Predictions of synthesizing element 119 and 120

The evaporation residue cross sections of synthesizing superheavy nuclei Z=119, 120 are calculated by different sets of master equations with different dynamical variables. Two methods basically predicted similar results that the ⁴⁸Ca induced hot fusion can produce element 119 easier than produce 120, and the evaporation residue cross sections for 119 are detectable by current advanced techniques, while the evaporation residue cross sections are below 0.1 pb for producing element 120.     

Shell Correction and Pairing Energies in the Dinuclear System Model

We investigate the dependences of the potential energy surfaces （PES） and the fusion probabilities for some cold fusion reactions leading to super-heavy elements on the nuclear shell effect and pairing energy. It is found that the shell effect plays an important role in the fusion of the super-heavy element while pairing energy＇s contribution is insignificant. The fusion probabilities and evaporation residue cross sections as functions of the Ge-isotope projectile bombarding ^208Pb are also investigated. It is found that evaporation residue cross sections do not always increase with the increasing neutron number of Ge-isotope.     

Influence of angular momentum on evaporation residue cross section as a probe of nuclear dissipation

By calculating the excess of the evaporation residue cross sections of the ²⁰⁰Pb     

Influence of angular momentum on evaporation residue cross section as a probe of nuclear dissipation

By calculating the excess of the evaporation residue cross sections of the 200pb nucleus over that predicted by the standard statistical model as a function of nuclear viscosity coefficient using a Langevin equation combined with a statistical decay model, it is found that high angular momentum not only amplifies the dissipation effects on the excess of the evaporation residue cross sections, but also considerably increases the sensitivity of this excess to the nuclear viscosity coefficient. These results suggest that on the experimental side, to accurately obtain the information of nuclear dissipation inside the saddle point by measuring the evaporation residue cross section, it had better populate those compound systems with high spins.     

Nuclear Potential and Fusion Cross Sections for Synthesizing Super-Heavy Elements in Di-nuclear Systems

A double foMing method with simplified Skyrme-type nucleon nucleon interaction is used to calculate the nuclear interaction potential between two nuclei. The calculation is performed in tip-to-tip orientation of the two nuclei if they are deformed. Based on this method, the potential energy surfaces~ the fusion probabilities and the evaporation residue cross sections for some cold fusion reactions leading to super-heavy elements within di-nuclear system model are evaluated. It is indicated that after the improvement, the exponential decreasing systematics of the fusion probability with increasing charge number of projectile on the Pb based target become better and the evaporation residue cross sections are in better agreement with the experimental data.     

Spin distribution of evaporation residue cross section within a stochastic approach

The Langevin equation including particle emission was used to reproduce the recently measured spin distribution of evaporation residue cross sections in the reaction 16O+184W at beam energies of 84, 92,100, 108, 116 and 120 MeV. By comparing the theoretical calculations with the experimental data, the validity of the stochastic approach to dissipative fission is verified. Moreover, a pre-saddle nuclear viscosity coefficient of 5 x 1021 S-1 is extracted.     

Application of the program LISE to fusion-evaporation

A new fusion-evaporation model LisFus for fast calculation of fusion residue cross sections has been developed in the framework of the code LISE. This model can calculate very small cross sections quickly compared to programs using the Monte Carlo method. Such type of the fast calculations is necessary to estimate fusion residue yields. Using this model, the program LISE now has the possibility of calculating the transmission of fusion residues through a fragment separator. It is also possible to use fusion residue cross sections calculated by the program PACE, which has been incorporated in the LISE package. The code PACE is a modified version of JULIAN—the Hillman-Eyal evaporation code using a Monte Carlo code coupling angular momentum. A comparison between PACE and the LisFus model is presented.     

Surveying the role of excitation energy in probing nuclear dissipation

A dynamical Langevin model is employed to calculate the excess of the evaporation residue cross sections of the 194pb nucleus over that predicted by the standard statistical model as a function of nuclear dissipation strength. It is shown that large excitation energy can increase the effects of nuclear dissipation on the excess of the evaporation residues and the sensitivity of this excess to the dissipation strength, and that more higher excitation energies have little contribution to further raising this sensitivity. These results suggest that on the experimental side, producing those compound systems with moderate excitation energy is sufficient for a good determination of the pre-saddle nuclear dissipation strength by measuring the evaporation residue cross section, and that forming an extremely highly excited system does not considerably improve the sensitivity of evaporation residues to the dissipation strength.     

Quasifission and difference in formation of evaporation residues in the O + W and F + Ta reactions

The role of the entrance channel has been studied to ascertain a cause of the observed difference between the evaporation residue cross sections normalized to the fusion cross sections in the ¹⁹F + ¹⁸¹Ta and ¹⁶O + ¹⁸⁴W reactions at high excitation energies. The theoretical analysis performed in the framework of the dinuclear system and advanced statistical models showed that the more intense yield of evaporation residues in the ¹⁶O + ¹⁸⁴W reaction in comparison with that in the ¹⁹F + ¹⁸¹Ta reaction was explained by the large capture and fusion cross sections in the former reaction, which is in agreement with the experimental data. The observed decrease in the evaporation residue cross section normalized to the fusion cross section in the ¹⁹F + ¹⁸¹Ta reaction, in comparison with one in the ¹⁶O + ¹⁸⁴W reaction at large excitation energies, is caused by the unintentional inclusion of the quasifission and fast fission contributions in the fissionlike fragment yields that were used in reconstructing the experimental fusion cross section in the normalizing procedure. The range of the angular momentum distribution for both systems was similar, but the partial cross sections are different, showing the presence of a difference in the hindrance to complete fusion in both reactions.     

超重核性质与合成机制的理论研究

探索原子核的电荷与质量极限,合成长寿命超重核是当前原子核物理研究的重要前沿问题之一。本文综述了我们近几年在超重原子核结构性质与合成机制方面取得的理论研究进展。在结构性质方面,利用处理对关联的粒子数守恒方法,基于推转壳模型,系统研究了锕系核与超镄核低激发谱,发展了多维形状约束的协变密度泛函理论并用于研究锕系核势能面和裂变位垒以及N=150同中子素中的非轴对称八极关联等。在超重核合成机制方面,系统研究了利用重离子熔合反应合成超重核的三步过程,包括俘获过程——提出了一个位垒穿透概率新公式、熔合过程——提出了一个基于动力学形变势能面的双核模型、存活过程——系统研究了激发态超重复合核存活概率等。系统研究了合成超重核的热熔合反应,得到的熔合蒸发截面与实验符合,并预言了合成119和120号超重元素的生成截面。     

Competition of fusion and quasifission in reactions leading to production of superheavy elements

The mechanism of fusion hindrance, an effect observed in the reactions of cold, warm, and hot fusion leading to production of superheavy elements, is investigated. A systematics of transfermium production cross sections is used to determine fusion probabilities. The mechanism of fusion hindrance is described as a competition of fusion and quasifission. Available evaporation residue cross sections in the superheavy region are reproduced satisfactorily. Analysis of the measured capture cross sections is performed and a sudden disappearance of the capture cross sections is observed at low fusion probabilities. A dependence of the fusion hindrance on the asymmetry of the projectile-target system is investigated using the available data. The most promising pathways for further experiments are suggested.     

不同理论模型预言合成超重核Z=119~122研究进展

通过双核系统模型与其他模型对$Z\leqslant$118元素的计算结果与实验数据的比较，证明了不同模型预测超重核的产生截面是可靠的。对比分析了不同模型对Z=119和Z=120超重核的预言结果，我们认为合成超重核Z=119和Z=120的最佳弹靶组合分别为反应$^{48}{\rm{Ca}} + ^{{\rm{252}}}{\rm{Es}}$和$^{40}{\rm{Ca}} + ^{{\rm{257}}}{\rm{Fm}}$，并且Z=119新核素很有可能会先于Z=120新核素在实验上被合成。由于实验上Z>100锕系靶的限制，人们正尝试寻找比48Ca更重的弹核来合成超重核Z=121和Z=122，超重核Z=121可以通过反应V+Cf来合成，而超重核Z=122的产生截面已经非常小，要求将来在实验上提高探测及鉴别技术。希望本文的讨论可以在将来为实验及理论核物理工作者们提供一些参考。     

MEASUREMENTS OF ANGULAR MOMENTUM EFFECT OF FISSION BARRIER

The fission cross sections and the evaporation residue cross sections for ¹²C+²⁰⁹Bi and ¹⁴N+Pb reactions were measured with the gold surface barrier silicon detectors and the mica nuclear track detectors.The critical angular momentum l_er was deduced from the measured evaporation residue cross section σ_er on the basis of the sharp-off model.The fission barrier with the particular angular momentum i derived from the condition Γ_f/Γ_n=1 at l=l_e The angular momentum effect of the fission barrier was studied experimentally.     

Dinuclear systems in complete fusion reactions

Formation and evolution of dinuclear systems in reactions of complete fusion are considered. Based on the dinuclear system concept, the process of compound nucleus formation is studied. Arguments confirming the validity of this concept are given. The main problems of describing the complete fusion in adiabatic approximation are listed. Calculations of evaporation residue cross sections in complete fusion reactions leading to formation of superheavy nuclei are shown. Isotopic trends of the cross sections of heavy nuclei formation in complete fusion reactions are considered.     

超重核合成的同位素依赖探讨

利用二参量Smoluchowski方程计算了54Fe+204Pb,56Fe+206Pb,58Fe+208Pb冷熔合和32,34,36S+238U热熔合的复合核形成截面和蒸发残余截面, 结果清楚地显示出超重核合成截面随同位素的变化。 由于较低的入射道库仑位垒、 较低的不对称裂变谷中的条件鞍点和较小的中子分离能, 一般地说, 丰中子同位素的超重核形成截面明显增强。 The cross sections of the compound nucleus formation and e vaporation residue for the 54Fe+204Pb, 56Fe+206Pb, 58Fe+208Pb cold fusion and 32,34, 36S+238U hot fusion have been calculated by using a two parameter Smoluchowski equation. Our results clearly show the isotope dependence of superheavy nucleus production. The formation cross sections of the neutron rich isotope are, generally speaking, obviously enhanced due to the lower Coloumb barrier , lower height of the conditional saddle point, and smaller neutron separation energy.     

Evidence of complete fusion in the subbarrier 16O+238U reaction

Evaporation residue cross sections in the ¹⁶O+²³⁸U reaction were measured for the energy range from above-to extreme subbarrier. We used a He-gas-jet system to transport the fusion products, and the α decay of the evaporation residues was measured by using a rotating wheel system. The measured cross sections for ^248,249,250Fm are reproduced by a statistical model calculation, for which partial cross sections are calculated by a coupled-channel model taking into account the prolate deformation of ²³⁸U. We conclude that complete fusion is the main process in the subbarrier energy region, and quasifission is not an important channel. The text was submitted by the authors in English. On leave from China Institute of Atomic Energy, Beijing, China.     

Activation cross sections for reactions of 109Ag induced with 40Ar ions

Activation cross sections for reactions of enriched targets of ¹⁰⁹Ag with ⁴⁰Ar ions of 288 and 228 MeV have been determined by γ-ray spectroscopy. Yields of evaporation residues and low-mass transfer products have been measured. Evaporation residue yield-mass distributions are compared with three sets of statistical model results. The calculation based on a rotating liquid drop model with fission competition is in very good agreement with experimental results. It is shown that the yields are inconsistent with a mechanism of incomplete fusion plus evaporation.     

Surveying the role of excitation energy in probing nuclear dissipation

A dynamical Langevin model is employed to calculate the excess of the evaporation residue cross sections of the ^194Pb nucleus over that predicted by the standard statistical model as a function of nuclear dissipation strength. It is shown that large excitation energy can increase the effects of nuclear dissipation on the excess of the evaporation residues and the sensitivity of this excess to the dissipation strength,and that more higher excitation energies have little contribution to further raising this sensitivity. These results suggest that on the experimental side,producing those compound systems with moderate excitation energy is sucient for a good determination of the pre-saddle nuclear dissipation strength by measuring the evaporation residue cross section,and that forming an extremely highly excited system does not considerably improve the sensitivity of evaporation residues to the dissipation strength.