Bright γ-ray source with large orbital angular momentum from the laser near-critical-plasma interaction

We propose a new laser-plasma-based method to generate bright γ-rays carrying large orbital angular momentum by interacting a circularly polarized Laguerre–Gaussian laser pulse with a near-critical hydrogen plasma confined in an over-dense solid tube. In the first stage of the interaction, it is found via fully relativistic three-dimensional particle-in-cell simulations that high-energy helical electron beams with large orbital angular momentum are generated. In the second stage, this electron beam interacts with the laser pulse reflected from the plasma disc behind the solid tube, and helical γ beams are generated with the same topological structure as the electron beams. The results show that the electrons receive angular momentum from the drive laser, which can be further transferred to the γ photons during the interaction. The γ beam orbital angular momentum is strongly dependent on the laser topological charge l and laser intensity a0, which scales as ${L}_{\gamma }\propto {a}_{0}^{4}$. A short (duration of 5 fs) isolated helical γ beam with an angular momentum of −3.3 × 10⁻¹⁴ kg m² s⁻¹ is generated using the Laguerre–Gaussian laser pulse with l = 2. The peak brightness of the helical γ beam reaches 1.22 × 10²⁴ photons s⁻¹ mm⁻² mrad⁻² per 0.1% BW (at 10 MeV), and the laser-to-γ-ray angular momentum conversion rate is approximately 2.1%.     

Angular distribution of radiation by relativistic electrons in a thin crystal

The results of theoretical investigation of angular distributions of radiation from a relativistic electron passing through a thin crystal at a small angle to the crystal axis are presented. The electron trajectories in crystal were simulated using the binary collision model which takes into account both coherent and incoherent effects at scattering. The angular distribution of radiation was calculated as a sum of radiation from each electron. It is shown that there are non-trivial angular distributions of the emitted photons, which is connected to the superposition of the coherent scattering of electrons by atomic rows (“doughnut scattering” effect) and the suppression of the radiation due to the multiple scattering effect (similar to the Landau–Pomeranchuk–Migdal effect in an amorphous matter). The orientation dependence of angular distribution of radiation is also analyzed.     

Fermi shuttle acceleration in atomic collisions: the case of ion induced electron emission

The “Fermi shuttle” acceleration of electrons in ion–atom collisions, i.e. multiple collision sequences of electrons bouncing off the projectile and target nuclei, can lead to the emission of very energetic electrons (i.e. with velocities higher than the binary encounter electrons). We performed measurements of the evolution of the Fermi shuttle electron yield with the induced perturbation and the target atomic number. The yield increases with the perturbation parameter (ratio of projectile charge and projectile velocity q/v_P), which was varied from q/v_P ≈ 0.2 (weak perturbation) to q/v_P ≈ 2 (strong perturbation). We also introduce a more realistic scaling parameter, which accounts for the re-bouncing of the electrons on target and projectile, and show that the yields increase as a function of this parameter. For a given projectile, the Fermi shuttle electron yield increases with the target atomic number. Furthermore, we show that the velocity distribution of the high-energy electrons is exponentially decreasing N(v)  exp(−nv) and exhibits the same evolution of the slope n with projectile velocity as in the case of Fermi accelerated deuterons.     

Landau damping of twisted waves in Cairns distribution with anisotropic temperature

The consideration of orbital angular momentum of an electric field(twisted mode) is applied to the kinetic theory of plasma. The linearized Vlasov–Poisson equation is solved for the anisotropic thermal distributed bi-Maxwellian and Cairns distributions of electrons to obtain the damping rates of twisted waves. The dispersion relation and Landau damping of Langmuir twisted modes are obtained. The presence of twisted modes opens up two more possibilities in Landau damping and dispersion relations. This may generate a mixture with ion sound waves. It seems to play the role of a control parameter of Landau damping.     

An algorithm for Monte Carlo simulation of bremsstrahlung emission by electrons

An algorithm for Monte Carlo simulation of bremsstrahlung emission by electrons based on the framework of SuperMC is presented in this paper with efficient and accurate methods to sample the angular distribution and energy of bremsstrahlung photons.The photon energy is sampled according to scaled energy-loss differential cross sections tabulated by Seltzer and Berger.A novel hybrid model for photon angular distribution by low-and high-energy incident electrons is developed.The model uses Tsai's full form of angular distribution function with atomic form factors for high-energy incident electrons.For electrons of ＜500 keV,a simple efficient and accurate analytical distribution function is developed,using adjustable parameters determined from the fitting of numerical values of the shape functions tabulated by Kissel et al.The efficiency of sampling photon energy is 80％.Our angular sampling algorithm for high-energy electron bremsstrahlung based on Tsai distribution function is very efficient (sampling efficiency ～ 70％) in the useful photon energy range.     

Carrier-envelope phase dependence in atomic ionization by short-laser pulses

In atomic ionization by few-cycle laser pulses doubly differential momentum distributions near-threshold exhibit a radial nodal structure that results from peaked partial-wave distributions near a particular angular momentum. We analyze the doubly differential momentum distribution for different carrier-envelope phases. We find that in the cases where the photoelectron reaches a minimum at threshold the angular momentum distribution is very sensitive to the CEP since small variations of the ponderomotive energy can lead to the opening or closing of a new channel.     

A closed-form solution for the two-dimensional Fokker–Planck equation for electron transport in the range of Compton Effect

In this paper we solve the Fokker–Planck (FP) equation, an alternative approach for the Boltzmann transport equation for charged particles in a rectangular domain. To construct the solution we begin applying the P_N approximation in the angular variable and the Laplace Transform in the x-variable, thus obtaining a first order linear differential equation in y-variable, which the solution is straightforward. The angular flux of electrons and the parameters of the medium are used for the calculation of the energy deposited by the secondary electrons generated by Compton Effect. The remaining effects will not be taken into account. The results will be presented under absorbed energy form in several points of interested. We present numerical simulations and comparisons with results obtained by using Geant4 (version 8) program which applies the Monte Carlo’s technique to low energy libraries for a two-dimensional problem assuming the screened Rutherford differential scattering cross-section.     

Charge evolution and energy loss associated with electron transmission through a macroscopic single glass capillary

Charge (time) evolution and the angular dependence of incident electrons in the range 300-1030 eV through a single macroscopic glass capillary was studied. Charge measurements were done at a sample tilt angle of ψ = 2? for observation angles θ = 0? and 0.5? (both ψ and θ were measured with respect to the incident beam direction) at incident energies of 520.7 and 824.5 eV using a parallel-plate spectrometer. After equilibrium of transmission, electrons had lower average centroid (mean) energies than the respective primary beam values. Centroid energies of transmitted electrons at the centroid of the angular distribution (where the observation angle θ is nearly equal to tilt angle of the sample ψ) were found to decrease exponentially with increasing sample tilt angles for all the measured electron energies. This energy loss is attributed to inelastic scattering of electrons with the inner wall of the sample close to the capillary entrance. Furthermore, the centroid energies of the transmitted electron angular distributions at 520.7 eV were found to lose energy for angular positions away from the capillary axis (angular centroid position) for all tilt angles, indicating a higher degree of inelastic scattering at the edges of the angular distributions.     

Investigation of isotope composition of nuclear fragments with angular momentum and Coulomb effects in peripheral ~(84)Kr+ ~(112,124)Sn collisons at 35 A MeV

New theoretical calculations are performed to investigate the Coulomb proximity and angular momentum effects on multifragmentation picture for84Kr+112,124 Sn collisions at an incident beam energy of 35 Me V/nucleon.Charge and isotopic distributions and the mean neutron-to-proton ratios of the fragments are reproduced within the microcanonical Markov chain calculations on the basis of Statistical Multifragmentation Model. It is shown that the Coulomb interactions and angular momentum effects are very important to reproduce isotopic composition of nuclear fragments in peripheral heavy-ion collisions at Fermi energies. Our results imply that it is possible to investigate in laboratories the modification of structure parameters of fragments, such as the symmetry energy coefficient, at subnuclear densities in dense environment of other species.     

X-rays generated by relativistic electrons in a waveguide radiator mounted inside a betatron

We have observed X-ray emission from an X-ray waveguide radiator excited by relativistic electrons in the experiments carried out at Tomsk betatron B-35. A stratified radiator of a new type was mounted on a goniometer head inside the betatron toroid. The radiator consisted of the W-C-W layers placed on Si substrate. The photographs of the angular distributions of the radiation generated in the radiator by 20 and 33 MeV electrons showed the waveguide effect of the three-layer structure on X-rays generated in the radiator. The effect appeared in the angular distribution of the radiation as an additional peak attributed to guided X-rays inside a wide cone of usual Bremsstrahlung.     

Defect creation caused by the decay of cation excitons and hot electron–hole recombination in wide-gap dielectrics

Insufficient radiation resistance of construction materials is the Achilles heel for thermonuclear energetics. In wide-gap dielectrics, Frenkel defects are created not only because of the knock-out (impact) mechanism but also because of the decay of the electronic excitations formed during the irradiation (i.e. due to nonimpact mechanisms). The processes of the defect creation at the irradiation of highly pure LiF single crystals at 6–8 K by 1–30-keV electrons, X-rays, or synchrotron radiation of 12–70 eV have been investigated. The annealing processes of these defects in a temperature range up to 200 K have been studied as well. In LiF, creation has been revealed for the following: (1) F–H pairs caused by the decay of anion excitons or by the recombination of electrons and holes, (2) F′–H–V_K and F–I–V_K defect groups at the decay of cation excitons (62 eV), or (3) 20-keV electron irradiation. The mechanism of defect creation at the recombination of hot holes and hot electrons has been discussed for -SiO₂ crystals with an energy gap between the subbands of a valence band. One of the possible ways to suppress this mechanism (“luminescent defence”) is doping a material by luminescent impurities able to capture a part of the energy of hot carriers before their relaxation and recombination (e.g. in MgO:Cr).     

Angular distributions of Diffracted Channeling Radiation from moderate energy axially channeled electrons in Si and LiF crystals

Numerical calculations of the Diffracted Channeling Radiation angular distributions from axially channeled electrons with relativistic factor γ = 20 in Si and LiF crystals are performed. The calculations are based on a newly developed effective quantum-mechanical calculations algorithm of the wave functions and electron energies at axial channeling in a crystal. The numerical results obtained exhibit some new features of the Diffracted Channeling Radiation from axially channeled electrons compared to that from planar channeled electrons, e.g. increase of the widths of peaks, which might be useful in future experiments on observation of Diffracted Channeling Radiation.     

A Monte Carlo investigation of secondary electron emission from solid targets: Spherical symmetry versus momentum conservation within the classical binary collision model

A Monte Carlo scheme is described where the secondary electron generation has been incorporated. The initial position of a secondary electron due to Fermi sea excitation is assumed to be where the inelastic collision took place, while the polar and azimuth angles of secondary electrons can be calculated in two different ways. The first one assumes a random direction of the secondary electrons, corresponding to the idea that slow secondary electrons should be generated with spherical symmetry. Such an approach violates momentum conservation. The second way of calculating the polar and azimuth angles of the secondary electrons takes into account the momentum conservation rules within the classical binary collision model. The aim of this paper is to compare the results of these two different approaches for the determination of the energy distribution of the secondary electrons emitted by solid targets.     

Young-type interferences using single-electron sources and an atomic-size two-center interferometer: Dependence with interferometer parameters

We present the realization of a Young-type double-slit experiment, in which single electrons, scattering on two protons, produce interference pattern. The electrons are produced by a Auger effect, following double capture process in low He²⁺ + H₂ collisions. Well-defined oscillations are visible in the angular distribution of the electrons emitted towards the receding protons. The presence of these oscillations is a clear demonstration that an electron interferes with itself. We also discuss the dependence of the interference pattern with interference parameters, such as the electron wavelength as well as the distance between the protons when the electron is ejected.     

基于伴随粒子的快中子成像系统角分辨研究

不同于传统的快中子成像系统,采用伴随粒子成像技术无需机械准直即可消除大部分γ射线和散射中子的干扰,实现对厚重物体的高对比度成像。角分辨是影响系统成像质量的一项重要参数。通过理论分析,研究了入射离子的初始动量、靶点尺寸和探测器空间分辨等多个因素对系统角分辨的影响。利用基于GEANT4的模拟程序,计算了不同参数下被标记中子出射角分布的二维图像。分析及模拟结果表明,靶点直径和α探测器空间分辨率是影响角分辨的重要因素。为满足系统角分辨小于1°的设计目标,入射离子的初始动量变化范围应较小,靶点直径应小于1 mm,同时α探测器的空间分辨率应小于0.5 mm。     

Dynamic model for control system design and simulation of a low temperature nuclear reactor

A lumped parameter dynamic model for the primary-loop and the U-tube steam generator of a low temperature power reactor is developed based on the fundamental conservation laws of fluid mass, energy and momentum. The dynamic model is formulated by coupling the point kinetics with reactivity feedback and the thermal-hydraulics of the reactor. The developed dynamic model is implemented on a personal computer using MATLAB/SIMULINK. Numerical simulation results for steady-state and transient responses are then presented, which show that the steady-state precision of the newly developed dynamic model is acceptable and the trend of the transient responses is correct. In addition, the “swell and shrink” behavior of the U-tube steam generator is also verified by numerical simulation. This newly established model can be utilized to control system design and simulation for the low temperature power reactor.     

Electron intensities obtained during backscattered-Mössbauer spectroscopy: II. Theoretical and experimental comparisons of emergent angular distributions

When gamma-rays from a ⁵⁷Co source impinge on an absorber containing ⁵⁷Fe, they can be resonantly absorbed, resulting in the generation of conversion electrons and subsequent Auger electrons, or they can be nonresonantly scattered via photoelectric and Compton processes. The ratio of resonant to nonresonant electrons escaping from the absorber represents the absolute peak height/area obtained from a backscattered-conversion electron Mössbauer spectrum and provides important information helpful in the depth-profiling of multilayer specimens.A simple theoretical model which accounts for resonant and nonresonant electrons emerging from the surface at angles from zero to 2π radians with energies above zero eV has been developed previously. The signal-to-background (S/B) ratios from 5 nm iron films and 2.5 × 10⁶ nm iron foils have been calculated using this model, and good agreement between calculated and experimental results has been obtained. The intensity of resonant and nonresonant electrons emerging from the absorber in specific angular intervals has also been calculated using this model for a 1.0 μm thick 92.8% enriched ⁵⁷Fe foil mounted on a stainless steel substrate. The simulated angular distributions for backscattered-resonant electrons and backscattered-nonresonant electrons both provide cosine-type curves and are similar to experimental data, resulting in an isotropic distribution of the S/B ratio. The predicted S/B ratio, however, is about twice the S/B ratio measured experimentally. This discrepancy may result from the neglect of: (i) electrons below 50 eV and (ii) multiple scattering events. Despite these shortcomings, the model proposed in this study is capable of comparing the relative magnitude of the resonant and nonresonant signals, thereby calibrating the resonant signal against the nonresonant background. This procedure appears to provide additional information helpful for nondestructive depth-profiling studies.     

Properties of Auger electrons following excitation of polarized atoms by polarized electrons

In non-relativistic approximation, the most general expression for differential cross sections describing the properties of Auger-electron emission induced in the excitation of polarized atoms by polarized electrons is obtained for the first time. The ways of the application of the general expressions suitable for the specific experimental conditions are outlined by deriving the expressions for the asymmetry parameters and the magnetic dichroism of the angular distribution of the Auger electrons as well as of the angular correlations between the scattered and Auger electrons.     

Some Problems in High-Energy Particle Detection and Analysis

1. The kinematic analysis of high-energy events requires, in general, particle momentum measurements with an absolute error of ±50-100 MeV/c, independent of momentum. Corresponding errors are 0.05% at 100 GeV/c, less at higher momenta. To minimize the cost of magnetic deflection of high-energy particles, improvements in angular precision, hence, in particle location accuracy, are desirable. The present analysis indicates that improving location accuracy from ±1/3 mm to ±0.1 mm is both desirable and feasible. The usefulness of still higher precision must await data on detailed systems performance. 2. For complex events, it appears likely that vidicon digitization can compete favorably in cost, convenience, and flexibility with large wire-array systems. 3. Neutral particle detectors that furnish accurate data on the location and direction of neutral particles (gamma rays, neutral hadrons) are likely to be important.