Self-consistent reaction field theory of solvent effects

A quantum-mechanical reaction field theory of solvent effects is proposed. It contains as a limiting case Onsager's model. It leads to an effective, non-linear, hamiltonian for the molecule in solution and hence to a tool for studying changes in charge distributions and molecular properties. Numerical examples are given.     

Space charge limiting current of an electron beam transported in a coaxial drift chamber

An expression for the space charge limiting current of an electron beam in a vacuum coaxial drift chamber is obtained in the strong field approximation. The expression is an analogue of the Bogdankevich-Rukhadze interpolation formula for a cylindrical drift chamber. The space charge limiting currents in the coaxial and cylindrical chambers are compared. The space charge limiting current in the vacuum coaxial chamber is numerically calculated and compared to analytical predictions.     

Coulomb potentials between spherical heavy ions

The Coulomb interaction between spherical nuclei having arbitrary radial nuclear charge distributions is calculated. All these realistic Coulomb potentials are given in terms of analytical expressions and are available for immediate application. So in no case a numerical computation of the Coulomb integral is required. The parameters of the charge distributions are taken from electron scattering analysis. The Coulomb self-energies of the charge distributions used are also calculated analytically in a closed form. For a number of nucleus-nucleus pairs, the Coulomb potentials derived from realistic charge distributions are compared with those normally used in various nucleus-nucleus optical model calculations. In this connection a detailed discussion of the problem how to choose consistently Coulomb parameters for different approximations is given.     

RESEARCH OF RELAXATION PROCESS INDUCED BY 80.6MeV 16O ON 27Al

The decreasing coefficients of the angular distributions, the charge distributions and the first and second moments of the charge distributions with variation of TKEL are deduced from the results of the measurement of projectile-like fragments produced by 80.6MeV ¹⁶O on ²⁷Al. The relaxation process of this reaction is analysed, the effects of the potential energy surface on the first and recond moments of charge distributions are discussed.     

Charge distributions in transverse coordinate space and in impact parameter space

We study the charge distributions of the valence quarks inside nucleon in the transverse coordinate space, which is conjugate to the transverse momentum space. We compare the results with the charge distributions in the impact parameter space.     

奇特原子核6,8He的电荷形状因子研究

应用相对论Eikonal近似计算了用不同模型给出的^6,8He的电荷半径和电荷分布的形状因子, 并将结果与⁶He和⁴He的实验结果进行了比较. 结果显示不同模型给出的电荷半径和电荷形状因子差别很大, 表明不同模型给出的晕中子与α核芯的关联有很大的差异. 计算和讨论结果为在下一代电子-原子核对撞机上可能进行的实验提供了理论参考, 同时, 也为现有讨论奇特原子核的理论模型提供了检验.     

Experimental tests of limiting fragmentation at the ISR

We present direct tests of the hypothesis of limiting fragmentation from an experiment at the CERN Intersecting Storage Rings at centre-of-mass energies between 31 and 53 GeV. Single-particle inclusive distributions and partial multiplicity distributions are observed for the case in which the energy of one beam is fixed while varying the energy of the other beam. Within cones around the beam, limiting fragmentation is shown to be valid.     

Theoretical study of the central depression of nuclear charge density distribution by electron scattering

The charge form factors of elastic electron scattering for isotones with N=20 and N=28 are calculated using the phase-shift analysis method, with corresponding charge density distributions from relativistic mean-field theory. The results show that there are sharp variations at the inner parts of charge distributions with the proton number decreasing. The corresponding charge form factors are divided into two groups because of the unique properties of the s-states wave functions, though the proton numbers change uniformly in two isotonic chains. Meanwhile, the shift regularities of the minima are also discussed, and we give a clear relation between the minima of the charge form factors and the corresponding charge radii. This relation is caused by the diffraction effect of the electron. Under this conclusion, we calculate the charge density distributions and the charge form factors of the A=44 nuclei chain. The results are also useful for studying the central depression in light exotic nuclei.     

Effect of external magnetic field on critical current for the onset of virtual cathode oscillations in relativistic electron beams

In this Letter we research the space charge limiting current value at which the oscillating virtual cathode is formed in the relativistic electron beam as a function of the external magnetic field guiding the beam electrons. It is shown that the space charge limiting (critical) current decreases with growth of the external magnetic field, and that there is an optimal induction value of the magnetic field at which the critical current for the onset of virtual cathode oscillations in the electron beam is minimum. For the strong external magnetic field the space charge limiting current corresponds to the analytical relation derived under the assumption that the motion of the electron beam is one-dimensional [D.J. Sullivan, J.E. Walsh, E. Coutsias, in: V.L. Granatstein, I. Alexeff (Eds.), Virtual Cathode Oscillator (Vircator) Theory, in: High Power Microwave Sources, vol. 13, Artech House Microwave Library, 1987, Chapter 13]. Such behavior is explained by the characteristic features of the dynamics of electron space charge in the longitudinal and radial directions in the drift space at the different external magnetic fields.     

Study on Charge Form Factors of the Exotic Nuclei 6,8He

Charge radius and charge form factors of different charge density distributions for ^6,8He are calculated with the relativistic Eikonal approximation. Detailed comparisons and discussions are presented. It is found that the charge form factors curves of ^6,8He are much lower than the experimental ones of ⁴He. This is, in principle, consistent with the experimental fact. Whereas detailed comparison among the charge form factors which correspond to different charge distributions show significant deviations. This indicates that the effects of the correlations between the halo neutrons and the α-core in ^6,8He with different charge density distributions are quite different. This result would provide a useful reference for the possible experiments on the next-generation electron-nucleus collider and for the tests of different theoretical models for the exotic nuclei ^6,8He.     

C-V Technique on Schottky contacts — Limitation of implanted profiles

Some limiting effects for the application of theC-V technique on Schottky contacts for measuring ion-implanted impurity distributions in semiconductors are studied. The basic demand for this technique lies in the possibility to measure the profile with sufficient accuracy within a wide range of depths. The depth of the space charge region for zero bias determines the minimum measurable distance. In greater depths the avalanche breakdown of the metal-semiconductor junction is a limiting factor. The region of applicable implanted doses and energies, in which it is possible to measure the profile within the vicinity of the maximum concentration, is determined by a numerical computation. At the same time the errors resulting from the application of the assumption of a Debye lenght equal to zero (depletion approximation) are examined. The regions in which it is possible to measure the impurity profile in the vicinity of the maximum with an error less than 5% are shown by the dose vs. implanted energy dependence. The computations are performed for As, B, P ions implanted into Si, and for Si, Te, Se ions implanted into GaAs with energies ranging from 0 to 200 keV.     

Quantitative analysis of proton imaging measurements of laser-induced plasmas

A method for obtaining quantitative information about electric field and charge distributions from proton imaging measurements of laser-induced plasmas is presented. A parameterised charge distribution is used as target plasma. The deflection of a proton beam by the electric field of such a plasma is simulated numerically as well as the resulting proton density, which will be obtained on a screen behind the plasma according to the proton imaging technique. The parameters of the specific charge distributions are delivered by a combination of linear regression and nonlinear fitting of the calculated proton density distribution to the measured optical density of a radiochromic film screen changed by proton exposure. It is shown that superpositions of spherical Gaussian charge distributions as target plasma are sufficient to simulate various structures in proton imaging measurements, which makes this method very flexible.     

On the sensitivity of β-transitions to the shape of the nuclear charge distribution

Fermi and other β-decay functions have been calculated using refined nuclear charge distributions like Gaussian and Fermi-type distributions. The sensitivity of the functions to the shape of the distribution is discussed in detail. The necessary changes in the beta-decay formulae are also explained. While these modifications are simple and straightforward, the usefulness of the β-decay formulae turns out to be questionable because of convergence difficulties introduced by the refined charge distributions.     

Limitations on Currents in Cylindrical Drift Tubes of Millimeter-Wave Vacuum Electronic Devices

The conditions providing the equilibrium stationary state of high-density solid and annular electron beams, transported in cylindrical drift tubes and focused by a homogeneous magnetic field, have been investigated. An efficient numerical model is proposed to determine the distributions of the electrostatic potential, charge density, and electron velocities in the stationary state of these beams, with allowance for the space-charge forces. The results of the numerical calculations for the limiting currents, determined by the longitudinal deceleration (caused by space-charge forces) and conditions for equilibrium transverse magnetic focusing for transported beams of different configurations on the length of a klystrontype vacuum electronic device, are presented. The limitations on the device working length, related to the excitation of diocotron instability, are estimated. The calculation results are compared with the data obtained using the quasi-three-dimensional program “Arsenal-MSU” and analytical formulas suggested by other researchers.

     

Processes limiting the rate of response in photorefractive composites

The processes limiting the rate of response of highly diffractive, reorientationally enhanced photorefractive polymer composite materials are identified from a series of degenerate four wave mixing and Mach–Zehnder interferometric measurements. In the regime of low intensity writing beams charge generation limits the rate of holographic grating formation, but at higher intensities charge transport or reorientation of dye molecules can restrict the rate of grating formation more strongly. A grating risetime of 540 ms is observed in a composite of high dye content with high reorientational mobility of the dye molecules. In this case it is proposed that the charge carrier mobility of the doped poly(N-vinylcarbazole):2,4,7-trinitro-9-fluorenone (PVK:TNF) matrix is the principal limiting factor in grating response rates.     

An analytical formula for fluctuations in nuclear charge density

The experimental charge densities of atomic nuclei show fluctuations in their distributions.This paper investigates the limits of accuracy of two-parameter Fermi and three-parameter Fermi distributions in describing the charge density.An improved analytical function for density distribution is proposed,which allows for density fluctuation.The experimental charge densities of~(40)Ca,~(60)Ni,~(100)Mo,~(152)Sm and~(208)Pb,representing the various shapes of density fluctuation,are used to assess the accuracy of the proposed formula.The proposed function reproduces the experimental charge densities with significant improvement in accuracy over other commonly used formulae.A compilation of charge density distribution parameters of 73 nuclei is presented based on the proposed formula.     

Distributions of ions in a cluster plasma created by a laser pulse

Energy and charge distributions of ions are calculated for a cluster beam irradiated by a high-power ultrashort laser pulse. It is shown that the self-consistent field of a cluster ionized by the laser beam strongly affects the characteristics of the ion distributions obtained after the cluster explodes. The mean concentration of atoms bound into clusters in a beam, the cluster size distribution, and the focal-spot diameter are found to have a weak effect on both energy and charge distributions of the ions, whereas the energy spectrum of the produced ions is determined by the mean cluster size.