3,789 research outputs found
A model for projectile fragmentation
A model for projectile fragmentation is developed whose origin can be traced
back to the Bevalac era. The model positions itself between the
phenomenological EPAX parametrization and transport models like "Heavy Ion
Phase Space Exploration" (HIPSE) model and antisymmetrised molecular dynamics
(AMD) model. A very simple impact parameter dependence of input temperature is
incorporated in the model which helps to analyze the more peripheral
collisions. The model is applied to calculate the charge, isotopic
distributions, average number of intermediate mass fragments and the average
size of largest cluster at different Z_{bound} of different projectile
fragmentation reactions at different energies.Comment: Talk given by Gargi Chaudhuri at the 11th International Conference on
Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1,
2012. 10 pages, 7 figure
Isoscaling in Peripheral Nuclear Collisions around the Fermi Energy and a Signal of Chemical Separation from its Excitation Energy Dependence
The isoscaling is investigated using the fragment yield data from fully
reconstructed quasi-projectiles observed in peripheral collisions of 28Si with
124,112Sn at projectile energies 30 and 50 MeV/nucleon. The excitation energy
dependence of the isoscaling parameter beta_prime is observed which is
independent of beam energy. For a given quasi-projectile produced in reactions
with different targets no isoscaling is observed. The isoscaling thus reflects
the level of N/Z-equilibration in reactions with different targets represented
by the initial quasi-projectile samples. The excitation energy dependence of
the isoscaling parameter beta_prime, corrected for the trivial 1/T temperature
dependence, does not follow the trend of the homogeneous system above 4
MeV/nucleon thus possibly signaling the onset of separation into isospin
asymmetric dilute and isospin symmetric dense phase.Comment: 4 pages, 4 figures, RevTeX, to appear in Physical Review
Ambiguities in statistical calculations of nuclear fragmentation
The concept of freeze out volume used in many statistical approaches for
disassembly of hot nuclei leads to ambiguities. The fragmentation pattern and
the momentum distribution (temperature) of the emanated fragments are
determined by the phase space at the freeze-out volume where the interaction
among the fragments is supposedly frozen out. However, to get coherence with
the experimental momentum distribution of the charged particles, one introduces
Coulomb acceleration beyond this freeze-out. To be consistent, we investigate
the effect of the attractive nuclear force beyond this volume and find that the
possible recombination of the fragments alters the physical observables
significantly casting doubt on the consistency of the statistical model.Comment: 11 pages+3 figure
Flow effects on multifragmentation in the canonical model
A prescription to incorporate the effects of nuclear flow on the process of
multifragmentation of hot nuclei is proposed in an analytically solvable
canonical model. Flow is simulated by the action of an effective negative
external pressure. It favors sharpening the signatures of liquid-gas phase
transition in finite nuclei with increased multiplicity and a lowered phase
transition temperature.Comment: 13 pages, 5 Post Script figures (accepted for publication in PRC
Symmetry energy and the isospin dependent equation of state
The isoscaling parameter , from the fragments produced in the
multifragmentation of Ni + Ni, Fe + Ni and
Fe + Fe reactions at 30, 40 and 47 MeV/nucleon, was compared with
that predicted by the antisymmetrized molecular dynamic (AMD) calculation based
on two different nucleon-nucleon effective forces, namely the Gogny and
Gogny-AS interaction. The results show that the data agrees better with the
choice of Gogny-AS effective interaction, resulting in a symmetry energy of
18-20 MeV. The observed value indicate that the fragments are formed at
a reduced density of 0.08 fm.Comment: 5 pages, 5 figures, Accepted for publication in Phys. Rev. C (Rapid
Communication
The Statistical Multifragmentation Model with Skyrme Effective Interactions
The Statistical Multifragmentation Model is modified to incorporate the
Helmholtz free energies calculated in the finite temperature Thomas-Fermi
approximation using Skyrme effective interactions. In this formulation, the
density of the fragments at the freeze-out configuration corresponds to the
equilibrium value obtained in the Thomas-Fermi approximation at the given
temperature. The behavior of the nuclear caloric curve at constant volume is
investigated in the micro-canonical ensemble and a plateau is observed for
excitation energies between 8 and 10 MeV per nucleon. A kink in the caloric
curve is found at the onset of this gas transition, indicating the existence of
a small excitation energy region with negative heat capacity. In contrast to
previous statistical calculations, this situation takes place even in this case
in which the system is constrained to fixed volume. The observed phase
transition takes place at approximately constant entropy. The charge
distribution and other observables also turn out to be sensitive to the
treatment employed in the calculation of the free energies and the fragments'
volumes at finite temperature, specially at high excitation energies. The
isotopic distribution is also affected by this treatment, which suggests that
this prescription may help to obtain information on the nuclear equation of
state
Symmetry energy and the isoscaling properties of the fragments produced in Ar, Ca + Fe, Ni reactions at 25 53 MeV/nucleon
The symmetry energy and the isoscaling properties of the fragments produced
in the multifragmentation of Ar, Ca + Fe, Ni
reactions at 25 - 53 MeV/nucleon were investigated within the framework of
statistical multifragmentation model. The isoscaling parameters , from
the primary (hot) and secondary (cold) fragment yield distributions, were
studied as a function of excitation energy, isospin (neutron-to-proton
asymmetry) and fragment symmetry energy. It is observed that the isoscaling
parameter decreases with increasing excitation energy and decreasing
symmetry energy. The parameter is also observed to increase with
increasing difference in the isospin of the fragmenting system. The sequential
decay of the primary fragments into secondary fragments, when studied as a
function of excitation energy and isospin of the fragmenting system, show very
little influence on the isoscaling parameter. The symmetry energy however, has
a strong influence on the isospin properties of the hot fragments. The
experimentally observed scaling parameters can be explained by symmetry energy
that is significantly lower than that for the ground state nuclei near
saturation density. The results indicate that the properties of hot nuclei at
excitation energies, densities and isospin away from the normal ground state
nuclei could be significantly different.Comment: 14 pages, 15 figure
Stochastic Interpolants: A Unifying Framework for Flows and Diffusions
A class of generative models that unifies flow-based and diffusion-based
methods is introduced. These models extend the framework proposed in Albergo &
Vanden-Eijnden (2023), enabling the use of a broad class of continuous-time
stochastic processes called `stochastic interpolants' to bridge any two
arbitrary probability density functions exactly in finite time. These
interpolants are built by combining data from the two prescribed densities with
an additional latent variable that shapes the bridge in a flexible way. The
time-dependent probability density function of the stochastic interpolant is
shown to satisfy a first-order transport equation as well as a family of
forward and backward Fokker-Planck equations with tunable diffusion. Upon
consideration of the time evolution of an individual sample, this viewpoint
immediately leads to both deterministic and stochastic generative models based
on probability flow equations or stochastic differential equations with an
adjustable level of noise. The drift coefficients entering these models are
time-dependent velocity fields characterized as the unique minimizers of simple
quadratic objective functions, one of which is a new objective for the score of
the interpolant density. Remarkably, we show that minimization of these
quadratic objectives leads to control of the likelihood for any of our
generative models built upon stochastic dynamics. By contrast, we establish
that generative models based upon a deterministic dynamics must, in addition,
control the Fisher divergence between the target and the model. We also
construct estimators for the likelihood and the cross-entropy of
interpolant-based generative models, discuss connections with other stochastic
bridges, and demonstrate that such models recover the Schr\"odinger bridge
between the two target densities when explicitly optimizing over the
interpolant
Influence of the Coulomb Interaction on the Chemical Equilibrium of Nuclear Systems at Break-Up
The importance of a Coulomb correction to the formalism proposed by Albergo
et al. for determining the temperatures of nuclear systems at break-up and the
ensities of free nucleon gases is discussed. While the proposed correction has
no effect on the temperatures extracted based on double isotope ratios, it
becomes non-negligible when such temperatures or densities of free nucleon
gases are extracted based on multiplicities of heavier fragments of different
atomic numbers
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