Nuclear stopping and compression in heavy-ion collisions at intermediate energies

The nuclear stopping and the radial flow are investigated with an isospin-dependent quantum molecular dynamics (IQMD) model for Ni + Ni and Pb + Pb from 0.4 to and 1.2 GeV/u. The expansion velocity as well as the degree of nuclear stopping are higher in the heavier system at all energies. The ratio between the flow energy and the total available energy in center of mass of the colliding systems exhibits a positive correlation to the degree of nuclear stopping. The maximum density (rho(max)) achieved in the compression is comparable to the hydrodynamics prediction only if the non-zero collision time effect is taken into account in the later. Due to the partial transparency, the growing of the maximum density achieved in the central region of the fireball with the increase of beam energy becomes gradually flat in the 1 GeV/u energy regime. (C) 2008 Elsevier B.V. All rights reserved.

Effects of elastic and inelastic NN scattering cross sections on pi(-)/pi(+) ratios in heavy-ion collisions at intermediate energies

Based on the isospin-dependent Boltzmann-Uehling-Uhlenbeck transport model and the scaling model according to nucleon effective mass, effects of elastic and inelastic NN scattering cross sections on pi(-)/pi(+) in the neutron-rich reaction of Ca-48 + Ca-48 at a beam energy of 400 MeV/nucleon are studied. It is found that cross-section effects of both NN elastic and inelastic scatterings affect Delta(1232), pi(-) and pi(+) production, as well as the value of pi(-)/pi(+).

Medium effects on pi(-)/pi(+) in heavy-ion collisions at intermediate energies

Based on the isospin-dependent transport model IBUU and on the scaling model according to nucleon effective mass, effects of elastic and inelastic NN scattering cross-sections on pi(-)/pi(+) in the neutron-rich reaction Ca-48 + Ca-48 at a beam energy of 400MeV/nucleon are studied. It is found that cross-section effects of both NN elastic and inelastic scatterings affect Delta(1232), pi(-) and pi(+) productions as well as the value of pi(-)/pi(+).

Multiple particle and cluster emissions from Zr following intermediate energies photo absorption

A new approach based on a N-a cluster photoabsorption model is proposed for the understanding of the puzzling steady increase behavior of the 90Zr (e, α) yield measured at the National Bureau of Standards (NBS) within the Giant Dipole Resonance and quasideuteron energy range. The calculation takes into account the pre-equilibrium emissions of protons, neutrons and alpha particles in the framework of an extended version of the multicollisional intranuclear cascade model (MCMC). Another Monte Carlo based algorithm describes the statistical decay of the compound nucleus in terms of the competition between particle evaporation (p, n, d, α, 3He and t) and nuclear fission. The results reproduce quite successfully the 90Zr (e,α) yield, suggesting that emissions of a particles are essential for the interpretation of the exotic increase of the cross sections.

Universal trend for heavy-ion total reaction cross-sections at energies above the Coulomb barrier

Heavy-ion total reaction cross-section measurements for more than 1100 reaction cases covering 61 target nuclei in the range (6)Li-(238)U and 158 projectile nuclei from (2)H to (84)Kr (mostly exotic ones) have been analyzed in a systematic way by using an empirical, three-parameter formula that is applicable to the cases of projectile kinetic energies above the Coulomb barrier. The analysis has shown that the average total nuclear binding energy per nucleon of the interacting nuclei and their radii are the chief quantities that describe the cross-section patterns. A great amount of cross-section data (87%) has been quite satisfactorily reproduced by the proposed formula; therefore, the total reaction cross-section predictions for new, not yet experimentally investigated reaction cases can be obtained within 25% (or much less) uncertainty.

Roy equation analysis of ππ scattering

We analyse the Roy equations for the lowest partial waves of elastic ππ scattering. In the first part of the paper, we review the mathematical properties of these equations as well as their phenomenological applications. In particular, the experimental situation concerning the contributions from intermediate energies and the evaluation of the driving terms are discussed in detail. We then demonstrate that the two S-wave scattering lengths a00 and a02 are the essential parameters in the low energy region: Once these are known, the available experimental information determines the behaviour near threshold to within remarkably small uncertainties. An explicit numerical representation for the energy dependence of the S- and P-waves is given and it is shown that the threshold parameters of the D- and F-waves are also fixed very sharply in terms of a00 and a20. In agreement with earlier work, which is reviewed in some detail, we find that the Roy equations admit physically acceptable solutions only within a band of the (a00,a02) plane. We show that the data on the reactions e+e−→ππ and τ→ππν reduce the width of this band quite significantly. Furthermore, we discuss the relevance of the decay K→ππeν in restricting the allowed range of a00, preparing the grounds for an analysis of the forthcoming precision data on this decay and on pionic atoms. We expect these to reduce the uncertainties in the two basic low energy parameters very substantially, so that a meaningful test of the chiral perturbation theory predictions will become possible.

Circumstantial Evidence for a Soft Nuclear Symmetry Energy at Suprasaturation Densities

Within an isospin- and momentum-dependent hadronic transport model, it is shown that the recent FOPI data on the pi(-)/pi(+) ratio in central heavy-ion collisions at SIS/GSI energies [Willy Reisdorf , Nucl. Phys. A 781, 459 (2007)] provide circumstantial evidence suggesting a rather soft nuclear symmetry energy E-sym(rho) at rho >= 2 rho(0) compared to the Akmal-Pandharipande-Ravenhall prediction. Some astrophysical implications and the need for further experimental confirmations are discussed.

DETERMINING THE DENSITY DEPENDENCE OF THE NUCLEAR SYMMETRY ENERGY USING HEAVY-ION REACTIONS

We review recent progress in the determination of the subsaturation density behavior of the nuclear symmetry energy from heavy-ion collisions as well as the theoretical progress in probing the high density behavior of the symmetry energy in heavy-ion reactions induced by high energy radioactive beams. We further discuss the implications of these results for the nuclear effective interactions and the neutron skin thickness of heavy nuclei.

Probing balance energy using momentum- and isospin-dependent potential

Using the momentum- and isospin-dependent Boltmann-Uehling-Uhlenbeck (BUU) model, we investigate the transverse flow and balance energy in two isotopic colliding systems Ca-48+Fe-58 and Cr-48+Ni-58 by adopting different symmetry potentials. By comparing the results between the two colliding systems, we find that the difference between the balance energies of two isotopic systems can be considered as a sensitive probe to the density dependence of symmetry energy.

Systematics of central heavy ion collisions in the 1A GeV regime

Using the large acceptance apparatus FOPI, we study central collisions in the reactions (energies in A GeV are given in parentheses): Ca-40 + Ca-40 (0.4, 0.6, 0.8, 1.0, 1.5, 1.93), Ni-58 + Ni-58 (0.15, 0.25, 0.4), Ru-96+Ru-96 (0.4, 1.0. 1.5), (96)zr+(96)zr 1.0, 1.5), Xe-129+CsI (0.15, 0.25, 0.4), Au-197 + Au-197 (0.09, 0.12, 0.15, 0.25, 0.4, 0.6, 0.8, 1.0, 1.2, 1.5). The observables include cluster multiplicities, longitudinal and transverse rapidity distributions and stopping, and radial flow. The data are compared to earlier data where possible and to transport model simulations. (C) 2010 Elsevier B.V. All rights reserved.

中能重离子碰撞中的同位旋效应及非对称核物质的状态方程研究

本论文介绍了放射性核束物理研究的现状以及当前常用的几种同位旋相关的重离子微观输运理论，系统描述了非对称核物质的状态方程、中能重离子碰撞中的同位旋效应以及中子星的性质。基于 Hartree-Fock 理论和扩展的 Skyrme 相互作用，在核物质近似下得到了一个非相对论性的密度、温度和同位旋相关的核物质状态方程 (IEOS)。系统研究了核物质状态方程的同位旋效应。讨论了核子的平均场、核物质的不可压缩系数、核子的有效质量以及核物质临界温度的同位旋相关性，并且给出了核物质饱笔点处的饱和密度、不可压缩系数以及单核子结合能的抛物线规律。同时，探讨了对称能的温度和密度相关性，给出了零温度时对称能的解析表达式，并提出了对称能温度相关性的抛物线规律，发现对称能随着温度的升高而减小。另外，基于以上的同位旋相关的核物质状态方程，对 ALADIN's Caloric Curve 给出了一种静态解释。在传统量子分子动力学 (QMD) 模型的基础上，通过在相互作用平均场、两体碰撞、泡利阻塞、初始化以及碎片构造过程中适当地考虑同位旋自由度，得到了一个同位旋相关的 QMD 模型 (IQMD 模型)。利用IQMD模型系统研究了中能重离子碰撞中的同位旋效应。例如，中能重离子碰撞中同位旋自由度的弛豫、重离子碰撞中核子前平衡发射的同位旋效应、重离子碰撞中的集体流（包括直接流、转动流、挤出流和径向流）及其同位旋相关性、原子核多重碎裂的同位旋效应及其消失、重离子碰撞中的化学不稳定性以及中能重离子碰撞中如何选取事件的碰撞参数及其同位旋效应等。同样，在传统的 Boltzmann-Langevin 方程中适当地考虑同位旋自由度，得到了同位旋相关的Boltzmann-Langevin方程 (IBLE)，利用IBLE研究了 ~(19)Na的产生截面。另外，利用IQMD模型探讨了多重碎裂的"neck" 机制以及重离子碰撞中局域势的有限程效应。基于前面给出的非相对论的核物质状态方程，系统研究了中子星的性质，如中子星的化学组份、质量、结合能、半径、密度剖面、转动惯量及表面红移等。结果表明，使用一些常用的 Skyrme 势参数能够给出与天文学观测相一致的结果。

中能重离子碰撞中激发热核性质同位旋效应的实验研究

中能重离子碰撞中的核反应机制及其形成的高激发热核的性质是中能重离子物理研究的重要领域，而高激发热核性质的同位旋效应研究是这一领域的热点之一。选取了具有不同N亿比的反应体系以研究激发热核性质的同位旋效应。本论文涉及的反应系统三对共六个反应体系：55MeV／u~(40)Ar+~(58.64)Ni、30MeV/u~(40)Ar+~(112,124)Sn、35Mev／u~(36)Ar+~(112,124)Sn，这六个反应体系的N／z比分别为1.13，1.26、1.24，1.41、1.18，1.35。分别从带电粒子多重性、相对态布居核温度、关联函数等角度研究了这三对反应体系高激发热核性质的同位旋效应。在55MeV／u 40Ar+58,64Ni核反应中，用兰州4π带电粒子探测器阵列测量带电粒子多重性，研究了He和中等质量碎片的产额与反应系统的同位旋的关系，以及这种同位旋效应与反应系统的碰撞参数（即碰撞的激烈程度）、系统的激发能的变化关系。对两个反应系统，观察到带电粒子多重性中He的比分随带电粒子多重性的增加而增大，带电粒子多重性中IMF的比分随带电粒子多重性的增加而先增大，后减小的规律。两个反应系统虽然具有相同的核电荷数，但轻粒子He和中等质量碎片在多重性中的比分有明显的同位旋相关性。在30Mev／u40Sn、35MeV/u~(40)Ar~(112，124)Sn、35Mev／u 36Ar＋112,124Sn反应中用13单元望远镜探测器阵列测量了小角关联粒子。由价a关联函数提取了30Mev／u 40Ar＋112，12Sn反应系统中激发热核的态布居核温。对于不同同位旋反应系统舜UAr＋112Sn和4VAr+124Sn，提取的相对杰布居核温度分别是4.18+0.28/0.21MEV和4.10士0.22/0.20MeV；考察态布居核温度和粒子能量的关薰时，观察到两个系统的发射温度均随着粒子能量的增加而降低，缺中子系统40Ar＋l12Sn中由低能时的5.13士.30/0.26MEV降低到高能时的3.87士0.37/0.29MeV，丰中子系统40Ar＋124Sn中由低能时的5．39士0.30/0.26MeV降低到高能时的3．32士MeV。讨论了这种布居态核温度的同位旋相关性。在35Mev／u 36Ar＋112，124Sn反应中提取了洲F（3‘25）的约化速度关联函数。相对丰中子36Ar＋124Sn系统的IMF关联函数在小约化速度处反关联程度更强，表明36Ai＋124Sn系统的发射IMF的平均时间更短。用MENEKA程序提取了两个系统IMF的平均发射时间，36Ai＋112sn反应中IMF的发射时间约为150fm/c，而36Ar十124Sn反应IME的发射时间稍短，约为120fm／c。以关联IMF的单核子总能量／动量为窗条件，发现低能IMF关联函数几乎没有差别，而高能IMF关联函数在小约化速度处的差别更为明显，表明两个系统IMF关联函数的同位旋效应可能来自于IMF的早期发射。为了得到进一步的信息，我们提取了高动量窗条件下的IMF发射时间，它们比平均发射时间短，36Ar＋112Sn反应中高能IMF的发射时间约为100蒯c，而36Ai＋124Sn反应中IMF的发射时间则更短，约为50fm/c。

中能区~(40)Ar + ~(197)Au/~(159)Tb/~(nat)Ag反应中高激发热核性质研究

通过对中能区Ar+Au/Tb/Ag反应中高激发核的发射时标、发射次序、发射机制、核温度、集体运动等衰变特性的研究，提取了轻粒子和中等质量碎片（IMF）的发射时间，IMF发射时间随束流能的升高而变短，发射机制逐渐由相继衰变过渡至多重碎裂。研究了轻粒子和碎片间的发射次序，对高能粒子和碎片，轻粒子先于碎片发射，而低能时，则为碎片先于轻粒子发射。IMF发射成分与角度和碰撞参数有关，前角区来自于弹核碎裂，后角区来自于类靶热核的蒸发。在平面和出平面研究表明，中速粒子和碎片为在平面发射占主导，即存在类转动效应；对轻粒子，转动效应随粒子质量增加而增加；对中速产物均观测到该效应随碰撞参数饿增大而增大。采用几种不同的方法提取了热核的核温度，研究了不同方法之间的区别。

Mieszanie stanów magnetycznych w skończonych układach spinowych opisywanych modelem Heisenberga

Wydział Fizyki: Zakład Fizyki Komputerowej

2-D R-matrix propagation: A large scale electron scattering simulation dominated by the multiplication of dynamically changing matrices

We examine the computational aspects of propagating a global R-matrix, R, across sub-regions in a 2-D plane. This problem originates in the large scale simulation of electron collisions with atoms and ions at intermediate energies. The propagation is dominated by matrix multiplications which are complicated because of the dynamic nature of R, which changes the designations of its rows and columns and grows in size as the propagation proceeds. The use of PBLAS to solve this problem on distributed memory HPC machines is the main focus of the paper.