Asymmetric fission of the pre-actinide nuclei

Study of α particles, protons and neutrons emission in reactions induced by¹⁶O on Sn targets. The threshold for direct a emission is found to be v/c?0.04 (v velocity of the projectile at the top of the Coulomb barrier) whereas direct protons and neutrons, if any, would be emitted for beam energies higher than 7.8MeV/n. (v/c>0.092). Atv/c 0.092, 90% of the direct a cross section is shown to be incomplete fusion. On¹¹⁶Sn target, besides the main exit channels α4n and αp4n, we observed 2α4n and α2p4n channels where the direct α is followed by evaporation particles. This implies a lower initial angular momentum than for the main channels.     

On the stability of superheavy nuclei against fission

We use the methods, which we developed in a previous paper, for the calculation of potential energy surfaces and lifetimes for superheavy nuclei. Two regions of relative stability against spontaneous fission, which are connected with the magic proton numbersZ=114 andZ=164 and which will both become accessible for experiments in the near future, are discussed. Especially the nuclei aroundZ=164 are investigated here at the first time. The lifetimes for α-decay are also estimated and appear to be long enough for experimental work. Furthermore, we have investigated the dependence of the fission barrier on the level-distributions at the fermi-surface. At the end we discuss the difficulties in the usual microscopic calculations for the fission process and show a way to overcome the limitations and inconsistencies of the usually used Strutinsky-type calculations.

     

On the critical size of pores

The critical sizes of pores, determined by surface tension forces, have been determined. It is shown that the pore size in a material cannot be lower than some critical value.     

Abrupt changes of the characteristics of the proton-induced fission of238U around 14-MeV excitation

An experiment combining the conventional radio-chemical stacked target technique and rapid measurements using a gas-jet transport system were carried out with a purpose of precise determination of charge dispersion and distribution for the proton-induced fission of²³⁸U at two incident energies of 9.4 and 12.4 MeV. We determined the charge dispersion width as 1.01±0.05 charge unit at 17.9-MeV excitation and 0.70±0.04 charge unit except for the symmetric mass region at 14.8-MeV excitation. Fundamental differences were found also in the mass distribution characteristics between 17.9-MeV and 14.8-MeV excitations. This abrupt change of the fission characteristics strongly supports the existence of different fission mechanisms below and above around 14-MeV excitation.     

瞬发超临界系统内持续裂变链的发展过程

根据基本的概率理论和瞬发临界附近有限裂变链的发展规律, 推导了单脉冲源条件下脉冲堆引发持续裂变链过程中的中子数与时间关系式, 并得到CFBR-Ⅱ堆上实验的验证. 该关系式进一步发展了Hansen理论模型, 除了可以描述裂变链后期的发展趋势, 还可以描述持续裂变链产生初期中子数的快速增长行为. 讨论了引发时间与源强的关系, 指出在弱源条件下, 引发时间几乎不依赖于源强. 关键词： 持续裂变链 脉冲堆 引发时间     

Kinetic energy distributions around symmetric thermal fission of U234 and U236

The kinetic energy distributions have been measured at LOHENGRIN for symmetric fission of U²³⁴ and U²³⁶. The pre-neutron emission values for the average total kinetic energy and its rms width have been deduced using a Monte Carlo simulation for neutron evaporation. The total kinetic energy dip between symmetric and asymmetric divisions is 24 MeV for U²³⁴ and 29 MeV for U²³⁶. A strong peak in the initial rms width has been observed for masses 111–123 in U²³⁴ and for 111–125 in U²³⁶. A static scission point model is used to understand the origin of these features.     

Anelastic relaxation and around the critical Sr content x = 0.02

Anelastic relaxation and ¹³⁹LaNQR relaxation rates in La_2–xSr_xCuO₄ for Sr content around 2 and 3 percent are discussed in terms of spin and lattice excitations and of the related ordering processes. It is argued how the phase diagram of La_2–xSr_xCuO₄ at the boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02) could be more complicate than previously thought, with a transition to a quasi-long range ordered state at K, as indicated by neutron scattering data. On the other hand, the ¹³⁹LaNQR spectra are compatible with a transition to an AF phase around K, in agreement with the phase diagram commonly accepted in literature. In this case the peaks in NQR and anelastic relaxation rates around 150 K and 80 K respectively in La_1.98Sr_0.02CuO₄, yield the first evidence of freezing process involving simultaneously lattice and spin excitations, possibly corresponding to motion of charged stripes. Received 18 May 2000 and Received in final form 11 July 2000     

On the conservation of electric charge around a monopole of finite size

In monopole-fermion dynamics, the boundary condition which is responsible for baryon number non-conservation also violates electric and color hypercharge conservation. We show by detailed calculations that actually the latter conservation laws are dynamically restored. It is shown that for a finite size monopole, there is a small but finite amplitude for the monopole ground state to make a virtual transition into a state containing a dyon and some fermions carrying equal and opposite charge as that of the dyon. But the amplitude for this state to make a virtual transition to a state carrying a net total charge is identically zero. The monopole ground state, as a result, is an eigenstate of electric charge even in the presence of massless fermions. We also calculate the four-body charge and chirality conserving but baryon number violating condensates, which exist independently of the existence of the anomaly and hence persist even in the presence of more generations of massless fermions.     

System size effect on the critical behavior in nuclear multifragmentation

Attempt has been made to examine the role of system size on the traditional signatures of critical behavior from a comparative study of Mg-Em at 4.5 A GeV and Kr-Em at 0.95 A GeV interactions. A number of relevant observables such as fluctuation in the sizes of the largest cluster, reduced variance and the mean value of second moment of charge distribution were estimated with the experimental data. From a comparison of our results with that of EOS collaboration for Au, La and Kr on carbon at 1 A GeV, a definite systematic variation in the heights and positions of the peaks could be observed with the change of fragmenting nuclei thereby confirming the effect of system size on MF mechanism.     

Finite size effects in critical dynamics and the renormalization group

Systems representable as a time-dependent Ginzburg-Landau model with nonconserved order parameter are considered in a block (V=L ^d) geometry with periodic boundary conditions, both for space dimensionalitiesd4 andd=4–. A systematic approach for studying finite size effects on dynamic critical behavior is developed. The method consists in constructing an effective reduced dynamics for the lowest-energy (q=0) mode by integrating out the remaining degrees of freedom, and generalizes recent analytic approaches for studying static finite size effects to dynamics. Above four dimensions, the coupling to the other (q0) modes is irrelevant and the probability densityP(,t) for the normalized order parameter=d^d x(x,t)/V satisfies a Fokker-Planck equation. The dynamics is equivalently described by the Langevin equation for a particle moving in a ||⁴ potential or by a supersymmetric quantum mechanical Hamiltonian. Dynamic finite size scaling is found to be broken, e.g. the order parameter relaxation rate varies at the bulk critical temperatureT _c, as (T _c, L)L ^–d/2 asL. By contrast, ford<4, the coupling to the other (q0) modes cannot be ignored and dynamic finite size scaling is valid. The asymptotic behavior of correlation and response functions can be studied within the framework of an expansion in powers of ^1/2. The scaling function associated with is computed to one-loop order. Finally, the many component (n) limit is briefly considered.     

Subtleties in data analysis related to the size of critical region

We comment on the analysis of the critical behavior of a layered driven diffusive system recently done by Achahbar and Marro. We discuss why we believe their method of taking the thermodynamic limit and determining the order-parameter exponent leads to unreliable estimates.     

Thermodynamic stability of equilibrium system and critical state

On the basis of general Gibbs conditions of stability, the requirements of critical state stability that are applied to adiabatic coefficient of stability (ACS) and their derivatives are obtained. The examination of these conditions with critical state definition establishes the existence of four alternative types of critical behaviour of the thermodynamic system. These types are classified by the critical behaviour of ACS and K_C, which is connected with ACS. Analysis of the asymptotic behaviour of adiabatic quantity (AQ), which is consistent with stability requirements, demonstrates that both analytic and nonanalytic behaviours are possible.     

半导体量子点形成临界尺寸的定量预测

半导体量子点是一类具有显著量子效应的零维量子结构,自组的模型系统,表现为Stranski-Krastanov型生长.其特征为,当超过3-4个Ge单原子层（浸润层）时,则由二维层状生长转变为三维岛状生长.Ge/Si量子点是初期形成的与衬底共格无位错的三维岛,岛表面由{105}晶面组成.文章作者利用第一性原理计算和介观理论模拟相结合的连续式多尺度（sequential multi-scale）方法,第一次对纯Ge和GeSi合金量子点在Si（001）表面的成核临界尺寸进行了定量的理论预测,同时研究了岛边缘的应力不连续对量子点稳定性的影响,实现了对Ge/Si量子点的形成和稳定性定量的理论研究。     

半导体量子点形成临界尺寸的定量预测

半导体量子点是一类具有显著量子效应的零维量子结构,自组的模型系统,表现为Stranski-Krastanov型生长.其特征为,当超过3—4个Ge单原子层（浸润层）时,则由二维层状生长转变为三维岛状生长.Ge/Si量子点是初期形成的与衬底共格无位错的三维岛,岛表面由{105}晶面组成.文章作者利用第一性原理计算和介观理论模拟相结合的连续式多尺度（sequential multi-scale）方法,第一次对纯Ge和GeSi合金量子点在Si(001)表面的成核临界尺寸进行了定量的理论预测,同时研究了岛边缘的应力不连续对量子点稳定性的影响,实现了对Ge/Si量子点的形成和稳定性定量的理论研究.