Search Results (12 total)

Light charged particles emitted at about 90^° in the frame of the projectile-like fragment in semiperipheral collisions of ⁹³Nb+⁹³Nb at 38A MeV give evidence for the occurrence, in the same class of events, of two different production mechanisms. This is demonstrated by differences in the kinetic energy spectra and in the isotopic composition of the particles. The emission with a softer kinetic energy spectrum and a low N/Z ratio for the hydrogen isotopes is attributed to an evaporation process. The harder emission, with a much higher N/Z ratio, can be attributed to a midvelocity process consisting of a nonisotropic emission, on a short time-scale, from the projectile-like fragment.

A systematic investigation of the average multiplicities of light charged particles and intermediate mass fragments emitted in peripheral and semiperipheral collisions is presented as a function of the beam energy, violence of the collision, and mass of the system. The data have been collected with the FIASCO setup in the reactions ⁹³Nb+⁹³Nb at (17,23,30,38)A MeV and ¹¹⁶Sn+¹¹⁶Sn at (30,38)A MeV. The midvelocity emission has been separated from the emission of the projectile-like fragment. This last component appears to be compatible with an evaporation from an equilibrated source at normal density, as described by the statistical code GEMINI at the appropriate excitation energy. On the contrary, the midvelocity emission presents remarkable differences in both the dependence of the multiplicities on the energy deposited in the midvelocity region and the isotopic composition of the emitted light charged particles.

The γ decay of the giant dipole resonance (GDR) in the ¹³²Ce compound nucleus with temperature up to ≈4  MeV has been measured, using the reaction ⁶⁴Ni+⁶⁸Zn at E_beam=300, 400, and 500 MeV. The γ and charged particles measured in coincidence with recoils are consistent with a fully equilibrated compound nucleus emission. The GDR width, obtained with the statistical model analysis, is found to increase almost linearly with temperature. This increase is rather well reproduced within a model including thermal shape fluctuations and the lifetime of the compound nucleus.

Peripheral and semiperipheral collisions have been studied in the system ⁹³Nb+⁹³Nb at 38A  MeV. The evaporative and midvelocity components of the light charged particle and intermediate mass fragment emissions have been carefully disentangled. In this way it was possible to obtain the average amount not only of charge and mass, but also of energy, pertaining to the midvelocity emission, as a function of an impact parameter estimator. This emission has a very important role in the overall balance of the reaction, as it accounts for a large fraction of the emitted mass and for more than half of the dissipated energy. As such, it may give precious clues on the microscopic mechanism of energy transport from the interaction zone toward the target and projectile remnants.

The emission pattern in the v_perp-v_par plane of intermediate mass fragments with Z  =  3–7 (IMF) has been studied in the collision ¹¹⁶Sn+ ⁹³Nb at 29.5A  MeV as a function of the total kinetic energy loss of the reaction. This pattern shows that for peripheral reactions most IMF’s are emitted at velocities intermediate between those of the projectile- and target-like products. Coulomb trajectory calculations show that these IMF’s are produced in the interaction zone in a short time interval at the end of the target-projectile interaction.

Light charged particles emitted by the projectilelike fragment were measured in the direct and reverse collision of ⁹³Nb and ¹¹⁶Sn at 25A MeV. The experimental multiplicities of hydrogen and helium particles as a function of the primary mass of the emitting fragment show evidence for a correlation with net mass transfer. The ratio of hydrogen and helium multiplicities points to a dependence of the angular momentum sharing on the net mass transfer.

A Reply to the Comment by S. Chattopadhyay.

Primary and secondary masses of heavy reaction products have been deduced from kinematics and energy–time-of-flight measurements, respectively, for the direct and reverse collisions of ¹⁰⁰Mo with ¹²⁰Sn at 14.1A MeV. Direct experimental evidence of the correlation of energy sharing with net mass transfer and model-independent results on the evolution of the average excitation from equal-energy to equal-temperature partition are presented.

The characteristics of the fission step following a binary deep-inelastic interaction have been reconstructed for three-body events detected in the reaction ¹⁰⁰Mo+¹⁰⁰Mo at 18.7A MeV and ^12-Sn+¹²⁰Sn at 18.4A MeV. The observed anisotropy of the in-plane angular distributions points to the fast decay of a rotating (and strongly deformed) nuclear object formed at the end of the deep-inelastic interaction. The derived time scale of the process indicates that asymmetric divisions are faster than symmetric ones.

The multiplicities of p and α particles detected in coincidence with fragments emitted in fully relaxed collisions in the reactions of 18.5A MeV ¹³⁶Xe+⁴⁸Ti have been measured for different exit channel mass asymmetries. A kinematic source analysis of the spectra and angular distributions of the light particles has been used to separate the total multiplicities into prescission and postscission contributions. From these results, the excitation energies at scission are determined using an empirical technique based upon previous measurements of light charged particle multiplicities observed in coincidence with evaporation residues. These excitation energies are found to decrease from ∼400 MeV to 110 MeV as the fragment mass asymmetry, A_H/A_L, varies from 4.8 to 1.0. A corresponding increase of the mean lifetime of the scissioning nucleus from ∼5×10^-22 s to ∼1×10^-20 s is derived using calculated statistical model decay widths. The extent to which this variation of lifetime with mass asymmetry may be attributed to completely damped deep inelastic collisions or to dynamic delays in the decay of a compound nucleus is discussed as is the need for inclusion of dynamics in the deexcitation calculations for hot nuclei. Observed three fragment events are also discussed.

High-energy γ rays have been measured in coincidence with heavy fragents in deeply inelastic reactions of ¹³⁶Xe+⁴⁸Ti at 18.5 MeV/nucleon. The giant dipole resonance (GDR) strength function is deduced from an analysis of the photon spectra within the statistical model. The GDR width Γ is studied as a function of the fragment excitation energy E^*. A saturation at about Γ=10 MeV is observed for E^*/A≥1.0 MeV/nucleon.

Exclusive measurements of two- and three-body events were performed for the system ¹²⁰Sn+¹⁰⁰Mo at 19.1 MeV/nucleon. Most ternary events are consistent with sequential processes in which one of the two deep-inelastic fragments fissions. For such events large differences are found between the fission probabilities of projectilelike and targetlike fragments of a given mass, this probability being larger for the nucleus which gained nucleons. This behavior demonstrates that there is a lack of equilibrium at the end of the deep-inelastic collision.