M. Louvel

Spallation neutron production by 0.8, 1.2, and 1.6 GeV protons on various targets

Spallation neutron production in proton induced reactions on Al, Fe, Zr, W, Pb and Th targets at 1.2 GeV and on Fe and Pb at 0.8, and 1.6 GeV measured at the SATURNE accelerator in Saclay is reported. The experimental double-differential cross-sections are compared with calculations performed with different intra-nuclear cascade models implemented in high energy transport codes. The broad angular coverage also allowed the determination of average neutron multiplicities above 2 MeV. Deficiencies in some of the models commonly used for applications are pointed out.

Time-scale analysis of events with three heavy fragments in the 22Ne + Au collisions at 60 MeV/u

Abstract Experimental results on the production of two or three heavy ( Z ⩾10) fragments in the 22 Ne + Au collisions at 60 MeV/u are presented; the three fragment requirement selects large linear momentum transfer (70%) and large excitation energy (3–5 MeV/u.m.a.) for the incomplete fusion system. An analysis of the relative velocities and angles in the centre of mass of the three fragments shows that a sequential process with short time span and mutual influence between the two binary decays is favoured. A confirmation is found in the comparison of the correlation functions V ij and Θ ij with the results of Coulomb trajectories calculat ions including various time scales.

Neck formation and decay in Pb + Au collisions at 29 MeV/u☆

Abstract Three fragment production has been studied in peripheral 208 Pb + 197 Au collisions at 29 MeV/u. The data suggests the formation and subsequent decay of necklike structures producing a small fragment emitted at mid-rapidity in between the two partners of a deep inelastic scattering. Data and trajectory calculations suggest also the existence of a competing process in which the neck is absorbed by one of the reactants, this latter decaying further by binary fission.

Complete energy damping in 29 MeV/nucleon Pb+Au two-body final-state reactions☆

Abstract Decay products emitted in highly dissipative Pb+Au reactions at 29 MeV/nucleon have been detected using a large area array. Multiplicities of fragments as large as 8 have been detected with a sizeable cross section. The m -fragment exit channels are fully compatible with the formation of a transient excited dinuclear system formed in damped collisions. The excitation energy function shows that fully damped collisions are achieved even for m = 2 indicating that heavy nuclei are able to sustain high excitation energy and end up as evaporation residues.

Nuclear disassembly time scales using space-time correlations

Abstract The lifetime, τ, with respect to multifragmentation of highly excited nuclei is deduced from the analysis of strongly damped Pb + Au collisions at 29 MeV/u. The method is based on the study of space-time correlations induced by “proximity” effects between fragments emitted by the two primary products of the reaction and gives the time between the reseparation of the two primary products and the subsequent multifragment decay of one partner. A lifetime significantly longer than the interaction time is found. It is compared with time scales corresponding to different types of dynamical instabilities which may be responsible for nuclear disassembly.

Incomplete fusion in nucleus-nucleus central collisions. Study of 40Ar on 27Al from 25 to 85 MeV/u☆

Abstract Light charged particles and fragments emitted in collisions between 40Ar between 25 and 85 MeV/u and 27Al target have been detected in a geometry close to 4π in the center of mass. An event by event analysis based on the use of the momentum sphericity tensor allowed to identify a significant fraction of the central collisions. The data show that an incomplete fusion nucleus is formed and decays via isotropic emission at all incident energies, although no heavy residue is left above 36 MeV/u. Fusion is less and less complete as the incident energy increases. Nevertheless, an increase in the multiplicity of charged particles shows that the average excitation energy increases.

Fission fragments in coincidence with quasi-projectile in the reactions of 16O on 232Th and 197Au at 31 MeV/nucleon

Abstract We studied the linear momentum transfer in the reaction 16 O on 232 Th and 197 Au at 31 MeV/u. For both targets, the most probable transferred linear momentum is 3. GeV/ c corresponding to 80% of the incident momentum. A linear dependence between the θ AB fission fragment correlation angle and the detected ejectile mass is observed. For projectile like fragments, the most probable value of θ AB is associated with beam velocity events. For light particles, we observe the dominance of large momentum transfer components whatever the measured energies.

Reaction mechanism in highly fragmented Pb + Au collisions at 29 MeV/u

Abstract Events in which at least 5 fragments with charge larger than 8 were emitted in dissipative Pb + Au collisions at 29 MeV/u incident energy are studied. An event by event shape analysis is performed and a prescription is proposed to select the most central collisions. For these latter, it is shown by comparing the data with the results of computer simulations that the di-nuclear system disassembles before re-separating, meaning that its lifetime is smaller than the interaction time. This conclusion is also supported qualitatively by studying light particle patterns.

Experimental evidence of highly energy relaxed products in the 35 MeV/u84Kr+197Au reaction

We have measured the mass, the atomic number and the kinetic energy of the fragments emitted between 8 and 23° in the 35 MeV/u Kr+Au reaction. We found few products with the velocity of the projectile and α lot of fragments with Z ~20–28. Their kinetic energy (≲ 1 GeV) is substantially smaller than the one corresponding to the beam energy. It is not clear that these products correspond to a two-body reaction and they could arise from a three-body mechanism.

Étude des réactions de protons de 400 MeV sur des noyaux de masse moyenne par une experience de coïncidences proton-gamma

Abstract A p-γ coincidence experiment has been performed with 400 MeV protons on medium nuclei. The angular and momentum analysis of the scattered protons demonstrates a nucleon primary quasi-free process. No indication of knockout reactions on clusters has been seen. The residual nuclei which have been identified from γ-ray spectra are mainly produced by evaporation processes. Intranuclear cascades followed by evaporation are sufficient to describe the multinucleon removal but residual nuclei near the target nucleus are sensitive to the nuclear structure.