Antisymmetrized Molecular Dynamics Approach to Multifragmentation

Abstract

It is discussed how we can describe various phenomena in medium energy nuclear collisions, such as multifragmentation, by using microscopic dynamical models. Basic ideas of Antisymmetrized Molecular Dynamics (AMD) are explained in order to show the important quantum features and their relation to various fragmentation mechanisms. Furthermore, an important problem about the nuclear equation of state and the observables in nuclear collisions is studied with AMD. The statistical property of molecular dynamics is also discussed. §1. IntroductionIn this lecture, I discuss the dynamics of nuclear collisions in the medium energy region, from several ten to several hundred MeV/nucleon, from a theoretical view point. It is generally expected that the nuclear system is compressed and/or excited in the early stage of the reaction. The compressed and/or excited matter will then expand and many clusters may be produced. One of the main aims of the study of heavy ion collisions is to explore the properties of nuclear matter in various densities and temperatures. The multifragmentation is interesting if it is related to the liquid gas phase transition in low-density nuclear matter. Although we hope to see some effect related to the equilibrated matter, it is difficult to produce equilibrated matter in dynamical collisions. Therefore, it is indispensable to study the properties of nuclear matter together with the reaction dynamics in microscopic dynamical approaches.Appearance of clusters or fragments, in addition to the properties of nuclear matter, is also an important aspect of heavy ion collisions in this energy region. It is known in experiment that fragments are produced copiously in various reactions, such as in relatively low energy region where the neck or midrapidity component may be created in dissipative binary reactions, 1) -3) in higher energy central collisions where clusters are produced copiously in expanding system, 4) and also in peripheral collisions where the excited projectile-like fragment breaks up into pieces. 5) The important part of the total nucleons in the system is contained in fragments in the final state. In typical cases, only about 20 % of the total protons in the system are emitted as isolated protons. 4), 6) Therefore, we cannot ignore the fragment formation dynamics in the microscopic description of collisions. It is not allowed to treat the fragmentation as a perturbative process.The word 'fragment' may be misleading since its usual meaning implies a special mechanism like breaking a solid object by an impact, where fragments are produced * )

Reaction dynamics and multifragmentation in Fermi energy heavy ion reactions

“…In the calculated results for 58 Ni, the two-peak structure is clearly observed in the simulation with the soft EOS, whereas a single broad peak is seen in that with the stiff EOS. This result is consistent with the calculations reported by Ono [26] for 40 Ca + 40 Ca at 35A MeV. This trend is also observed for 64 Zn + 92 Mo in Fig.…”

“…This interaction gives a soft equation of state (EOS) with an incompressibility value K of 228 MeV for infinite nuclear matter and has a momentum dependent mean field. In the present work, a modified Gogny interaction with a stiff EOS with K=360 MeV is also applied [26]. The parameter set adopted for K=360 MeV was that used by Haddad et al [27] and labeled as D1-G3.…”

“…[MeV fm 3(1+σ) present work, a modified Gogny interaction with a stiff EOS with K=360 MeV is also applied [26]. The parameter set adopted for K=360 MeV was that used by Haddad et al [27] and labeled as D1-G3.…”

Effects of cluster correlations on fragment emission in 12C + 12C at 50 MeV/nucleon

“…In order to improve the reproduction of the experimental data for the LCP yields, a stochastic coalescence process [10,23] is introduced in AMD/D to take into account nucleonnucleon correlations among N nucleons with N ≤ 4 under Pauli principle. This treatment is referred as coalescence treatment in this article and the program is called AMD/D-COALS.…”

“…The basic idea [23] is the following. For each subsystem with a proton and a neutron, for example this stochastic process moves the centroids Z a and Z b to the same point 1 2 (Z a + Z b ).…”