Evolution of the decay mechanisms in central collisions of XeXe + SnSn from E/AE/A = 8 to 29 MeVMeV

Collisions of Xe+Sn at beam energies of $E/A$ = 8 to 29 $MeV$ and leading to fusion-like heavy residues are studied using the $4\pi$ INDRA multidetector. The fusion cross section was measured and shows a maximum at $E/A$ = 18-20 $MeV$. A decomposition into four exit-channels consisting of the number of heavy fragments produced in central collisions has been made. Their relative yields are measured as a function of the incident beam energy. The energy spectra of light charged particles (LCP) in coincidence with the fragments of each exit-channel have been analyzed. They reveal that a composite system is formed, it is highly excited and first decays by emitting light particles and then may breakup into 2- or many- fragments or survives as an evaporative residue. A quantitative estimation of this primary emission is given and compared to the secondary decay of the fragments. These analyses indicate that most of the evaporative LCP precede not only fission but also breakup into several fragments.Comment: Invited Talk given at the 11th International Conference on Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1, 2012. To appear in the NN2012 Proceedings in Journal of Physics: Conference Series (JPCS

New approach of fragment charge correlations in 129Xe+(nat)Sn central collisions

A previous analysis of the charge (Z) correlations in the $\Delta Z-$ plane for Xe+Sn central collisions at 32 MeV/u has shown an enhancement in the production of equally sized fragments (low $\Delta Z$) which was interpreted as an evidence for spinodal decomposition. However the signal is weak and rises the question of the estimation of the uncorrelated yield. After a critical analysis of its robustness, we propose in this paper a new technique to build the uncorrelated yield in the charge correlation function. The application of this method to Xe+Sn central collision data at 32, 39, 45 and 50 MeV/u does not show any particular enhancement of the correlation function in any $\Delta Z$ bin.Comment: 23 pages, 9 figures, revised version with an added figure and minor changes. To appear in Nuclear Physics

Sequential fissions of heavy nuclear systems

In Xe+Sn central collisions from 12 to 20 MeV/A measured with the INDRA 4$\pi$ multidetector, the three-fragment exit channel occurs with a significant cross section. In this contribution, we show that these fragments arise from two successive binary splittings of a heavy composite system. Strong Coulomb proximity effects are observed in the three-fragment final state. By comparison with Coulomb trajectory calculations, we show that the time scale between the consecutive break-ups decreases with increasing bombarding energy, becoming compatible with quasi-simultaneous multifragmentation above 18 MeV/A.Comment: 6 pages, 5 figures, contribution to conference proceedings of the Fifth International Workshop on Nuclear fission and Fission-Product Spectroscop

Principal and Teacher Percetipons of Performance Evaluation and Professional Growth (PE/PG) System Implementation

A study to investigate the challenges and successes schools are having as they strive to fully implement Maine’s Performance Evaluation and Professional Growth policy since last year

Fragment size correlations in finite systems - application to nuclear multifragmentation

We present a new method for the calculation of fragment size correlations in a discrete finite system in which correlations explicitly due to the finite extent of the system are suppressed. To this end, we introduce a combinatorial model, which describes the fragmentation of a finite system as a sequence of independent random emissions of fragments. The sequence is accepted when the sum of the sizes is equal to the total size. The parameters of the model, which may be used to calculate all partition probabilities, are the intrinsic probabilities associated with the fragments. Any fragment size correlation function can be built by calculating the ratio between the partition probabilities in the data sample (resulting from an experiment or from a Monte Carlo simulation) and the 'independent emission' model partition probabilities. This technique is applied to charge correlations introduced by Moretto and collaborators. It is shown that the percolation and the nuclear statistical multifragmentaion model ({\sc smm}) are almost independent emission models whereas the nuclear spinodal decomposition model ({\sc bob}) shows strong correlations corresponding to the break-up of the hot dilute nucleus into nearly equal size fragments

Spinodal decomposition of expanding nuclear matter and multifragmentation

Density fluctuations of expanding nuclear matter are studied within a mean-field model in which fluctuations are generated by an external stochastic field. Fluctuations develop about a mean one-body phase-space density corresponding to a hydrodinamic motion that describes a slow expansion of the system. A fluctuation-dissipation relation suitable for a uniformly expanding medium is obtained and used to constrain the strength of the stochastic field. The distribution of the liquid domains in the spinodal decomposition is derived. Comparison of the related distribution of the fragment size with experimental data on the nuclear multifragmentation is quite satisfactory.Comment: 19 RevTex4 pages, 6 eps figures, to appear in Phys. Rev.

Probing pre-formed alpha particles in the ground state of nuclei

In this Letter, we report on alpha particle emission through the nuclear break-up in the reaction 40Ca on a 40Ca target at 50A MeV. It is observed that, similarly to nucleons, alpha particles can be emitted to the continuum with very specific angular distribution during the reaction. The alpha particle properties can be understood as resulting from an alpha cluster in the daughter nucleus that is perturbed by the short range nuclear attraction of the collision partner and emitted. A time-dependent theory that describe the alpha particle wave-function evolution is able to reproduce qualitatively the observed angular distribution. This mechanism offers new possibilities to study alpha particle properties in the nuclear medium.Comment: 4 pages, 3 figure

Isospin diffusion in semi-peripheral 58Ni^{58}Ni + 197Au^{197}Au collisions at intermediate energies (I): Experimental results

Isospin diffusion in semi-peripheral collisions is probed as a function of the dissipated energy by studying two systems $^{58}Ni$ + $^{58}Ni$ and $^{58}Ni$ + $^{197}Au$, over the incident energy range 52-74\AM. A close examination of the multiplicities of light products in the forward part of phase space clearly shows an influence of the isospin of the target on the neutron richness of these products. A progressive isospin diffusion is observed when collisions become more central, in connection with the interaction time

Isospin diffusion from 40,48Ca + 40,48Ca experimental data at Fermi energies: Direct comparisons with transport model calculations

This article presents an investigation of isospin equilibration in cross-bombarding 40,48 Ca + 40,48 Ca reactions at 35 MeV/nucleon, by comparing experimental data with filtered transport model calculations. Isospin diffusion is studied using the evolution of the isospin transport ratio with centrality. The asymmetry parameter δ = (N − Z )/A of the quasiprojectile (QP) residue is used as isospin-sensitive observable, while a recent method for impact parameter reconstruction is used for centrality sorting. A benchmark of global observables is proposed to assess the relevance of the antisymmetrized molecular dynamics (AMD) model, coupled to GEMINI++, in the study of dissipative collisions. Our results demonstrate the importance of considering cluster formation to reproduce observables used for isospin transport and centrality studies. Within the AMD model, we prove the applicability of the impact parameter reconstruction method, enabling a direct comparison to the experimental data for the investigation of isospin diffusion. For both, we evidence a tendency to isospin equilibration with an impact parameter decreasing from 9 to 3 fm, while the full equilibration is not reached. A weak sensitivity to the stiffness of the equation of state employed in the model is also observed, with a better reproduction of the experimental trend for the neutron-rich reactions.Departamento de Física Aplicad

Nuclear multifragmentation time-scale and fluctuations of largest fragment size

Distributions of the largest fragment charge, Zmax, in multifragmentation reactions around the Fermi energy can be decomposed into a sum of a Gaussian and a Gumbel distribution, whereas at much higher or lower energies one or the other distribution is asymptotically dominant. We demonstrate the same generic behavior for the largest cluster size in critical aggregation models for small systems, in or out of equilibrium, around the critical point. By analogy with the time-dependent irreversible aggregation model, we infer that Zmax distributions are characteristic of the multifragmentation time-scale, which is largely determined by the onset of radial expansion in this energy range.Comment: Accepted for publication in Physical Review Letters on 8/4/201