Langevin analysis of fission excitation functions induced by protons

The stochastic Langevin approach to fission is applied to analyze fission excitation functions measured in p+²06Pb and p+²09Bi systems. A presaddle friction strength of（3–5）×10²¹s^-1is extracted by comparing theoretical predictions with experimental data. Furthermore, the small distortion of the formed compound nuclei with respect to the spherical shape under the condition of low angular momentum suggests that experimentally, populating an excited compound system via light-ion induced reactions favors a more accurate determination of presaddle friction with a fission cross section.     

Two-dimensional Langevin modeling of fission dynamics of the excited compound nuclei ~(188)Pt,~(227)Pa and ~(251)Es

A stochastic approach based on one-and two-dimensional Langevin equations is applied to calculate the pre-scission neutron multiplicity,fission probability,anisotropy of fission fragment angular distribution,fission cross section and the evaporation cross section for the compound nuclei ~(188)Pt,~(227)Pa and ~(251)Es in an intermediate range of excitation energies.The chaos weighted wall and window friction formula are used in the Langevin equations.The elongation parameter,c,is used as the first dimension and projection of the total spin of the compound nucleus onto the symmetry axis,K,considered as the second dimension in Langevin dynamical calculations.A constant dissipation coefficient of K,γk=0.077(MeV zs)~(-1/2),is used in two-dimensional calculations to reproduce the above mentioned experimental data.Comparison of the theoretical results of the pre-scission neutron multiplicity,fission probability,fission cross section and the evaporation cross section with the experimental data shows that the results of two-dimensional calculations are in better agreement with the experimental data.Furthermore,it is shown that the two-dimensional Langevin equations together with a dissipation coefficient of K,γk=0.077(MeV zs)~(-1/2),can satisfactorily reproduce the anisotropy of fission fragment angular distribution for the heavy compound nucleus(251)~Es.However,a larger value of γk=0.250(MeV zs)~(-1/2)is needed to reproduce the anisotropy of fission fragment angular distribution for the lighter compound nucleus(227)Pa.     

基于ImIQMD模型对近垒重系统准裂变反应的研究

利用改进的同位旋相关的量子分子动力学(ImIQMD)模型程序,对能量在库仑位垒附近的48Ca+208Pb反应系统进行了模拟。 计算得到的俘获截面、 准裂变截面和碎片分布及其发生的时间分布等结果与实验数据符合较好。 同时还得到了准裂变过程中发射第三个较大碎片的截面, 并简单讨论了它们的发射机制。Based on the improved isospin dependent quantum molecular dynamics (ImIQMD) model,the reaction of 48Ca+208Pb at energies near the Coulomb barrier was investigated systematically. The capture cross section,the quasi fission cross section, the mass distribution of the quasi fission fragments and the time for quasi fission happened were obtained. According to the comparison with experimental data,the calculated capture cross sections and the fragment distributions of the quasi fission were reasonable. Additionally,we got the cross sections of the third largest fragment,and the emission mechanism was also discussed briefly.     

Fission studies of gold induced by (1665 MeV) π- using a CR-39 detector

The fission cross section and fission probability of ¹⁹⁷Au, induced by (1665 MeV) π^-, have been studied using CR-39 track detectors. A 4π-geometry was used to count track statistics. A beam of negative pions of 1665 MeV was produced at AGS of Brookhaven National Laboratory, USA, and allowed to fall normally on the stack. Two detectors from the stack were scanned for fission fragment tracks after etching in 6N NaOH at 70℃. The statistics of fission fragment tracks in both detectors were obtained. It was found that there was a marked asymmetry of registered tracks with respect to the forward and backward hemispheres. This asymmetry could be partly accounted for on the basis of momentum transfer to the struck nucleus. On the basis of counting statistics fission cross section was measured, and fission probability was determined by dividing the fission cross section with the reaction cross section. The fission cross-section and fission probability were compared with the computed values using the cascade-exciton model code CEM95.     

激发能相关的能级密度参数和重核衰变性质(英文)

基于裂变的随机模型,研究了能级密度参数在鞍点与其基态处的比值a_f/a_n在热核衰变过程中的作用。计算表明,a_f/a_n的大小能显著影响蒸发剩余截面及其自旋分布的大小以及裂变过程中热核的耗散行为。发现,能级密度参数对激发能的依赖性a_f/a_n(E*) 能改变断点处的激发能对摩擦强度的敏感性。最后指出了a_f/a_n随E* 的变化对超重核的稳定性和散裂反应退激阶段描述的意义。Based on the stochastic approach to fission, we have demonstrated that the ratio of level-density parameters at saddle to that at ground-state configuration, af/an, plays a significant role in the decay of an excited nucleus. It modifies not only the sensitivity of evaporation residue cross section and its spin distributionsto nuclear friction, but also the dissipation properties of hot nuclei. Furthermore, we find that af/an as a function of excitation energy appreciably affects the evolution of excitation energy at scission with the friction strength. A consequence of a change of af /an with E* on the stability of superheavy nuclei and on modelling de-excitation processes of spallation reactions is indicated.     

重离子周边碰撞和中心碰撞引起的不同裂变行为

本实验同时研究了4 0Ar+209Bi反应中周边碰撞和中心碰撞两者产生的关联裂变碎片,以及其与α粒子的再关联.对裂变碎片质量分布和能量分布随热裂变核初始温度演化的系统分析,发现中心碰撞和周边碰撞所形成的热核存在着不同的裂变行为. Correlated fission fragments from the 40Ar+209Bi reaction and their further correlation with α particles have been studied for peripheral and central collisions simultaneously. The existence of different fission behavior of hot nuclei formed in central and peripheral collisions was found from the systematic analysis of the mass and energy distributions of fission fragments as a function of the initial temperature of hot fissioning nuclei.     

Roles of Isospin in Evaporation Residue Cross Section as a Probe of Nuclear Dissipation

The influence of isospin on the excess of evaporation residue cross section over its standard statistical-model value for nuclei ^194pb, ^200Pb, and ^206pb is studied via a Langevin equation coupled with a statistical decay model. The magnitude of this excess for a low-isospin fissioning nucleus is shown to be larger and its dependence on the nuclear viscosity coefficient to be stronger than those of a high-isospin fissioning nucleus. These results suggest that to obtain a more accurate information of viscosity coefficient inside the saddle point by measuring evaporation residue cross sections, we had better choose those compound systems with small isospin.     

Study of pre-scission particle emissions and fission probability of the l78W produced in fusion reactions

A dynamical model based on one-dimensional Langevin equations was used to calculate the average pre-fission multiplicities of neutrons, light charged particles, and the fission probability for compound nucleus 17Sw produced in fusion reactions. The pre-seission multiplicities of particles and fission probability are calculated and compared with the experimental data over a wide range of excitation energy. A modified wall and window dissipation with a reduction coefficient, ks, has been used in the Langevin equations for reproducing experimental data. It was shown that the results of the calculations are in good agreement with the experimental data by using values of ks in the range 0.24〈 ks 〈0.47.     

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.     

Identification of Vibrational States in 230T（n,f） Reaction

The excitation function for the fission of 230Th induced by neutrons has an unusual maximum for neutron energies in the vicinity of 700KeV,It has been suggested that this maximum may be associated with the vibrational-mode resonance states,The unusual peak in the excitation function is interpreted in terms of a vibrational-model resonance state in a two-humpted fission barrier.From theoretical fits to the fission cross sections and angular distributions,it is shown that the resonance has K=1/2.     

角动量对基于裂变截面探测核耗散的影响（英文）

核耗散降低了裂变截面值(σdrop f)。基于Langevin模型,研究了角动量对σdrop f作为鞍点前摩擦强度β函数的影响。发现在低自旋,摩擦对σdrop1f的影响显著增强。通过考察系统95Bi,201Bi和207Bi的σdrop f随β的演化,发现摩擦效应随着Bi核中质比(N/Z)的增加而变大。这些结果建议,为了更好地在实验上用裂变截面探测β,应产生那些具有低自旋、高N/Z的热核系统。     

Track detector study of fission of bismuth nuclei induced by He, radioactive He and Li ions

The cross-sections for fission of ²⁰⁹Bi induced by secondary beams of ⁶He by ⁴He and ⁷Li were measured. For the registration of fission fragments polyterephtalate plastic and muscovite mica detectors were used. In the case of the reaction of helium ions with bismuth the cross-section of the fission of ²⁰⁹Bi induced by ⁶He are significantly higher than the corresponding -particle-induced fission cross-section.     

The Role of Potential Structure in Excitation Functions of Synthesizing Superheavy Nuclei

The excitation functions of two very similar reaction channels, 58Fe＋ 208pb→ 265Hs ＋ 1n and 58Fe＋ 209Bi → 266Mt ＋1n are studied in the framework of the dinuclear system conception. The fusion probabilities are found to be strongly subject to the structure of the driving potential. Usually the fusion probability is hindered by a barrier from the injection channel towards the compound nuclear configuration. The barrier towards the mass symmetrical direction, however, also plays an important role for the fusion probability, because the barrier hinders the quasi-fission, and therefore helps fusion.     

The Bi(d,γ)Po reaction

The yield of the ²⁰⁹Bi(d, γ)^211g.s.Po and ^211mPo (T_1/2 = 25.2s) reaction was measured for deuteron energies E_d = 8–11.5 MeV. The reaction was identified by the -activities of the Po isotope. At E_d = 10.43 MeV, the (d, γ) cross section for the population of the ground state of ²¹¹Po is σ_g.s. = 16 ± 3 μb, the ratio relative to the cross section for the metastable state is σ_g.s./σ_m = 25.4 ± 0.9. These values and the yield curves were compared with calculations using a simple model for the population of the two states. In the excitation region E* = 15–19 MeV, the branching ratio of γ- to particle emission is nearly constant and has a value of about 0.4 × 10⁻⁴.     

Cross-section measurements for the reaction B(p, He)Be

Cross sections have been measured for the transitions to the ground state and first excited state of ⁸Be and to the two 2⁺ levels at 16.63 MeV and 16.93 MeV excitation in the reaction ¹⁰B(p, ³He) at E_p = 49.5 MeV. The angular distributions of the cross section were compared with DWBA predictions.     

Ionizing radiation effect on single event upset sensitivity of ferroelectric random access memory

The impact of ionizing radiation effect on single event upset(SEU) sensitivity of ferroelectric random access memory(FRAM) is studied in this work. The test specimens were firstly subjected to ~(60)Co γ-ray and then the SEU evaluation was conducted using ~(209)Bi ions. As a result of TID-induced fatigue-like and imprint-like phenomena of the ferroelectric material, the SEU cross sections of the post-irradiated devices shift substantially. Different trends of SEU cross section with elevated dose were also found, depending on whether the same or complementary test pattern was employed during the TID exposure and the SEU measurement.     

Dominant angular momentum of gross structure in inelastic C+C(2) scattering

The gross structures observed at the CM energies of 24, 30 and 36 MeV in the angle-integrated ¹²C+¹²C(2⁺) cross section are attributed to contributions of the dominant angular momenta of 16, 18 and 20, respectively, based on analyses of the angular distributions. These assignments are shown to be favorable also for predictions of other available data such as particle-γ correlations, alignments of the spin substates and the elastic excitation functions.     

Coulomb Effect on Inelastic Scattering of Proton on Deformed Nuclei at High Energies

In this paper, by using the collective coordinate of A. Bohr and phenomenological deformed optical potential and Coulomb interaction, the scattering amplitude of two composite particle systems can be obtained. The differential cross sections for the 0⁺, 2⁺, 4⁺ and 6⁺, p(0.8 GeV)+¹⁵⁴Sm and p (0.8 Gev)+¹⁷⁶Yb are calculated. The results show that the Coulomb effect is quite important.     

CVD法单层MoS2在209Bi离子辐照下的损伤效应研究

通过光学显微镜、拉曼光谱确定了CVD（化学气相沉积法）制备的不同厚度MoS₂的层数,采用拉曼分析结合原子力显微镜观测分析了由HIRFL提供的高能209Bi离子辐照CVD制备的单层MoS₂样品随辐照注量的损伤规律。随辐照注量增加,E_2g1和A_1g两种声子振动模式对应的拉曼峰逐渐蓝移,且拉曼特征峰强度减弱,这是由于带正电荷的209Bi辐照产生潜径迹型晶格缺陷吸附空气中氧分子而引入p型掺杂引起的。同时,在辐照注量为5×1010 ions/cm2的单层MoS₂的AFM图像中观察到潜径迹主要以凹坑形式出现,与机械剥离法观测到的凸起径迹明显不同,分析了不同制备工艺对径迹形貌的影响。比较了机械剥离法制备MoS₂样品的拉曼光谱和AFM成像的实验数据和结果,认为不同制备方法会影响单层或少层MoS₂的电子密度。The layer number of MoS₂ with different thickness was determined by the optical microscope and Raman spectra. And the damage effect of the CVD (chemical vapor deposition) prepared single-layer MoS₂ sample irradiated by 209Bi ions was analyzed by the combination of Raman analysis and AFM observations. With the increase of irradiation fluence, the Raman characteristic peaks of E_2g1 and A_1g corresponding to both phonon vibration modes gradually bluely shift, and the intensity of the peaks obviously decreased. This is due to the fact that the 209Bi ion irradiation results in the latent track type lattice defects and they adsorb the oxygen molecules in the air ended with the p-type doping. Meanwhile, from the AFM image of the mono-layer of irradiated MoS₂ under the 5×1010 ions/cm2, it can been seen that latent tracks mainly occur in the form of pits, which different from the hillock tracks observed by mechanical stripping method. The influence of different preparation technology to the track morphology is analyzed. Compared with the Raman and AFM results of MoS₂ prepared by mechanical stripping, it is considered that different preparation methods will affect the electron density in single or few layers of MoS₂.     

Studies on the track formation mechanism of the heavy ions in CR-39

The bulk etch rate for two types of CR-39 detector was measured as a function of temperature and the activation energies of bulk etching was determined. Experimental values of track etch rate were derived directly from the function of the succesive measured track length vrs. etching time for ²⁰⁹Bi, ¹²⁹Xe and ²⁰Ne ions.

The maximum etchable length of 13 MeV/u ²⁰⁹Bi and 13.04 MeV/u ¹²⁹Xe ions have been measured at and below these energies. A comparison of the measured and calculated track length data is presented.