Search Results (14 total)

Two-proton decay from ¹⁸Ne excited states has been studied by complete kinematical reconstruction of the decay products. The ¹⁸Ne nucleus has been produced as a radioactive beam by ²⁰Ne primary projectile fragmentation at 45  AMeV incident energy on a Be target. The ¹⁸Ne at 33  AMeV incident energy has been excited via Coulomb excitation on a ^natPb target. The obtained results unambiguously show that the 6.15 MeV ¹⁸Ne state two-proton decay proceeds through a ²He diproton resonance (31%) and democratic or virtual sequential decay (69%). The quoted branching ratio has been deduced from relative angle and momentum correlations of the emitted proton pairs.

Signatures of isospin effects were investigated for neutron-rich (¹²⁴Sn+⁶⁴Ni) and neutron-poor (¹¹²Sn+⁵⁸Ni) 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 ¹H and ³He particles in the neutron-poor system are strongly enhanced with respect to the neutron-rich system, and the yields of ³H, ⁶He, and ^7,8Li are suppressed at all impact parameters, (ii) the yields of ²H, ⁴He, and ⁶Li 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 ¹²⁴Sn+⁶⁴Ni 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.

In the present paper we have studied γ-ray emission in coincidence with charged particles measured in binary processes for the collisions ⁴⁰Ca+⁴⁸Ca, ⁴⁶Ti at 10 MeV/nucleon. The comparison between γ-ray yields, obtained under identical conditions on the charged particles detected in coincidence, shows an extra yield of around 15 MeV for the ⁴⁰Ca+⁴⁸Ca collision. The analysis of the γ-charged fragment coincidence events, along with the study performed with the dynamical model CoMD-II, explains this effect as the result of a giant dipole resonance emission from the intermediate system, characterized by a high degree of coherence. The performed study aims to establish a link among pre-equilibrium γ-ray emission, initial charge/mass ratio memory effects in the intermediate system for the ⁴⁰Ca+⁴⁸Ca system at short time, and the achievement of a substantial charge/mass ratio or isospin equilibration of the primary fragments formed at longer time. Moreover, the presence of a remarkable extra yield in a restricted range of the γ-ray spectra suggests that this equilibration process evolves through a quasiresonant mechanism.

Some properties of fast, nonequilibrium splitting of projectiles in the ¹²⁴Sn+⁶⁴Ni reaction at 35A MeV were determined using the 4π 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.

Semiperipheral collisions in the ¹²⁴Sn+⁶⁴Ni reaction at 35 MeV/nucleon were studied using the forward part of the Charged Heavy Ion Mass and Energy Resolving Array. Nearly completely determined ternary events involving projectilelike fragments (PLF), targetlike fragments (TLF), and intermediate mass fragments (IMF) were selected. A new method of studying the reaction mechanism, focusing on the analysis of the correlations between relative velocities in the IMF+PLF and IMF+TLF subsystems, is proposed. The relative velocity correlations provide information on the time sequence and time scale of the neck fragmentation processes leading to production of IMFs. It is shown that the majority of light IMFs are produced within 40–80 fm/c after the system starts to reseparate. Heavy IMFs are formed at times of about 120 fm/c or later and can be viewed as resulting from two-step (sequential) neck rupture processes.

The reaction ⁶He+⁶⁴Zn was studied in order to investigate the effects of the projectile neutron-halo structure on the reaction mechanism at energies around the Coulomb barrier. Elastic scattering angular distributions, transfer∕breakup angular distributions, and fusion excitation functions have been measured. Due to the low-recoil energy of the evaporation residues and the low intensity of the ⁶He beam, the fusion cross section was measured by detecting off-line the atomic x-ray emission which follows the electron capture decay of the evaporation residues. For comparison the reaction ⁴He+⁶⁴Zn was studied using the same technique. The data for the reaction ⁶He+⁶⁴Zn show that the transfer and breakup mechanisms account for almost 80% of the total reaction cross section, moreover we do not observe an enhancement of the fusion cross section when compared with the ⁴He+⁶⁴Zn reaction.

We measured γ-ray spectra in coincidence with charged particles for the reactions ⁴⁰Ca+⁴⁸Ca,⁴⁶Ti at beam energy of 25  MeV∕nucleon. Events corresponding to reaction mechanisms from quasielastic to fusion were observed. In fusion and in inelastic reactions an enhancement in the γ-ray spectra is present in the energy region around 10  MeV for the ⁴⁸Ca target with respect to ⁴⁶Ti data. The analysis of the γ-ray spectra relative to ⁴⁸Ca and ⁴⁶Ti targets shows the persistence of the GDR up to an excitation energy around 4−5  MeV∕nucleon for the mass region around 60. This result was obtained by including, in the statistical calculations, corrections due to the open decay channels, the mass dependence of the GDR parameters, and isospin effects.

Coherent and incoherent dipolar γ-ray emission is studied in a fully dynamical approach by means of the constrained molecular dynamics model. The study is focused on the system ⁴⁰Ca+⁴⁸Ca for which recently experimental data have been collected at 25 MeV/nucleon. The approach allows us to explain the experimental results in a self-consistent way without using statistical or hybrid models. Moreover, calculations performed at higher energy show interesting correlations between the fragment formation process, the degree of collectivity, and the coherence degree of the γ-ray emission process.

The structure of neutron-rich ^40,42,44S nuclei has been investigated through in-beam γ-ray spectroscopy using the fragmentation reaction of a 60 MeV A ⁴⁸Ca beam on a thin Be target. E_γ, I_γ, γγ-coincidence, and γ-ray angular distributions were measured for each produced fragment. The level schemes previously containing only a single γ transition were extended, and spin values were proposed for the new states. The experimental results were interpreted by use of microscopic collective-model and large-scale shell-model calculations. The results of the model calculations are consistent with each other, and give a reasonable description of the experimental results. Both models predict an erosion of the N=28 shell closure at Z=16 and suggest a deformed ground state for ^40,42S and a spherical-deformed mixed configuration for ⁴⁴S.

The neutron-rich ^66,68Ni have been produced at GANIL via interactions of a 65.9A MeV ⁷⁰Zn beam with a ⁵⁸Ni target. Their reduced transition probability B(E2;0₁⁺→2⁺) has been measured for the first time by Coulomb excitation in a ²⁰⁸Pb target at intermediate energy. The B(E2) value for ⁶⁸Ni₄₀ is unexpectedly small. An analysis in terms of large scale shell model calculations stresses the importance of proton core excitations to reproduce the B(E2) values and indicates the erosion of the N  =  40 harmonic-oscillator subshell by neutron-pair scattering.

The fluctuations in the excitation functions of the dissipative collision induced on the ²⁷Al+²⁷Al system have been studied in the laboratory energy range 114.2–123 MeV. The amplitude of the fluctuations, the average angular distributions, the energy autocorrelation functions, and the angular correlation functions have been studied in the framework of the partially overlapped molecular level model. The comparisons with the model allow us to estimate the characteristic times involved in the process, the degree of coherent rotations, and the density of molecular states through which the collision evolves. In particular the study of the angular correlations also reveals the coupling effects between rotational and intrinsic degree of freedom of the intermediate dinuclear system.

The six-α decay of ²⁴Mg has been studied via the reaction ¹³N+¹¹B at two beam energies, 29.5 MeV and 45 MeV. Large detector arrays allowed this reaction to be studied with low intensity radioactive ¹³N beam. The excitation energy in the ²⁴Mg^* was in the region of the 46 MeV resonance in ¹²C+¹²C scattering. From the six-α particles, events of interest were selected by associating two of the α particle pairs as the decay products of ⁸Be. The c.m. energy spectra for two ⁸Be+two α events have been extracted. The analysis revealed that most of these events originate from ¹²C_31^-+¹²C_31^-. The total cross section for these six-α events has been extracted at the two beam energies.

γ rays emitted in coincidence with charged particles have been measured in the collision ¹²C+⁶⁴Ni at 94.7 MeV. The γ spectrum detected in coincidence with evaporation residues has been fitted using the statistical code CASCADE and does not show clear evidence for nonstatistical emission. γ spectra measured in coincidence with fast, forward emitted α particles show a contribution around 10 MeV which has been interpreted as being due to preequilibrium dipole emission in the first stages of the collision. The presence of this nonstatistical yield has been explained performing theoretical calculations based on the Boltzmann-Nordeim-Vlasov equation.