Youssef El Masri

Experimental determination of the symmetry energy of a low density nuclear gas

Experimental analyses of moderate-temperature nuclear gases produced in the violent collisions of 35 MeV/nucleon Zn-64 projectiles with Mo-92 and Au-197 target nuclei reveal a large degree of alpha particle clustering at low densities. For these gases, temperature- and density-dependent symmetry energy coefficients have been derived from isoscaling analyses of the yields of nuclei with A <= 4. At densities of 0.01 to 0.05 times the ground-state density of symmetric nuclear matter, the temperature- and density-dependent symmetry energies range from 9.03 to 13.6 MeV. This is much larger than those obtained in mean-field calculations and reflects the clusterization of low-density nuclear matter. The results are in quite reasonable agreement with calculated values obtained with a recently proposed virial equation of state calculation.

Transparency effects in heavy-ion collisions over the energy range 100–300 MeV/nucleon

Abstract Direct measurements of total reaction cross sections between 100 and 300 MeV/nucleon indicate that σ R reaches a minimum around 300 MeV/nucleon corresponding to the maximum of the (surface) transparency effects in heavy ion collisions. Data are well reproduced by simple microscopic calculations and are in agreement with the new parametrization we proposed earlier.

3 Fragment Sequential Decay of Heavy-nuclei Around 3-mev U Excitation-energy

A new method is proposed to identify the sequential decay of hot nuclei into large fragments. This method is based on a minimization procedure. It is illustrated for the Ar + Au reaction at 30 MeV/u. In this case, corresponding to about 3 MeV/u excitation energy, it is shown that the three fragment production can be explained by sequential binary splittings.

Lifetimes of rotational levels in170W

Lifetimes of the first 2+, 4+ and 6+ levels in170W have been measured by the recoil-distance method based on the Doppler effect, using the155Gd (20Ne, 5n)170W reaction. The following conclusions can be drawn from the results: if there are any deviations from the predictions of the rotational model for the lifetimes of the 4+ and 6+ states, they are smaller than 8.5 %; if this model accurately describes the nucleus170W, the intrinsic quadrupole moment of this nucleus is: Q0=5.93±0.06 barns, and its deformation is: β=0.240±0.003. The value of Q0 is compared with the predictions of various calculations for the tungsten isotopes.

Directed collective flow and azimuthal distributions in Ar-36+Al-27 collisions from 55 to 95 MeV/u

A 4 pi charged particle detector array with a low velocity threshold has been used to detect the products from reactions induced by Ar-36 on Al-27 at energies ranging from 55 to 95 MeV/u. Well characterized events were selected and sorted as a function of the impact parameter. Two methods were used for sorting these events with respect to their impact parameters and three methods were compared to determine the reaction plane. The transverse momentum analysis has been found to be the best method to extract the direction of the reaction plane for this system and for the experimental set-up used here. The energy of vanishing flow for central collisions has been found to be around 90-95 MeV/u. The azimuthal distributions of mid-rapidity particles exhibit a preferential in-plane emission and no squeeze-out effect.

Gamma-ray Multiplicity Measurements for the Determination of the Initial Angular-momentum Ranges in Normal and Fast Fission Processes

Abstract Gamma-ray multiplicities (first and second moments) have been measured, in the 220 MeV 20 Ne + nat Re and 315 MeV 40 Ar+ 165 Ho reactions, as a function of fission fragment masses and centre-of-mass total kinetic energies. The two reactions lead to the same fusion nucleus, 205 At, at the same excitation energy (167 MeV). The experimental critical angular momentum for the fission process in the Ne + Re system (91 ± 3) ℏ is close to l Bf =0 (~80ℏ) while in the Ar + Ho reaction this critical angular momentum (136 ± 4) ℏ is much larger than the l Bf =0 value, favoring the occurrence of the fast fission process. The observed widths of the fission fragment mass distribution: (42 ± 2) u in the Ne + Re system and (56 ± 4) u in the Ar + Ho reaction strengthen this hypothesis. For both compound nucleus fission and fast fission components in Ar + Ho, the total spin values obtained in absolute magnitude and in their dependence on the mass asymmetry are well described by assuming rigid rotation of the fissioning complex and statistical excitation of some collective rotational modes such as “Bending” and “Wriggling” according to the Schmitt-Pacheco model. These modes, however, are not all fully excited, their degrees of excitation are approximately the same for both fission components. From theoretical estimates of equilibration times, one anticipates the “Tilting” mode to be by far the last to be excited, and from its non-excitation in the present data together with the excitation of bending and wriggling, a time interval of about 10 −21 s to 2 × 10 −20 s can be derived for the reaction time of both normal fission and fast fission. The γ-ray multiplicity as a function of the c.m. total kinetic energy decreases in the Ne + Re system, while it increases in the Ar + Ho system even for symmetric splitting, which indicates experimentally for the first time that fast fission populates the whole mass range. This difference between the two reactions is in agreement with the normal-fission-fast-fission distinction in angular momentum space.

Highest temperatures sustainable in heavy nuclei produced in Ar+Au collisions at 60 AMeV

The specific decay mode of hot nuclei surviving fission or fragmentation and leading in the final state to the production of a large residue has been studied in Ar+Au collisions at 60 A MeV. The temperature of these residues has been obtained by analysing the kinematical characteristics (angular and kinetic energy distributions) of the coincident neutrons detected by the DEMON neutron detector, A total cross section of 50 mb has been measured for the production of residues with an initial temperature of 6 MeV or more. This result is compared with the predictions of a microscopic transport model

Azimuthal Dependence of the Directed Flow in Collisions Up To 95 Mev/u - From Inplane To Out-of-plane Enhancement

Nucleus-nucleus reaction mechanisms at incident energies below 10 MeV/u are governed by mean field effects. Nucleon-nucleon collisions begin to become important as the energy increases above the Fermi energy. This evolution manifests itself experimentally in the modification of the directed flow of the emitted participant (mid-rapidity) nucleons. In the reaction plane, it is expressed by the flow parameter [1]. Another aspect lies in the azimuthal distribution of particles relative to the reaction plane. At high energies, a maximum in the direction perpendicular to the reaction plane on both sides was named squeeze-out-effect [2]. Conversely, below 100 MeV/u, maxima in the reaction plane have been observed for the light systems of Ar + V : rotation-like effect [3]. It was studied as a Function of energy, impact parameter and emitted particle mass in the collisions of Ar-40 on Al-27 at energies from 36 to 65 MeV/u [4]. The latter study has been extended up to 95 MeV/u and measurements have been made for the Zn-64 + Ni-58 system at 5 energies from 35 to 79 MeV/u.

Evidence for Nonequilibrium Particle-emission Before Symmetrical Disintegration of a Composite System Formed in the O-16+ca-40 Reaction At 230 Mev

Abstract Measurement of fragment-fragment correlations in the reactions of 230 MeV 16 O with 40 Ca and 280 MeV 32 S with 24 Mg have been used to isolate processes in which symmetric decay follows nonequilibrium emission of one or two alpha particles. At the higher energy per nucleon. in contrast to previous observations for lower velocity projectiles, nonequilibrium emission followed by symmetric decay has approximately the same probability as the symmetric fission following complete fusion.

Electromagnetic-transitions in the Ground-state Rotational Band of Tb-159

The ground-state band of159Tb has been Coulomb excited up to spin 23/2 by 151-MeV40Ar ions. The lifetimes of the 9/2 to 23/2 levels have been determined by combining results obtained with the RD and DSA methods, theE2/M1 mixing ratios for ΔI=1 transitions have been measured using angular correlation techniques and the branching ratios for the levels up to spin 17/2 have been determined. The energies of the levels and the reducedM1 andE2 transition probabilities for their decay have been compared with the predictions of the rotational model, without and with ΔK=1 admixtures. No satisfactory agreement could be achieved for all available experimental data simultaneously.