形变相关的壳修正对N=126处断前中子发射的同位素效应的影响

利用动力学加统计模型就形变相关的壳修正（DDSC）对中子126壳层附近209,213,217Fr断前中子发射的影响进行了研究。模拟结果表明,DDSC抬升了复合核的裂变位垒,且213Fr的升高约为209,217Fr的2倍,但却没有改变鞍点位置。尽管位垒升高延缓了系统的裂变,但其动力学过程受热力学驱动力（TDF）和核阻尼间竞争的主导,因此准确提取壳修正的作用还需考虑核耗散的形变关系。在裂变第一阶段,当核耗散取一体耗散（OBD）参数时,壳修正没有改变断前中子发射的同位素效应,然而当核耗散取标准参数设置（SPS）时,由于213Fr的TDF存在着异常增强,故该规律未能展现。在裂变第二阶段,位垒升高引起的断前中子发射的增强受到了TDF与核阻尼间竞争的反制,故断前中子发射的同位素效应仍未能显现。综合两阶段情况,DDSC对N=126处断前中子发射的同位素效应的影响受第一阶段规律的支配。The effect of deformation-dependent shell correction (DDSC) on the emission of prescission neutron (EPN) is studied within a dynamical and statistical model for three isotopes of 209,213,217Fr near the neutron 126 closure-shell. The results show that the fission barriers are enhanced with DDSC, and the increment of 213Fr is almost 2 times those of 209,217Fr, but those saddle points are not changed. Although the enhancement of fission barrier delays nuclear fission, the fission dynamics process is controlled by the competition between thermodynamic driving force (TDF) and nuclear damping, so the deformation-dependence of nuclear dissipation must be considered in order to extract the role of shell correction. The shell correction doesn't alter isotope effect of EPN with OBD nuclear dissipation in the first phase of nuclear fission, but the rule does not been exhibited because that there is abnormal enhancement of TDF using SPS nuclear dissipation. The increment of EPN caused by the rise of fission barrier is countered by the competition between TDF and nuclear damping in the second phase of nuclear fission, hence the effect of EPN cannot exhibit. The effect of DDSC on EPN near the neutron 126 closure-shell is dominated by the rules in the first phase of nuclear fission.     

曲率能对热力学驱动力的影响

为了研究曲率能对核裂变热力学驱动力（TDF）的影响,首先利用包含曲率能的截断版小液滴模型计算了200Pb和224Th的位垒和熵垒,对比液滴模型的计算结果表明:曲率能未改变224Th的位垒鞍点,却将200Pb的位垒鞍点向后推移。能级密度参数的形变关系越强则两系统的熵垒鞍点越靠近基态。为了进一步探究曲率能如何通过位势和熵势影响TDF,以断前中子多重性（PNM）为探针,通过两种方案进行了模拟,结果表明:曲率能降低了两系统的位势驱动力,而增强了其熵势驱动力。结合PNM的计算表明,前一种效应要比后一种效应明显,因此曲率能总体减弱了200Pb和224Th的TDF,进而延缓了两系统的核裂变进程。In order to study the effect of curvature energy on the thermodynamic driving force (TDF) of nuclear fission, the potential and entropy barrier of 200Pb and 224Th systems are calculated by using the truncated droplet model including curvature energy, respectively. Compared with the liquid drop model, the results show that curvature energy does not affect the saddle point of 224Th, but pushes the saddle point of 200Pb backwards the ground state. The stronger the deformation dependence of the level density parameter is, the closer the saddle point of entropy barrier for these systems is to the ground state. In order to further investigate how curvature energy affects TDF through nuclear potential and entropy, respectively, the prescission neutron multiplicity (PNM) is selected as the probe, some simulations based on two schemes are carried out. The results show that curvature energy reduces the potential driving force of 200Pb and 224Th, and enhances the entropy potential driving force. Combined with the calculations and analyses of PNM, the former effect is more obvious than the latter, so curvature energy weakens TDF of two systems on whole, thus delaying the nuclear fission process of two systems.     

Deexcitation Energy of Superdeformed Secondary Minima as a Probe to Density Dependence of Symmetry Energy

Deexcitation energies of superdeformed secondary minima of odd-odd A u and T1 isotopes are investigated with the relativistic mean field （RMF） model where the isoscalar-isovector coupling is included to change the symmetry energy. It is verified by the theoretical analysis and numerical results that the deexcitation energies of superdeformed secondary minima relative to the ground states in these heavy nuclei are sensitive to differences in the symmetry energy. In particular, the linear correlation between the deexeitation energies of odd-odd Au and T1 isotopes and the neutron skin thickness in 208Pb is established. Moreover, explorations are extended to superdeformed candidates of other mass regions. It is found that the linear correlation can even be established between the deexcitation energies and the symmetry pressure at subsaturation density. These indicate that deexcitation energies can serve as a probe to the density dependence of the symmetry energy.     

Energy dissipation of a ring-like metal rubber isolator

Metal rubber （MR） is a kind of homogeneous poroelastic damping material made of metal wire. In this paper, by ana- lyzing the forces on the MR isolator and the MR element, the hysteresis loops of the force and deformation are studied and verified by experiments. The results show that the force and displacement hysteresis loop of the MR isolator is described by the force and deformation hysteresis loops of the MR elements. In addition, the relationship between the energy dissipation coefficient of the MR element and that of the MR isolator is derived. The energy dissipation coefficient is programmed and calculated by MATLAB using experimental data, and the results are compared with the theoretical value. It is the basis for the design and applied research of the MR isolator in a future study.     

Proton Fraction in Neutron Star Matter and Three-Body Force Effects

The three-body force effects on the equation of state and its iso-spin dependence of asymmetric nuclear matter and on the proton fraction in neutron star matter have been investigated[1] within Brueckner Hartree-Fock (BHF) approach by using a microscopic three-body force. It is shown that, even in the presence of the three-body force, the empirical parabolic law of the energy per nucleon vs. isospin asymmetry β=(N-Z)/A is fulfilled in the whole asymmetry range 0 ≤β ≤1 and also up to high density. The three-body force provides a strong enhancement of symmetry energy at high density in agreement with relativistic approaches. Thecalculated proton fraction in β stable neutron star matter is given in Fig.l, where the thick solid line is the BHF prediction using AV18 TBF, while the dotted one is the BHF result using AVis. The thin solid line is the prediction of the Dirac-Brueckner approach taken from Ref.[2]. The results extracted according to the phenomenological paramete rizations of the symmetry energy suggested in Ref.[3] are also plotted. It is seen from     

高激发核相继裂变的限制

高温核衰变是当前中能重离子核物理中研究最热烈的课题.理论上对高温核的衰变进行了各种预言,相继裂变就是其中之一.本文从有关核裂变的时间尺度、裂前中子发射、裂变截面等随激发能的变化和多重碎裂的实验结果以及复杂碎片发射时间的实验结果分析了对高温核相继裂变的限制,提出了从复杂粒子蒸发向多重碎裂过渡的可能性. Sequential fission is one of the several kinds of theoratical production for hot nucleus de-cay.By analysing relevant experimental results such as fission time scale,prescission neutron emission,fission cross sections and their variation with excitation energy,the results of multifragmentation andthe time scale of complex fragment emission,a limit to the sequential fission of the hot nucleus is sug-gested in this paper.A transition from intermediate mass fragment evaporation to..     

Calculation of prompt fission neutron spectra for 235U(n,f)

The prompt fission neutron spectra for the neutron-induced fission of ²³⁵U at E_n < 5 MeV are calculated using nuclear evaporation theory with a semi-empirical model, in which the nonconstant and constant temperatures related to the Fermi gas model are taken into account. The calculated prompt fission neutron spectra reproduce the experimental data well. For the n(thermal)+²³⁵U reaction, the average nuclear temperature of the fission fragment, and the probability distribution of the nuclear temperature, are discussed and compared with the Los Alamos model. The energy carried away by γ rays emitted from each fragment is also obtained and the results are in good agreement with the existing experimental data.     

Multi-modal analysis for low energy neutron-induced fission of 235U

Low energy neutron induced fission of 235U is studied in the framework of the multi-modal fission model. The fission fragment properties, such as the yields, the average total kinetic energy distribution and the average neutron separation energy, are investigated for incident neutron energies from thermal to 6.0 MeV. The multi-modal fission approach is also used to evaluate the prompt fission neutron multiplicity and spectra for the neutron-induced fission of 235U with an improved version of the Los Alamos model for incident neutrons below the （n, nf） threshold. The three most dominant fission modes are taken into account. The model parameters are determined on the basis of experimental data. The calculated results are in good agreement with the experimental data.     

Calculation of prompt fission neutron spectrum for 233U（n,f） reaction by the semi-empirical method

The prompt fission neutron spectra for the neutron-induced fission of 233U for low energy neutrons （below 6 MeV） are calculated using nuclear evaporation theory with a semi-empirical method, in which the partition of the total excitation energy between the fission fragments for the nth＋233U fission reactions is determined by the available experimental and evaluation data. The calculated prompt fission neutron spectra agree well with the experimental data. The proportions of high-energy neutrons of prompt fission neutron spectrum versus incident neutron energies are investigated with the theoretical spectra, and the results are consistent with the systematics. The semi-empirical method could be a useful tool for the prompt evaluation of fission neutron spectra.     

The spin-isospin decomposition of the nuclear symmetry energy from low to high density

The energy per particle BA in nuclear matter is calculated up to high baryon density in the whole isospin asymmetry range from symmetric matter to pure neutron matter.The results,obtained in the framework of the Brueckner-Hartree-Fock approximation with two-and three-body forces,confirm the well-known parabolic dependence on the asymmetry parameterβ=(N?Z)/A(β^2 law)that is valid in a wide density range.To investigate the extent to which this behavior can be traced back to the properties of the underlying interaction,aside from the mean field approximation,the spin-isospin decomposition of BA is performed.Theoretical indications suggest that theβ^2 law could be violated at higher densities as a consequence of the three-body forces.This raises the problem that the symmetry energy,calculated according to theβ^2 law as a difference between BA in pure neutron matter and symmetric nuclear matter,cannot be applied to neutron stars.One should return to the proper definition of the nuclear symmetry energy as a response of the nuclear system to small isospin imbalance from the Z=N nuclei and pure neutron matter.     

Study of fission dynamics with the three-dimensional Langevin equations

The dynamics of fission has been studied by solving one- and three-dimensional Langevin equations with dissipation generated through the chaos weighted wall and window friction formula. The average prescission neutron multiplicities, fission probabilities and the mean fission times have been calculated in a broad range of the excitation energy for compound nuclei ²¹⁰Po and ²²⁴Th formed in the fusion-fission reactions ⁴He + ²⁰⁶Pb , ¹⁶O + ²⁰⁸Pb and results compared with the experimental data. The analysis of the results shows that the average prescission neutron multiplicities, fission probabilities and the mean fission times calculated by one- and three-dimensional Langevin equations are different from each other, and also the results obtained based on three-dimensional Langevin equations are in better agreement with the experimental data.     

曲率能及压缩能对缺中子裂变系统断前中子发射的影响

用改进了的动力学与统计相结合的模型, 研究了曲率能及压缩能对\,4\,个缺中子裂变系统: ¹⁸⁸Pt, ²⁰⁰Pb, ²¹³Fr和²²⁴Th的断前中子发射的影响. 计算结果显示用改进模型得到的断前中子多重性相比原来模型的低估要更靠近实验数据. 对于目前工作考虑压缩能的原因进行了讨论.     

Determination of the fission fragment excitation energy with allowance for neutron multiplicity

The study of the excitation energy distribution of fission fragments as a function of their mass and charge is important for revealing the nuclear fission mechanism and useful for many applications. To measure directly the excitation energy of primary fission fragments (before emission of neutrons) is a great problem. A method of obtaining these excitation energies from calculated neutron multiplicities and experimental values for differential yields of fragment pairs after emission of neutrons is considered. The Empire-II code was used to calculate neutron multiplicities as a function of various characteristics of the nuclear structure, fission process, and fission fragment deexcitation.     

Probabilities for the emission of light particles and their energy and angular distributions for true quaternary nuclear fission

On the basis of quantum-mechanical fission theory, the features of true quaternary nuclear fission are studied by treating this fission process as a sequence of three processes following one another in the course of time. The first two processes are the escape of the first and then the second of the two light particles emitted from the neck of a fissioning nucleus because of a nonadiabatic character of the collective deformation motion of this nucleus. Finally, the third process is the separation of the fissioning nucleus into two rather heavy fission fragments. The differences that arise in the emission probabilities and in the angular and energy distributions upon going over from the first emitted to the second emitted prescission third and fourth particles are analyzed by invoking experimental data on the spontaneous and thermalneutron-induced fission of nuclei, and it is shown that these differences are caused by the changes both in the geometric configuration of the fissioning nucleus and in the shell structure of its neck after the first prescission particle is emitted from it.     

Prescission neutrons in the fission of 235U and 252Cf nuclei

Experimental data obtained previously for the energy-angular distribution of neutrons originating from the fission of ²⁵²Cf (spontaneous fission) and ²³⁵U (thermal-neutron-induced fission) nuclei are analyzed, the angle being measured with respect to the direction of fission-fragment motion. A regularity common to all independent experiments is revealed: at an angle of about 90°, there exists an excess of neutrons (30% for ²⁵²Cf and 60% for ²³⁵U) that does not admit explanation within the model of neutron emission from fully accelerated fragments. Two possible explanations of this experimental fact—neutron emission during the acceleration process and the existence of an additional source of neutrons (predominantly, prescission neutrons)—are considered. It is shown that the latter conjecture describes the observed features for both nuclei more adequately. The total yield of prescission neutrons and their energy and angular distributions are determined.     

Description of true and delayed ternary nuclear fission accompanied by the emission of various third particles

The mechanisms and the features of the main types of nuclear ternary fission (that is, true ternary fission, in which a third particle is emitted before the rupture of the fissioning nucleus into fragments, and delayed ternary fission, in which a third particle is emitted from fission fragments going apart) are investigated within quantum-mechanical fission theory. The features of T-odd asymmetry in true ternary nuclear fission induced by cold polarized neutrons are investigated for the cases where alpha particles, prescission neutrons, and photons appear as third particles emitted by fissioning nuclei, the Coriolis interaction of the spin of the polarized fissioning nucleus with the spin of the third particle and the interference between the fission amplitudes for neutron resonances excited in the fissioning nucleus in the case of projectile-neutron capture being taken into account. For the cases where third particles emitted by fission fragments are evaporated neutrons or photons, T-odd asymmetries in delayed ternary nuclear fission induced by cold polarized neutrons are analyzed with allowance for the mechanism of pumping of large fission-fragment spins oriented orthogonally to the fragment-emission direction and with allowance for the interference between the fission amplitudes for neutron resonances.