Search Results (15 total)

Invariant transverse-velocity spectra of intermediate-mass fragments were measured with the 4π multidetector system INDRA for collisions of ¹⁹⁷Au on ¹⁹⁷Au at incident energies between 40 and 150 MeV per nucleon. Their scaling properties as a function of incident energy and atomic number Z are used to distinguish and characterize the emissions in (i) peripheral collisions at the projectile and target rapidities, and in (ii) central and (iii) peripheral collisions near midrapidity. The importance of dynamical effects is evident in all three cases and their origin is discussed.

Sideward flow values have been determined with the INDRA multidetector for Ar+Ni, Ni+Ni, and Xe+Sn systems studied at GANIL in the 30A to 100A MeV incident energy range. The balance energies found for Ar+Ni and Ni+Ni systems are in agreement with previous experimental results and theoretical calculations. Negative sideward flow values have been measured. The possible origins of such negative values are discussed. They could result from a more important contribution of evaporated particles with respect to the contribution of promptly emitted particles at midrapidity. But effects induced by the methods used to reconstruct the reaction plane cannot be totally excluded. Complete tests of these methods are presented and the origins of the “autocorrelation” effect have been traced back. For heavy fragments, the observed negative flow values seem to be mainly due to the reaction plane reconstruction methods. For light charged particles, these negative values could result from the dynamics of the collisions and from the reaction plane reconstruction methods as well. These effects have to be taken into account when comparisons with theoretical calculations are done.

We report the first results of the beam-spin asymmetry measured in the reaction e→p→epγ at a beam energy of 4.25 GeV. A large asymmetry with a sinφ modulation is observed, as predicted for the interference term of deeply virtual compton scattering (DVCS) and the Bethe-Heitler process. The amplitude of this modulation is α  =  0.202±0.028. In leading-order and leading-twist perturbative QCD, the α is directly proportional to the imaginary part of the DVCS amplitude.

Intermediate velocity emissions of light charged particles are studied for the Ar+Ni system at 95A MeV. Experimental parallel velocity and transverse energy distributions are compared to those of a calculation based on intranuclear cascades followed by percolation and evaporation steps. The trends of the distributions are very similar, confirming the importance of prompt emissions in the experimental data.

This paper introduces a new method for the selection of central single-source events, based on classical multivariate techniques. The resulting discriminating variable is shown to be valid for different hypotheses on the nuclear source deexcitation mechanism. It enables the selection of events which are representative of the whole set of single-source events. Application to the Ni+Ni at 32A MeV system measured with the INDRA multidetector has allowed the determination of the fusion probability as a function of the impact parameter and the evaluation of the corresponding cross section.

Experimental data obtained with the 4π multidetector system INDRA are used to study midvelocity emissions of light charged particles (LCP) and intermediate mass fragments for peripheral and semicentral collisions of Xe and Sn at energies between 25 and 50 MeV/nucleon. The analysis is performed as a function of incident energy and of impact parameter, defined through the total transverse energy of LCP. The onset of midvelocity emissions is found to be close to 25 MeV/nucleon. Evaporative processes are also identified and are found to be sensitive to the impact parameter but show, for a given impact parameter, little dependence on the incident energy. A chemical analysis of the midvelocity component is performed. Compared to the evaporative process, midvelocity matter is found to be more neutron rich. Results are compared with the predictions of a dynamical model (CHIMERA). The general trends are well reproduced but some interesting differences are observed, notably in the amount of relative energy dissipation, showing that midvelocity emissions could be sensitive to the finer details of the interaction.

For 50 MeV/nucleon ¹²⁹Xe+^natSn multifragmentation events, we deduced, by means of correlation techniques, the multiplicities of the hydrogen and helium isotopes which were emitted by the hot primary excited fragments produced at the stage of the disassembly of an equilibrated hot source. We also derived the relative kinetic energy distributions between the primary clusters and the light charged particles that they evaporate. From the comparison between the secondary multiplicities observed experimentally and the multiplicities predicted by the GEMINI model, we concluded that the source breaks into primary fragments which are characterized by the same N/Z ratio as the combined system. Knowing the secondary light charged particle multiplicities and kinetic energies, we reconstructed the average charges of the hot fragments and we estimated their mean excitation energies. The fragment excitation energies are equal to 3.0 MeV/nucleon for the full range of intermediate mass fragment atomic number. This global constancy indicates that, on the average, thermodynamical equilibrium was achieved at the disassembly stage of the source.

Experimental data obtained with the 4π multidetector system INDRA are used to study the light charged particle (LCP, Z<~2) and intermediate mass fragment (IMF, Z>~3) production in peripheral and semicentral collisions of Xe and Sn at 50 MeV/nucleon. It is found that a sizable fraction of the detected LCP’s and IMF’s originates from the midvelocity region. These fragments can be seen to come either from a prompt (preequilibrium) mechanism or from a slower but dynamically influenced emission process. The relative magnitude of the dynamically influenced emission relative to the isotropic statistical evaporation is presented as a function of the transverse energy of light particles, used as an impact parameter selector. The results are compared to dynamical models with which a good agreement is obtained.

The experimental signature of the formation of a necklike structure, with a velocity between that of the projectilelike emitter and that of the targetlike emitter, is investigated with the same beam and experimental setup for targets lighter and heavier than the projectile. The reactions are ³⁵Cl on ¹²C and on ¹⁹⁷Au at 43 MeV/nucleon. Particle velocity distributions are compared with two-source statistical simulations and the presence of a necklike structure is inferred from the data. In the second part of the paper, dynamical model simulations with the formation of a necklike structure are presented for the ³⁵Cl+¹²C system at 43 MeV/nucleon.

Fragment production has been studied as a function of the source mass and excitation energy in peripheral collisions of ³⁵Cl+¹⁹⁷Au at 43 MeV/nucleon and ⁷⁰Ge+^natTi at 35 MeV/nucleon. The results are compared to the Au+Au data at 600 MeV/nucleon obtained by the ALADIN Collaboration. A mass scaling, by A_source∼35 and 190, strongly correlated to excitation energy per nucleon, is presented, suggesting a thermal fragment production mechanism. Comparisons to a standard sequential decay model and the lattice-gas model are made. Fragment emission from a hot, rotating source is unable to reproduce the experimental source size scaling.

Characteristics of ³⁵Cl+¹²C collisions at 43 MeV/nucleon have been studied for events with total charge detected ( ΣZ  =  23). It is shown that, while single-source events are present in the data, the binary nature of the collision is dominant. For binary events, the emitting sources (projectilelike and targetlike) were reconstructed independently allowing a direct measurement of the total dissipated energy. It is found that up to 75% of the available energy is dissipated and the significant momentum transfer of the selected events leads to the “equal temperature limit.”

The same hot nuclear system (ΣZ=18) has been studied for two different entrance channels with reaction products detected in a forward array of scintillators: central collisions of ²⁴Mg on a ¹²C target at 25A and 35A MeV and peripheral pickup reactions of ³⁵Cl on a ¹⁹⁷Au target at 43A MeV. The detection-efficiency-corrected charge distributions, multiplicity of charged particles and cross sections as a function of excitation energy are compared. The reaction mechanism is investigated, through comparison to simulations with statistical observables. The central reaction ²⁴Mg+¹²C at 35A MeV is well characterized by a dissipative binary collision scenario. Data at 25A MeV show less evidence of such dynamical characteristics. The intermediate-mass fragments (3≤Z≤8) production for each reaction is compared to model calculations for different values of excitation energy. The systems formed in the central collision at 25A MeV and the pickup reaction at 43A MeV show similar source characteristics, both statistically and in momentum space. However, the yields of the various exit channels, from evaporation and/or fission to multifragmentation and vaporization, differ for the two reactions. © 1996 The American Physical Society.

Light charged particle emission has been investigated as a function of excitation energy in exclusive experiments on the decay of ¹⁶O, ²⁴Mg, and ³⁵Cl projectiles between 25A and 70A MeV. The systematics of excitation energy removed by Z=1 and Z=2 particles were deduced. The results are similar to a previous study of proton and α-particle evaporation in compound nucleus reactions at beam energies below 20A MeV, supporting the idea of a common statistical process.

The relation between the folding angle distributions observed in the decay of ¹⁶O into four alphas and the final alpha-alpha interactions is discussed and inferred from model calculations. Likewise, the excitation energy dependence of the four-alpha decay channel probability is studied. A lack of alpha-alpha interactions is deduced from the analysis and the observed energy dependence is found to be characteristic of a statistical decay. This reveals the statistical nature of the disassembly and suggests a sequential breakup as the decay method.