Table of contents

Editors: Giorgio Giardina (University of Messina) Avazbek K Nasirov (Joint Institute for Nuclear Research) Sara Pirrone (Istituto Nazionale di Fisica Nucleare Sezione di Catania)

Organizing Committee: Chairman: G Giardina (Messina – Italy) Co-Chairman: A K Nasirov (Dubna – Russia) Co-Chairman: S Pirrone (Catania – Italy) Scientific Secretary: G Mandaglio (Messina – Italy) and A Di Pietro (Catania – Italy)

Organizing Institutions: University of Messina    Fondazione Bonino-Pulejo (Messina)

Topics:

• Capture and Deep Inelastic Collision

• Quasifission

• Complete Fusion

• Fast Fission

• Fusion-Fission

• Ternary Fission

• Synthesis of Heavy and Superheavy Elements

• Pre-equilibrium emission

Local Organizing Committee: G Fazio (Messina), G Giardina (Messina), G Mandaglio (Messina), M Manganaro (Messina), M Romaniuk (Messina), C Saccà (Messina), A Di Pietro (Catania), S Pirrone (Catania), A Nasirov (Dubna)

Sponsored by: University of Messina, Fondazione Bonino-Pulejo (Messina), INFN Sezione di Catania, INFN Laboratori Nazionali del Sud Catania.

Website:http:/nucleo.unime.it/symp2010/

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

With the quantum diffusion approach the behavior of capture cross sections and mean-square angular momenta of captured systems are revealed in the reactions with deformed and spherical nuclei at sub-barrier energies. The calculated results are in a good agreement with existing experimental data. With decreasing bombarding energy under the barrier the external turning point of the nucleus-nucleus potential leaves the region of short-range nuclear interaction and action of friction. Because of this change of the regime of interaction, an unexpected enhancement of the capture cross section is expected at bombarding energies far below the Coulomb barrier. This effect is shown its worth in the dependence of mean-square angular momentum of captured system on the bombarding energy. From the comparison of calculated capture cross sections and experimental capture (fusion) cross sections, the importance of quasifission near the entrance channel is shown for the actinide-based reactions and reactions with medium-heavy nuclei at sub-barrier energies.

The possibilities for producing neutron-rich isotopes ^82,84,86Zn and ^86,88,90,92Ge are demonstrated in the reactions ⁴⁸Ca+²³⁸U, ²⁴⁴Pu at incident energies near the Coulomb barrier. The production cross sections of new neutron-rich isotopes of nuclei with charge numbers Z=64–80 are estimated as well. The dynamics of the binary reaction is considered as the diffusive multinucleon transfer between the interacting nuclei in the collisions when the excitation energy of the produced exotic isotope is lower than the threshold for the neutron emission. In the quasifission reactions ⁴⁸Ca+^244,246,248Cm at beam energies close to the corresponding Coulomb barriers one can produce new isotopes of superheavies with Z = 103–108, which mainly undergo fission.

Three decades ago, it was first recognized that the observation of fission fragments in heavy-ion induced reactions does not necessarily mean that they originate from the fission decay of a compound nucleus formed by the fusion of the projectile and the target nuclei. This conclusion was based on several different observations. First, it was recognized that the fission cross section exceeded the upper bound imposed by the existence of a stabilizing pocket in the ion-ion potential thus some of the fission cross section must originate from partial waves that do not proceed through a compound nucleus and, in addition, that the fission mass distribution in these cases was observed to be wider than expected on the basis of a compound nucleus model. Concurrently, it was noted that the fission fragment anisotropy in heavy-ion induced fission substantially exceed expectations based on the transition state model. Subsequent studies of the two-dimensional mass-angle distribution of fission fragments clearly demonstrated that these fragments are the result of a dynamic process, in which the system evolves toward mass symmetry on a time scale that is commensurate with the rotational period of the complex. This process is now referred to as "quasi-fission" although the terms "fission without a barrier" and fast fission were also used. Recently, much progress in the theoretical description of this process has been achieved and further precise experiments have been conducted, which provide further constraints on our understanding of these complex processes that also play a critical role in attempts to synthesize heavy and super-heavy nuclei via heavy-ion fusion processes. In this talk I will discuss some of the history and recent developments in the study of the quasi-fission process.

Heavy ion induced reactions offer a wide range of possibilities in exploring the collective behavior of nuclei and nuclear processes. Recent years have seen a spurt in the experimental and theoretical studies to understand the mechanism of the fission process in heavy target projectile systems, arising from the interest to produce elements in the superheavy region. A number of new features are observed experimentally with respect to the kinetic energy, mass and angular distributions of fission fragments and their correlations, implying prevalence of non-equilibrium phenomena in the fission process in many target projectile systems. There are many theoretical attempts both in terms of static potential energy considerations and dynamics to understand the important degrees of freedom that govern the evolution of the nuclear system from initial interaction to the final fission stages. Depending on the degree and nature of equilibration, various processes such as fast-fission, quasi-fission and pre-equilibrium fission have been invoked that can compete with the fully equilibrated compound nuclear fission process. In what we call a C-T Fissility plot, we show the remarkable dependence of the onset of these non-equilibrium processes on the entrance channel parameters.

Substantial experimental and theoretical efforts have been made in the last decade to study into detail the transition from quasi-elastic to deep inelastic processes in heavy ion collisions. The understanding of this transition is relevant not only to probe which degrees of freedom play a role in the multinucleon transfer process but also for its connection with near barrier fusion reactions. A boost in the field has been given with the implementation of large solid angle magnetic spectrometers coupled to γ arrays with which extensive investigations have been carried out for nuclear structure and reaction dynamics. In the present paper aspects of these studies will be presented, with emphasis on processes undergoing significant energy loss in binary collisions.

The hindrance to complete fusion is a phenomenon presenting in the most part of the capture events in reactions with massive nuclei. This phenomenon is due to the onset of the quasifission process which competes with complete fusion during the evolution of the composed system formed at capture stage. The branching ratio between quasifission and complete fusion strongly depends from different characteristics of reacting nuclei in the entrance channel. The experimental and theoretical investigations of reaction dynamics connected with the formation of composed system is nowadays the main subject of the nuclear reactions. There is ambiguity in establishment of the reaction mechanism leading to the observed binary fissionlike fragments. The correct estimation of the fusion probability is important in planning experiments for the synthesis of superheavy elements. The experimental determination of evaporation residues only is not enough to restore the true reaction dynamics. The experimental observation of fissionlike fragments only cannot assure the correct distinguishing of products of the quasifission, fast fission, and fusion-fission processes which have overlapping in the mass (angular, kinetic energy) distributions of fragments. In this paper we consider a wide set of reactions (with different mass asymmetry and mass symmetry parameters) with the aim to explain the role played by many quantities on the reaction mechanisms. We also present the results of study of the ⁴⁸Ca+²⁴⁹Bk reaction used to synthesize superheavy nuclei with Z = 117 by the determination of the evaporation residue cross sections and the effective fission barriers < B_f > of excited nuclei formed along the de-excitation cascade of the compound nucleus.

We studied multi-nucleon transfer reactions in the region of heavy and superheavy nuclei. The goal was to investigate these reactions as possibility to create new superheavy neutron-rich isotopes, which cannot be produced in fusion reactions. The experiments have been performed at the velocity filter SHIP at GSI. At SHIP we can detect and identify the heavy, target-like, transfer products. Due to the low background at the focal plane detector and the isotope identification via radioactive decays, the setup allows to reach an upper cross-section limit of 10 pb/sr within one day of beamtime. We investigated the systems ^58,64Ni + ²⁰⁷Pb and ⁴⁸Ca + ²⁴⁸Cm at beam energies below and up to 20% above the Coulomb barrier. At all energies we observed a massive transfer of protons and neutrons, where transfer products with up to eight neutrons more than the target nucleus could be identified.

The main aim of the present study is to evaluate the fusion probabilities and investigate competing quasifission process in the reactions with heavy ions leading to the formation of superheavy composite systems. The mass-energy distributions of binary fragments as well as their cross sections have been measured for a wide range of composite systems with Z=82–122 formed in the reactions with ²²Ne, ²⁶Mg, ⁴⁸Ca, ⁵⁸Fe and ⁸⁶Kr ions at energies around the Coulomb barrier. The experiments were carried out using a double-arm time-of-flight spectrometer of binary reaction products CORSET. The results of the experimental investigation of the influence of the entrance channel properties on the competition between fusion-fission and quasifission for the "warm" fusion reactions is discussed.

The formation of compound nuclei with Z = 120 and Z = 124 has been evidenced from their very long fission times measured by the blocking technique in single crystals. A possible explanation for the long measured fission times might be found in the temperature dependence of the fission barriers, as predicted by Hartree-Fock-Bogolubov calculations at finite temperature in this super-heavy nucleus domain.

A variety of phenomena connected with the formation of a dinuclear complex is observed in the heavy ion collisions at low energies. The dinuclear system model allows us to analyze the experimental data and to interpret them by comparison of the partial capture, fusion and evaporation residue cross sections measured for the different reactions leading to the same compound nucleus. The comparison of theoretical and experimental values of the mass and angular distributions of the reaction products gives us a detailed information about reaction mechanism forming the observed yields. The observed very small cross sections of the evaporation residues may be explained by the strong fusion hindrance and/or instability of the heated and rotating compound nucleus and smallness its survival probability. The fusion hindrance arises due to competition between complete fusion and quasifission while the smallness of survival probability is connected with the decrease of the fission barrier at large excitation energy and angular momentum of compound nucleus.

Fragment mass distributions for fission after full momentum transfer (FMT) were measured for the ³⁰Si,³¹P,^34,36S,⁴⁰Ar + ²³⁸U reactions at bombarding energies around the Coulomb barrier. The experiment was carried out at the JAEA tandem accelerator facility. We observed strong variation of the mass distribution on beam energy and projectile nucleus. In the reaction of ³⁶S +²³⁸U we observed a transition from symmetry to asymmetry mass distributions when the beam energies were decreased from the above-barrier to sub-barrier values. The mass asymmetry was A_L /A_H = 74/200, which corresponds to the fission valley leading to the nuclei close to the doubly closed-shell nuclei ⁷⁸Ni /²⁰⁸Pb. The fission channel is populated by quasifission, which is the disintegration without forming the compound nucleus. The incident-energy dependence is strongly correlated with the prolate deformation of ²³⁸U and the orientation at the initial impact. The results indicate that the reaction starting at the polar collisions on ²³⁸U has larger quasifission probability. The mass distributions are nicely reproduced by a model calculation using Langevin equation. The ⁴⁰Ar + ²³⁸U reaction also has the similar mass asymmetry (A_L/A_H ≈ 78/200) in quasifission. In the reactions of ³¹P + ²³⁸U and ³⁰Si + ²³⁸U, mass asymmetries are A_L/A_H 81/188 and 90/178, respectively. The results suggest that the system produced by the reaction using lighter projectile approaches closer to the shape of the compound nucleus even when quasifission occurs. In the reactions of ³⁰Si +²³⁸U and ³⁴S+ ²³⁸U, we also measured the evaporation residue (ER) cross sections to obtain information on the fusion probability. From the ER cross sections for the ³⁴S+²³⁸U reaction it was suggested that the symmetric fission is also dominated by quasifission.

Systems of intermediate fissility are characterized by an evaporation residues cross section comparable or larger than the fission cross section, and by a relatively higher probability for charged particle emission in the pre-scission channel. In a theoretical framework in which time scale estimates of the fission process rely on statistical model calculations, the analysis of particle emission in the evaporation residues channel is the source of additional constraints on statistical and dynamical models. This contribution will focus on our statistical and dynamical analysis of a more complete set of data from the system ³²S + ¹⁰⁰Mo at E_Lab = 200 MeV. Statistical model fails in reproducing the whole set of data and no convincing estimate is possible of the fission time scale. In particular, while pre-scission multiplicities can be reproduced without delay, the model strongly overestimates proton and alpha particle multiplicities in the evaporation residues channel irrespective of the statistical model input parameters and prescriptions used for the level density and the transmission coefficients. The analysis of the same set of data with a dynamical model produces a very good agreement with the full set of data and indicates that one-body dissipation plays a dominant role in the fission process, implying a fission delay of 23–25×10⁻²¹s.

The angular distribution of fission fragments for the ³²S+¹⁸⁴W reaction at center-of-mass energies of 118.8, 123.1, 127.3, 131.5, 135.8, 141.1 and 144.4 MeV were measured. The experimental fission excitation function is obtained. The fragment angular anisotropy is found by extrapolating the fission angular distributions. The measured fission cross sections are decomposed into fusion-fission, quasifission and fast-fission contributions by the dinuclear system model. The total evaporation residue and fusion-fission excitation functions are calculated in the framework of the advanced statistical model. The hindrance to complete fusion at small collision energies increases due to the increase of quasifission events and it is explained by the elongated shape of the dinuclear system which is formed in collisions with small orientation angles to the beam direction. An increase of the hindrance to complete fusion at large beam energies is explained by the dependence of the quasifission and intrinsic fusion barriers of dinuclear system on its angular momentum: at large angular momentum the quasifission barrier decreases and the intrinsic fusion barrier increases. In this reaction the contributions of fusion-fission and quasifission fragments are comparable.

The structure of the halo nuclei is expected to influence the fusion mechanism at energies around and below the Coulomb barrier. Here new data of ⁴He+⁶⁴Zn at sub-barrier energies are presented which cover the same energy region of previous measurements of ⁶He+⁶⁴Zn. The fusion cross section was measured by using an activation technique where the radioactive evaporation residues produced in the reaction were identified by the X-ray emission which follows their electron capture decay. By comparing the two system, we observe an enhancement on the fusion cross section in the reaction induced by ⁶He, at energy below the Coulomb barrier. It is shown that this enhancement seems to be due to static properties of halo 2n ⁶He nucleus.

The effects of shell closure in nuclei via cluster decay have been investigated. In this context,the Preformed Cluster Model (PCM) based on Quantum Mechanical Fragmentation Theory is used. The key point in the cluster radioactivity is that it involves the interplay of close shell effects of parent and daughter nucleus. Small half life for a parent indicates shell stabilized daughter and long half life indicates the stability of the parent against the decay. Cluster decays of rare-earth nuclei are studied with a view to look for neutron magic shells for the ₅₀Sn nucleus as the daughter product always, to find the most probable cluster decays and the possibility, if any, of new neutron shells. The recently observed α-decay chains ^293–294117 which were produced by the fusion reactions with target ²⁴⁹Bk and projectile ⁴⁸Ca at Dubna, in Russia. The α-decay calculations are performed for all the parents of the two alpha decay chains and compared with the experimental data and with other theoretical model.

We present the first results on the bremsstrahlung emission of photons accompanying ternary spontaneous fission of the ²⁵²Cf nucleus. We also compare our calculations on the basis of quantum model with preliminary experimental data and find a good agreement between theory and experiment for photon energies up to 500 keV, when the α-particle emission is in presence of the field of two fission fragments of the daughter nucleus.

The high energy bremsstrahlung γ-rays accompanying the spontaneous fission of ²⁵² Cf were measured in the 10–70 MeV energy range. The photons were detected by two BGO scintillator detectors (ø7.6cm × 7.6cm) in coincidence with neutrons detected by plastic scintillator detector, in the 90° and 180° geometry of the two BGO detectors with respect to the axis of the plastic scintillator. The distance from the ²⁵²Cf source to the BGO detectors was 10 cm, and the one to the plastic detector was 50 cm. The fast digital shape analysis technique and the time-of-flight method were used to reject pile-up effects and cosmic ray background. The γ-ray emission probability of 3 × 10⁻⁸photon/(MeV × fission) at E_γ = 70 MeV was obtained.

We investigate the reaction path followed by Heavy Ion Collisions with exotic nuclear beams at low energies. We focus on the interplay between reaction mechanisms, fusion vs. break-up (fast-fission, deep-inelastic), that in exotic systems is expected to be influenced by the symmetry energy term at densities around the normal value. The method described here, based on the event by event evolution of phase space quadrupole collective modes, will nicely allow to extract the fusion probability at relatively early times, when the transport results are reliable. Fusion probabilities for reactions induced by ¹³²Sn on ^64,58Ni targets at 10 AMeV are evaluated. We obtain larger fusion cross sections for the more n-rich composite system, and, for a given reaction, with a soft symmetry term above saturation. A collective charge equilibration mechanism (the Dynamical Dipole Resonance, DDR) is revealed in both fusion and break-up events, depending on the stiffness of the symmetry term just below saturation. Finally we investigate the effect of the mass asymmetry in the entrance channel for systems with the same overall isospin content and similar initial charge asymmetry. As expected we find reduced fusion probabilities for the more mass symmetric case, while the DDR strength appears not much affected. This is a nice confirmation of the prompt nature of such collective isovector mode.

With a view to study onset and strength of incomplete fusion at low projectile energies (i.e., ~ 4–7 MeV/nucleon) three sets of experiments have been performed in ¹²C,¹⁶O+¹⁶⁹Tm systems. In first set of experiments, spin-distributions and feeding intensity profiles for xn,αxn/2αxn-channels have been measured to figure out associated ℓ-values. The spin-distributions for direct-α-emitting channels (associated with incomplete fusion) have been found to be distinctly different than that observed for fusion-evaporation (complete fusion) channels. The mean value of driving input angular momenta associated with direct-α-emitting-channels have been found to be higher than that observed for fusion-evaporation xn/α-emitting-channels, and increases with direct-α-multiplicity in forward cone. The second set of experiments has been performed to understand influence of incomplete fusion on complete fusion at these energies. Incomplete fusion strength function has been deduced from the analysis of experimental excitation functions. The third set of experiments deals with the validation of data reduction procedure used to deduce incomplete fusion fraction, and to confirm the fusion incompleteness at slightly above barrier energies. Forward-recoil-ranges of heavy reaction products have been measured and analysed on the basis of break-up fusion model. More than one linear-momentum-transfer components associated with full- and/or partial-fusion of projectile with target nucleus have been observed. Experimental ranges of forward-recoils are found to be in good agreement with that estimated using range-energy formulation. The relative strengths of complete and incomplete fusion components deduced from the analysis of forward-recoil-ranges and excitation functions complement each other. Result presented in this paper conclusively demonstrate substantial incomplete fusion contribution at energy as low as 7% above the barrier.

The study of fusion in collision around the Coulomb barrier, induced by radioactive or stable weakly bound nuclei, has been the subject of many experiments in the last years. From a semiclassical point of view, direct processes like break-up or transfer may be favoured by the low binding energies and one might also expect suppressed fusion cross section due to the competition with the break-up. However, according to the Coupled Channel calculations (CC), it is well known that the presence of strong open reaction channels can be responsible for a fusion cross-section enhancement with respect to the single barrier penetration calculations and the break-up should be included in such calculations. In order to further investigate on this topic, the ⁶Li+⁶⁴Zn collision has been studied at several energies around the Coulomb barrier, to extract the total fusion and the total reaction cross sections and to study the energy dependence of the optical model potential.

The revival of transfer reaction studies benefited from the construction of the new generation large solid angle spectrometers based on trajectory reconstruction that reached an unprecedented efficiency and selectivity. The coupling of these spectrometers with large γ arrays allowed the identification of individual excited states, their population pattern and decay modes via particle-γ coincidences.

In the present paper aspects of fragment-γ coincidence studies measured with the Prisma-Clara set up in ⁴⁰Ca+⁹⁶Zr and ⁴⁰Ar+²⁰⁸Pb are discussed. In particular, we report about states of particle-phonon character, supporting the idea that the relevant degrees of freedom acting in the reaction dynamics define the final yield distributions.

We study the effect of isospin degree of freedom on the balance energy (E_bal) as well as its mass dependence throughout the mass range for two different sets of isobaric systems with N/Z = 1 and 1.4 at different colliding geometries ranging from the central to peripheral ones. Our findings reveal the dominance of Coulomb repulsion in isospin effects on E_bal as well as its mass dependence throughout the range of the colliding geometry. Our results also indicate that the effect of symmetry energy on the energy of vanishing flow is uniform throughout the mass range and throughout the colliding geometry.

We demonstrate the role of the mass asymmetry on the balance energy (E_bal) by studying asymmetric reactions throughout the periodic table and over entire colliding geometry. Our results, which are almost independent of the system size and as well as of the colliding geometries indicate a sizeable effect of the asymmetry of the reaction on the balance energy.