3,812 research outputs found
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
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
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
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
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
An improved method for estimating the neutron background in measurements of neutron capture reactions
The relation between the neutron background in neutron capture measurements
and the neutron sensitivity related to the experimental setup is examined. It
is pointed out that a proper estimate of the neutron background may only be
obtained by means of dedicated simulations taking into account the full
framework of the neutron-induced reactions and their complete temporal
evolution. No other presently available method seems to provide reliable
results, in particular under the capture resonances. An improved neutron
background estimation technique is proposed, the main improvement regarding the
treatment of the neutron sensitivity, taking into account the temporal
evolution of the neutron-induced reactions. The technique is complemented by an
advanced data analysis procedure based on relativistic kinematics of neutron
scattering. The analysis procedure allows for the calculation of the neutron
background in capture measurements, without requiring the time-consuming
simulations to be adapted to each particular sample. A suggestion is made on
how to improve the neutron background estimates if neutron background
simulations are not available.Comment: 11 pages, 9 figure
Compensation effect between deaths from Covid-19 and crashes: The Italian case
Emergencies such as the Covid-19 pandemic pose several decision-making issues, while clear evidence of successful strategies are still unavailable, different policies may be identified. However, in such emergencies, the preservation of public health, by firstly reducing human loss of life may be prioritized and then restrictive measures are implemented. The trade-off between damage due to the threat and the decrease in damage due to the lockdown is largely unexplored. Here we show that there is a degree of compensation between damage from epidemic deaths and from traffic deaths, especially in the case of immediate restrictive measures imposed by governments. Based on the Italian case, we found that damage from loss of human Capital and health care costs could have been fully compensated if the lockdown had been imposed ten days earlier. Considering only one Italian region (Puglia), where the epidemic was delayed and then restrictions were timely, damage due to loss of human Capital was largely compensated in the real scenario. However, damage due to loss of welfare could not have been fully compensated for, since Covid-19 deaths largely outnumber traffic deaths in the simulated epidemic period and loss of welfare damage is scarcely dependent on the age-at-death. From a broader perspective, societies seem to react to external threats as a whole organism, thus tending to restore the original equilibrium. Governmental decisions could accelerate this process. However, in the case of similar threats, some wounds cannot be compensated for, such as the incalculable damage due to loss of welfare
Analysis of Boltzmann-Langevin Dynamics in Nuclear Matter
The Boltzmann-Langevin dynamics of harmonic modes in nuclear matter is
analyzed within linear-response theory, both with an elementary treatment and
by using the frequency-dependent response function. It is shown how the source
terms agitating the modes can be obtained from the basic BL correlation kernel
by a simple projection onto the associated dual basis states, which are
proportional to the RPA amplitudes and can be expressed explicitly. The source
terms for the correlated agitation of any two such modes can then be extracted
directly, without consideration of the other modes. This facilitates the
analysis of collective modes in unstable matter and makes it possible to asses
the accuracy of an approximate projection technique employed previously.Comment: 13 latex pages, 4 PS figure
Time development of a density perturbation in the unstable nuclear matter
We present the solution of the time development of an unstable initial
density perturbation in the linearized Vlasov equation, completing the previous
analysis in the literature. The additional contributions found are usually
damped and can be neglected at large times in the unstable region. The work
clarifies also the problem of the normalization of the solution with respect to
the initial perturbation of the density.Comment: revision of the discussion, different initial perturbation, 9 pages,
4 figures included, uses epsfi
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