核粘滞性对裂变过程的影响

根据裂变扩散模型,把裂变核的形变运动看成是一种准布朗粒子在裂变势作用下的扩散过程.通过Monte-Carlo方法模拟这种布朗粒子在两维相空间内的运动,研究了核粘滞性对裂变过程的影响;解释了裂变过程中的耗散现象.在各种粘滞系数下,对于裂变速率,断点动能等动力学置的数值计算结果是合理的.此法易于推广用来研究多维扩散问题.     

核粘滞性对裂变过程的影响

根据裂变扩散模型,把裂变核的形变运动看成是一种准布朗粒子在裂变势作用下的扩散过程.通过Monte-Carlo方法模拟这种布朗粒子在两维相空间内的运动,研究了核粘滞性对裂变过程的影响;解释了裂变过程中的耗散现象.在各种粘滞系数下,对于裂变速率,断点动能等动力学量的数值计算结果是合理的.此法易于推广用来研究多维扩散问题.     

裂变的布朗运动模型

本文将裂变过程看成为一多维的布朗运动,求得普朗克-福克方程适合于这一核过程的稳定解,并计算了单位时间内的裂变几率,将计算结果与玻尔-惠勒理论作了比较。对²³⁶U裂变所作的数值计算表明:此模型主要特点在于考虑了粘滞性对裂变几率的影响,据现有对裂变粘滞性的估计,这种影响将使裂变几率减小20％-40％。     

10.6MeV/u 84Kr+27Al反应中角动量对断点前粒子多重性的影响

用推广的裂变扩散模型研究了角动量对10.6MeV/u ⁸⁴Kr(²⁷Al,二体裂变)反应断点前粒子多重性的影响,发现阶点前粒子多重性和断点时间随着角动量的减小逐渐增大,而且绝大多数粒子是在鞍点前发射的. 对实验中二体裂变反应发生的角动量范围进行权重平均,得到了与实验一致的断点前粒子多重性. 同时还研究了影响断点前粒子多重性和断点时间的诸多因素.     

核裂变速率的解析解模型及计算

本文利用福克-普朗克方程在谐振子位场中得到的解析解形式, 类比于克拉麦(Kramers)早期提出的定态理论, 给出了计算裂变速率的解析解模型计算公式. 以Pu²⁴⁰为例对不同粘滞系数下的裂变宽度进行了计算, 观察分析了裂变宽度随时间的变化, 并与跃迁态方法和定态理论的结果进行了比较.     

中子裂变链统计涨落问题的数值计算方法

研究了弱中子源驱动下,裂变系统中子裂变链统计涨落问题的数值计算方法。进行了数值计算建模、数值方法分析、数值计算检验、一类问题概率分布函数的统计涨落特征量的数值计算示范。特例数值检验表明:只要数值解方程组阶数(截断) N足够大,数值解满足归一(守恒) 律、指数增长律,并与精确解析解一致。对于非定常裂变系统中子裂变链统计涨落问题提出了一维等效模型下数值模拟的方法。     

裂变路径对断点前粒子发射的影响

用扩散模型研究了裂变系统从鞍点到断点的时间演化, 以及在此间的粒子发射与鞍点前粒子发射的比随着裂变系统质量的变化. 解释了实验上发现的断点前粒子多重性随裂变碎片质量不对称性的变化趋势. 结果表明上述两种变化趋势都可以归因于裂变演化路程长短的影响.     

用福克-普朗克方程严格解裂变速率问题

本文应用数值方法严格求解二维福克-普朗克(Fokker-Planck)方程,研究在鞍点上的速度分布、坐标和速度的二次矩以及核裂变速率随时间的演变.在相当大的粘滞性范围内,研究核裂变速率与粘滞性的依赖关系,观察到在一定粘滞性下出现裂变速率极大值的现象.对计算的各种结果进行了解释。     

确定裂变断点构形的一个模型

确定裂变核的断点构形是裂变物理中的一个重要问题.本文给出一个对一有缩颈的裂变核形状判断其是否会立即断裂的能量判据:在保持裂变核体积守恒和形状基本不变的条件下,如果核的形变能随其颈部变细而增加,则认为裂变核暂时不会断裂;反之,如果形变能随颈部变细而单调下降,则认为裂变核立即发生断裂.并采用适于描写颈部消失过程的形状参数化方法对²³⁵U（n,f）体系的几种质量分配的断后现象进行了统计模型的计算（入射中子能量从0—14Mev）,这包括碎片的平均动能;电荷分布和质量分布.计算结果均与实验符合较好,证明了所提断点模型的合理性.     

从四维Langevin方程的Monte Carlo模拟计算裂变碎片动能分布

本文将一个关于对称裂变的两个集体坐标(碎片质心距和颈部参数)及其共轭动量的四维Langevin方程作为描述布朗粒子从鞍点下降到断点这一扩散过程的动力学方程.通过计算机模拟随机力冲量,用Monte Carlo方法求解了Langevin方程组.计算了322/A<40区间的八个核裂变碎片的平均动能、动能涨落宽度,结果与实验值相符.     

EFFECT OF NUCLEAR VISCOSITY ON FISSION PROCESS

According to the fission diffusion model,the deformation motion of fission nucleues is regarded as a diffusion process of quasi-Brownian particles under fission potential,Through simulating such Brownian motion in two dimensional phase space by Monte-Carlo method,the effect of nuclear viscosity on Brownian particle diffusion is studied,Dynamical quantities,suchas fission rate,kinetic energy distribution on scission,and so on are numerally calculated for various viscosity coefficients,The results are reasonable in physics,This method can be easily extended to deal with multi-dimensional diffusion problems.     

Thermal Fission Rate Calculated Numerically by Particles Multi-passing over Saddle Point

Langevin simulation of the particles multi-passing over the saddle point is proposed to calculate thermal fission rate. Due to finite friction and the corresponding thermal fluctuation, a backstreaming exists in the process of the particle descent from the saddle to the scission. This leads to that the diffusion behind the saddle point has influence upon the stationary flow across the saddle point. A dynamical correction factor, as a ratio of the flows of multi- and first-overpassing the saddle point, is evaluated analytically. The results show that the fission rate calculated by the particles multi-passing over the saddle point is lower than the one calculated by the particle firstly passing over the saddle point, and the former approaches the results at the scission point.     

Deformation parameter changes in fission mass yields within the systematic statistical scission-point model

The fission fragment mass-yields are evaluated for pre-actinide and actinide isotopes using a systematic statistical scission point model. The total potential energy of the fissioning systems at the scission point is presented in approximate relations as functions of mass numbers,deformation parameters and the temperature of complementary fission fragments. The collective temperature, Tcoll, and the temperature of fission fragments, Ti, are separated and the effect of collective temperature on mass yields results is investigated. The fragment temperature has been calculated with the generalized superfluid model. The sum of deformation parameters of complementary fission fragments has been obtained by fitting the calculated results with the experimental data. To investigate the transitions between symmetric and asymmetric modes mass yields for pre-actinide and heavy actinides are calculated with this model. The transition from asymmetric to symmetric fission is well reproduced using this systematic statistical scission point model. The calculated results are in good agreement with the experimental data with Tcoll= 2 Me V at intermediate excitation energy and with T_(coll)= 1MeV for spontaneous fission.Despite the Langevin model, in the scission point model, a constraint on the deformation parameters of fission fragments has little effect on the results of the mass yield.     

Study of Fission Barrier Heights of Uranium Isotopes by the Macroscopic-Microscopic Method

Potential energy surfaces of uranium nuclei in the range of mass numbers 229 through 244 are investigated in the framework of the macroscopic-microscopic model and the heights of static fission barriers are obtained in terms of a double-humped structure. The macroscopic part of the nuclear energy is calculated according to Lublin-Strasbourg-drop (LSD) model. Shell and pairing corrections as the microscopic part are calculated with a folded-Yukawa single-particle potential. The calculation is carried out in a five-dimensional parameter space of the generalized Lawrence shapes. In order to extract saddle points on the potential energy surface, a new algorithm which can effectively find an optimal fission path leading from the ground state to the scission point is developed. The comparison of our results with available experimental data and others' theoretical results confirms the reliability of our calculations.     

高温下复合核的热裂变速率

在核裂变的动力学描述中,考虑到质量随坐标的连续变化,提出了自由能有效势,导出了一个新的裂变速率公式.改进后的公式对于核温度与势能位垒高度相当的热核裂变同样适用,其结果比Kramers公式更接近于断点裂变速率的朗之万模拟值.     

Potential energy surfaces and lifetimes for spontaneous fission of heavy and superheavy elements from a variable density dependent mass formula

The potential energy surface for spontaneous fission is calculated using realistic density distributions for finite nuclei. Particular emphasis is placed on the region of the potential between the saddle and scission point. The method involves computing the energy of the system using an energy density functional consistent with varible density distributions and nuclear masses and obtained from a statistical many body theory. The results show that there exists an external or scission barrier to the fission process. Lifetimes and mass distributions which are computed using these potential energy surfaces are found to be in adequate agreement with observations for ²³⁴U, ²³⁶U, ²⁴⁰Pu, ²⁴⁴Cm, ²⁴⁸Cf, and ²⁵²Cf. Our predicted upper limit for the spontaneous fission half-lives of elements 112 and 114 is one year but the calculation indicates that these could be considerably shorter than a year.     

THE SHAPE OF FISSION SCISSION POINT

A shape, rotating cassin's oval body with symmetry deformation parameter of 1.0, in nuclear fission scission point is suggested. By use of the shape, simple calculations on average total kinetic energy of fission fragments in both symmetry and asymmetry cases show that the results are in agreement with semiempirical formula due to V.E. Viola.     

Transition fission states for heated nuclei

Induced fission reactions of fissioning compound nuclei that result from the capture of various incident particles (nucleons, γ rays, multiply charged ions) by target nuclei are investigated using the generalized nucleus model and the Wigner random matrix method. The effect produced on the fission widths of the compound nucleus by the competition between the excitation energies of its collective vibrational degrees of freedom that lead to its scission into fission fragments and its rotational and multi-quasiparticle states is analyzed. Bohr’s concept of transition fission states developed for near-barrier nuclear fission is generalized to the induced fission of nuclei with the excitation energies noticeably higher than the fission barriers. The temperature of the fissioning nucleus in the vicinity of the point of its scission into fission fragments is estimated.