arXiv subjects

G. Pastore

Publications and source records attributed to G. Pastore.

17 recordsLinked to original sources

arXiv2020

Dynamical fission of the quasiprojectile and isospin equilibration for the system 80Kr+ 48 Ca at 35 MeV/nucleon

Experimental results concerning the dynamical fission of quasiprojectiles in semiperipheral collisions for the system 80 Kr+ 48 Ca at 35 MeV/nucleon are presented. Data have been collected with four blocks of the FAZIA setup in the first physics experiment of the FAZIA Collaboration. The degree of isospin equilibration between the two fission fragments and its dependence on their charge asymmetry is investigated. The data are compared with the results of the AMD model coupled to GEMINI as an afterburner, in order to get hints about the timescale of the process.

nucl-ex↗

arXiv2018

The FAZIA setup: a review on the electronics and the mechanical mounting

In this paper the technological aspects of the FAZIA array will be explored. After a productive commissioning phase, FAZIA blocks started to measure and give very useful data to explore the physics of Fermi energy heavy-ion reactions. This was possible thanks to many technical measures and innovations developed in the commissioning phase and tuned during the first experimental campaigns. This paper gives a detailed description of the present status of the FAZIA setup from the electronic and mechanical point of view, trying also to trace a path for new improvements and refinements of the apparatus.

physics.ins-det↗

arXiv2018

Experimental study of precisely selected evaporation chains in the decay of excited $^{25}$Mg

The reaction $^{12}$C + $^{13}$C at 95 MeV bombarding energy is studied using the GARFIELD + Ring Counter apparatus located at the INFN Laboratori Nazionali di Legnaro. In this paper we want to investigate the de-excitation of $^{25}$Mg aiming both at a new stringent test of the statistical description of nuclear decay and a direct comparison with the decay of the system $^{24}$Mg formed through $^{12}$C+$^{12}$C reactions previously studied. Thanks to the large acceptance of the detector and to its good fragment identification capabilities, we could apply stringent selections on fusion-evaporation events, requiring their completeness in charge. The main decay features of the evaporation residues and of the emitted light particles are overall well described by a pure statistical model; however, as for the case of the previously studied 24Mg, we observed some deviations in the branching ratios, in particular for those chains involving only the evaporation of $\alpha$ particles. From this point of view the behavior of the $^{24}$Mg and $^{25}$Mg decay cases appear to be rather similar. An attempt to obtain a full mass balance even without neutron detection is also discussed.

nucl-ex↗

arXiv2016

Semi-automatic charge and mass identification in two-dimensional matrices

This article presents a new semi-automatic method for charge and mass identification in two-dimensional matrices. The proposed algorithm is based on the matrix's properties and uses as little information as possible on the global form of the identification lines, making it applicable to a large variety of matrices, including Particular attention has been paid to the implementation in a suitable graphical environment, so that only two mouse-clicks are required from the user to calculate all initialization parameters. Example applications to recent data from both INDRA and FAZIA telescopes are presented.

physics.ins-det↗

arXiv2014

Energy measurement and fragment identification using digital signals from partially depleted Si detectors

A study of identification properties of a Si-Si DE-E telescope exploiting an underdepleted residual-energy detector has been performed. Five different bias voltages have been used, one corresponding to full depletion, the others associated with a depleted layer ranging from 90% to 60% of the detector thickness. Fragment identification has been performed using either the DE-E technique or Pulse Shape Analysis (PSA). Both detectors are reverse mounted: particles enter from the low field side, to enhance the PSA performance. The achieved charge and mass resolution has been quantitatively expressed using a Figure of Merit (FoM). Charge collection efficiency has been evaluated and the possibility of energy calibration corrections has been considered. We find that the DE-E performance is not affected by incomplete depletion even when only 60% of the wafer is depleted. Isotopic separation capability improves at lower bias voltages with respect to full depletion, though charge identification thresholds are higher than at full depletion. Good isotopic identification via PSA has been obtained from a partially depleted detector whose doping uniformity is not good enough for isotopic identification at full depletion.

physics.ins-det↗

arXiv2013

The european FAZIA initiative: a high-performance digital telescope array for heavy-ion studies

The european Fazia collaboration aims at building a new modular array for charged product identification to be employed for heavy-ion studies. The elementary module of the array is a Silicon-Silicon-CsI telescope, optimized for ion identification also via pulse shape analysis. The achievement of top performances imposes specific electronics which has been developed by FAZIA and features high quality charge and current preamplifiers, coupled to fully digital front-end. During the initial R&D phase, original and novel solutions have been tested in prototypes, obtaining unprecedented ion identification capabilities. FAZIA is now constructing a demonstrator array consisting of about two hundreds telescopes arranged in a compact and transportable configuration. In this contribution, we mainly summarize some aspects studied by FAZIA to improve the ion identification. Then we will briefly discuss the FAZIA program centered on experiments to be done with the demonstrator. First results on the isospin dynamics obtained with a reduced set-up demonstrate well the performance of the telescope and represent a good starting point towards future investigations with both stable and exotic beams.

physics.ins-det↗

arXiv2009

Dynamics and energy landscape in a tetrahedral network glass-former: Direct comparison with models of fragile liquids

We report Molecular Dynamics simulations for a new model of tetrahedral network glass-former, based on short-range, spherical potentials. Despite the simplicity of the forcefield employed, our model reproduces some essential physical properties of silica, an archetypal network-forming material. Structural and dynamical properties, including dynamic heterogeneities and the nature of local rearrangements, are investigated in detail and a direct comparison with models of close-packed, fragile glass-formers is performed. The outcome of this comparison is rationalized in terms of the properties of the Potential Energy Surface, focusing on the unstable modes of the stationary points. Our results indicate that the weak degree of dynamic heterogeneity observed in network glass-formers may be attributed to an excess of localized unstable modes, associated to elementary dynamical events such as bond breaking and reformation. On the contrary, the more fragile Lennard-Jones mixtures are characterized by a larger fraction of extended unstable modes, which lead to a more cooperative and heterogeneous dynamics.

cond-mat.stat-mech↗

arXiv2007

Understanding fragility in supercooled Lennard-Jones mixtures. I. Locally preferred structures

We reveal the existence of systematic variations of isobaric fragility in different supercooled Lennard-Jones binary mixtures by performing molecular dynamics simulations. The connection between fragility and local structures in the bulk is analyzed by means of a Voronoi construction. We find that clusters of particles belonging to locally preferred structures form slow, long-lived domains, whose spatial extension increases by decreasing temperature. As a general rule, a more rapid growth, upon supercooling, of such domains is associated to a more pronounced super-Arrhenius behavior, hence to a larger fragility.

cond-mat.stat-mech↗

arXiv2007

Understanding fragility in supercooled Lennard-Jones mixtures. II. Potential energy surface

We numerically investigated the connection between isobaric fragility and the properties of high-order stationary points of the potential energy surface in different supercooled Lennard-Jones mixtures. The increase of effective activation energies upon supercooling appears to be driven by the increase of average potential energy barriers measured by the energy dependence of the fraction of unstable modes. Such an increase is sharper, the more fragile is the mixture. Correlations between fragility and other properties of high-order stationary points, including the vibrational density of states and the localization features of unstable modes, are also discussed.

cond-mat.stat-mech↗

arXiv2006

Are there localized saddles behind the heterogeneous dynamics of supercooled liquids?

We numerically study the interplay between heterogeneous dynamics and properties of negatively curved regions of the potential energy surface in a model glassy system. We find that the unstable modes of saddles and quasi-saddles undergo a localization transition close to the Mode-Coupling critical temperature. We also find evidence of a positive spatial correlation between clusters of particles having large displacements in the unstable modes and dynamical heterogeneities.

cond-mat.stat-mech↗

arXiv2004

Computer simulation study of the closure relations in hard sphere fluids

We study, using Monte Carlo simulations, the cavity and the bridge functions of various hard sphere fluids: one component system, equimolar additive and non additive binary mixtures. In particular, we numerically check the assumption of local dependency of the bridge functions from the indirect correlation functions, on which most of the existing integral equation theories hinge. We find that this condition can be violated either in the region around the first and second neighbors shell, or inside the hard core, for the systems here considered. The violations manifest themselves clearly in the so called Duh-Haymet plots of the bridge functions versus the indirect correlation functions and become amplified as the coupling of the system increases.

cond-mat.stat-mech↗

arXiv2003

A fully polarizable and dissociable potential for water

A new classical interaction potential for water simulations is presented. Water is modeled as a fully dissociable set of atoms with a point dipole, determined self-consistently, on every oxygen atom. The oxygen polarizability is not fixed but depends on the geometry of the system. We show that, in spite of the limited number of free parameters, the model reproduces the geometrical and vibrational properties of microclusters in a satisfactory way.

cond-mat↗

arXiv2003

Direct correlation functions of the Widom-Rowlinson model

We calculate, through Monte Carlo numerical simulations, the partial total and direct correlation functions of the three dimensional symmetric Widom-Rowlinson mixture. We find that the differences between the partial direct correlation functions from simulation and from the Percus-Yevick approximation (calculated analytically by Ahn and Lebowitz) are well fitted by Gaussians. We provide an analytical expression for the fit parameters as function of the density. We also present Monte Carlo simulation data for the direct correlation functions of a couple of non additive hard sphere systems to discuss the modification induced by finite like diameters.

cond-mat.stat-mech↗

arXiv2003

Stability of the iterative solutions of integral equations as one phase freezing criterion

A recently proposed connection between the threshold for the stability of the iterative solution of integral equations for the pair correlation functions of a classical fluid and the structural instability of the corresponding real fluid is carefully analyzed. Direct calculation of the Lyapunov exponent of the standard iterative solution of HNC and PY integral equations for the 1D hard rods fluid shows the same behavior observed in 3D systems. Since no phase transition is allowed in such 1D system, our analysis shows that the proposed one phase criterion, at least in this case, fails. We argue that the observed proximity between the numerical and the structural instability in 3D originates from the enhanced structure present in the fluid but, in view of the arbitrary dependence on the iteration scheme, it seems uneasy to relate the numerical stability analysis to a robust one-phase criterion for predicting a thermodynamic phase transition.

cond-mat↗

arXiv2003

Generating functionals, consistency, and uniqueness in the integral equation theory of liquids

We discuss and illustrate through numerical examples the relations between generating functionals, thermodynamic consistency (in particular the virial-free energy one), and uniqueness of the solution, in the integral equation theory of liquids. We propose a new approach for deriving closures automatically satisfying such characteristics. Results from a first exploration of this program are presented and discussed.

cond-mat↗

arXiv1997

Optimized random phase approximations for arbitrary reference systems: extremum conditions and thermodynamic consistence

The optimized random phase approximation (ORPA) for classical liquids is re-examined in the framework of the generating functional approach to the integral equations. We show that the two main variants of the approximation correspond to the addition of the same correction to two different first order approximations of the homogeneous liquid free energy. Furthermore, we show that it is possible to consistently use the ORPA with arbitrary reference systems described by continuous potentials and that the same approximation is equivalent to a particular extremum condition for the corresponding generating functional. Finally, it is possible to enforce the thermodynamic consistence between the thermal and the virial route to the equation of state by requiring the global extremum condition on the generating functional.

physics.chem-ph↗

arXiv1996

Ab initio simulations of liquid systems: Concentration dependence of the electric conductivity of NaSn alloys

Liquid NaSn alloys in five different compositions (20, 40, 50, 57 and 80% sodium) are studied using density functional calculations combined with molecular dynamics(Car-Parrinello method). The frequency-dependent electric conductivities for the systems are calculated by means of the Kubo-Greenwood formula. The extrapolated DC conductivities are in good agreement with the experimental data and reproduce the strong variation with the concentration. The maximum of conductivity is obtained, in agreement with experiment, near the equimolar composition. The strong variation of conductivity, ranging from almost semiconducting up to metallic behaviour, can be understood by an analysis of the densities-of-states.

mtrl-th↗