Engineering topics

Colonna, M

Publications and source records attributed to Colonna, M.

6 records

DOE OSTI2011

Dynamical Dipole Mode in Heavy-Ion Fusion-Evaporation and Fission Reactions in the {sup 192}Pb Mass Region

The prompt {gamma}-ray emission related with the dynamical dipole mode decay was investigated in the {sup 192}Pb mass region by means of the {sup 40}Ca+{sup 152}Sm and {sup 48}Ca+{sup 144}Sm fusion-evaporation and fission reactions at E{sub lab} = 11 and 10.1 MeV/nucleon, respectively. The two reactions populate, through entrance channel having different charge asymmetries, the {sup 192}Pb compound nucleus at an excitation energy of 236 MeV with identical spin distribution. Preliminary results of this experiment show that the dynamical dipole mode survives in collisions involving heavier mass reaction partners than those studied previously. As a fast cooling mechanism on the fusion path, the prompt dipole {gamma} radiation could be of interest for the synthesis of super-heavy elements through ''hot'' fusion reactions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2011

Hadron-quark phase transition in asymmetric matter with dynamical quark masses

The two-equation-of-state model is used to describe the hadron-quark phase transition in asymmetric matter formed at high density in heavy-ion collisions. For the quark phase, the three-flavor Nambu-Jona-Lasinio effective theory is used to investigate the influence of dynamical quark mass effects on the phase transition. At variance to the MIT-Bag results, with fixed-current quark masses, the main important effect of the chiral dynamics is the appearance of an end point for the coexistence zone. We show that a first-order hadron-quark phase transition may take place in the region T subset of (50-80) MeV and {rho}{sub B} subset of (2-4){rho}{sub 0}, which is possible to be probed in the new planned facilities, such as FAIR at GSI-Darmstadt and NICA at JINR-Dubna. From the isospin properties of the mixed phase, some possible signals are suggested. The importance of chiral symmetry and dynamical quark mass on the hadron-quark phase transition is stressed. The difficulty of an exact location of a critical end point comes from its appearance in a region of competition between chiral symmetry breaking and confinement, where our knowledge of effective QCD theories is still rather uncertain.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2011

Transverse collective flow and midrapidity emission of isotopically identified light charged particles

The transverse flow and relative midrapidity yield of isotopically identified light charged particles (LCPs) has been examined for the 35 MeV/nucleon {sup 70}Zn+{sup 70}Zn, {sup 64}Zn+{sup 64}Zn, and {sup 64}Ni+{sup 64}Ni systems. A large enhancement of the midrapidity yield of the LCPs was observed relative to the yield near the projectile rapidity. In particular, this enhancement was increased for the more neutron-rich LCPs demonstrating a preference for the production of neutron-rich fragments in the midrapidity region. Additionally, the transverse flow of the LCPs was extracted, which provides insight into the average movement of the particles in the midrapidity region. Isotopic and isobaric effects were observed in the transverse flow of the fragments. In both cases, the transverse flow was shown to decrease with an increasing neutron content in the fragments. A clear inverse relationship between the transverse flow and the relative midrapidity yield is shown. The increased relative midrapidity emission produces a decreased transverse flow. The stochastic mean-field model was used for comparison to the experimental data. The results showed that the model was able to reproduce the general isotopic and isobaric trends for the midrapidity emission and transverse flow. The sensitivity of these observables to the density dependence of the symmetry energy was explored. The results indicate that the transverse flow and midrapidity emission of the LCPs are sensitive to the denisty dependence of the symmetry energy.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2011

Symmetry energy effects on fusion cross sections

We investigate the reaction path followed by heavy ion collisions with exotic nuclear beams at low energies. We focus on the interplay between reaction mechanisms, fusion vs. breakup (fast-fission, deep-inelastic), that in exotic systems is expected to be influenced by the symmetry energy term at densities around the normal value. The evolution of the system is described by a stochastic mean-field transport equation, where two parametrizations for the density dependence of symmetry energy (Asysoft and Asystiff) are implemented, allowing one to explore the sensitivity of the results to this ingredient of the nuclear interaction. The method described here, based on the event-by-event evolution of phase-space quadrupole collective modes, nicely allows us to extract the fusion probability at relatively early times, when the transport results are reliable. Fusion probabilities for reactions induced by {sup 132}Sn on {sup 64,58}Ni targets at 10 A MeV are evaluated. We obtain larger fusion cross sections for the more n-rich composite system, and, for a given reaction, in the Asysoft choice. Finally a collective charge equilibration mechanism (the dynamical dipole) is revealed in both fusion and breakup events, depending on the stiffnsess of the symmetry term just below saturation.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2011

Symmetry energy effects on the mixed hadron-quark phase at high baryon density

The phase transition of hadronic to quark matter at high baryon and isospin density is analyzed. Relativistic mean-field models are used to describe hadronic matter, and the MIT bag model is adopted for quark matter. The boundaries of the mixed phase and the related critical points for symmetric and asymmetric matter are obtained. Due to the different symmetry term in the two phases, isospin effects appear to be rather significant. With increasing isospin asymmetry the binodal transition line of the (T,{rho}{sub B}) diagram is lowered to a region accessible through heavy-ion collisions in the energy range of the new planned facilities (e.g., the FAIR/NICA projects). Some observable effects are suggested, in particular an isospin distillation mechanism with a more isospin asymmetric quark phase, to be seen in charged meson yield ratios, and an onset of quark number scaling of the meson-baryon elliptic flows. The presented isospin effects on the mixed phase appear to be robust with respect to even large variations of the poorly known symmetry term at high baryon density in the hadron phase. The dependence of the results on a suitable treatment of isospin contributions in effective QCD Lagrangian approaches, at the level of explicit isovector parts and/or quark condensates, is discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2010

Fragmentation paths in dynamical models

We undertake a quantitative comparison of multifragmentation reactions, as modeled by two different approaches: the antisymmetrized molecular dynamics (AMD) and the momentum-dependent stochastic mean-field (SMF) model. Fragment observables and pre-equilibrium (nucleon and light cluster) emission are analyzed, in connection with the underlying compression-expansion dynamics in each model. Considering reactions between neutron-rich systems, observables related to the isotopic properties of emitted particles and fragments are also discussed, as a function of the parametrization employed for the isovector part of the nuclear interaction. We find that the reaction path, particularly the mechanism of fragmentation, is different in the two models and reflects on some properties of the reaction products, including their isospin content. This should be taken into account in the study of the density dependence of the symmetry energy from such collisions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗