D. Santonocito

Isospin diffusion and equilibration for Sn+Sn collisions at E/A=35 MeV

Equilibration and equilibration rates have been measured by colliding Sn nuclei with different isospin asymmetries at beam energies of E/A=35 MeV. Using the yields of mirror nuclei of {sup 7}Li and {sup 7}Be, we have studied the diffusion of isospin asymmetry by combining data from asymmetric {sup 112}Sn+{sup 124}Sn and {sup 124}Sn+{sup 112}Sn collisions with those from symmetric {sup 112}Sn+{sup 112}Sn and {sup 124}Sn+{sup 124}Sn collisions. We use these measurements to probe isospin equilibration in central collisions where nucleon-nucleon collisions are strongly blocked by the Pauli exclusion principle. The results are consistent with transport theoretical calculations that predict a degree of transparency in these collisions, but inconsistent with the emission of intermediate mass fragments by a single chemically equilibrated source. Comparisons with quantum molecular dynamics calculations are consistent with results obtained at higher incident energies that provide constraints on the density dependence of the symmetry energy.

Particle identification via pulse shape analysis for large-area silicon detectors of the CHIMERA array

Mass and atomic-number identification (ID) of reaction products is a fundamental requirement of any nuclear reaction study. An effective particle-ID method is demonstrated, based on pulse shape analysis/discrimination (PSD) applied to large-area, single-element silicon detectors. This technique uses commercial electronic modules and achieves atomic number resolution rivaling that typically obtained with multi-element (/spl Delta/E-E) detector telescopes. The method is applied to the CHIMERA detector system without compromising its time-of-flight (TOF) resolution. In-beam tests of the PSD method have been performed with large-area, 300-/spl mu/m thick CHIMERA silicon detectors, measuring particles from the /sup 19/F+/sup 12/C reaction at Tandem energies. Performance of a simple PSD set up is discussed, for front and rear particle injection.

Isospin transport in 84 Kr + 112,124 Sn collisions at Fermi energies

Isospin transport phenomena in dissipative heavy ion collisions have been in- vestigated at Fermi energies with a beam of 84 Kr at 35AMeV. A comparison of theh Ni /Z of light and medium products forward-emitted in the centre of mass frame when the beam impinges on two different targets, the n-poor 112 Sn and the n-rich 124 Sn, is presented. Data were collected by means of a three-layer telescope with very good performances in terms of mass identification (full isotopic resolution up to Z�20 for ions punching through the first detector layer) built by the FAZIA Collaboration and located just beyond the grazing angle for both reactions. Theh Ni /Z of the products detected when the n-rich target is used is always higher than that associated to the n-poor one; since the detector was able to measure only fragments coming from the QuasiProjectile decay and/or neck emission, the observed behaviour can be ascribed to the isospin diffusion process, driven by the isospin gradient between QuasiProjectile and QuasiTarget. Moreover, for light fragments theh Ni /Z as a function of the lab velocity of the fragment is observed to increase when we move from the QuasiProjectile velocity to the centre of mass (neck zone). This effect can be interpreted as an evidence of isospin drift driven by the density gradient between the QuasiProjectile zone (at normal density) and the more diluted neck zone.

On the origin of fast proton emission in intermediate energy heavy ion collisions

Abstract Exclusive measurements of multiplicity distributions of fast protons emitted in 40Ar and 132Xe induced reactions at 44 MeV/u bombarding energy have been performed using MEDEA 4π detection sytem. The data reflect the impact parameter dependence of the overlap region size and scale from system to system as the number of participant protons. The hypothesis of pre-equilibrium emission following first order p-N collisions is consistent with the data.

Measurement of neutron yield by 62 MeV proton beam on a thick Beryllium target

In the framework of research on IVth generation reactors and high intensity neutron sources a low-power prototype neutron amplifier was recently proposed by INFN. It is based on a low-energy, high current proton cyclotron, whose beam, impinging on a thick Beryllium converter, produces a fast neutron spectrum. The world database on the neutron yield from thick Beryllium target in the 70 MeV proton energy domain is rather scarce. The new measurement was performed at LNS, covering a wide angular range from 0 to 150 degrees and an almost complete neutron energy interval. In this contribution the preliminary data are discussed together with the proposed ADS facility.

Persistence of odd-even staggering in charged fragment yields from the

Odd-even staggering effects on charge distributions are investigated for fragments produced in semiperipheral and central collisions of 112Sn+58Ni at 35 MeV/nucleon. For fragments with Z<16 one observes a clear overproduction of even charges, which decreases for heavier fragments. In peripheral collisions staggering effects persist up to Z about 40. For light fragments, staggering appears to be substantially independent of the centrality of the collisions, suggesting that it is mainly related to the last few steps in the decay of hot nuclei.

^{112}Sn + ^{58}Ni

Abstract The prompt γ-ray emission is investigated in the 16 A MeV energy region by means of the 36,40Ar + 96,92Zr fusion reactions leading to a compound nucleus in the vicinity of 132Ce. The dynamical nature of this radiation is confirmed. We show that the prompt γ radiation has an angular distribution pattern consistent with a dipole oscillation along the symmetry axis of the dinuclear system. The data are compared with calculations based on a collective bremsstrahlung analysis of the reaction dynamics.

collision at 35 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.  2001 Elsevier Science B.V. All rights reserved.

Prompt dipole radiation in fusion reactions

The odd-even staggering of the yield of final reaction products has been studied as a function of proton (Z) and neutron (N ) numbers for the collisions 84 Kr + 112 Sn and 84 Kr + 124 Sn at 35 MeV/nucleon in a wide range of elements (up to Z ≈ 20). The experimental data show that staggering effects rapidly decrease with increasing size of the fragments. Moreover the staggering in N is definitely larger than the one in Z. Similar general features are qualitatively reproduced by the GEMINI code. Concerning the comparison of the two systems, the staggering in N is in general rather similar, being slightly larger only for the lightest fragments produced in the n-rich system. In contrast the staggering in Z, although smaller than that in N , is sizably larger for the n-poor system with respect to the n-rich one.

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

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.