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We present results of barrier distribution measurements for $^{24}\mathrm{Mg}+^{90,92}\mathrm{Zr}$ systems by the backscattering method. The measurements were done at near-barrier beam energies of 68--88.5 MeV (in the laboratory frame) at six angles. A strong discrepancy between experimental results and the predictions of the standard coupled channels calculations was observed. To test the hypothesis that this discrepancy may be due to partial energy dissipation caused by coupling to many target noncollective levels, we performed calculations using the coupled channels plus random matrix theory ($\mathrm{CC}+\mathrm{RMT}$) model. The comparison of calculations with experimental results gives qualitative support of the hypothesis; however, quantitatively the agreement is not as good as in the case of the $^{20}\mathrm{Ne}$ projectile.

International audience; Background: Near-threshold α-clustered states in light nuclei have been postulated to have a structure consisting of a diffuse gas of α particles which condense into the 0s orbital. Experimental evidence for such a dramatic phase change in the structure of the nucleus has not yet been observed. Purpose: To understand the role of α condensation in light nuclei experimentally. Method: To examine signatures of this α condensation, a compound nucleus reaction using 160-, 280-, and 400-MeV O16 beams impinging on a carbon target was used to investigate the C12(O16,7α) reaction. This permits a search for near-threshold states in the α-conjugate nuclei up to Mg24. Results: Events up to an α-particle multiplicity of seven were measured and the results were compared to both an extended Hauser-Feshbach calculation and the Fermi breakup model. The measured multiplicity distribution exceeded that predicted from a sequential decay mechanism and had a better agreement with the multiparticle Fermi breakup model. Examination of how these 7α final states could be reconstructed to form Be8 and C12(02+) showed a quantitative difference in which decay modes were dominant compared to the Fermi breakup model. No new states were observed in O16, Ne20, and Mg24 due to the effect of the N-α penetrability suppressing the total α-particle dissociation decay mode. Conclusion: The reaction mechanism for a high-energy compound nucleus reaction can only be described by a hybrid of sequential decay and multiparticle breakup. Highly α-clustered states were seen which did not originate from simple binary reaction processes. Direct investigations of near-threshold states in N-α systems are inherently impeded by the Coulomb barrier prohibiting the observation of states in the N-α decay channel. No evidence of a highly clustered 15.1-MeV state in O16 was observed from [Si★28,C12(02+)]O16(06+) when reconstructing the Hoyle state from three α particles. Therefore, no experimental signatures for α condensation were observed.

Directed and elliptic flows of neutrons and light charged particles were measured for the reaction 197Au+197Au at 400 MeV/nucleon incident energy within the ASY-EOS experimental campaign at the GSI laboratory. The detection system consisted of the Large Area Neutron Detector LAND, combined with parts of the CHIMERA multidetector, of the ALADIN Time-of-flight Wall, and of the Washington-University Microball detector. The latter three arrays were used for the event characterization and reaction-plane reconstruction. In addition, an array of triple telescopes, KRATTA, was used for complementary measurements of the isotopic composition and flows of light charged particles. From the comparison of the elliptic flow ratio of neutrons with respect to charged particles with UrQMD predictions, a value \gamma = 0.72 \pm 0.19 is obtained for the power-law coefficient describing the density dependence of the potential part in the parametrization of the symmetry energy. It represents a new and more stringent constraint for the regime of supra-saturation density and confirms, with a considerably smaller uncertainty, the moderately soft to linear density dependence deduced from the earlier FOPI-LAND data. The densities probed are shown to reach beyond twice saturation., Comment: 25 pages, 22 figures, accepted for publication in Physical Review C

Within the dinuclear system model the charge, mass, and isotopic distributions of the products in the reactions $^{78}\mathrm{Kr}+^{40}\mathrm{Ca}$ and $^{86}\mathrm{Kr}+^{48}\mathrm{Ca}$ are predicted at bombarding energy 10 MeV/nucleon. The heavy-ion phase-space exploration code is applied to take into consideration the pre-equilibrium emission of light particles. The competition is treated between complete fusion followed by the decay of compound nucleus and quasifission channels. The possible explanation of the odd-even staggering in the yield of the final reaction products at high excitation energies is discussed.

Be10 and C16 spectroscopy has been investigated by analyzing their breakup events on CH2 and CD2 targets. Breakup fragments have been detected by means of the CHIMERA detector. In particular, we investigated cluster decays of Be10 in He4+He6 and of C16 in He6+Be10 and He4+He6+He6. From the relative energy analysis of breakup fragments, we investigate the spectroscopy of excited states of projectile nuclei. In the Be10 case we observe known states at 9.51, 10.16, 10.6, and 11.8 MeV. Further, we suggest the existence of a new state at 13.5 MeV, possibly 6+ as indicated from angular correlation analysis. The relative energy (Erel+Eth) spectrum of C16, reconstructed starting from He6+Be10 correlations, shows a peak at about 20.6 MeV, probably related to the existence of an high-lying excited state. Non-vanishing yields are also seen in the triple coincidences He4+He6+He6.

Data from the experiment on the 197Au + 197Au reaction at 23 AMeV are analyzed with an aim to find signatures of exotic nuclear configurations such as toroid-shaped objects. The experimental data are compared with predictions of the ETNA code dedicated to look for such configurations and with the QMD model. A novel criterion of selecting events possibly resulting from the formation of exotic freeze-out configurations, "the efficiency factor", is tested. Comparison between experimental data and model predictions may indicate for the formation of flat/toroidal nuclear systems.

Background: In heavy-ion collisions at intermediate energies the production of light charged particles and intermediate mass fragments (IMFs) is due to different reaction mechanisms and different time scales, ranging from fast dynamical processes to statistical emission from the equilibrated system.Purpose: We compare the IMF statistical and dynamical emission probabilities in collisions of a neutron-rich $^{124}\mathrm{Sn}+^{64}\mathrm{Ni}$ system and a neutron-poor $^{112}\mathrm{Sn}+^{58}\mathrm{Ni}$ system at the laboratory energy of 35 $A$ MeV.Method: The IMFs production mechanism in semiperipheral reactions has been investigated in our previous works. In this paper, the analysis is expanded for the same set of data and production cross sections have been evaluated for dynamical and statistical emission in a coherent way for light and heavy fragments.Results: The data analysis has evidenced a strong competition between dynamical and statistical emission mechanisms. Probability of the dynamical emission of IMFs is strongly influenced by the ($N/Z$) ratio of the colliding system.Conclusions: It is demonstrated that the statistical emission is equally probable for the two systems, while the dynamical emission is enhanced for the neutron-rich system, especially for heavy fragments ($Z\ensuremath{\ge}6$). The observed effect points at a high sensitivity of the prompt-dynamical emission to the ($N/Z$) ratio of a given system.

Fast electron velocity spectra have been measured by means of the multidetector ARGOS (actually located at Laboratorio Nazionale del Sud, Catania) in a large angular range for atomic collisions induced by a $77A\ensuremath{-}\mathrm{MeV}$ ${}^{40}\mathrm{Ar}$ beam on an Al target. The results are in a fair agreement with a recently proposed relativistic billiard ball model and quasielastic scattering approximation calculations. However, the experimental binary-encounter peak velocities are found to be significantly lower than theoretically predicted. The data also show evidence for an excess of fast electrons in the backward direction.

Velocity spectra of evaporation residues produced in the {sup 35}Cl+{sup 12}C reaction have been measured at bombarding energies of 125, 140, and 154 MeV using time-of-flight techniques. These distributions were used to identify evaporation residues and to separate complete fusion and incomplete fusion components. The results show the presence of small contributions of incomplete fusion components which appear to be due to a cluster transfer reaction mechanism. Angular distributions and total and complete fusion evaporation residue cross sections were extracted at 70, 90, 110, 125, 140, and 154 MeV. The complete fusion cross sections and the deduced critical angular momenta are compared with other experimental data and the predictions of existing models. {copyright} {ital 1997} {ital The American Physical Society}

Mass and charge identified ejectiles, spanning from {sup 11}B to {sup 22}Ne, have been detected in the 123 MeV {sup 19}F+{sup 56}Fe reaction. The coexistence of deep inelastic collision (DIC) and incomplete fusion (IF) mechanisms has been observed. The shape of the energy spectra and their behavior with angle allowed us to identify two components: The less dissipative one was dominating near the grazing angle. For both components experimental optimum {ital Q} values were derived. Two approaches based on the sum rule (SR) model of Wilczy{acute n}ski were used to calculate DIC and IF contributions to the complex fragment cross sections. Both prescriptions fit reasonably well experimental ejectile cross sections and {ital Q} optimum values. Results of the present investigation support the idea that the DIC can be treated on the same footing as IF in the SR model once the first process is confined in an inner angular momentum window, starting from the maximum fusion angular momentum, with respect to quasielastic processes. {copyright} {ital 1996 The American Physical Society.}

Sideward flow values have been determined with the INDRA multidetector for Ar+Ni, Ni+Ni and Xe+Sn systems studied at GANIL in the 30 to 100 A.MeV incident energy range. The balance energies found for Ar+Ni and Ni+Ni systems are in agreement with previous experimental results and theoretical calculations. Negative sideward flow values have been measured. The possible origins of such negative values are discussed. They could result from a more important contribution of evaporated particles with respect to the contribution of promptly emitted particles at mid-rapidity. But effects induced by the methods used to reconstruct the reaction plane cannot be totally excluded. Complete tests of these methods are presented and the origins of the ``auto-correlation'' effect have been traced back. For heavy fragments, the observed negative flow values seem to be mainly due to the reaction plane reconstruction methods. For light charged particles, these negative values could result from the dynamics of the collisions and from the reaction plane reconstruction methods as well. These effects have to be taken into account when comparisons with theoretical calculations are done., Comment: 27 pages, 15 figures

Odd-even staggering effects on charge distributions are investigated for fragments produced in semiperipheral and central collisions of ${}^{112}\text{Sn}+{}^{58}\text{Ni}$ at 35 MeV/nucleon. For fragments with $Z\ensuremath{\le}16$ one observes a clear overproduction of even charges, which decreases for heavier fragments. Staggering persists up to $Z\ensuremath{\sim}30$. The staggering appears to be substantially independent of the centrality of the collisions, suggesting that it is mainly related to the last few steps in the decay of hot nuclei.

We present a new experimental method to correlate the isotopic composition of intermediate mass fragments (IMF) emitted at midrapidity in semiperipheral collisions with the emission timescale: IMFs emitted in the early stage of the reaction show larger values of $\ensuremath{\langle}N/Z\ensuremath{\rangle}$ isospin asymmetry, stronger angular anisotropies, and reduced odd-even staggering effects in neutron to proton ratio $\ensuremath{\langle}N/Z\ensuremath{\rangle}$ distributions than those produced in sequential statistical emission. All these effects support the concept of isospin ``migration'', that is sensitive to the density gradient between participant and quasispectator nuclear matter, in the so called neck fragmentation mechanism. By comparing the data to a stochastic mean field (SMF) simulation we show that this method gives valuable constraints on the symmetry energy term of nuclear equation of state at subsaturation densities. An indication emerges for a linear density dependence of the symmetry energy.

Velocity spectra, angular, and mass distributions of the evaporation residues produced in the reaction [sup 28]Si+[sup 12]C at [ital E]([sup 28]Si)=104, 115, and 154 MeV have been measured in the angular range 2.5[degree][le][var theta][sub [ital L]][le]14[degree]. The results show for all the studied bombarding energies the presence of small contributions of incomplete fusion reactions. These small incomplete fusion components present in the evaporation residue cross sections appear to be due to a cluster transfer reaction mechanism. Similar results have been obtained bombarding a [sup 13]C target. The obtained results are compared with earlier measurements and the predictions of some existing models.

Even-odd effects in $Z$ and $N$ distributions of light fragments emitted at forward angles in nuclear collisions ${}^{40}$Ca $+$ ${}^{40}$Ca, ${}^{40}$Ca $+$ ${}^{48}$Ca, and ${}^{48}$Ca $+$ ${}^{48}$Ca at 25 MeV/nucleon and identified in charge and mass with the Chimera multidetector have been analyzed. The amplitude of even-odd staggering effects seems to be related to the neutron to proton ratio $N/Z$ of the entrance channels. A qualitative explanation of this effect, taking into account the deexcitation phase of primary excited fragments, is discussed.

The slowing down of fast electrons emitted from insulators [Mylar, polypropylene (PP)] irradiated with swift ion beams (C, O, Kr, Ag, Xe; 20--64 MeV/u) was measured by the time-of-flight method at LNS, Catania and GANIL, Caen. The charge buildup, deduced from both convoy- and binary-encounter electron peak shifts, leads to target material-dependent potentials (6.0 kV for Mylar, 2.8 kV for PP). The number of projectiles needed for charging up (charging-up time constant) is inversely proportional to the electronic energy loss. After a certain time, a sudden decharging occurs. For low beam currents, charging-up time, energy shift corresponding to maximum charge buildup, and time of decharging are regular. For high beam currents, the time intervals become irregular (chaotic).

Equilibration and equilibration rates have been measured by colliding Sn nuclei with different isospin asymmetries at beam energies of E/A=35 MeV. Using the yields of mirror nuclei of {sup 7}Li and {sup 7}Be, we have studied the diffusion of isospin asymmetry by combining data from asymmetric {sup 112}Sn+{sup 124}Sn and {sup 124}Sn+{sup 112}Sn collisions with those from symmetric {sup 112}Sn+{sup 112}Sn and {sup 124}Sn+{sup 124}Sn collisions. We use these measurements to probe isospin equilibration in central collisions where nucleon-nucleon collisions are strongly blocked by the Pauli exclusion principle. The results are consistent with transport theoretical calculations that predict a degree of transparency in these collisions, but inconsistent with the emission of intermediate mass fragments by a single chemically equilibrated source. Comparisons with quantum molecular dynamics calculations are consistent with results obtained at higher incident energies that provide constraints on the density dependence of the symmetry energy.

An interesting process of violent reseparation of a heavy nuclear system into three or four fragments of comparable size was recently observed in {sup 197}Au+{sup 197}Au collisions at 15 MeV/nucleon. Combined analysis of the binary deep inelastic events and the ternary and quaternary breakup events demonstrates that the newly observed ternary and quaternary reactions belong to the same wide class of deep inelastic collisions as the conventional (binary) damped reactions. It is shown that the ternary and quaternary breakup reactions occur at extremely inelastic collisions corresponding to small impact parameters, while more peripheral collisions lead to well-known binary deep inelastic reactions.

A quantitative comparison is made between the absolute cross sections associated with statistical and dynamical emission of heavy fragments in the {sup 124}Sn+{sup 64}Ni and {sup 112}Sn+{sup 58}Ni collisions experimentally investigated at 35A MeV beam energy using the multidetector CHIMERA. The result shows that the dynamical process is about twice as probable in the neutron-rich {sup 124}Sn+{sup 64}Ni system as in the {sup 112}Sn+{sup 58}Ni neutron-poor one. This unexpected and significant difference indicates that the reaction mechanism is strongly dependent on the entrance-channel isospin (N/Z) content.

Collisions of a very heavy nonfusing nuclear system $^{197}\mathrm{Au}$$+$$^{197}\mathrm{Au}$ were studied at an energy of $15$ MeV/nucleon. An interesting process of violent reseparation of this heavy system into three or four fragments of comparable size was observed. In the case of ternary partitioning, either the projectile-like fragment (PLF) or target-like fragment (TLF) breaks up almost collinearly with the PLF-TLF separation axis. In the case of quaternary reactions, both PLF and TLF were observed breaking up along this direction. By comparison with a dynamical model of deep inelastic collisions it was concluded that the ternary and quaternary reactions occur in semiperipheral collisions, in a range of angular momenta corresponding to about $0.5--0.7$ of the maximum $L$ value for grazing collisions. The time elapsing from the scission of the binary PLF $+$ TLF system to the secondary scission of PLF or TLF was estimated to be of about $70--80$ fm/$c$ for the ternary reactions and $80--100$ fm/$c$ for the quaternary reactions.

40Ca+;{40,48}Ca,46Ti reactions at 25 MeV/nucleon have been studied using the 4pi CHIMERA detector. An isospin effect on the competition between fusionlike and binarylike reaction mechanisms has been observed. The probability of producing a heavy residue is lower in the case of N approximately Z colliding systems as compared to the case of reactions induced on the neutron rich 48Ca target. Predictions based on constrained molecular dynamics II calculations show that the competition between fusionlike and binary reactions in the selected centrality bins can constrain the parametrization of the symmetry energy and its density dependence in the nuclear equation of state.

A new reaction mechanism of violent reseparation of a heavy nucleus-nucleus system, {sup 197}Au+{sup 197}Au, into three or four massive fragments in collisions at 15 MeV/nucleon has been observed. After reseparation, the fragments are almost exactly aligned, thus showing a very short time scale of the reseparation process, of about 70-80 fm/c.

The masses of neutron-deficient nuclides near the $N=Z$ line with $A=64\text{\ensuremath{-}}80$ have been determined using a direct time-of-flight technique which employed a cyclotron as a high-resolution spectrometer. The measured atomic masses for $^{68}\mathrm{Se}$ and $^{80}\mathrm{Y}$ were 67.9421(3) u and 79.9344(2) u, respectively. The new values agree with the 2003 Atomic Mass Evaluation. The result for $^{68}\mathrm{Se}$ confirms that this nucleus is a waiting point of the rp-process, and that for $^{80}\mathrm{Y}$ resolves the conflict between earlier measurements. Using the present results and the 2003 Atomic Mass Evaluation compilation, the empirical interaction between the last proton and the last neutron in $N=Z$ nuclei has been revisited and extended.

Signatures of isospin effects were investigated for neutron-rich ({sup 124}Sn+{sup 64}Ni) and neutron-poor ({sup 112}Sn+{sup 58}Ni) systems at 35 MeV/nucleon for noncentral collisions. The centrality dependence of these signatures was tested for several impact parameter estimators. Our main observations are (i) the yields of {sup 1}H and {sup 3}He particles in the neutron-poor system are strongly enhanced with respect to the neutron-rich system, and the yields of {sup 3}H, {sup 6}He, and {sup 7,8}Li are suppressed at all impact parameters, (ii) the yields of {sup 2}H, {sup 4}He, and {sup 6}Li particles are almost the same for both systems, (iii) the N/Z ratio of intermediate mass fragments is correlated with the neutron richness of the system and is weakly dependent on the centrality of the collision, and (iv) the neutron richness of the detected fragments increases strongly with decreasing rapidity in the range from that of the projectile-like fragment to the c.m. region. The gross features of experimental data are reproduced by quantum molecular dynamics model calculations. A comparison between model calculations and the data indicates that the fragments produced in the c.m. regions are weakly excited.

Multi-breakup processes for the {sup 124}Sn+{sup 64}Ni system at 35 MeV/nucleon have been studied with the forward part of the CHIMERA detector. An extensive comparison between experimental data corresponding to almost complete ternary events and constrained molecular dynamics (CoMD-II) calculations suggests different characteristic times in the selected processes. This is in agreement with previous studies of the same reaction already published concerning the prompt intermediate-mass-fragment emission. Stimulated by CoMD-II calculations, we investigate the existence of more complex dynamical multi-breakup processes occurring on the same time scale. A detailed study of the rotational dynamics leading to slower dynamical fission processes is also presented.

Some properties of fast, nonequilibrium splitting of projectiles in the $^{124}\mathrm{Sn}+^{64}\mathrm{Ni}$ reaction at 35A MeV were determined using the $4\ensuremath{\pi}$ CHIMERA detector system. In particular the charge distributions, in- and out-of-plane angular distributions, and relative velocities of projectilelike fragments were measured. The time scale of the process was estimated and it turned out that the process is sequential but much faster than the ordinary, equilibrated fission.

We report absolute cross sections for binary encounter and convoy electron emission. Fast-electron velocity spectra were measured for atomic collisions induced by a 45 MeV/u ${}^{58}\mathrm{Ni}$ beam impinging on solid targets by means of the multidetector ARGOS in a large angular range (from 1.5\ifmmode^\circ\else\textdegree\fi{} to 165\ifmmode^\circ\else\textdegree\fi{}). Different conducting elemental targets ${(}^{12}\mathrm{C},$ ${}^{27}\mathrm{Al},$ ${}^{58}\mathrm{Ni}$ and ${}^{64}\mathrm{Ni},$ ${}^{\mathrm{nat}}\mathrm{Ag},$ ${}^{197}\mathrm{Au})$ and polystyrene were used. Characteristics of electrons with a velocity close to the beam velocity (convoy electrons) were found to be very sensitive to the incoming charge state of the projectile. Their yield increases with the target atomic number. The yield of binary encounter electrons with a velocity of almost twice the beam velocity at small ejection angles is roughly proportional to the area density of encountered target electrons. The velocity spectra centroids of these electrons are shifted towards lower velocities than predicted by simple two-body relativistic kinematics. Surprisingly, this effect seems to be mostly independent of target material and thickness. With increasing target atomic number, the high-energy tails of the spectra exhibit an unexpectedly large number of very fast electrons. A multiple-scattering mechanism involving multiple collision sequences of electrons between projectile and target atoms is invoked to explain this effect. The data also show evidence for an excess of fast electrons in the backward direction.