into many small fragments. We also give the neutron multiplicity and decay pattern for the reaction {sup 208}Pb on {sup 197}Au at 29 MeV per nucleon, for which some preliminary data is available.

We show that the inclusion of the dynamics after contact in a heavy-ion collision can completely modify the predictions of superheavy element production through fusion induced by neutron-rich-projectile nuclei.

Standard arguments used to justify the adiabatic approximation in barrier penetration problems at extremely low bombarding energies are critically examined. A simple model calculation is used to illustrate how the relative strength of the coupling to the intrinsic degrees of freedom may affect the validity of the adiabatic approach.

A detailed analysis of the Coulomb excitation of γ-unstable bands in transitional nuclei is made. Approximate analytic expressions valid in the sudden limit are supplemented with coupled-channel calculations. The population patterns obtained with the quantal or semiclassical treatments are interpreted making use of the stationary phase approximation. A quantitative description of the excitation process is also developed in terms of a classical formalism which incorporates both relative motion and the intrinsic γ degree of freedom. Applications are presented for the reaction 208 Pb + 196 Pt.

We investigate the transfer of nucleon pairs to a deformed superfluid system under the strong-coupling conditions which also favour the population of relatively large angular-momentum states. To this end we study the consequences of a model hamiltonian in which the deformation variables in ordinary space are linked with those which establish the superfluid character of the target in gauge space. We find that reaction cross sections defined in the I , ΔN plane exhibit marked correlation patterns. We thus identify these quantities as specially suited to explore the rich spectrum of possibilities which result from the combined excitation of pairing and rotational bands. A discussion of the different ways in which quantal interference effects manifest themselves under these circumstances is also given.