489 research outputs found
Inhomogeneity growth in two-component fermionic systems
The dynamics of fermionic many-body systems is investigated in the framework
of Boltzmann-Langevin (BL) stochastic one-body approaches. Within the recently
introduced BLOB model, we examine the interplay between mean-field effects and
two-body correlations, of stochastic nature, for nuclear matter at moderate
temperature and in several density conditions, corresponding to stable or
mechanically unstable situations. Numerical results are compared to analytic
expectations for the fluctuation amplitude of isoscalar and isovector
densities, probing the link to the properties of the employed effective
interaction, namely symmetry energy (for isovector modes) and incompressibility
(for isoscalar modes). For unstable systems, clusterization is observed. The
associated features are compared to analytical results for the typical length
and time scales characterizing the growth of unstable modes in nuclear matter
and for the isotopic variance of the emerging fragments. We show that the BLOB
model is generally better suited than simplified approaches previously
introduced to solve the BL equation, and it is therefore more advantageous in
applications to open systems, like heavy ion collisions.Comment: 19 pages, 13 figure
Frustrated fragmentation and re-aggregation in nuclei: a non-equilibrium description in spallation
Heavy nuclei bombarded with protons and deuterons in the 1 GeV range have a
large probability of undergoing a process of evaporation and fission; less
frequently, the prompt emission of few intermediate-mass fragments can also be
observed.
We employ a recently developed microscopic approach, based on the
Boltzmann-Langevin transport equation, to investigate the role of mean-field
dynamics and phase-space fluctuations in these reactions.
We find that the formation of few IMF's can be confused with asymmetric
fission when relying on yield observables, but it can not be assimilated to the
statistical decay of a compound nucleus when analysing the dynamics and
kinematic observables: it can be described as a fragmentation process initiated
by phase-space fluctuations, and successively frustrated by the mean-field
resilience. As an extreme situation, which corresponds to non-negligible
probability, the number of fragments in the exit channel reduces to two, so
that fission-like events are obtained by re-aggregation processes.
This interpretation, inspired by nuclear-spallation experiments, can be
generalised to heavy-ion collisions from Fermi to relativistic energies, for
situations when the system is closely approaching the fragmentation threshold
Bifurcations in Boltzmann-Langevin One Body dynamics for fermionic systems
We investigate the occurrence of bifurcations in the dynamical trajectories
depicting central nuclear collisions at Fermi energies. The quantitative
description of the reaction dynamics is obtained within a new transport model,
based on the solution of the Boltzmann-Langevin equation in three dimensions,
with a broad applicability for dissipative fermionic dynamics. Dilute systems
formed in central collisions are shown to fluctuate between two energetically
favourable mechanisms: reverting to a compact shape or rather disintegrating
into several fragments. The latter result can be connected to the recent
observation of bimodal distributions for quantities characterising
fragmentation processes and may suggest new investigations
Bifurcations in dissipative fermionic dynamics
The Boltzmann-Langevin One-Body model (BLOB), is a novel one-body transport
approach, based on the solution of the Boltzmann-Langevin equation in three
dimensions; it is used to handle large-amplitude phase-space fluctuations and
has a broad applicability for dissipative fermionic dynamics. We study the
occurrence of bifurcations in the dynamical trajectories describing heavy-ion
collisions at Fermi energies.
The model, applied to dilute systems formed in such collisions, reveals to be
closer to the observation than previous attempts to include a Langevin term in
Boltzmann theories. The onset of bifurcations and bimodal behaviour in
dynamical trajectories, determines the fragment-formation mechanism. In
particular, in the proximity of a threshold, fluctuations between two
energetically favourable mechanisms stand out, so that when evolving from the
same entrance channel, a variety of exit channels is accessible.
This description gives quantitative indications about two threshold
situations which characterise heavy-ion collisions at Fermi energies. First,
the fusion-to-multifragmentation threshold in central collisions, where the
system either reverts to a compact shape, or splits into several pieces of
similar sizes. Second, the transition from binary mechanisms to neck
fragmentation (in general, ternary channels), in peripheral collisions.Comment: Conf. proc. ECHIC November 6-8, 2013 Messina (Italy
Mean-field instabilities and cluster formation in nuclear reactions
We review recent results on intermediate mass cluster production in heavy ion
collisions at Fermi energy and in spallation reactions. Our studies are based
on modern transport theories, employing effective interactions for the nuclear
mean-field and incorporating two-body correlations and fluctuations. Namely we
will consider the Stochastic Mean Field (SMF) approach and the recently
developed Boltzmann-Langevin One Body (BLOB) model. We focus on cluster
production emerging from the possible occurrence of low-density mean-field
instabilities in heavy ion reactions. Within such a framework, the respective
role of one and two-body effects, in the two models considered, will be
carefully analysed. We will discuss, in particular, fragment production in
central and semi-peripheral heavy ion collisions, which is the object of many
recent experimental investigations. Moreover, in the context of spallation
reactions, we will show how thermal expansion may trigger the development of
mean-field instabilities, leading to a cluster formation process which competes
with important re-aggregation effects
Spinodal instability growth in new stochastic approaches
Are spinodal instabilities the leading mechanism in the fragmentation of a
fermionic system? Numerous experimental indications suggest such a scenario and
stimulated much effort in giving a suitable description, without being
finalised in a dedicated transport model.
On the one hand, the bulk character of spinodal behaviour requires an
accurate treatment of the one-body dynamics, in presence of mechanical
instabilities. On the other hand, pure mean-field implementations do not apply
to situations where instabilities, bifurcations and chaos are present. The
evolution of instabilities should be treated in a large-amplitude framework
requiring fluctuations of Langevin type.
We present new stochastic approaches constructed by requiring a thorough
description of the mean-field response in presence of instabilities. Their
particular relevance is an improved description of the spinodal fragmentation
mechanism at the threshold, where the instability growth is frustrated by the
mean-field resilience.Comment: Conf. proc. IWM2014-EC, Catania, 6-9 May 201
Ising analogue to compact-star matter
By constructing an Ising analogue of compact-star matter at sub-saturation
density we explored the effect of Coulomb frustration on the nuclear liquid-gas
phase transition. Our conclusions is twofold. First, the range of temperatures
where inhomogeneous phases form expands with increasing Coulomb-field strength.
Second, within the approximation of uniform electron distribution, the limiting
point upon which the phase-coexistence region ends does not exhibit any
critical behaviour. Possible astrophysics consequences and thermodynamical
connections are discussed.Comment: 4 pages, 3 figure
A non-equilibrium microscopic description of spallation
We investigate the prompt emission of few intermediate-mass fragments in spallation reactions induced by protons and deuterons in the 1GeV range. Such emission has a minor contribution to the total reaction cross section, but it may overcome evaporation and ïŹssion channels in the formation of light nuclides. The role of mean-ïŹeld dynamics and phase-space ïŹuctuations in these reactions is investigated through the Boltzmann-Langevin transport equation. We found that a process of frustrated fragmentation and re-aggregation is a prominent mechanism of production of IMFs which can not be assimilated to the statistical decay of a compound nucleus. Very interestingly, this process may yield a small number of IMF in the exit channel, which may even reduce to two, and be wrongly confused with ordinary asymmetric ïŹssion. This interpretation, inspired by nuclear-spallation experiments, can be generalised to heavy-ion collisions approaching the fragmentation threshold
Optical properties of highly n-doped germanium obtained by in situ doping and laser annealing
High n-type doping in germanium is essential for many electronic and optoelectronic applications especially for high performance Ohmic contacts, lasing and mid-infrared plasmonics. We report on the combination of in situ doping and excimer laser annealing to improve the activation of phosphorous in germanium. An activated n-doping concentration of 8.8ââĂââ1019 cmâ3 has been achieved starting from an incorporated phosphorous concentration of 1.1ââĂââ1020 cmâ3. Infrared reflectivity data fitted with a multi-layer Drude model indicate good uniformity over a 350ânm thick layer. Photoluminescence demonstrates clear bandgap narrowing and an increased ratio of direct to indirect bandgap emission confirming the high doping densities achieved
Nuclear expansion and symmetry energy of hot nuclei
The decrease in the symmetry energy of hot nuclei populated in Ni +
Ni, Fe + Ni and Fe + Fe reactions at beam
energies of 30, 40, and 47 MeV/nucleon, as a function of excitation energy is
studied. It is observed that this decrease is mainly a consequence of
increasing expansion or decreasing density rather than the increasing
temperature. The results are in good agreement with the recently reported
microscopic calculation based on the Thomas-Fermi approach. An empirical
relation to study the symmetry energy of finite nuclei in various mass region
is proposed.Comment: 10 pages, 2 figure
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