G. V. Margagliotti

Different isotopic composition as a signature for different processes leading to fragment production in midperipheral 58Ni+58Ni collisions at 30 MeV/nucleon

We report on the results of experiments performed to investigate the 58 Ni + 58 Ni 30 MeV/nucleon reaction. In midperipheral collisions a massive fragment (4 Z 12) production has been observed. The emission patterns exhibit features consistent with dynamical fragmentation of a neck zone between the interacting nuclei, while in addition and at the same time, the projectile-like and targetlike residues are subject to statistical decay. The nature of the fragments produced via the two different mechanisms differ both for what concerns charge distribution and isotopic composition. In particular, neutron rich fragments can be produced in dynamical processes, even if the starting nuclear matter presents a N/Z ratio close to the unity. uf6d9 2001 Elsevier Science B.V. All rights reserved.

Contemporary presence of dynamical and statistical production of intermediate mass fragments in midperipheral 58Ni+58Ni collisions at 30 MeV/nucleon

Abstract The 58Ni + 58Ni reaction at 30xa0MeV/nucleon has been experimentally investigated at the Superconducting Cyclotron of the INFN Laboratori Nazionali del Sud. In midperipheral collisions the production of massive fragments (4⩽Z⩽12), consistent with the statistical fragmentation of the projectile-like residue and the dynamical formation of a neck, joining projectile-like and target-like residues, has been observed. The fragments coming from these different processes differ both in charge distribution and isotopic composition. In particular it is shown that these mechanisms leading to fragment production act contemporarily inside the same event.

Strong Enhancement of Extremely Energetic Proton Production in Central Heavy Ion Collisions at Intermediate Energy

The energetic proton emission has been investigated as a function of the reaction centrality for the system (58)Ni + (58)Ni at 30A MeV. Extremely energetic protons (E(NN)(p) > or = 130 MeV) were measured and their multiplicity is found to increase almost quadratically with the number of participant nucleons, thus indicating the onset of a mechanism beyond one- and two-body dynamics.

Experience with the Test and Qualification of Double-Sided Silicon Microstrip Sensors for the ALICE Inner Tracking System

The two outermost layers of the ALICE inner tracking system are composed of about 1800 AC-coupled double-sided silicon microstrip detectors. More than two thousand sensors have been delivered by three manufacturers, which amount to the largest production of this type of devices undertaken so far. Sensor testing and quality control has been performed at INFN Trieste. By careful optimization of the testing procedure, a complete static characterization of the large number of sensors has been possible without making use of fully automatic test stations, which are difficult to adapt to handling diced double-sided detectors. The paper presents some aspects of the experience gained from the testing and qualification of the sensors. The test procedures and the statistics of results will be briefly illustrated, and some problems encountered will be discussed.

Hard photons and energetic protons as probes of the nuclear dynamics at intermediate energy

SummaryThe emission of hard photons and pre-equilibrium protons has been investigated in exclusive experiments performed with MEDEA detector. The observation of the γ-proton anticorrelation indicates that the dominant production mechanism is the first-chance neutron-proton (np) collisions. The anisotropic angular distributions point out the presence of screening effects and are reminiscent of the free np scattering thus giving information of the space-time origin of the energetic protons. Protons, with energy more than twice the kinematical limit for nucleon-nucleon (NN) collisions, have been observed in several reactions around the Fermi energy.

Towards a better understanding of hard photon emission in intermediate energy heavy ion collisions

High energy photon spectra have been measured in several 58Ni induced reactions at 30A MeV incident energy. A two source analysis of the data has been performed using a two exponential parameterization of the associated gamma spectra. The relative intensity of the two components has been deduced as a function of the total mass of the interacting system and of the impact parameter. To attempt a characterization of the emission sources, correlations between photons and IMFs have been measured for the first time.

Size and asymmetry of the reaction entrance channel: Influence on the probability of neck production

Abstract The results of experiments performed to investigate the Nixa0+xa0Al, Nixa0+xa0Ni, Nixa0+xa0Ag reactions at 30 MeVxa0/xa0nucleon are presented. From the study of dissipative midperipheral collisions, it has been possible to detect events in which intermediate mass fragments (IMF) production takes place. The decay of a quasi-projectile has been identified; its excitation energy leads to a multifragmentation totally described in terms of a statistical disassembly of a thermalized system ( T ≃ 4 xa0MeV , E ∗ ≃ 4 xa0MeV / nucleon ). Moreover, for the systems Nixa0+xa0Ni, Nixa0+xa0Ag, in the same nuclear reaction, a source with velocity intermediate between that of the quasi-projectile and that of the quasi-target, emitting IMF, is observed. The fragments produced by this source are more neutron rich than the average matter of the overall system, and have a charge distribution different, with respect to those statistically emitted from the quasi-projectile. The above features can be considered as a signature of the dynamical origin of the midvelocity emission. The results of this analysis show that IMF can be produced via different mechanisms simultaneously present within the same collision. Moreover, once fixed the characteristics of the quasi-projectile in the three considered reactions (in size, excitation energy and temperature), one observes that the probability of a partner IMF production via dynamical mechanism has a threshold (not present in the Nixa0+xa0Al case) and increases with the size of the target nucleus.

The reaction entrance channel and isotopic composition of fragments in a dynamical and statistical multifragmentation regime

From the study of the Ni+Ni,Ag 30 MeV/nucleon dissipative midperipheral collisions, it has been possible to detect events in which Intermediate Mass Fragments (IMF) are emitted by two different sources with different mechanisms. The sources are: a) a quasi-projectile (QP) (and a quasi-target (QT)), with an excitation energy that leads to multifragmentation totally described in terms of a statistical disassembly of a thermalized system (T 4. MeV, E* 4 MeV/nucleon); b) an intermediate source, emitting both light particles and IMF. In this second case, fragments are more neutron rich than the average matter of the overall system, and have a very different charge distribution, with respect to those statistically emitted from the QP. The above features can be considered as a signature of the dynamical origin of the midvelocity emission. The results of this analysis show that IMF can be produced via different mechanisms simultaneously present within the same collision. They also show that the probability of IMF production via dynamical reaction increase with the size of the target nucleus.

Hard gammas and energetic protons as probes of nuclear dynamics

The reaction dynamics has been investigated in the 58Ni+58Ni at 30 MeV/u reaction studying energetic proton emission and heavy residue features. Moreover, measuring the hard gamma multiplicity associated to different classes of heavy residues we achieve a quantitative measurement of the reaction centrality. Experimental data have been also compared with dynamical calculations. Protons with energy exceeding more than five times the incident energy per nucleon were measured and their impact parameter dependence seems to indicate the presence of cooperative processes.