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同位旋依赖的量子分子动力学模型IQMD[量子分子动力学之家]
同位旋依赖的量子分子动力学模型IQMD
 更新时间:2009/11/4 19:54:05  点击数:708
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IQMD 模型是在标准的量子分子动力学(QMD)基础上考虑核体系质子-中子不对称自由度的影响。

http://www-subatech.in2p3.fr/~theo/qmd/versions/qmdvers/node1.html

Isospin-QMD  is a QMD realization based on the VUU approach (like original QMD was based on BUU). It includes isospin degrees of freedom for nucleons, deltas and pions. IQMD has been used for the analysis of collective effects of nucleons  and pions. Experimental comparisons with this model has been performed It is numerically completely independent of the original QMD. The isospin degrees of freedom enter into the cross sections (here cross sections of VUU  similar to the parametrizations of VerWest and Arndt  have been taken) as well as in the Coulomb interactions. The elastic and inelastic cross sections for proton-proton and proton-neutron used in IQMD can be found in [Phys. Rev. C 51, 3320 (1995)]. The cross section for neutron-neutron are assumed to be equal to the proton-proton cross sections.

 

Potentials used in IQMD

As already quoted the Coulomb-interaction used in IQMD is treated explicitely using the real changes of nucleons, deltas and pions. Additionally a symmetry potential between protons and neutrons corresponding to the Bethe-Weizsaecker mass formula has been included

where and denote the isospin projections of particles i and j. Other baryonic potentials like and are defined isospin-independent in similar manner like it is done in QMD and HQMD. In IQMD the range of is chosen to be fm, where QMD and BQMD use a value of fm. Like QMD, IQMD does not employ subtraction of a mean Yukawa term in the two-particle interactions.

 

Pions in IQMD

Free pions are moving under the influence of the Coulomb interactions. Pions may be produced by the decay of a delta and may be reabsorbed by a nucleon forming a delta again. IQMD and HQMD differ concerning the pion production in the production cross sections (HQMD uses OBE cross sections), the included resonances (HQMD has additionally ) and the angular distribution of inelastic collisions (HQMD has more realistic non-isotropic distributions obtained from OBE calculations which is not present in original IQMD). More recent versions of IQMD (e.g. [44,45]) also use the inelastic angular distributions of HQMD. The effect of this modification is quite small.

 

Initialisation in IQMD

An important difference is the initialisation. In IQMD the centroids of the Gaussians in a nucleus are randomly distributed in a phase space sphere ( and ) with fm corresponding to a ground state density of . The Fermi momentum depends on the ground state density and has for a value of about MeV/c. BQMD and HQMD (in Wood Saxon as well as in hard sphere initialisation) assume ground state densities of . While in BQMD and HQMD the maximum momentum is determined by the local binding energy (which causes an effective reduction of the total Fermi energy to about 10 -12 MeV), in IQMD the momenta are uniformly distributed within a momentum sphere without further local constraints. Therefore it may happen that nucleons are barely bound even from the beginning, a possibility which is explicitely forbidden in BQMD and HQMD. On the other hand this ansatz allows to make available the full fermi energy calculated from the Skyrme ansatz while in BQMD and HQMD the mean kinetic energie of the Fermi motion is strongly reduced. Finally it should be noted that IQMD proceeds a Lorentz contraction of the nucleus coordinate distribution which is not done by BQMD and HQMD and which becomes important for higher energies GeV.

Furthermore it should be stated that the default version of IQMD uses a system dependent Gaussian width, which has for the regarded Au+Au cases the values of (and e.g. for Ca+Ca ) while BQMD and HQMD normally use independent of the system size.

 Some publications with IQMD model

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