B. Tamain

A hot expanding source in 50 A MeV Xe+Sn central reactions

The INDRA multidetector has been used to study multifragmentation processes in central collisions for the Xe + Sn reaction at 50 A MeV. A single isotropic source formed at an excitation energy of 12 A MeV exhausting most of the emitted charged products has been isolated in such collisions. The fragment kinetic energy spectra indicate a fast disintegration of the system with a radial collective motion of about 2 A MeV. The light charged particle characteristics within this scenario are also discussed.

A plastic multidetector for light nuclei identification at GANIL

Abstract A multidetector consisting of 96 plastic scintillators is now in operation at GANIL. It can identify in charge all fast light nuclei emitted between 3° and 30°.

A possible mechanism in heavy ion induced reactions: "Fast fission process"

The influence of the orbital angular momentuml on the mass distribution of fission fragments is studied, both on previously available data on heavy ion induced fission and in new specifically planned experiments: systems40Ar+165Ho and24Mg+181Ta at bombarding energies ranging from 180 up to 391 MeV and leading to the same fissioning nucleus205At with differentl distributions. Whenl values corresponding to a vanished fission barrier are reached, the mass distribution broadens. This suggest the existence of a specific process, “fast fission”, atl-values intermediate betweenl-values leading to compound nucleus formation and deep inelastic collisions, respectively. This process and its conditions of occurence are discussed; of special interest are the correlated differences between the limitations to the fusion cross-section and the fission mass distributions broadenings, respectively, for the Ar+Ho and Mg+Ta systems.

Influence of angular momentum on the mass distribution width of heavy ion induced fission: What is the frontier between fission and quasi-fission?

Abstract The 124 and 206 MeV 20 Ne+ nat Re and the 192 and 297 MeV 40 Ar+ 165 Ho systems have been used to form the 205 At nucleus with different angular momentum populations. The mass distributions of the fission fragments have been measured. Removing the effect of the temperature, it has been found that their width strongly increases with angular momentum especially when the fission barrier has vanished.

Calculation of interaction barriers using the energy density formalism

Abstract Interaction barriers are calculated within the framework of the energy density formalism using the sudden approximation with no adjustable parameters. The experimental data are well reproduced up to Z 1 Z 2 ≈ 2700–2800; above this value, only an estimate can be given, due to changes in the interaction energy curves. A graphical method allowing one to calculate the interaction barriers is proposed.

Charge and current-sensitive preamplifiers for pulse shape discrimination techniques with silicon detectors

New charge- and current-sensitive preamplifiers coupled to silicon detectors and devoted to studies in nuclear structure and dynamics have been developed and tested. For the first time shapes of current pulses from light charged particles and carbon ions are presented. Capabilities for pulse shape discrimination techniques are demonstrated.

Evidence for the formation of highly excited compound-like nuclei (T⋍5 MeV) in collisions of 720 MeV C projectiles with 238U, 232Th and 197Au targets

Abstract In collisions of 720 MeV C projectiles with U, Th and Au targets the study of low energy light charged particles detected in coincidence with fission fragments gives strong support for the formation of highly excited compound-like nuclei having reached a large degree of relaxation before decaying.

Surveying the nuclear caloric curve

Abstract The 4π array INDRA was used to detect nearly all charged products emitted in Ar + Ni collisions between 52 and 95 MeV/u. The charge, mass and excitation energy E ∗ of the quasi-projectiles have been reconstructed event by event. Excitation energies up to 25 MeV per nucleon are reached. Apparent temperatures obtained from several double isotopic yield ratios Tr0 show different dependences upon E ∗ . T6Li7Li3Heα0 yields the highest values, as well as the high energy slopes Ts of the kinetic energy spectra. Two statistical models, sequential evaporation and gas in complete equilibrium, taking into account side feeding and discrete excited states population, show that the data can be explained by a steady increase of the initial temperature with excitation energy without evidence for a liquid-gas phase transition.

The decay of primary products in binary highly damped 208Pb + 197Au collisions at 29 MeV/u☆

Abstract Events with fragment multiplicities up to eight have been detected with a large detector array in Pb + Au reactions at 29 MeV/u. All collisions show a binary character irrespective of a possible further disassembly of the two highly excited primary partners. For the most violent collisions, dissipative orbiting is observed as well as full energy damping corresponding to excitation energies as high as 6 MeV/u.

Characteristics of the fragments produced in central collisions of129Xe+natSnfrom 32Ato 50AMeV

Characteristics of the primary fragments produced in central collisions of 129Xe + natSn from 32 to 50 AMeV have been obtained. By using the correlation technique for the relative velocity between light charged particles (LCP) and fragments, we were able to extract the multiplicities and average kinetic energy of secondary evaporated LCP. We then reconstructed the size and excitation energy of the primary fragments. For each bombarding energy a constant value of the excitation energy per nucleon over the whole range of fragment charge has been found. This value saturates at 3 AMeV for beam energies 39 AMeV and above. The corresponding secondary evaporated LCP represent less than 40% of all produced particles and decreases down to 23% for 50 AMeV. The experimental characteristics of the primary fragments are compared to the predictions of statistical multifragmentation model (SMM) calculations. Reasonable agreement between the data and the calculation has been found for any given incident energy. However SMM fails to reproduce the trend of the excitation function of the primary fragment excitation energy and the amount of secondary evaporated LCPs.