Electron capture processes in ion-atom collisions

The development of the theory of electron capture processes in ion-atom collisions is reviewed in the present work. The formal theory of scattering is used to obtain the Born and the distorted wave Born series for the rearrangement scattering matrix. On the basis of these series, the applications of the first and second Born approximation, the OBK approximation, the DWBA and the Padé approximants to electron capture processes are discussed. The impulse approximation, the Faddeev method and the second order potential method are also explained. The results of the investigations of the high energy behaviour of the charge transfer cross section based on the Faddeev-Watson multiple scattering expansion are analysed.The methods of molecular and atomic eigenfunction expansions are presented and their relative merits discussed. The semi-classical treatment for determining the differential electron capture probability is reviewed. Results obtained in works utilising expansions in Sturmian, Gaussian and pseudo-state wave functions are also presented. An integral equation approach to the close coupling approximation is described. The field theoretic approach to investigate the electron capture process is discussed in the last section.The results obtained by different theoretical methods are compared with available experimental results.     

基于LabVIEW的塞曼效应实验改进

介绍了一种利用LabVIEw改进塞曼效应实验的方法.在LabVlEW开发环境中采用Activex技术进行实验图像采集,并应用数字图像处理技术来减少多种对实验图像干扰的影响.实现了电子比荷、波数差的计算机辅助测量,提高了测量结果的精确度,改善了实验教学效果.     

Directional blood flow imaging in volumetric optical microangiography achieved by digital frequency modulation

An effective digital frequency modulation approach to achieve directional blood flow imaging within microcirculations in tissue beds in vivo for optical microangiography is presented. The method only requires the system to capture one three-dimensional data set within which the interferograms are modulated by a constant frequency modulation that gives one directional flow information. The result is that the imaging speed is doubled and the computational load is halved. The method is experimentally validated by a flow phantom and is tested for imaging of cerebral vascular blood perfusion in a live mouse with the cranium left intact.     

真实场景的三维视频采集及显示

提出了一种真实场景三维视频采集及彩色显示的方法.设计了一种采用条纹投影的实时三维成像系统及采用液晶空间光调制器的实时全息彩色三维显示系统.在三维成像系统中采用π相移正弦条纹与编码图案结合实现绝对相位测量,从而可以测量孤立物体.同时对采用数字微镜的投影仪进行改造,实现高速投影,并与高速摄像机配合实现三维视频采集.首先利用实时三维成像系统同时获取三维场景的彩色强度像和距离像;然后根据这些三维成像数据, 设计和制作计算菲涅耳全息图;最后在实时全息彩色三维显示系统中再现.三维信息的采集和显示速度达到了60帧每秒.     

Accurate treatment of size distribution effects in polydisperse spray diffusion flames: multi-fluid modelling,computations and experiments

An Eulerian multi-fluid model is validated by comparison with experimental measurements in the test case of laminar spray counterflow diffusion flames. Special attention is devoted, both from the modelling and experimental point of view, to the treatment of the droplet distribution tail, characterized by the rare occurrence of relatively large droplets carrying a non-negligible amount of mass. The Eulerian multi-fluid approach is shown to capture the dynamics, evaporation and heating of the droplets with a limited number of sections and, thus, at a modest cost. This simplification will be essential for the use of multi-fluid methods in multi-dimensional problems.     

On the classical-trajectory eikonal approximation for electron capture into multicharged ions

The eikonal approach to electron capture developed in earlier publications is analyzed with respect to its physical content using the Born series as a reference expansion. In the language of perturbation theory, the eikonal approach describes a close collision between electron and projectile followed by multiple distant collisions between electron and target nucleus. The distant encounters have a large weight in phase space and therefore dominate capture at intermediate energies. The post-prior discrepancy is shown to be fairly small for nonresonant 1s-1s capture.     

Effects of kinematical constraints in nuclear muon capture

Muon capture is discussed in the framework of linear response theory. This approach allows a general discussion of the role played by kinematical constraints in determining which features of nuclear dynamics are actually probed in the capture event. It is argued that the muon momentum spread can increase the capture rate at large energy transfer.     

On the valence model for radiative capture

We give several parametrizations for the elastic scattering and radiative capture cross sections for low neutron bombarding energy and discuss the relationship between the corresponding resonance parameters. We then perform an extensive investigation of the valence radiative capture model of Lane and Lynn. This model is formulated here in the frame of the shell-model approach. We exhibit the similarities and differences between our results and those derived from the R-matrix approach by Lane and Lynn on the one hand and from the optical-model approach by Lane and Mughabghab on the other hand. Particular attention is paid to the choice of the average potential well in the shell-model approach, in relation to the proper way to identify theoretical quantities and phenomenological parameters. We show that practically equivalent results can be obtained from a complex average potential well and from a suitably chosen real potential well, respectively. The following topics are investigated formally and numerically: dependence of the various theoretical expressions on the choice of the (real or complex) average potential well; relative importance of external and internal capture; dependence of photon widths and background cross section on mass number (for thermal energy and for E = 100 keV); dependence of the resonance parameters and background cross sections on energy, for A = 60; comparison between experimental data and theoretical values for radiative capture on ⁵⁶Fe and ⁶⁰Ni. We discuss the conditions of validity of the valence capture model. In particular, we investigate the role of the giant dipole resonance and of the closed channels. We argue that the success of the valence capture model is intimately related to the importance of external capture. The contribution of the low-lying excited target states is investigated formally and numerically; it increases with mass number and tends to diminish the correlation between neutron and photon widths, which is implied by the valence capture model.     

Application of the theory of open quantum systems to nuclear physics problems

Quantum diffusion equations with transport coefficients explicitly depending on time are derived from the generalized non-Markovian Langevin equations. The asymptotic behavior of the friction and diffusion coefficients is investigated in the case of the FC and RWA couplings between the collective and internal subsystems. An asymptotic expression is obtained for the propagator of the density matrix of the open quantum system with the general quadratic Hamiltonian, linearly coupled (in coordinate and momentum) to internal degrees of freedom. The effect of different sets of transport coefficients on the decoherence and decay rate of the metastable state is investigated using the master equation for the reduced density matrix of open quantum systems. The developed approach is used to study the capture of the projectile nucleus by the target nucleus at energies near the Coulomb barrier. Capture cross sections in asymmetric reactions are well described with allowance for the calculated capture probabilities. Particular cases where dissipation favors penetration through the potential barrier are found. The generalized Kramers formula for the quasi-stationary decay rate of the quantum metastable systems is analytically derived.     

Fusion of 60Ni + 100Mo near and below the Coulomb barrier

Various sub-barrier capture reactions with ^16,18O and ^40,48Ca are treated within the quantum diffusion approach. The role of neutron transfer in these capture reactions is discussed. The quasielastic and capture barrier distributions are analyzed and compared with the recent experimental data.     

Radiative capture of low-energy neutrons in the shell-model approach to nuclear reactions

The shell-model approach to photonuclear reactions is applied to the radiative capture of low-energy neutrons. The direct, compound and channel resonant contributions to the collision matrix appear in a natural way. We show that the experimental data in the vicinity of the 3s giant resonance can be quantitatively interpreted in terms of the channel resonant capture. The relationship with the R-matrix approach of Lane and Lynn is discussed.     

Charge transfer in H~ H and H~ He~ collisions at intermediate and high energies using an asymptotic two-state atomic expansion method

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Resonance and background interference in 54Fe neutron capture

Asymmetric s-wave resonances observed in the neutron capture cross section of ⁵⁴Fe can be explained by interference between resonance valence components. An upper limit is obtained for the background capture cross section which is much smaller than that expected from the large correlation observed between s-wave reduced neutron widths and radiative widths.     

钍俘获反应率离线伽马测量方法

为了测定聚变-裂变反应堆模型钍包层中的钍俘获率以及钍-铀转化率, 探索了一种新的钍活化离线γ测量法. 利用测量²³²Th俘获反应产物²³³Th衰变链中²³³Pa衰变放出的311.98 keV 特征γ射线, 来反推计算并最终确定²³²Th(n,γ)²³³Th的反应率, 测试实验中不确定度约6% (²³³Th/²³²Th量级为10^-17情况下). 详细介绍了此方法的背景和原理方法, 并进行简单的校验实验, 证明其能够较好地得到模拟装置中的俘获率. 与瞬发γ测量法以及质谱分析法进行对比, 本方法更适合用于聚变-裂变反应堆模型钍包层中的钍俘获率以及钍-铀转化率测量, 并有望进一步测量其他相关参数.     

Coulomb capture of muons and atomic radius

An earlier theoretical treatment of the Coulomb capture of muons has been extended to take the radius of the (electronic) atom into account. Smaller radii are shown to favor the capture, in agreement with earlier experimental findings.     

Integrated endoscopy system for simultaneous imaging and spectroscopy for early lung cancer detection

An integrated endoscopy system for simultaneous imaging and spectroscopy was developed to facilitate more accurate and convenient detection of early lung cancers. A specially designed three-CCD camera in combination with a dedicated light source permits capture of both white-light color images and tissue autofluorescence images without the need to switch between two different cameras. A mirror with an optical fiber at its center, placed at an interim imaging plane inside the camera unit, facilitates simultaneous imaging and spectroscopy measurements in either white-light reflectance mode or fluorescence mode. The system has been successfully tested in a clinic, demonstrating a practical approach to improve both diagnostic sensitivity and specificity at the same time.     

Lattice-Boltzmann simulations of dynamic interfacial tension due to soluble amphiphilic surfactant

An amphiphilic Lattice-Boltzmann approach is adopted to model dynamic interfacial tension due to non-ionic surfactant. In the current system, the surfactant adsorption kinetics is diffusion dominated and the interface separates two immiscible fluids. A rotational relaxation time and a diffusive/viscous relaxation time are associated with the surfactant. The model results are compared with experimental data for the dynamic interfacial tension of a pendant oil droplet in water, with oil soluble surfactant. We demonstrate how to adapt and calibrate the model to capture the adsorption timescale of the surfactant and the magnitude of interfacial tension reduction due to surfactant. A scheme to overcome numerical instabilities due to the relatively low surfactant concentration, is devised. We are able to qualitatively match the Frumkin equation of state for the interfacial tension.