723 research outputs found
Theoretical predictions of experimental observables sensitive to the symmetry energy: Results of the SMF transport model
In the framework of mean-field based transport approaches, we discuss recent
results concerning heavy ion reactions between charge asymmetric systems, from
low up to intermediate energies. We focus on isospin sensitive observables,
aiming at extracting information on the density dependence of the isovector
part of the nuclear effective interaction and of the nuclear symmetry energy.
For reactions close to the Coulomb barrier, we explore the structure of
collective dipole oscillations, rather sensitive to the low-density behavior of
the symmetry energy. In the Fermi energy regime, we investigate the interplay
between dissipation mechanisms, fragmentation and isospin effects. At
intermediate energies, where regions with higher density and momentum are
reached, we discuss collective flows and their sensitivity to the momentum
dependence of the isovector interaction channel, which determines the splitting
of neutron and proton effective masses. Finally, we also discuss the isospin
effect on the possible phase transition from nucleonic matter to quark matter.
Results are critically reviewed, also trying to establish a link, when
possible, with the outcome of other transport models.Comment: A contribution to the upcoming EPJA Special Volume on Nuclear
Symmetry Energ
Theory of Single Charge Exchange Heavy Ion Reactions
The theory of heavy ion single charge exchange reactions is reformulated. In
momentum space the reaction amplitude factorizes into a product of projectile
and target transition form factors, folded with the nucleon-nucleon isovector
interaction and a distortion coefficient which accounts for initial and final
state ion-ion elastic interactions. The multipole structure of the transition
form factors is studied in detail for Fermi-type non-spin flip and
Gamow-Teller-type spin flip transitions, also serving to establish the
connection to nuclear beta decay. The reaction kernel is evaluated for central
and rank-2 tensor interactions. Initial and final state elastic ion-ion
interaction are shown to be dominated by the imaginary part of the optical
potential allowing to evaluate the reaction coefficients in the strong
absorption limit, realized by the black disk approximation. In that limit the
distortion coefficient is evaluated in closed form, revealing the relation to
the total reaction cross section and the geometry of the transition form
factors. It is shown that at small momentum transfer distortion effects reduce
to a simple scaling factor, allowing to define reduced forward-angle cross
section which is given by nuclear matrix elements of beta decay-type. The
response function formalism is used to describe nuclear charge changing
transitions. Spectral distributions obtained by a self-consistent HFB and QRPA
approach are discussed for excitations of and ,
respectively, and compared to spectroscopic data. The interplay of nuclear
structure and reaction dynamics is illustrated for the single charge exchange
reaction at MeV
The Multifaceted Activity of the VirF Regulatory Protein in the Shigella Lifestyle
Shigella is a highly adapted human pathogen, mainly found in the developing world and causing a severe enteric syndrome. The highly sophisticated infectious strategy of Shigella banks on the capacity to invade the intestinal epithelial barrier and cause its inflammatory destruction. The cellular pathogenesis and clinical presentation of shigellosis are the sum of the complex action of a large number of bacterial virulence factors mainly located on a large virulence plasmid (pINV). The expression of pINV genes is controlled by multiple environmental stimuli through a regulatory cascade involving proteins and sRNAs encoded by both the pINV and the chromosome. The primary regulator of the virulence phenotype is VirF, a DNA-binding protein belonging to the AraC family of transcriptional regulators. The virF gene, located on the pINV, is expressed only within the host, mainly in response to the temperature transition occurring when the bacterium transits from the outer environment to the intestinal milieu. VirF then acts as anti-H-NS protein and directly activates the icsA and virB genes, triggering the full expression of the invasion program of Shigella. In this review we will focus on the structure of VirF, on its sophisticated regulation, and on its role as major player in the path leading from the non-invasive to the invasive phenotype of Shigella. We will address also the involvement of VirF in mechanisms aimed at withstanding adverse conditions inside the host, indicating that this protein is emerging as a global regulator whose action is not limited to virulence systems. Finally, we will discuss recent observations conferring VirF the potential of a novel antibacterial target for shigellosis
Investigation of collective radial expansion and stopping in heavy ion collisions at Fermi energies
We present an analysis of multifragmentation events observed in central Xe+Sn
reactions at Fermi energies. Performing a comparison between the predictions of
the Stochastic Mean Field (SMF) transport model and experimental data, we
investigate the impact of the compression-expansion dynamics on the properties
of the final reaction products. We show that the amount of radial collective
expansion, which characterizes the dynamical stage of the reaction, influences
directly the onset of multifragmentation and the kinematic properties of
multifragmentation events. For the same set of events we also undertake a shape
analysis in momentum space, looking at the degree of stopping reached in the
collision, as proposed in recent experimental studies. We show that full
stopping is achieved for the most central collisions at Fermi energies.
However, considering the same central event selection as in the experimental
data, we observe a similar behavior of the stopping power with the beam energy,
which can be associated with a change of the fragmentation mechanism, from
statistical to prompt fragment emission.Comment: 15 page
Beyond the random phase approximation with a local exchange vertex
With the aim of constructing an electronic structure approach that systematically goes beyond the GW and random phase approximation (RPA) we introduce a vertex correction based on the exact-exchange (EXX) potential of time-dependent density functional theory. The EXX vertex function is constrained to be local but is expected to capture similar physics as the Hartree-Fock vertex. With the EXX vertex, we then unify different beyond-RPA approaches such as the various resummations of RPA with exchange and the second-order screened exchange approximation. The theoretical analysis is supported by numerical studies on the hydrogen dimer and the electron gas, and we discuss the role of including the vertex correction in both the screened interaction and the self-energy. Finally, we give details on our implementation within the plane-wave pseudo potential framework and demonstrate the excellent performance of the different RPA with exchange methods in describing the energetics of hydrogen and van der Waals bonds
Validation of Geant4 nuclear reaction models for hadrontherapy and preliminary results with SMF and BLOB
Reliable nuclear fragmentation models are of utmost importance in hadrontherapy, where Monte Carlo (MC) simulations are used to compute the input parameters of the treatment planning software, to validate the deposited dose calculation, to evaluate the biological effectiveness of the radiation, to correlate the bĂľ emitters production in the patient body with the delivered dose, and to allow a non- invasive treatment verification.
Despite of its large use, the models implemented in Geant4 have shown severe limitations in reproducing the measured secondaries yields in ions interaction below 100 MeV/A, in term of production rates, angular and energy distributions [1–3]. We will present a benchmark of the Geant4 models with double-differential cross sec- tion and angular distributions of the secondary fragments produced in the 12C fragmentation at 62 MeV/A on thin carbon target, such a benchmark includes the recently implemented model INCL++ [4,5]. Moreover, we will present the preliminary results, obtained in simulating the same interaction, with SMF [6] and BLOB [7]. Both, SMF and BLOB are semiclassical one-body approaches to solve the Boltzmann-Langevin equation. They include an identical treatment of the mean-field propagation, on the basis of the same effective interaction, but they differ in the way fluctuations are included.
In particular, while SMF employs a Uehling-Uhlenbeck collision term and introduces fluctuations as projected on the density space, BLOB introduces fluctuations in full phase space through a modified collision term where nucleon-nucleon correlations are explicitly involved. Both of them, SMF and BLOB, have been developed to sim- ulate the heavy ion interactions in the Fermi-energy regime. We will show their capabilities in describing 12C fragmentation foreseen their implementation in Geant4
Clustering effects in the Li(p,He)He reaction at astrophysical energies
Background: The understanding of nuclear reactions between light nuclei at
energies below the Coulomb barrier is important for several astrophysical
processes, but their study poses experimental and theoretical challenges. At
sufficiently low energies, the electrons surrounding the interacting ions
affect the scattering process. Moreover, the clustered structure of some of
these nuclei may play a relevant role on the reaction observables.
Purpose: In this article, we focus on a theoretical investigation of the role
of clustered configurations of Li in reactions of astrophysical interest.
Methods: The Li(p,He)He reaction cross section is described
considering both the direct transfer of a deuteron as a single point-like
particle in Distorted Wave Born Approximation (DWBA), and the transfer of a
neutron and a proton in second-order DWBA. A number of two- and three-cluster
structure models for Li are compared.
Results: Within the two-cluster structure model, we explore the impact of the
deformed components in the Li wave-function on the reaction of interest.
Within the three-cluster structure model, we gauge the degree of -d
clustering and explicitly probe its role on specific features of the reaction
cross section. We compare the energy trend of the astrophysical factor
deduced in each case.
Conclusions: Clustered Li configurations lead in general to a significant
enhancement of the astrophysical factor in the energy region under study. This
effect only originates from clustering, whereas static deformations of the
ground-state configuration play a negligible role at very low energies
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