Photofission accompanied by pion emission

For the nuclear-fission process induced by photons of energy in the range 150 < E_γ < 600 MeV and accompanied by pion emission, the total cross section; the angular and differential distributions of pions; the excitation-energy, mass, and charge distributions of compound nuclei; and the mass distribution of the fission fragments are predicted on the basis of the cascade-evaporation-fission model. These features are compared for the cases of nuclear fission induced by photons and protons of initial energy in the same range.     

Calculation of atomic spontaneous emission rate in 1D finite photonic crystal with defects

We derive an expression for the spontaneous-emission rate in finite one-dimensional photonic crystals with arbitrary defects, using the effective resonator model to describe electromagnetic field distributions in the structure. We obtain explicit formulae for contributions from different modes, i.e. radiation, substrate and guided modes. Formal calculations are illustrated by a few numerical examples, which demonstrate that application of the effective resonator model simplifies the interpretation of results.     

Induced nuclear fission accompanied by pion emission

Basic features of the nuclear-fission process induced by protons of incident energy in the range 150<E _p<600 MeV and accompanied by pion emission are predicted on the basis of the cascade-evaporation-fission model. Specific calculations are performed for the total cross section; and the angular and double-differential distributions of pions; excitation-energy,mass number, and charge-number distributions of compound nuclei; and the mass-energy distributions of fission fragments. Various lines of possible experimental investigations into this fission channel are discussed, including searches for the pionic channel of nuclear decay induced by protons of energy close to the meson-production threshold, advancements to the energy region E _p<100 MeV in order to study of new mechanisms of pion production in nuclear fission, and an extension of investigations to the case of nuclear fission accompanied by kaon emission.     

Azimuthal distributions of final-state particles and fragments and transverse structure of emission source in high-energy nucleus-nucleus collisions

The azimuthal distributions of final-state particles and fragments produced in high-energy nucleus--nucleus collisions are described by a modified multisource ideal gas model which contains the expansions and movements of the emission sources. The transverse structures of the sources are given in the transverse plane by momentum components p_x and p_y,and described by parameters in the model. The results of the azimuthal distributions, calculated by the Monte Carlo method, are in good agreement with the experimental data in nucleus--nucleus collisions at high energies.     

The surface potential barrier in secondary emission from semiconductors

A model for the calculation of secondary electron energy distributions due to bombardment of an amorphous surface by electrons is presented. It takes into account the possibilities of reflexion and tunneling at the surface potential barrier. The general shape of the energy distributions obtained are compared with some typical experimental data. The advantages of the expression obtained for the secondary electron energy distribution are its simplicity and the fact that it can fit the experimental data of a great number of different materials, thus rendering it useful in calculations relating to electron multipliers or other secondary emission devices.     

Systematics of complex fragment emission in niobium-induced reactions

Complex fragments of 3<Z⪅35 have been detected in the reverse-kinematics reactions of ⁹³Nb plus ⁹Be, ¹²C and ²⁷Al at bombarding energies of E/A=11.4, 14.7 and 18.0 MeV. Velocity spectra and angular distributions show the presence of projectile and target-like components along with a component isotropic (in the reaction plane). This latter component aappears as a Coulomb ring in the invariant cross section plots indicating the presence of a binary decay which is confirmed by the coincidence data. Statistical model calculations indicate that for the Nb+Be and C reactions, the isotropic component is associated with the binary decay of compound nuclei formed in complete fusion reactions. The charge distributions for these two systems are consistent with the conditional barriers predicted with the rotating finite-range model. For the Nb+Al reactions, there is an additional isotropic component besides compound nucleus decay, which may arise from fast fission.     

Experimental determination of distributions of soot particle diameter and number density by emission and scattering techniques

A diagnostics method was presented that uses emission and scattering techniques to simultaneously determine the distributions of soot particle diameter and number density in hydrocarbon flames. Two manta G-504 C cameras were utilized for the scattering measurement, with consideration of the attenuation effect in the flames according to corresponding absorption coefficients. Distributions of soot particle diameter and number density were simultaneously determined using the measured scattering coefficients and absorption coefficients under multiple wavelengths already measured with a SOC701 V hyper-spectral imaging device, according to the Mie scattering theory. A flame was produced using an axisymmetric laminar diffusion flame burner with 194 mL/min ethylene and 284 L/min air, and distributions of particle diameter and number density for the flame were presented. Consequently, the distributions of soot volume fraction were calculated using these two parameters as well, which were in good agreement with the results calculated according to the Rayleigh approximation,demonstrating that the proposed diagnostic method is capable of simultaneous determination of the distributions of soot particle diameter and number density.     

The moon as a cosmic source of linearly polarized radio emission

We analyze parameters of the partially linearly polarized thermal radio emission from the Moon taking the effects of radiative heat transfer and surface roughness into account. The distributions of the Stokes parameters I, Q, and U over the visible lunar disk are considered. The polarization parameters of the integral radio emission as functions of the frequency and the Moon phase are obtained by integrating the Q and U distributions. We consider the possibility of using the Moon as a reference source of partially linearly polarized radiation for space-borne and ground-based projects aimed at studying polarization of the Galactic radio emission and the cosmic microwave background. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 7, pp. 593–606, July 2007.     

Ion-induced secondary electron and negative ion emission from Mo/O

Absolute yields of secondary electrons and negative ions resulting from collisions of Na⁺ with Mo(100) and a polycrystalline molybdenum surface have been measured as a function of the oxygen coverage of the surface for impact energies below 500 eV. The sputtered negative ions have been identified with mass spectroscopy, and O⁻ is found to be the dominant sputtered negative ion for the surfaces at all oxygen coverages and impact energies. Both the electron and O⁻ yields have an impact energy threshold at about 50 eV and exhibit a strong dependence on oxygen coverage. The kinetic energy distributions of the secondary electrons and sputtered O⁻ were determined as functions of the oxygen coverage and impact energy. The distributions for O⁻ are characterized by a narrow low-energy peak (at 1–2 eV) followed by a low-level high-energy tail. The secondary electrons have a narrow (FWHM 1–2 eV) kinetic energy distribution, centered approximately at 1–2 eV. The shapes of the distributions and their most probable energies are essentially invariant with impact energy, oxygen coverage and the nature of the Mo surface. The emission is explained and analyzed in terms of a simple model which involves a collision-induced electronic excitation of the MoO⁻ surface state. The decay of this excited state leads to the production of both secondary electrons and O⁻ with energy distributions and yields comparable to those observed.     

Bound-pion absorption followed by emission of two nucleons

The bound-pion absorption reaction, viz. ¹²C(π^?, NN), is studied using Hartree-Fock (HF) wave functions obtained with the unitary-model-operator approach starting with the hard-core nucleon-nucleon (NN) interaction. The inequality of the energies of the two outgoing nucléons is treated exactly and calculations are done using the “1N model” for π-absorption. Other effects taken into account are: NN scattering in the final state, contributions of all excited states of ¹⁰B and ¹⁰Be with E < 5 MeV, and effects of the strong π-nucleus interaction and the finite nuclear size on the bound-π wave function. Branching ratios and angular distributions of absorption rates are in better agreement with experimental data. The correct order of magnitude of the total absorption rate is reproduced. Whatever the effects of short-range correlations present in the HF wave functions, they are not masked by the NN final-state interaction. The contribution of excited states in ¹⁰B and ¹⁰Be is found to be quite large. Absorption rates obtained with the HF and oscillator wave functions differ significantly both in size and shape.     

Multicharged particle emission in the disintegration of aluminium induced by 800 mev electrons

The disintegration of aluminium nuclei by 800 MeV electrons is studied using plastic detectors. Angular and energy distributions of emitted Li, Be and B isotopes are presented. The compatibility of the results with a two-stage process (cascade and evaporation) is studied with the aid of a Monte Carlo calculation.     

Monte Carlo simulation of ion-induced kinetic electron emission statistics from solids

Abstract

Electron emission statistics (ES) of proton-induced electron emission from Au at keV energy are investigated by applying a Monte Carlo model to describe the transport of electrons. Apparent deviations of ES from Poisson distributions are found in the same manner as experimental ES: larger probabilities for no electron emission and for emission of 2 or more electrons. At low energy, electron cascades produce an important deviation from the Poisson distribution, in addition with deviations due to the backscattering of incident ions.     

Interference model of femtosecond laser pulse conical emission in dispersive medium

A simple interference model is proposed for conical emission frequency-angular spectrum formation during the filamentation of femtosecond laser pulse in a nonlinear dispersive medium. The model allows to obtain analytical expressions for frequency-angular distributions of the supercontinuum spectral components of pulses at different wavelengths in media with arbitrary material dispersion law. The model reproduces the supercontinuum frequency-angular spectrum transformation for the case of laser pulse splitting into several subpulses and for multiple refocusing of the light field in filament. Frequency-angular spectra analytically calculated from the proposed interference model are in good agreement with the results of numerical simulations performed for the filamentation of femtosecond laser pulses in fused silica.     

3.7 A GeV 16O与乳胶作用相对论性α粒子的双源发射

对3.7 A GeV 16O与原子核乳胶作用α射弹碎片的发射进行了研究, 发现α射弹碎片的角分布不能用单一的旁观体-反应体模型来解释，所以假设α射弹碎片来源于两个温度不同的发射源。     

Control of spontaneous emission by complex nanostructures

Spontaneous emission in the presence of complex nanostructures is discussed by use of a calculational scheme that permits us to deal with interfaces of arbitrary shape. Control over the field associated with the emission is shown to be attainable. In particular, decay rates are offered for geometries that lead to focusing and collimation of near- and far-field distributions. Emission from axially symmetric gratings is shown to lead to narrow angular distributions of emission, and focusing at the foci of dielectric ellipsoids is achieved for dimensions comparable with the wavelength. In the latter case the total emission rate for two atoms in an ellipsoidal cavity is shown to be enhanced in a way that deviates from the predictions of the Dicke effect by means of intermediate- and far-field contributions.     

The role of the plasmon mechanism in the secondary electron emission in LiF

Energy distributions of secondary electrons emitted by LiF monocrystals were measured for various energies and angles of incidence of the primary electron beam. The analysis has shown the dominating role of the plasmon mechanism of secondary electron generation. On this base a simple model to calculate the secondary electron yield has been developed. This model made it possible to estimate the contribution of each mechanism of secondary electron generation to the total yield. Good agreement between experimental and calculated data was found. It has been also shown that in LiF the plasmon mechanism can produce up to 90 percent of the total number of escaped secondary electrons.     

Size selected growth of nanodots: effects of growth kinetics and energetics on the formation of stationary size distributions

The size selection of nanodots during the growth is studied by using a reaction kinetic model, where reaction rates depend on the dot size. The characteristic feature of the reaction rates is the energetics, where the free energy of dots has a minimum at the certain dot size. The model equations are solved by using a particle coalescence simulation method. We find phenomenologically three distinct stages of growth. First, during the initial deposition stage, distributions with high density of small dots occur. Second, there is an intermediate and short-lived stationary state, which is controlled by kinetics of growth. Third, a long-lived stationary state is obtained, with nearly Gaussian size distributions, mostly determined by the energetics of the growth but also significantly affected by the kinetics. In the final stage, size selection and narrowing of the distributions occur. It is also shown that in the final stage of growth the Fokker-Planck type continuum model describes well the evolution of the distributions and the size selection.