Angular Distribution Of Emission Research Articles

Investigation on the intensity contrast of Kα line emission from laser–matter interactions

The intensity contrast and its angular distribution of Kα line emission originated from the difference of angular distributions of Kα and bremsstrahlung emissions from copper foil targets bombarded by electrons similar to the hot electrons generated in laser–matter interactions are investigated by Monte Carlo simulations. For mono-energetic electron incidences, a higher contrast Kα emission is generated at large detection angles relative to the incident electron direction and for higher electron energy. The Kα emission contrast is decreased with the increase in target thickness. When the areal density of targets is fixed, the contrast is almost unchanged with the change of target density and thickness. For incident electrons with a Boltzmann energy distribution, a higher contrast Kα emission can also be generated at large detection angles and for higher electron temperatures, but the contrast is lower compared to that for mono-energetic electron incidences, and it is changed only slightly with the increase in target thickness. These results help to understand the contrast of Kα emissions in previous experiments. Suggestions are proposed for future laser–matter interaction experiments for higher contrast Kα emissions.

Electron–ion collision and polarization of X-ray fluorescence radiation under hot quantum plasma conditions

In the current article, the spectral properties and electron collision (total and magnetic) excitation cross sections of ions taking placed in quantum plasmas are investigated. These cross sections are further used to study the polarization and angular distribution characteristics of the de-excitation radiation X-ray spectra, which play an important role in basic theoretical research, the diagnosis of the plasma environment, and the design of optical devices. To do so, a distorted wave method within the relativistic Dirac–Coulomb atomic structure scheme is suggested. The effective interaction potential between electrons and particles in hot quantum plasmas in the method is determined using a quantum approach that incorporates the influence of effective plasma screening effects caused by collective plasma oscillations. This potential replaces the traditional Coulomb interaction potential and is used in solving the modified Dirac equation to obtain the bound and continuum electron wave functions. Higher-order relativistic effects, such as the Breit interaction and the dominant quantum electrodynamics corrections, are added to enhance the accuracy of the method. Detailed calculations for the relativistic atomic structure and collision excitation dynamics process are carried out, taking the highly stripped H-like O7+ ion of astrophysical importance as an illustrative example. Detailed investigations are also conducted on the variation of energies, collision cross sections, and fluorescence polarizations as functions of the plasma parameters. Our results suggest that the joint effects of shielding and plasma coupling lead the energies, cross sections and fluorescence polarizations decrease (compared with the isolated case). The angular distribution of the X-ray fluorescence emission shows large change, suggesting their sensitivity to these effects. This study not only offers a valuable approach to investigating the plasma shielding and plasmon coupling effects in quantum plasmas but also holds significant relevance for applications in controlled nuclear fusion, astrophysical plasmas and so on.

Theoretical triple differential cross sections for ionization of N 2 and CH 4 molecules by electron impact

We present the angular distribution of electron emission by calculating the triple differential cross sections for (e, 2e) process on N 2 and CH 4 by using the three-body formalism of molecular first Born approximation (MFBA), two-Coulomb wave (M2CW), and three-Coulomb wave (M3CW) models, respectively. In these models, a Coulomb distorted wave is considered for the motion of the incident electron. We have considered the continuum-continuum correlation effect by choosing the final state as the three-Coulomb and two-Coulomb wave functions in M3CW and M2CW models, whereas the ejected electron is affected by a single centre field of the target in the MFBA model. In the M2CW model, the interaction between scattered electron-residual target ion has been described as a plane wave. The distinguishing feature among the three models has been noted in the TDCS as a strong binary peak with and without a recoil peak for several electron emission energies at fixed scattering angle. In the case of N 2 molecule, the TDCS shows oscillatory behaviour with the variation of the electron emission angle. The positions of the binary peak obtained by our theoretical models are well established by the experimental findings, but a large deviation is found in the region of the recoil peak. The contributions of TDCS for different molecular orbitals of the molecules to the spectrum of angular distributions at different electron emission energies have also been analyzed. Finally, a comparison is made with the measurement in coplanar asymmetry geometry. Overall, good agreement was found between experiments and M3CW theory.

A numerical study of the anisotropic distribution of γ-ray emission from oriented 129m,131m,133mXe

Abstract Spin-polarized radioactive nuclei emit radioactive decay products in an anisotropic manner that is characteristic of their degree of nuclear orientation. This property can be utilized for nuclear magnetic resonance spectroscopy and imaging. In this manuscript, we present Python and MATLAB numerical simulations that investigate the angular distribution and measurable asymmetry of γ-ray emission from spin-oriented nuclei of the metastable isotopes 129 m , 131 m , 133 m Xe. We examine several cases that represent different degrees of spin alignment and detection geometries. The results of our simulations provide a benchmark for experiments that use γ-decay anisotropy, such as the GAMMA-MRI project, which aims to develop a novel medical-imaging technique combining the strengths of magnetic resonance and nuclear medicine imaging.

Ionization of uracil molecule in collisions with 2.5-MeV/u Si12+ ions

The energy and angular distribution of electron emission from a RNA base molecule, uracil (C 4 H 4 N 2 O 2), are investigated in collisions with 2.5-MeV/u Si12+ ions under high perturbation strength. The absolute double differential cross sections (DDCS) are measured using electron spectroscopy for emission angles between 20° to 160° in the electron energy range 1-620 eV. The single differential cross sections (SDCS) are evaluated by integrating the DDCS over emission angles or energies. The measured cross sections (DDCS and SDCS) are compared with the state-of-the-art continuum distorted wave-eikonal initial state (CDW-EIS) theoretical model. The DDCS obtained by the CDW-EIS is found to provide better agreement with the measurements in the backward angles. With an increase in perturbation strength from 0.5 to 1.19 a.u., the DDCS is found to increase by 10 times in comparison to the earlier reported uracil data for 3.5-MeV/u bare C-ions [Phys. Rev. A 87, 032716 (2013)]. The significant enhancement in the TCS is found for the total cross section in case of Si12+ ions over that predicted by a scaling law as derived for lower charge state projectiles. The forward-backward angular asymmetry is found to increase monotonically with the velocity of emitted electrons and shows very good agreement with the model. For a comparative study, the DDCS is also measured for oxygen molecule using same ion impact, which is found to be about ten times lower than that for the uracil. The forward-backward angular asymmetry for oxygen is found to be almost the same as uracil.

Field emission angular distribution from single molecules

We demonstrate that the field emission from a single molecule at the apex of a metallic tip exhibits a well-defined angular distribution pattern (field emission angular distribution, FAD) with a spherical harmonics shape, regardless of the species of the molecule. From carefully controlled simultaneous measurements of the emission pattern, the emission current and emission energy, we deduced the formation mechanism of the FAD pattern in which the electron from the metallic tip resonantly tunnels into vacuum via a molecule-induced state situated at the Fermi level of the tip. The characteristic FAD patterns were consistent with the Fourier transform of the superatom molecular orbitals (SAMOs). Thus, the present method is unique for real-time imaging of the SAMOs, or the low-lying Rydberg states, of single molecules.

Preequilibrium cluster emission in massive transfer reactions near the Coulomb barrier energy

Within the framework of the dinuclear system model, the preequilibrium emission of neutron, proton, deuteron, triton, $^{3}\mathrm{He}$, $\ensuremath{\alpha}$, $^{6}\mathrm{Li}$, $^{7}\mathrm{Li}$, $^{8}\mathrm{Be}$, and $^{9}\mathrm{Be}$ in the transfer reactions of $^{12}\mathrm{C}$ $+$ $^{209}\mathrm{Bi}$, $^{40,48}\mathrm{Ca}+^{238}\mathrm{U}$, $^{238}\mathrm{U}+^{238}\mathrm{U}$, and $^{238}\mathrm{U}+^{248}\mathrm{Cm}$ has been systematically investigated. The production rate, kinetic energy spectra, and emission angular distribution are calculated. It is found that the preequilibrium emission mechanism is associated with the reaction system and beam energy. The preequilibrium cross sections of proton, deuteron, triton, and alpha are comparable in magnitude. The reaction with $^{40}\mathrm{Ca}$ is favorable for the cluster emission in comparison with $^{48}\mathrm{Ca}$ on $^{238}\mathrm{U}$ at the near barrier energy. A broad angular distribution of the preequilibrium cluster is found in the heavy systems $^{238}\mathrm{U}+^{238}\mathrm{U}$ and $^{238}\mathrm{U}+^{248}\mathrm{Cm}$. The method is also possible for the weakly bound nuclei induced reactions. Future experiments are discussed for the preequilibrium cluster measurements.

Cyclotron radiation from shaped electron wavefunctions

Recent years have shown increasing interest in understanding the role of the wavefunction of a quantum source on the characteristics of its emitted radiation. In this work, we demonstrate that shaping the wavefunction of the source can drastically change the instantaneous emission. We exemplify this concept by examining an electron in cyclotron motion, calculating the angular power distribution of emission by an electron in a Schrodinger cat state. The emitted cyclotron radiation reveals a breakdown of classical–quantum correspondence. The short-time dynamics of the radiation process shows deviations in the power and electron trajectory that disappear at long times, where the predictions of classical electrodynamics are recovered.

Directivity Control of Terahertz Radiation from Single-Color Filament Plasma with Polypropylene Pipe

Two-dimensional angular distribution of terahertz emission from single-color filamentation of laser pulses is experimentally studied. The use of a polypropylene pipe makes it possible to transform the angular pattern of terahertz radiation from the original hollow-cone one to a more preferable unimodal structure within the terahertz frequency range. It has been shown that propagation in a dielectric pipe can significantly (up to several orders of magnitude) increase the terahertz radiation amplitude at a required distance. We have also experimentally demonstrated that bending the pipe allows us to control the direction of the terahertz radiation propagation.

Gamma Rays and Neutrinos from the Galactic Plane at the PeV frontier

The Tibet ASγ experiment recently reported the observation of a diffuse gamma-ray emission from the Galactic plane with energy up to the PeV. This finding seems to be confirmed by LHAASO preliminary results. Both measurements provide the first evidence of a diffuse gamma-ray emission throughout the Galaxy up to such high energies. These results have relevant implications for neutrino astronomy since they strengthen the expectation that a neutrino diffuse emission from the Galactic plane could soon be discovered by IceCube and KM3NeT. To explore this possibility we use physically motivated numerical models which reasonably describe the observed gamma-ray diffuse emission angular distribution and spectral energy distribution from few GeV up to the PeV under the hypothesis that is mostly originated by the cosmic ray population of the Galaxy. We will discuss the possible detectability of the associated neutrino emission and the valuable implications it may have for understanding the origin and propagation of cosmic rays.

Observation of Nondipole-Induced Asymmetry in the Angular Emission Distribution of Photoelectrons from Fixed-in-Space CO Molecules.

We investigate experimentally and theoretically the C and O 1s photoionization of fixed-in-space CO molecules at a photon energy of 905eV. We find a significant dependence of the photoelectron angular distributions on the direction of propagation of the ionizing radiation. It results from an interplay of nondipole effects, on one hand, and molecular effects, on the other. The nondipole effects lead to an increase of the emission probability in the forward direction along the light propagation, and the photoelectron wave being scattered by the molecular potential gives rise to a strong peak in the direction of the atom neighboring the emitter site. These effects can either conspire or extenuate each other, depending on the photoelectron emission direction and molecular orientation in space.

A liquid hydrogen tube moderator arrangement for SNS second target station.

Liquid hydrogen filled tubes arranged in a triangular shape surrounded by light-water premoderators were investigated as cold moderators coupled to a neutron production zone of a tungsten target fed by a 1.3 GeV proton beam in a short-pulse mode. A moderator concept optimized for the tube length, premoderator thickness, and target position was found to deliver the highest pulse-integrated neutron brightness suitable to send cold neutron beams for neutron scattering instrumentation geared toward medium resolution and small samples. Reducing the tube diameter from 50 to 10mm boosts the cold neutron brightness (E < 10meV) by a factor of 2.25, also with enhancements seen by downsizing the height of cylindrical moderators in earlier studies. Attempts were made to characterize this novel moderator system with regard to pulse shapes, local brightness distribution, and angular distribution of emission at the neutron emission port.

Unraveling Atomic and Electronic Surface Structure and Dynamics from Angular Photoelectron Distributions

Angle-resolved photoelectron spectroscopy (ARPES) is a powerful tool in solid state sciences. Beside the direct measurement of the energy-momentum dispersion relation, the angular distribution of the photoelectron current reveals the structural environment of the emitting atoms via photoelectron diffraction effects. Moreover, in the case of molecular layers, the angular distribution of emission from molecular orbitals can be directly related to their charge density distribution via so-called orbital tomography. In the present paper we summarize our efforts undertaken over the past 12 years to add the dimension of time to these two methods via pump-probe experiments with femtosecond resolution. We give a comprehensive introduction to standard ARPES and time-resolved two photon photoemission and then focus on our efforts towards time-resolved versions of photoelectron diffraction and orbital tomography. Both, optimization of experimental parameters and data acquisition procedures, as well as new numerical tools are needed in order to realize such challenging full stop missing after experiments.

K-LL Auger electron angular distribution and absolute cross section measurement upon keV energy electron impact ionization of CH[formula omitted], N[formula omitted] and Ne

We report on the measurement of absolute cross sections and angular distribution of electron emission in the K-LL Auger region, upon electron impact ionization of molecular (CH4 and N2) and atomic (Ne) targets. Differential cross sections in the K-LL Auger energy range were measured at 6 keV and 8 keV impact energies. The angular distribution of the K-LL Auger yield is shown to be isotropic for all the three targets. Total K-LL Auger emission cross sections were estimated by integrating the measured differential cross sections over the entire energy and angular range. The estimated cross sections are in good agreement with the theoretical estimates.

Accurate dose measurements using Cherenkov emission polarization imaging.

Cherenkov radiation carries the potential of direct in-water dose measurements, but its precision is currently limited by a strong anisotropy. Taking advantage of polarization imaging, this work proposes a new approach for high-accuracy Cherenkov emission dose measurements. Cherenkov radiation produced in a 15×15×20-cm3 water tank is imaged with a cooled charge-coupled device (CCD) camera from four polarizer transmission axes [0, 45, 90, 135°]. The water tank is positioned at the isocenter of a 5×5-cm2 , 6-, and 18-MV photon beam. Using Malus' law, the polarized portion of the signal is extracted. Corrections are applied to the polarized signal following azimuthal and polar Cherenkov emission angular distributions extracted from Monte Carlo simulations. Projected percent depth dose and beam profiles are measured and compared with the prediction from a treatment planning system (TPS). Corrected polarized signals on the central axis reduced deviations at depth (mean ± standard deviation) from 8%±5% to 0.8%±1% at 6MV and 8%±7% to 1%±3% at 18MV. For the profile measurement, differences between the corrected polarized signal and the TPS calculations are 1%±3% and 2%±3% on the central axis at 6 and 18MV respectively. In these conditions, Cherenkov emission is shown to be partly polarized. This work proposes a novel polarization imaging approach enabling high-precision water-based dose measurements using the Cherenkov radiation. The method allows a correction of the Cherenkov emission anisotropy within 4% on the beam central axis and in depth.

Angular Distribution of Ion Beam Emission From Dense Plasma Focus Device: An Investigation by the Vlasov–Maxwell Equations

The Vlasov–Maxwell (VM) equations have been solved by the forward semi-Lagrangian (FSL) method to calculate the argon ion beam angular distribution in the dense plasma focus (DPF) device. The validity of the VM employment was investigated through the Dreicer field, the mean free path, and the Debye length of the plasma pinch. The simulations have shown that the ion beams were emitted in a direct way at the initial pressures of 10 and 20 Pa. The ion beam angular distribution expanded for pressures bigger than 20 Pa. It has been shown that this was because the radial component of the electric field inside the pinch was increased and the axial component of the electric field was decreased by increasing the initial pressure of the working gas. The places of the ion beam micro-sources (hotspots) have been illustrated by means of the ion and electron density profiles.

Symmetry Violation in Bichromatic Ionization by a Free-Electron Laser: Photoelectron Angular Distribution and Spin Polarization

A fundamental phenomenon of coherent control is investigated theoretically using the example of neon photoionization by the bichromatic field of a free-electron laser. A system exposed to coherent fields with commensurable frequencies loses some symmetry, which manifests itself in the angular distribution and spin polarization of the electron emission. We predict several such effects, for example, the violation of symmetry with respect to the plane perpendicular to the polarization vector of the second harmonic and the appearance of new components of spin polarization. Furthermore, we predict a very efficient control of spin polarization via manipulation of the phase between the harmonics. Experimental observation of these effects is accessible with modern free-electron lasers operating in the extreme ultraviolet wavelength regime.

Numerical study of Langmuir wave coalescence in laser-plasma interaction

Type-III-burst radio signals can be mimicked in the laboratory via laser-plasma interaction. Instead of an electron beam generating Langmuir waves (LWs) in the interplanetary medium, the LWs are created by a laser interacting with a millimeter-sized plasma through the stimulated Raman instability. In both cases, the LWs feed the Langmuir decay instability which scatters them in several directions. The resulting LWs may couple to form electromagnetic emission at twice the plasma frequency, which has been detected in the interplanetary medium, and recently in a laboratory laser experiment [Marquès et al., Phys. Rev. Lett. 124, 135001 (2020)]. This article presents the first numerical analysis of this laser configuration using particle-in-cell simulations, providing details on the wave spectra that are too difficult to measure in experiments. The role of some parameters is addressed, with a focus on laser intensity, in order to illustrate the behavior of the electromagnetic emission's angular distribution and polarization.

Design of modular multi-channel electron spectrometers for application in laser matter interaction experiments at Prague Asterix Laser System

This paper describes design, development, and implementation of a multi-channel magnetic electron spectrometer for the application in laser-plasma interaction experiments carried out at the Prague Asterix Laser System. Modular design of the spectrometer allows the setup in variable configurations to evaluate the angular distribution of hot electron emission. The angular array configuration of the electron spectrometers consists of 16 channels mounted around the target. The modules incorporate a plastic electron collimator designed to suppress the secondary radiation by absorbing the wide angle scattered electrons and photons inside the collimator. The compact model of the spectrometer measures electron energies in the range from 50keV to 1.5MeV using ferrite magnets and from 250keV to 5MeV using stronger neodymium magnets. An extended model of the spectrometer increases the measured energy range up to 21MeV or 35MeV using ferrite or neodymium magnets, respectively. Position to energy calibration was obtained using the particle tracking simulations. The experimental results show the measured angularly resolved electron energy distribution functions from interaction with solid targets. The angular distribution of hot electron temperature, the total flux, and the maximum electron energy show a directional dependence. The measured values of these quantities increase toward the target normal. For a copper target, the average amount of measured electron flux is 1.36 × 1011, which corresponds to the total charge of about 21nC.

Calculation and analysis of key physical problems: Fluorescence yield, deexcitation time, scattering and polarization in X-ray fluorescence CT imaging

X-ray fluorescence computed tomography (XFCT) is a molecular imaging technique with great potential applications in biomedical imaging, in which used is the primary X-ray to excite element probes with high atomic number inside samples or tissues for functional imaging. However, owing to the limitation of molecular sensitivity and spatial resolution, the XFCT has not been widely used in the molecular imaging. A large number of Compton scattering photons are produced as the broadband primary X-ray passes through the samples or tissues, forming a strong noise background in the collected XRF signal, which is a major cause of limited molecular sensitivity. Therefore, eliminating the Compton scattering noise is very important for improving molecular sensitivity. In this paper, we summarize the main physical processes involved in the imaging process of XFCT, including the angle distribution and polarization state of the fluorescence and Compton scattering photons, fluorescence yield and deexcitation time of K-shell vacancy. The above physical processes are the main limitations of the imaging quality of XFCT. Through the derivation and analysis of physical processes, we explore the possibility of using these physical effects to reduce the Compton scattering noise and draw some conclusions below. 1) The deexcitation time of K-shell vacancy of the element with high atomic number is very short, consequently the scattered light and fluorescence cannot be distinguished between each other under the time resolution condition of the existing detector. 2) The angular distribution of the K-shell fluorescence emission is isotropic, and the differential cross section of Compton scattering reaches a minimum value near the polarization direction of the incident light of which the minimum decreases as the linear polarization degree of the incident light increases. Therefore, the polarized light source is beneficial to reducing the Compton scattering noise. 3) The linear degree of polarization of K-shell fluorescence is zero, while the Compton scattering photons possess a certain linear degree of polarization in some scattering directions, so the difference between polarization states may be helpful in reducing the Compton scattering noise.