40,48Ca+90,96Zr融合反应的动力学模型研究

应用改进的量子分子动力学模型,在严格挑选初始核考虑弹靶结构效应的基础上,研究了近垒和垒上融合反应^40,48Ca+^90,96Zr. 研究表明: 4个反应的理论计算截面与实验值很好符合; 丰中子反应⁴⁰Ca+⁹⁶Zr的垒下融合截面比其他3个反应有明显增强的现象.为了理解丰中子反应⁴⁰Ca+⁹⁶Zr与⁴⁰Ca+⁹⁰Zr相比垒下融合截面增强,而Ca+⁹⁶Zr垒下融合截面没有明显增强的原因, 进一步分析了484个反应的融合位垒,中子转移与融合位垒的关系、中子转移与Q值的关系,结果表明: 正反应Q值会引起核子(特别是中子)转移的增强,从而导致动力学融合位垒的下降和垒下融合截面增强.     

Near-barrier quasi-elastic cross sections and barrier distribution of 32S+90,96Zr

Quasi-elastic cross sections have been measured for ³²S+^90,96Zr with high accuracy near the Coulomb barrier at backward angles and the barrier distributions are extracted from them. We find that barrier distribution of ³²S+⁹⁶Zr is flat and extends to low energies. The sub-barrier fusion cross sections are strongly enhanced because of such barrier distribution. Compared with the reaction of ³²S+⁹⁰Zr, ³²S+⁹⁶Zr has stronger neutron transfers up to the six-neutron pickup, which is due to the positive Q values of neutron transfers in the latter. This indicates that the neutron transfer may play a role in the enhancement of sub-barrier fusion for ³²S+⁹⁶Zr.     

近垒能区32S+90,96Zr势垒分布的研究

精确测量了³²S+^90,96Zr两系统的背角准弹散射激发函数, 总体误差小于1%, 从中分别抽取了它们的势垒分布, 对比观测到³²S+⁹⁶Zr的势垒结构扁平且向低能区展宽, 这种 势垒可导致在垒下能区该体系的熔合截面的大大增强. ³²S+⁹⁶Zr与³²S+⁹⁰Zr相比 存在较强的中子转移反应, 且转移Q值为正. ³²S+⁹⁶Zr势垒的扁平结构可能是中子转移道耦合所致, 这会导致³²S+⁹⁶Zr垒下熔合截面大的增强.     

Transfer reactions and sub-barrier fusion in 40Ca + 90, 96Zr

The two systems ⁴⁰Ca + ^90,96Zr have been studied by measuring nucleon transfer reactions at two energies near the Coulomb barrier, thus complementing the available sub-barrier fusion cross-sections. Angular distributions for various transfer channels have been determined. Significantly larger neutron transfer cross-sections are found for the target ⁹⁶Zr that exhibits the larger enhancement in the sub-barrier fusion cross-sections. All data have been analyzed with a new model for heavy-ion collisions that calculates simultaneously transfer cross-sections, fusion excitation functions and barrier distributions. The model gives a good account of both transfer and fusion data. Received: 2 May 2002 / Accepted: 4 June 2002 / Published online: 26 November 2002 RID="a" ID="a"e-mail: montagnoli@pd.infn.it, Fax +39049 8277102, Tel. +39049 8277117. RID="b" ID="b"On leave from the China Institute for Atomic Energy, 102413 Beijing, China. Communicated by C. Signorini     

Theoretical study of fusion reactions 32S + 94,96Zr and 40Ca + 94,96Zr and quadrupole deformation of 94Zr

The dynamic coupling effects on fusion cross sections for reactions~(32)S + ~(94,96)Zr and ~(40)Ca + ~(94,96)Zr are studied with the universal fusion function formalism and an empirical coupled channel(ECC) model. An examination of the reduced fusion functions shows that the total effect of couplings to inelastic excitations and neutron transfer channels on fusion in ~(32)S +~(94)Zr(~(40)Ca +~(94)Zr) is almost the same as that in ~(32)S +~(96)Zr(~(40)Ca +~(96)Zr). The enhancements of the fusion cross section at sub-barrier energies due to inelastic channel coupling and neutron transfer channel coupling are evaluated separately by using the ECC model. The results show that effect of couplings to inelastic excitations channels in the reactions with94 Zr as target should be similar as that in the reactions with ~(96) Zr as target. This implies that the quadrupole deformation parameters β_2of ~(94)Zr and~(96) Zr should be similar to each other.However, β_2 's predicted from the finite-range droplet model, which are used in the ECC model, are quite different. Experiments on~(48) Ca +~(94)Zr or~(36) S +~(94)Zr are suggested to solve the puzzling issue concerning β_2for~(94)Zr.     

The influence of Isospin Dependence of In-Medium NN Cross Sections on the Ratio of Emitted Neutrons to Protons in HICs

The influence of isospin dependence of in-medium nucleon-nucleon cross sections on the nip ratios for emitted nucleons in reactions 96Zr-j-96Zr and 96Ru-j-96RH at Eb z 400 AMeV is investigated by means of an improved quantum molecular dynamics model. Our results show that the high energy part of the spectra of the n//p ratios for emitted nucleons is sensitive to the isospin dependence of in-medium nucleon-nucleon cross sections for neutron-rich reaction systems. Therefore, we propose that the nip ratio of emitted high energy nucleons in a very neutron-rich reaction system at several hundreds of AMeV can be taken as sensitive observables to constrain the isospindependence of in-medium nucleon-nucleon cross sections.     

Dynamics of low-energy heavy-ion fusion

Preliminary data on the fusion of ³⁶S+⁹⁶Zr are reported; the excitation function near the barrier is intermediate between those of ⁴⁰Ca+^90,96Zr. The peculiar role of the strong 3⁻ octupole vibration of ⁹⁶Zr is pointed out, in addition to the couplings to neutron transfer channels with positive Q-values. Recent data on ⁴⁰Ca+¹²⁴Sn are also shown; for that system the fusion barrier distribution is wide without separated peaks, similar to the case of ⁴⁰Ca+⁹⁶Zr. Simplified coupled-channel calculations have been performed, including surface vibrations and sequential neutron pick-up channels, with form factors that fit the single- and multi-nucleon transfer data for ⁴⁰Ca+¹²⁴Sn. A good agreement with the data is found.     

Radial Expansion Flow in Reactions of 40Ca+58Fe and 40Ca+58Ni

By means of using an isospin-dependent Boltzmann-Langevin equation which includes isospin-dependent symmetry energy, Coulomb energy, isospin-dependent nucleon-nucleon cross sections, Pauli blocking, and initialization, the radial expansion flow of reaction systems ⁴⁰Ca+⁵⁸Ni and ⁴⁰Ca+⁵⁸Fe at 53, 100, 150, and 200 MeV/u in the central collisions were studied. It has shown that the more neutron rich system exhibits smaller radial expansion flow. It was found that the neutron rich system had smaller threshold energy which may provide a new method to determine the isospin dependent nuclear equation of state from calculated result and linear fitting result.     

MEASUREMENT OF COMPLETE FUSION CROSS SECTION OF 12C+159Tb,12C+165Ho

Complete fusion cross sections have been measured for ¹²C+¹⁵⁹Tb and ¹²C+¹⁶⁵Ho reactions by using K-X rays of evaporation residues with Si(Li)spectrometer.The half-lives of evaporation residues and its yield distributions as a function of incident energy have also been obtained.The experimental values for the complete fusion cross section were compared with the theoretical ones.     

Cross Section Measurements of Neutron-Rich Hg Isotopes in Reaction of 28MeV/u 18O with Thick Natural Lead Target

By using 28MeV/u ¹⁸O beam,an on-line melting lead target equipment and 4π-ΔEβ-γcoincident technique,the relative cross section ratios and absolute cross sections of neutron-rich nuclides ^205—208Hg have been measured. The production cross section of ²⁰⁹Hg is extrapolated.The cross section of ²⁰⁷Hg produced by-2p +1n from target decreases suddenly,which may be explained by change of reaction Q value.     

Complete and Incomplete Fusion in the Reaction of 115In with 16O Ions

Angular distributions and differential range distributions have been measured with nuclear chemistry techniques for 20 target residues from the reactions of ¹¹⁵In with 92 and 71MeV ¹⁶O ions.In terms of the increase of impact parameter,the results indicate continuous evolution of the reaction mechanisms from complete fusion,incomplete fusion to direct reaction involving steadily decreasing transfer of mass and momentum.Contribution of incomplete fusion in ¹⁶O+¹¹⁵In reaction is significantly larger than that in ¹⁶O+⁶⁵Cu reaction.     

Measurement of Cross Sections for Pb(n,x)203Hg,W(n,x)182Ta and W(n,x)183Ta Reactions Induced by 14 MeV Neutrons

The cross sections for Pb(n,x)²⁰³Hg,W(n,x)^l82Ta and W(n,x)¹⁸³Ta reactions have been measured by the activation method. The neutron fluences were determined by the cross sections of ⁹³Nb (n,2n)^92mNb reaction. The neutron energies in the measurements were determined by the cross section ratios of Zr(n,2n)^89m+gZr and Nb(n,2n)^92mNb reactions.     

New Neutron-Rich Isotope 208Hg

The new neutron-rich isotope ²⁰⁸Hg has been identified for the first time from the reaction products in fully-stopping thick ^nat.Pb target bombarded by 30MeV/u ¹²C beam provided by the Heavy Ion Research Facility in Lanzhou.Assignment of the nuclide ²⁰⁸Hg was based on the measurements of the deacy γ energy and halflife of its daughter ²⁰⁸Ti,which was grown up with ²⁰⁸Hg β-deacy.The half-life of ²⁰⁸Hg β-deacy was determined to be 42+23－12 min.The average production cross section of ²⁰⁸Hg over the incident energy region from Coulomb barrier to 30MeV/u is found to be 1.1±0.5μb.     

Fragmentation Cross Sections of 12C on Different Targets at Beam Energies from 50 to 100MeV/Nucleon

The fragmentation cross sections of reactions ^12C＋^2H, ^12C, ^14N, ^16O at beam energies from 50 to 100 MeV/nucleon are investigated using the isospin-dependent Boltzmann-Langevin equation model. It is found that fragment species increase approximately with the increasing target mass. The fragment species and some fragments production cross sections in reactions of ^12C＋^12C, ^14N, ^16O show an obvious variation at the beam energies from 50 to 80 MeV/nucleon. However the calculated fragment production cross sections do not change much when the incident energy increases from 80 to 100 MeV/nucleon.     

40, 48Ca+90, 96Zr近垒熔合反应的动力学研究

发展了一种改进的量子分子动力学模型,并用这一模型研究了40,48Ca+90,96Zr的近垒熔合反应.改进的量子分子动力学模型能很好地描述一系列核,从6Li到208Pb,的基态性质及它们的时间演化.在相同参数下计算得到的40Ca+90Zr以及40Ca+96Zr这两个熔合反应的激发函数与实验结果都符合得相当好.在分析丰中子核熔合截面增强机制中,发现丰中子核熔合反应初始阶段颈部的N/Z值明显偏大,促使反应中动态位垒降低,从而引起了丰中子核反应熔合截面的增强. An improved Quantum Molecular Dynamics (QMD) model is proposed and the fusion reactions 40, 48Ca+90, 96Zr are studied by using this model. With our improved QMD model, the ground state properties and their time evolution of nuclei from 6Li to 208Pb can be reproduced reasonably well and the excitation functions of fusion cross section for reactions40Ca+96Zr and 40Ca+96Zr at near barrier can be reproduced remarkably well with the same set of parameters...     

Fusion of 60Ni + 100Mo near and below the Coulomb barrier

Various sub-barrier capture reactions with ^16,18O and ^40,48Ca are treated within the quantum diffusion approach. The role of neutron transfer in these capture reactions is discussed. The quasielastic and capture barrier distributions are analyzed and compared with the recent experimental data.     

A SEMI-CLASSICAL ANALYSIS ON THE REACTION 12C+209Bi AT BOMBARDING ENERGIES SLIGHTLY ABOVE THE COULOMB BARRIER

For reactions induced by light heavy ion such as ¹²C, at bombarding energy slightly above the coulomb barrier, using semi-classical theory, neglecting macroscopic frictional force, the reaction mechnisms are sorted by the orbital angular momentum of the entrance channel from the classical deflection functions fitting elastic scattering. Moreover, inserting a transfer probability factor representing the Q-Window effect into the semi-classical angular distribution formula of Kalinkin and Grabowski, we obtain a formula for the differential cross-section per unit energy interval. The calculated results, including the elastic scattering angular distributions, energy spectrum and angular distributions of emitted α-particles in quasi-elastic process, complete fusion cross-section, fusion-fission cross-sections and the exitation functions of evaporated neutron, agree with the experimental data on the reaction ¹²C+²⁰⁹Bi reported by our Institute.     

SINGLE PROTON TRANSFER REACTION INDUCED BY HEAVY ION 40Ar(11B, 10Be)41K

The angular distribution has been measured for elastic scattering on ⁴⁰Ar with ¹¹B and for the single proton transfer reaction ⁴⁰Ar(¹¹B, ¹⁰Be)⁴¹K at the incident beam energy of 50 MeV. The elastic scattering cross section has been fitted in terms of the optical model. The EFR-DWBA aproach with recoil effect has been used to analyze the differential cross section. The spectroscopic factor was extracted.