Search Results (20 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.

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 ratio of the total charge bound in fragments with Z between 2 and 15 to the hydrogen yield, (tsum₂¹⁵M_iZi)/M_H, has been measured, and the neutron-to-proton ratio n/p has been estimated from the data of central Au + Au reactions between 100 and 400A MeV, measured with the phase I setup of the detector system FOPI and GSI, Darmstadt, in the polar-angle range between 7° and 30°. These two quantities were used to determine the entropy per nucleon S/A by comparing them with the predictions of the freesco code. The analysis allows the simulataneous extraction of the values of the baryonic entropy, temperature, and collective flow. The extracted values are in good agreement with the values obtained in earlier FOPI studies, and, for the baryonic entropy, with recent hydrodynamic calculations.

Using the FOPI facility at GSI Darmstadt complete data of Au on Au collisions at 150A MeV were collected for charged products (Z=1–15) at laboratory angles 1°≤Θ_lab≤30°. Central collisions were selected by applying various criteria. The kinetic energy spectra of fragments from an isolated midrapidity source are investigated in detail for center-of-mass angles 25°≤Θ_c.m.≤45°. The heavy products (Z≥3) are used to determine the collective energy which is found to be at least 10A MeV.

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.

Charged particles have been observed in collisions of Au on Au at an incident energy of 150A MeV using a high-granularity detector system covering approximately the forward hemisphere in the center-of-mass system. Highly central collisions have been studied using a double selection criterion which combines large charged-particle multiplicities with small transverse-momentum directivities. In this class of events about one quarter of the total nuclear charge emerges as intermediate-mass fragments with nuclear charges Z>2. These fragments are centered at midrapidity and are produced with large transverse velocities.

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.

Photons with energies from 2 to 60 MeV have been measured in coincidence with binary fragments in the reaction ⁹²Mo+⁹²Mo at an incident energy of 19.5A MeV. The rapid change of the γ-ray spectrum and multiplicity with the fragment total kinetic energy in the exit channel indicates that the γ rays are emitted statistically by the highly excited fragments. Temperatures as high as 6 MeV are inferred.

The triply differential cross sections d³σ / dA dE dθ of binary exit channels in the reactions ⁹²Mo + ⁹²Mo and ¹⁰⁰Mo + ¹⁰⁰Mo have been measured at energies between 12 and 18.8 MeV/u. Complete relaxation is reached at energy losses up to 650 MeV, corresponding to temperatures in the fragments of up to 5 MeV; the primary mass distributions tend to spread over the full range of mass asymmetries, indicating a loss of the initial target and projectile identity and hence the disappearance of an essential feature of the deep-inelastic process.

The reactions of ¹⁴⁴Sm on ¹⁴⁴Sm and ¹⁵⁴Sm on ¹⁵⁴Sm have been studied at energies 30% in excess of the barrier. Whereas the number of exchanges nucleons is similar in both reactions, the number of exchanged protons is considerably larger at small energy losses in the ¹⁴⁴Sm system. On the basis of the shell-corrected liquid-drop potential energy surface these observations are attributed to the closed N=82 neutron shell which, for ¹⁴⁴Sm, hinders the neutron exchange and leads to a preferential transfer of protons.

Fusion reactions are initiated by capture of two incident nuclei to form a quasistable dinuclear complex. Excitation functions for such capture reactions have been measured in bombardments with ²⁰⁸Pb on targets of ²⁶Mg, ²⁷Al, ⁴⁸Ca, ⁵⁰Ti, ⁵²Cr, and ⁵⁸Fe. Compared with standard fusion models, based on the interaction of rigid spheres, the experiments show major deviations. These are explained by a dynamical model that takes deformations induced upon contact into account.

A measurement of the projectilelike fragments is reported for the collision of ⁸⁶Kr on ¹⁶⁶Er at a bombarding energy of 12.1 MeV/u. The results seem understandable only if a dynamical splitting of the projectilelike fragments is assumed.

The average γ-ray multiplicity M has been measured as a function of the scattering angle, energy loss, and nuclear charge of the projectile fragment produced in the ⁸⁶Kr-¹²⁰Sn and ⁸⁶Kr-¹⁶⁶Er reactions at a bombarding energy of 5.99 MeV/N. M increases steeply as a function of energy loss for the partly damped collisions but remains nearly constant for the fully damped events. M appears to depend mainly on energy loss rather than on mass transfer. The results are discussed in terms of a classical sticking model for the reaction mechanism.