关于重核强阻尼反应的研究

讨论了重核碰撞形成巨复合体系破裂的强阻尼反应产生超重核的问题。 评述了两种主要的理论模型: 唯象的多维朗之万模型和本课题组发展的微观输运理论模型。 概括阐述了微观输运理论研究反应 197Au+197Au , 238U+238U和244Pu+244Pu等的主要结果。 给出了反应中产生的超重碎块的几率与入射能的关系, 复合体系和超重碎块的衰变机制以及所形成的超重碎块的结合能和形状的分布。 In this paper, the possibility of producing superheavy fragments through strongly damped reactions in very massive nuclear collisions is investigated. Two theoretical models, which are the phenomenologically multi dimensional Langevin equation model and improved quantum molecular dynamics model developed recently by our group, are briefly reviewed. The dependence of the production probability of superheavy fragments on the incident energy, the decay mechanism of the composite systems and superheavy fragments, and the distributions of the binding energy and shapes of superheavy fragments are iscussed for reactions of 244Pu+244Pu, 238U+238U, 197Au+197Au based on the improved quantum molecular dynamics model.     

The Average Lifetime of Giant Composite Systems Formed in Strongly Damped Collisions

The dynamic, adiabatic and diabat ic entrance potentials in strongly damped reactions of ^238 U＋^238 U, ^232 Th ＋ ^250Cf are calculated and compared. The feature of the dynamical potential implies that it is possible for the composite systems to stick together for a period of time. By means of the improved quantum molecular dynamics model the time evolution of the density and charge distributions of giant composite systems and their fragments for reactions ^238U＋^238U, ^232Th＋^250Cf are investigated, from which the lifetimes of giant composite systems at different energies are obtained. The longest average lifetime of ^238U＋^238U is found when the incident energy is about Ec.m =1080 MeW, which is about 1200 fm/c.     

Probing the Dissipation Mechanism in Ternary Reactions of ^197Au＋^197Au by Mean Free Path of Nucleons

the collision of very heavy nuclei ^197Au＋^197Au at 15AMeV has been studied within the improved quantum molecular dynamics model. A class of ternary events satisfying nearly complete balance of mass numbers is selected. The experimental mass distributions for the system ^197Au＋^197Au ternary fission fragments, the heaviest（A1）, the intermediate （A2） and the lightest （A3）, are reproduced well. The mean free path of nucleons in the reaction system is studied and the shorter mean free path is responsible for the ternary fission with three mass comparable fragments, in which the two-body dissipation mechanism plays a dominant role.     

Determination of the nucleon-nucleon interaction in the ImQMD model by nuclear reactions at the Fermi energy region

The nucleon-nucleon interaction is investigated by using the improved quantum molecular dynamic （ImQMD） model with three sets of parameters IQ1, IQ2 and IQ3, in which the corresponding incompressibility coefficients of nuclear matter are different. The charge distributions of fragments are calculated for various reaction systems at different incident energies. The parameters strongly affect the charge distributions and the fragment multiplicity spectrum below the threshold energy of nuclear multifragmentation. The fragment multiplicity spectrum for 238U＋197Au at 15 A MeV and the charge distributions for 129Xe＋12~Sn at 32 and 45 A MeV, and 197Au＋197Au at 35 A MeV are reproduced by the ImQMD model with the set of parameter IQ3. It is found that： 1） The charge distribution of the fragments and the fragment multiplicity spectrum are good observables for testing the model and the parameters. 2） The Fermi energy region is a sensitive energy region for studying nucleon-nucleon interaction.     

变形核强阻尼碰撞中巨复合体系的形成和衰变

利用改进的量子分子动力学（ImQMD）模型研究了质心系入射能量为980 MeV时变形核238U+238U体系的3种不同碰撞模式（腹对腹、 尖对腹和尖对尖）下强阻尼反应过程中初级超重碎块的产生几率和复合体系的寿命、 形状和拉长取向等性质, 发现尖对腹碰撞模式有利于初级超重碎块的产生, 还发现这种碰撞模式产生的复合体系有最长的寿命。Three different collision modes ( Belly belly, Belly tip and Tip tip) of the deformed uranium nucleus 238U bombarding the deformed 238U in strongly damped reaction at the incident energy 980 MeV has been investigated based on the improved quantum molecular dynamics (ImQMD) model. The formation and properties of the formed composite system during the reaction process were carefully studied including the lifetime, the shape configuration and the angular distribution of primary fragment. It was found that the Belly tip mode is suitable for producing the super heavy fragments, and the composite system has the longest lifetime in this mode.     

Pion Production in Heavy-Ion Collisions in the 1 A GeV Region

Within the framework of the improved isospin dependent quantum molecular dynamics （ImIQMD） model, pion emission in heavy-ion collisions in the region 1 A GeV is investigated systematically, in which the pion is considered to be mainly produced by the decay of resonances △（1232） and N^＊（1440）. The in-medium dependence and Coulomb effects of pion production are included in the calculation. Total pion multiplicity and π^-/π^＋ yields are calculated for the reaction ^197Au＋^197Au in central collisions for selected Skyrme parameters SkP, SLy6, Ska, SⅢ and compared with the measured data of the FOPI collaboration.     

Understanding of the dissipation mechanism in ternary fission for the system 197Au+197Au

The mass number distributions of three fragments from the ternary fission of the system ¹⁹⁷Au+¹⁹⁷Au are reproduced rather well by using the improved quantum molecular dynamics (ImQMD) model without any adjusting parameter. It is found that the probability of ternary fission evidently depends on the incident energy and the impact parameter, and the two-body dissipation is the main mechanism responsible for the formation of the third fragment with comparable mass.     

Possible Mechanisms of Ternary Fission in the 197Au+197Au System at 15 A MeV

Ternary fission in ^197Au＋^197Au collisions at 15 A MeV is investigated by using the improved quantum molecular dynamical （ImQMD） model. The experimental mass distributions for each of the three fragments are reproduced for the first time without any freely adjusting parameters. The mechanisms of ternary fission in central and semicentral collisions are dynamically studied. In direct prolate ternary fission, two necks are found to be formed almost simultaneously and rupture sequentially in a very short time interval. Direct oblate ternary fission is a very rare fission event, in which three necks are formed and rupture simultaneously, forming three equally sized fragments along space-symmetric directions in the reaction plane. In sequential ternary fission a binary division is followed by another binary fission event after hundreds of fm/c.     

Development of the Neck in Fusion Reactions ^40Ca＋^90,96Zr

The neck dynamics and nucleon transfer through the neck in fusion reactions ^40Ca ^90,96Zr are studied by applyingthe improved quantum molecular dynamics model. A special attention is paid to the dynamic behaviour of the neck development at touching point and to the contribution of excess neutrons in a neutron-rich target (or projectile) to neck formation and nucleon transfer.     

Determination of the nucleon-nucleon interaction in the ImQMD model by nuclear reactions at the Fermi energy region

The nucleon-nucleon interaction is investigated by using the improved quantum molecular dynamic (ImQMD) model with three sets of parameters IQ1,IQ2 and IQ3,in which the corresponding incompressibility coefficients of nuclear matter are different. The charge distributions of fragments are calculated for various reaction systems at different incident energies. The parameters strongly affect the charge distributions and the fragment multiplicity spectrum below the threshold energy of nuclear multifragmentation. The fragment multiplicity spectrum for ²³⁸U+¹⁹⁷Au at 15 A MeV and the charge distributions for ¹²⁹Xe+¹²⁰Sn at 32 and 45 A MeV,and ¹⁹⁷Au+¹⁹⁷Au at 35 A MeV are reproduced by the ImQMD model with the set of parameter IQ3. It is found that: 1) The charge distribution of the fragments and the fragment multiplicity spectrum are good observables for testing the model and the parameters. 2) The Fermi energy region is a sensitive energy region for studying nucleon-nucleon interaction.     

Superheavy fragments produced in the asymmetric strongly damped collision

The strongly damped collisions of very heavy nuclei 232Th+250Cf at the energy range of 680-1880 MeV have been studied within the improved quantum molecular dynamics model. The production probability of primary superheavy fragments with Z ≥ 114 (SHFs) for the asymmetric reaction 232Th+250Cf is higher than that for the symmetric reaction 244Pu+244Pu and 238U+238U. The calculated results show that the mass and charge distributions of primary fragments, the excitation energy distribution of SHFs depend on the incident energies strongly. Two stages of the decay process of composite systems are distinguished by very different decay slopes, which imply different decay mechanisms of the composite system. The first stage is for the decay of giant composite systems and the second one corresponds to the decay of fragments of giant composite systems including SHFs through emitting neutron, proton or other charged particles, and also through fission or fragmentation. The slow reduction of SHFs in the second stage seems to be helpful for the survival of primary superheavy fragments.     

Understanding of the dissipation mechanism in ternary fission for the system 197Au+197Au

The mass number distributions of three fragments from the ternary fission of the system 197Au+197Au are reproduced rather well by using the improved quantum molecular dynamics (ImQMD) model without any adjusting parameter. It is found that the probability of ternary fission evidently depends on the incident energy and the impact parameter, and the two-body dissipation is the main mechanism responsible for the formation of the third fragment with comparable mass.     

Possibilities of producing superheavy nuclei in multinucleon transfer reactions based on radioactive targets

The multinucleon transfer(MNT) process has been proposed as a promising approach to produce neutronrich superheavy nuclei(SHN).MNT reactions based on the radioactive targets ~(249)Cf,~(254)Es,and ~(257)Fm are investigated within the framework of the improved version of a dinuclear system(DNS-sysu) model.The MNT reaction~(238)U+~(238)U was studied extensively as a promising candidate for producing SHN.However,based on the calculated cross-sections,it was found that there is little possibility to produce SHN in the reaction ~(238)U+~(238)U.In turn,the production of SHN in reactions with radioactive targets is likely.     

Light charged particle emission and fission in238U+238U collisions at 1,740 MeV

Inclusive⁴He and⁴H energy spectra and heavy fragment coincidence correlations have been measured for reactions of 7.31 MeV/u²³⁸U with²³⁸U and^?197Au targets. The H/He production cross sections are in the range 15–26 mb, and their emission spectra are very similar for the two systems. The observed strong kinematic shifts with angle are reproduced in shape and magnitude by Monte Carlo simulations of particle evaporation from projectile-like and target-like fragments, indicating competition between charged particle emission and sequential fission. No evidence is found for high energy charged particle emission associated with ultra-highZ composite systems. Heavy fragment measurements indicate an abundance of quasielastic and deeply inelastic reaction fragments, as well as sequential fission of target and projectile nuclei. For²³⁸U nuclei, the fission occurs predominantly in an asymmetric mode, reminiscent of fission at low excitation energy. For²³⁸+²³⁸U reactions in the vicinity of the grazing angle, the frequency of single sequential fission (with survival of the partner fragment) is twice as large as double sequential fission in which both the target and projectile undergo fission. In²³⁸U+¹⁹⁷Au reactions, the survival probability of the heavy fragments is even greater. The surprisingly high survival probabilities of high-Z fragments imply a preponderance of very soft collisions in these very-heavy-ion reactions, at least at energies not very far over the Coulomb barrier.     

QMD模型的改进及其在低能重离子反应中的应用

丰中子核以及重核熔合机制的研究以及中能重离子碰撞中多重碎裂的研究都迫切需要一个统一的、自洽的微观动力学模型.经过对量子分子动力学模型进行根本的改进,发展了一个新的、适用于低能以及中能重离子反应的统一描述的微观动力学模型.改进的量子分子动力学(ImQMD)模型能够将整个熔合反应过程中的动力学效应、同位旋效应以及弹靶质量不对称效应等比较全面地、自洽地考虑进来,从而给熔合反应的研究提供了一个新的途径.ImQMD模型能够很好地再现一系列核的基态性质以及10多个熔合反应的激发函数(包括丰中子核熔合体系以及实验最新测量的132Sn+64Ni熔合体系).此外还运用该模型初步探索了重核熔合过程中复合体系的寿命与体系的入射能量、体系大小以及体系的中子质子比的依赖关系. We have developed a new microscopic dynamical model called improved quantum molecular dynamical model (ImQMD). This model can describe the fusion process at energies near the Coulomb barrier as well as the multifragmentation process at intermediate energies in heavy-ion collision (HIC) uniformly. By using this model, fusion cross sections (including some of neutron-rich nuclei reactions and that of newly measured~(132)Sn+~(64)Ni fusion reaction) of tens reactions can be reproduced remarkably well. In fusi...     

Study on the formation of the composite system of 238U+238U

Strongly damped reactions of 238U+238U, at Ecm = 680-1880 MeV have been studied based on the improved quantum molecular dynamics model. We find that at a certain energy region the entrance channel potential is weakly repulsive and the dissipation is very strong after touching configuration, these two effects make the time delay of re-separation for colliding system. The single particle potential well of the transiently formed composite system has Coulomb barrier about 15-20 MeV high at the surface, which makes the excited unbound protons being still embedded in the potential well and moving in a common mono-single particle potential for a period of time and thus restrains from quick decay of the composite system.     

原子核的大质量转移反应和丰中子超重核的产生(英文)

迄今为止,人们合成的超重核都是缺中子的,无论是熔合-裂变反应还是碎化过程都无法使产物达到周期表的“东北区域”。而重核之间(如U核之间) 的近垒大质量转移反应则可能是目前生成丰中子超重核和达到未知丰中子重核区域的唯一途径。在改进的量子分子动力学(ImQMD) 模型结合统计模型框架内,研究了U+U等反应体系的大质量转移反应,计算了反应产生的初生态碎片的质量和电荷分布,并成功再现了产物的终态质量和电荷分布。通过比较3 个反应136Xe+248Cm,48Ca+248Cm和238U+248Cm 产生的106号元素的截面大小,揭示了U+U等重核大质量转移反应对产生丰中子超重核是非常有利的。For elements with Z > 100 only neutron deficient isotopes have been synthesized so far. The “northeast area” of the nuclear map can be reached neither in fusion-fission reactions nor in fragmentation processes.The large mass transfer reactions in near barrier collisions of heavy (U-like) ions seem to be the only reaction mechanism allowing us to produce neutron rich heavy nuclei including those located at the superheavy(SH) island of stability and unexplored area of heavy neutron-rich nuclides. This study is extremely important for nuclear astrophysical investigations and, in particular, for the understanding of the r process. In this paper within the Improved Quantum Molecular Dynamics (ImQMD) model combining with the statistical-evaporation model, the large mass transfer reactions, like 238U+238U have been studied. The charge and mass distributions of transiently formed primary fragments are investigated within the ImQMD model and de-excitation processes of those primary fragments are described by the statistical decay model. The mass distribution of the final products in 238U+238U collisions is obtained and compared with the recent experimental data. Through compared the formation cross sections of transfermium element 106 by three reactions of 136Xe+248Cm, 48Ca+248Cm and 238U+248Cm, it is explored that the large mass transfer reactions, like U+U are very benefit for the production of SH nuclei.