丰质子核反应中轻带电粒子发射的同位旋效应

采用同位旋相关的Boltzmann-Langevin方程计算了核素12—15N和17—20Ne反应中轻带电粒子发射的同位旋效应。12—15N与28Si靶的反应结果显示轻带电粒子的产生截面有明显的同位旋效应,12N的轻带电粒子产生截面突然增大,与实验得出的结论相同,由此检验了所采用的计算方法的可行性。同时还计算了17—20Ne与9Be靶的反应,发现17Ne的轻带电粒子产生截面也是突然增大,并且其质子分布有较大的弥散,据此认为17Ne可能具有晕结构。

轻丰中子核反应中子集团产生截面的理论研究

采用Boltzmann-Langevin方程研究了能量为35MeV／u的14Be,8He,6He,11Li,17B,11Be,19C与 12C靶的反应,计算了产生中子集团的截面,发现14Be与12C靶反应产生4n的截面与实验值符合得很好．通过这几个入射核与12C靶形成中子集团截面的对比,发现核的晕中子越多产生中子集团的截面越大,晕中子数相同时,质量数越大产生中子集团的截面越大．中子集团可能主要来自晕核子．

~(40)Ca+~(58)Fe,~(58)Ni反应中的径向膨胀流研究

利用同位旋相关的Boltzmann Langevin方程研究了40 Ca + 5 8Fe和40 Ca +5 8Ni两个反应系统在 53 ,1 0 0 ,1 50和 2 0 0MeV/u入射能量下对心碰撞的径向膨胀流 .发现对于丰中子系统40 Ca + 5 8Fe的径向膨胀流系统性地小于稳定系统40 Ca+ 5 8Ni的径向膨胀流 .在假定轰击能量与反应体系的压缩密度呈抛物线关系时 ,能够解释入射能量和径向膨胀流之间呈现的直线关系 .提取了出现径向膨胀流的轰击能量阈值 ,发现对丰中子系统40 Ca + 5 8Fe得到的能量阈值小于稳定系统40 Ca+ 5 8Ni所得到的能量阈值

28.7MeV/u的~(14,16,18)O+~(7,9)Be反应中的同位素分布研究

用同位旋相关的Boltzmann Langevin方程研究了在入射能量为2 8 7MeV/u下 ,不同弹核 14O ,16 O和 18O轰击不同靶核 7Be和 9Be的反应 ,计算了生成碎片的产生截面 ,发现用丰中子 (缺中子 )炮弹或丰中子 (缺中子 )靶进行反应 ,所得到的产物均有丰中子 (缺中子 )的碎片出现 .同位素分布宽度和峰位与入射体系密切相关 ,产生碎片的电荷数越接近入射弹核的电荷数 ,则同位素分布的宽度越大 ,峰位偏离β稳定线值越远 ,其同位旋效应越明显 .

放射性核素引起反应中同位素分布的同位旋效应及19Na产生截面的ya研究

本论文介绍了放射性核束物理研究的现状以及当前常用的几种同位旋相关的重离子微观输运理论，对传统的 Boltzmann-Langevin 方程（BLE）考虑了同位旋相关的平均场、核子-核子碰撞截面和泡里阻塞，面且在初始化相空间的抽样中区分了中子和质子，并合模型也考虑了同位旋效应，建立了同位旋相关的 Boltzmann-Langevin 方程（IBLE）。利用IBLE对放射性核引起反应中的同位素分布，~(19)Na 的产生截面，以及中能重离子碰撞中的径向膨胀流进行了系统的研究，并对超重核的合成进行了一些初步的讨论。利用 IBLE 分别研究了不同弹核 ~(14)O，~(16)O 和 ~(18)O 在入射能量为 28.7MeV/u 下轰击不同靶核~7Be 和 ~9Be的反应，计算生成碎片的产生截面，发现用丰中子（缺中子）炮弹或丰中子（缺中子）靶进行反应，所得到的产物均有丰中子（缺中子）的碎片出现。同位素分布宽度和峰位入射体系密切相关，产生碎片的电荷数越接近于入射弹核的电荷数，则同位素分布的宽度越大，峰位偏离β稳定线值越远，其同位旋效应越明显。在28.7 MeV/u入射能量下，对反应系统 ~(17-20,22)Ne + ~(12)C 和 ~(20)Ne + ~9 进行了研究。对核素 ~(19)Na 产生截面进行计算和比较后，发现缺中子核引起的反应，具有更大~(19)Na的产生截面。通过研究反应系统 ~(40)Ca + ~(58)Ni 和 ~(40)Ca + ~(58)Fe 的径向膨胀流随入射能量的变化关系，发现径向膨胀流存在着强烈的同位旋相关性。利用径向膨胀流随入射能量的变化关系和拟和结果，从理论上证实了存在使径向膨胀流为零的特定入射能量，发现对于不同的反应体系这个能量是不同的，它随同位旋自由度的变化而变化。

6He奇异核结构性质及反应动力学研究

本论文通过在兰州放射性束流线（RIBLL）上进行的6He同Si靶和Pb靶反应实验测量得到了6He与Si靶、Pb靶反应总截面和去除双中子截面以及6He与Si靶、Pb靶反应弹核碎裂产生4He碎片的纵向动量分布。采用能够很好描述正常核反应总截面的半经验的Shen公式计算了4，6 He同Be，C，Al，Si靶的反应总截面。对于4He，从低能到高能理论计算和实验数据符合很好。但是对于6He，理论计算和实验数据符合不好。通过SHF理论、RMF理论、RDDH理论、各种核子密度分布形式（2PHO-tyPe，HO-tyPe，2PFM-tyPe及新提出的修正的费米密度分布MFM-type）计算出6He的各种密度分布（包括晕核密度、皮核密度及正常核密度），带入Glotlber模型计算了6He同Be，C，Al，Si靶的反应总截面以及双中子去除截面，只有使用晕核密度理论计算才能够很好地符合实验数据，进一步证实6He具有双中子晕核结构，确定其基本构型。比较了轻靶（Si革巴）和重靶（Pb靶）电磁离解效应刘一反应总截面和去除双中子截面的影响，重靶电磁离解效应明显。采用能够很好描述稳定核的动量分布宽度的Goldllaber理论、Morrissey经验公式、W.A.Friedman的重离子弹核碎裂模型，以及Lise小组发展的Lise程序，计算了6He与Si靶和Pb靶反应弹核碎裂产生的4He碎片的纵向动量分布，也同样证实了6He具有双中子晕核结构。论文最后，利用同位旋相关的Boltzmann-Langevin方程（IBLE）计算稳定核4He和奇异核6He同c靶的反应系统来研究6He反应动力学特性，研究反应产生的核素产生截面，以及4，5 He产生截面随碰撞参数的变化。

Self-assembly of star block copolymers by dynamic density functional theory

The dynamic mean-field density functional method, driven from the generalized time-dependent Ginzburg-Landau equation, was applied to the mesoscopic dynamics of the multi-arms star block copolymer melts in two-dimensional lattice model. The implicit Gaussian density functional expression of a multi-arms star block copolymer chain for the intrinsic chemical potentials was constructed for the first time. Extension of this calculation strategy to more complex systems, such as hyperbranched copolymer or dendrimer, should be straightforward. The original application of this method to 3-arms block copolymer melts in our present works led to some novel ordered microphase patterns, such as hexagonal (HEX) honeycomb lattice, core-shell HEX lattice, knitting pattern, etc. The observed core-shell HEX lattice ordered structure is qualitatively in agreement with the experiment of Thomas [Macromolecules 31, 5272 (1998)].

Dilute nitride semiconductors : band structure, scattering and high field transport

The substitution of a small fraction x of nitrogen atoms, for the group V elements in conventional III-V semiconductors such as GaAs and GaSb strongly perturbs the conduction band of the host semiconductor. In this thesis we investigate the effects of nitrogen states on the band dispersion, carrier scattering and mobility of dilute nitride alloys. In the supercell model we solve the single particle Hamiltonian for a very large supercell containing randomly placed nitrogen. This model predicts a gap in the density of states of GaNxAs1−x, where this gap is filled in the Green’s function model. Therefore we develop a self-consistent Green’s function (SCGF) approach, which provides excellent agreement with supercell calculations and reveals a gap in the DOS, in contrast with the results of previous non-self-consistent Green’s function calculations. However, including the distribution of N states destroys this gap, as seen in experiment. We then examine the high field transport of carriers by solving the steadystate Boltzmann transport equation and find that it is necessary to include the full distribution of N levels in order to account for the small, low-field mobility and the absence of a negative differential velocity regime observed experimentally with increasing x. Overall the results account well for a wide range of experimental data. We also investigate the band structure, scattering and mobility of carriers by finding the poles of the SCGF, which gives lower carrier mobility for GaNxAs1−x, compared to those already calculated, in better agreement with experiments. The calculated optical absorption spectra for InyGa1−yNxAs1−x and GaNxSb1−x using the SCGF agree well with the experimental data, confirming the validity of this approach to study the band structure of these materials.

Double-electron above threshold ionization of helium

We present calculations of intense-field multiphoton ionization processes in helium at XUV wavelengths. The calculations are obtained from a full-dimensional integration of the two-electron time-dependent Schrödinger equation. A momentum-space analysis of the ionizing two-electron wavepacket reveals the existence of double-electron above threshold ionization (DATI). In momentum-space two distinct forms of DATI are resolved, namely non-sequential and sequential. In non-sequential DATI correlated electrons resonantly absorb and share energy in integer units of Ïlaser.

High-energy cutoff in the spectrum of strong-field nonsequential double ionization

Electron energy distributions of singly and doubly ionized helium in an intense 390 nm laser field have been measured at two intensities (0.8 PW/cm(2) and 1.1 PW/cm(2), where PW equivalent to 10(15) W/cm(2)). Numerical solutions of the full-dimensional time-dependent helium Schrodinger equation show excellent agreement with the experimental measurements. The high-energy portion of the two-electron energy distributions reveals an unexpected 5U(p) cutoff for the double ionization (DI) process and leads to a proposed model for DI below the quasiclassical threshold.

Dynamics of Gompertzian tumour growth under environmental fluctuations

We investigate the effect of correlated additive and multiplicative Gaussian white noise oil the Gompertzian growth of tumours. Our results are obtained by Solving numerically the time-dependent Fokker-Planck equation (FPE) associated with the stochastic dynamics. In Our numerical approach we have adopted B-spline functions as a truncated basis to expand the approximated eigenfunctions. The eigenfunctions and eigenvalues obtained using this method are used to derive approximate solutions of the dynamics under Study. We perform simulations to analyze various aspects, of the probability distribution. of the tumour cell populations in the transient- and steady-state regimes. More precisely, we are concerned mainly with the behaviour of the relaxation time (tau) to the steady-state distribution as a function of (i) of the correlation strength (lambda) between the additive noise and the multiplicative noise and (ii) as a function of the multiplicative noise intensity (D) and additive noise intensity (alpha). It is observed that both the correlation strength and the intensities of additive and multiplicative noise, affect the relaxation time.

Studies on the dynamics and rheology of dilute suspensions of Brownian particles in simple shear flow under the action of an external force

We present a novel approach to computing the orientation moments and rheological properties of a dilute suspension of spheroids in a simple shear flow at arbitrary Peclct number based on a generalised Langevin equation method. This method differs from the diffusion equation method which is commonly used to model similar systems in that the actual equations of motion for the orientations of the individual particles are used in the computations, instead of a solution of the diffusion equation of the system. It also differs from the method of 'Brownian dynamics simulations' in that the equations used for the simulations are deterministic differential equations even in the presence of noise, and not stochastic differential equations as in Brownian dynamics simulations. One advantage of the present approach over the Fokker-Planck equation formalism is that it employs a common strategy that can be applied across a wide range of shear and diffusion parameters. Also, since deterministic differential equations are easier to simulate than stochastic differential equations, the Langevin equation method presented in this work is more efficient and less computationally intensive than Brownian dynamics simulations.We derive the Langevin equations governing the orientations of the particles in the suspension and evolve a procedure for obtaining the equation of motion for any orientation moment. A computational technique is described for simulating the orientation moments dynamically from a set of time-averaged Langevin equations, which can be used to obtain the moments when the governing equations are harder to solve analytically. The results obtained using this method are in good agreement with those available in the literature.The above computational method is also used to investigate the effect of rotational Brownian motion on the rheology of the suspension under the action of an external force field. The force field is assumed to be either constant or periodic. In the case of con- I stant external fields earlier results in the literature are reproduced, while for the case of periodic forcing certain parametric regimes corresponding to weak Brownian diffusion are identified where the rheological parameters evolve chaotically and settle onto a low dimensional attractor. The response of the system to variations in the magnitude and orientation of the force field and strength of diffusion is also analyzed through numerical experiments. It is also demonstrated that the aperiodic behaviour exhibited by the system could not have been picked up by the diffusion equation approach as presently used in the literature.The main contributions of this work include the preparation of the basic framework for applying the Langevin method to standard flow problems, quantification of rotary Brownian effects by using the new method, the paired-moment scheme for computing the moments and its use in solving an otherwise intractable problem especially in the limit of small Brownian motion where the problem becomes singular, and a demonstration of how systems governed by a Fokker-Planck equation can be explored for possible chaotic behaviour.

Relativistic ab initio calculations for ion-atom collisions

Within the independent particle model we solve the time-dependent single-particle equation using ab initio SCF-DIRAC-FOCK-SLATER wavefunctions as a basis. To reinstate the many-particle aspect of the collision system we use the inclusive probability formalism to answer experimental questions. As an example we show an application to the case of S{^15+} on Ar where experimental data on the K-K charge transfer are available for a wide range of impact energies from 4.7 to 90 MeV. Our molecular adiabatic calculations and the evaluation using the inclusive probability formalism show good results in the low energy range from 4.7 to 16 MeV impact energy.

Mean-field model for Josephson oscillation in a Bose-Einstein condensate on an one-dimensional optical trap

Using the axially-symmetric time-dependent Gross-Pitaevskii equation we study the phase coherence in a repulsive Bose-Einstein condensate (BEC) trapped by a harmonic and an one-dimensional optical lattice potential to describe the experiment by Cataliotti et al. on atomic Josephson oscillation [Science 293, 843 (2001)]. The phase coherence is maintained after the BEC is set into oscillation by a small displacement of the magnetic trap along the optical lattice. The phase coherence in the presence of oscillating neutral current across an array of Josephson junctions manifests in an interference pattern formed upon free expansion of the BEC. The numerical response of the system to a large displacement of the magnetic trap is a classical transition from a coherent superfluid to an insulator regime and a subsequent destruction of the interference pattern in agreement With the more recent experiment by Cataliotti et al. [New J. Phys. 5, 71 (2003)].

Josephson oscillation and induced collapse in an attractive Bose-Einstein condensate

Using the axially symmetric time-dependent Gross-Pitaevskii equation we study the Josephson oscillation of an attractive Bose-Einstein condensate (BEC) in a one-dimensional periodic optical-lattice potential. We find that the Josephson frequency is virtually independent of the number of atoms in the BEC and of the interatomic interaction (attractive or repulsive). We study the dependence of the Josephson frequency on the laser wave length and the strength of the optical-lattice potential. For a fixed laser wave length (795 nm), the Josephson frequency decreases with increasing strength as found in the experiment of Cataliotti [Science 293, 843 (2001)]. For a fixed strength, the Josephson frequency remains essentially unchanged for a reasonable variation of laser wave length around 800 nm. However, the Josephson oscillation is disrupted with the increase of laser wave length beyond 2000 nm leading to a collapse of a sufficiently attractive BEC. These features of a Josephson oscillation can be tested experimentally with present setups.