arXiv subjects

A. Ono

Publications and source records attributed to A. Ono.

30 records · Page 2Linked to original sources

arXiv2006

Comment on "Symmetry energy and the isospin dependent equation of state"

In a recent paper, Shetty et al [Phys. Rev. C 70, 011601R (2004)] reported that the experimental isoscaling parameters aexp from several reactions favor the Gogny-AS effective nucleon-nucleon interaction over Gogny interaction. This conclusion is reached by comparing data to Antisymmetrized Molecular Dynamics (AMD) predictions for collisions of calcium isotopes. The specific simulations produce excited fragments at t=300 fm/c before they decay. Sequential decay calculations of the excited fragments suggest that the isoscaling parameter apri from the AMD calculations could be reduced by as much as 50%. We also explore the linear equation used to relate other reactions to the AMD calculations for Ca+Ca collisions. The uncertainty in the relation is larger than the differences between the predicted apri when using Gogny and Gogny-AS interactions.

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arXiv2006

Properties of the Initial Participant Matter Interaction Zone in Near Fermi-Energy Heavy Ion Collisions

The sizes, temperatures and free neutron to proton ratios of the initial interaction zones produced in the collisions of 40 MeV/nucleon $^{40}$Ar + $^{112}$Sn and 55 MeV/nucleon$^{27}$Al + $^{124}$Sn are derived using total detected neutron plus charged particle multiplicity as a measure of the impact parameter range and number of participant nucleons. The size of the initial interaction zone, determined from a coalescence model analysis, increases significantly with decreasing impact parameter. The temperatures and free neutron to proton ratios in the interaction zones are relatively similar for different impact parameter ranges and evolve in a similar fashion.

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arXiv2005

Short timescale behavior of colliding heavy nuclei at intermediate energies

An Antisymmetrized Molecular Dynamics model is used to explore the collision of $^{114}$Cd projectiles with $^{92}$Mo target nuclei at E/A=50 MeV over a broad range in impact parameter. The atomic number (Z), velocity, and emission pattern of the reaction products are examined as a function of the impact parameter and the cluster recognition time. The non-central collisions are found to be essentially binary in character resulting in the formation of an excited projectile-like fragment (PLF$^*$) and target-like fragment (TLF$^*$). The decay of these fragments occurs on a short timescale, 100$\le$t$\le$300 fm/c. The average excitation energy deduced for the PLF$^*$ and TLF$^*$ `saturates for mid-central collisions, 3.5$\le$b$\le$6 fm, with its magnitude depending on the cluster recognition time. For short cluster recognition times (t=150 fm/c), an average excitation energy as high as $\approx$6 MeV is predicted. Short timescale emission leads to a loss of initial correlations and results in features such as an anisotropic emission pattern of both IMFs and alpha particles emitted from the PLF$^*$ and TLF$^*$ in peripheral collisions.

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arXiv2005

Tracing the Evolution of Temperature in Near Fermi Energy Heavy Ion Collisions

The kinetic energy variation of emitted light clusters has been employed as a clock to explore the time evolution of the temperature for thermalizing composite systems produced in the reactions of 26A, 35A and 47A MeV $^{64}$Zn with $^{58}$Ni, $^{92}$Mo and $^{197}$Au. For each system investigated, the double isotope ratio temperature curve exhibits a high maximum apparent temperature, in the range of 10-25 MeV, at high ejectile velocity. These maximum values increase with increasing projectile energy and decrease with increasing target mass. The time at which the maximum in the temperature curve is reached ranges from 80 to 130 fm/c after contact. For each different target, the subsequent cooling curves for all three projectile energies are quite similar. Temperatures comparable to those of limiting temperature systematics are reached 30 to 40 fm/c after the times corresponding to the maxima, at a time when AMD-V transport model calculations predict entry into the final evaporative or fragmentation stage of de-excitation of the hot composite systems. Evidence for the establishment of thermal and chemical equilibrium is discussed.

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arXiv2004

A Ghoshal-like Test of Equilibration in Near-Fermi-Energy Heavy Ion Collisions

Calorimetric and coalescence techniques have been employed to probe equilibration for hot nuclei produced in heavy ion collisions of 35 to 55 MeV/u projectiles with medium mass targets. Entrance channel mass asymmetries and energies were selected in order that very hot composite nuclei of similar mass and excitation would remain after early stage pre-equilibrium particle emission. Inter-comparison of the properties and de-excitation patterns for these different systems provides evidence for the production of hot nuclei with decay patterns relatively independent of the specific entrance channel.

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arXiv2003

Reaction Dynamics and Multifragmentation in Fermi Energy Heavy Ion Reactions

The reaction systems, 64Zn + 58Ni, 64Zn + 92Mo, 64Zn + 197Au, at 26A, 35A and 47A MeV, have been studied both in experiments with a 4$π$ detector array, NIMROD, and with Antisymmetrized Molecular Dynamics model calculations employing effective interactions corresponding to soft and stiff equations of state (EOS). Direct experimental observables, such as multiplicity distributions, charge distributions, energy spectra and velocity spectra, have been compared in detail with those of the calculations and a reasonable agreement is obtained. The velocity distributions of $α$ particles and fragments with Z >= 3 show distinct differences in calculations with the soft EOS and the stiff EOS. The velocity distributions of $α$ particle and Intermediate Mass Fragments (IMF's) are best described by the stiff EOS. Neither of the above direct observables nor the strength of the elliptic flow are sensitive to changes in the in-medium nucleon-nucleon (NN) cross sections. A detailed analysis of the central collision events calculated with the stiff EOS revealed that multifragmentation with cold fragment emission is a common feature predicted for all reactions studied here. A possible multifragmentation scenario is presented; after the preequilibrium emission ceases in the composite system, cold light fragments are formed in a hotter gas of nucleons and stay cold until the composite system underdoes multifragmentation. For reaction with 197Au at 47A MeV a significant radial expansion takes place. For reactions with 58Ni and 92Mo at 47A MeV semi-transparency becomes prominent. The differing reaction dynamics drastically change the kinematic characteristics of emitted fragments. This scenario gives consistent explanations for many existing experimental results in the Fermi energy domain.

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arXiv2002

Sequential Decay Distortion of Goldhaber Model Widths for Spectator Fragments

Momentum widths of the primary fragments and observed final fragments have been investigated within the framework of an Antisymmetrized Molecular Dynamics transport model code (AMD-V) with a sequential decay afterburner (GEMINI). It is found that the secondary evaporation effects cause the values of a reduced momentum width, $σ_0$, derived from momentum widths of the final fragments to be significantly less than those appropriate to the primary fragment but close to those observed in many experiments. Therefore, a new interpretation for experiemental momentum widths of projectile-like fragments is presented.

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arXiv2002

Fragment production in central heavy-ion collisions: reconciling the dominance of dynamics with observed phase transition signals through universal fluctuations

Fragment production in central collisions of Xe+Sn has been systematically studied with the INDRA multidetector from 25 to 150 AMeV. The predominant role of collision dynamics is evidenced in multiple intermediate mass fragment production even at the lowest energies, around the so-called "multifragmentation threshold". For beam energies 50 AMeV and above, a promising agreement with suitably modified Antisymmetrised Molecular Dynamics calculations has been achieved. Intriguingly the same reactions have recently been interpreted as evidence for a liquid-gas phase transition in thermodynamically equilibrated systems. The universal fluctuation theory, thanks to its lack of any equilibrium hypothesis, shows clearly that in all but a tiny minority of carefully-selected central collisions fragment production is incompatible with either critical or phase coexistence behaviour. On the other hand, it does not exclude some similarity with aggregation scenarios such as the lattice-gas or Fisher droplet models.

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arXiv1999

Examination of self interaction correction methods for Na clusters

We examine whether the self interaction correction method by Harrison, which does not introduce the spherical single particle density approximation to energy functional, can be applied to Na clusters. We show that it does not work well, especially, for large clusters, though it works well for atomic systems. We suggest that it is better to apply this method only to the Hartree term. We also show that the effects of non-diagonal Lagrange multiplier originating from the orthonormality of single particle orbitals are negligible.

cond-mat↗

arXiv1998

Bremsstrahlung in $α$ Decay

A quantum mechanical analysis of the bremsstrahlung in $α$ decay of $^{210}$Po is performed in close reference to a semiclassical theory. We clarify the contribution from the tunneling, mixed, outside barrier regions and from the wall of the inner potential well to the final spectral distribution, and discuss their interplay. We also comment on the validity of semiclassical calculations, and the possibility to eliminate the ambiguity in the nuclear potential between the alpha particle and daughter nucleus using the bremsstrahlung spectrum.

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arXiv1994

Flow of Nucleons and Fragments in 40Ar + 27Al Collisions Studied with AMD

Collective transverse momentum flow of nucleons and fragments in intermediate energy 40Ar + 27Al collisions is calculated with the antisymmetrized molecular dynamics (AMD). The observed flow and its balance energy are reproduced very well by the calculation with the Gogny force which corresponds to the soft EOS of the nuclear matter. Especially the calculated absolute value of the fragment flow is larger than that of the nucleon flow in the negative flow region, which can be explained by the existence of two components of flow. In addition to many similarities, difference in the deuteron flow is found between 12C + 12C and 40Ar + 27Al collisions, and its origin is investigated by paying attention to the production mechanism of light fragments. We also investigate the dependence of the flow of nucleons and fragments on the stochastic collision cross section and the effective interaction, and conclude that the stiff EOS without momentum dependence of the mean field is not consistent to the experimental data.

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arXiv1993

Momentum distribution of fragments in heavy ion reactions: Dependence on the stochastic collision process

Momentum distribution of fragments produced in 12C+12C reaction at 28.7 MeV/nucleon is analyzed with antisymmetrized version of molecular dynamics (AMD). Calculations are made for several cases of stochastic collisions and the projectile fragmentation peak in momentum distribution is reproduced by the incorporation of many-body nature in stochastic collision process. Furthermore the value of the cross section is found to be reflected in the low momentum part of fragments such as alpha particles and 9Be.

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