Initial Charge Distribution Research Articles

Validity of spherical approximations of initial charge cloud shape in silicon detectors

Spherical approximation has been used extensively in low-energy X-ray imaging to represent the initial charge cloud produced by photon interactions in silicon detectors, mainly because of its simplicity. However, for high-energy X-rays, where the initial charge distribution is as important as the diffusion process, the spherical approximation will not result in a realistic detector response. In this paper, we present a bubble-line model that simulates the initial charge cloud in silicon detectors for photons in the energy range of medical imaging. An initial charge cloud can be generated by sampling the center of gravity and the track size from statistical distributions derived from Monte Carlo generated tracks and by distributing a certain proportion of photon energy into a bubble (68%) and a line portion uniformly. The simulations of detector response demonstrate that the new model simulates the detector response accurately and corresponds well to Monte Carlo simulation.

Correlated development of a (2 × 2) reconstruction and a charge accumulation layer on the InAs(111)–Bi surface

We have studied the formation of a Bi-induced (2 × 2) reconstruction on the InAs(111) B surface. In connection to the development of the (2 × 2) reconstruction, a two dimensional charge accumulation layer located at the bottom of the InAs conduction band appears as seen through a photoemission structure at the Fermi level. Not well ordered Bi layers do not induce a charge accumulation. The Bi-induced reconstruction reduces the polarization of the pristine surface and changes the initial charge distribution. InAsBi alloying occurs below the surface where Bi acts as charge donor leading to the charge accumulation layer.

Conformation specific and charge directed reactivity of radical cation intermediates of α-substituted (amino, hydroxy, and keto) bioactive carboxylic acids

The reactivity of radical cation carboxylic acids is investigated on the basis of mass spectrometry, infrared-vacuum ultraviolet (IR-VUV) photoionization spectroscopy, and high level correlated ab initio calculations. Their reactivity is found to be highly conformation specific and is governed by their initial charge distribution following ionization. In the present work, the radical cations of lactic acid, pyruvic acid, glycine, and valine are studied to probe their stability and conformation specific reactivity following single photon, vertical ionization at 10.5 eV. For lactic acid, glycine, and valine, the localization site of the hole following sudden removal of an electron depends on their specific intramolecular hydrogen bonding network. Lactic acid, glycine, and valine undergo complete fragmentation following vertical ionization at 10.5 eV; however, pyruvic acid does not completely dissociate following vertical ionization. Only 45% of the pyruvic acid parent ions undergo C(alpha)-C(carboxylic) bond dissociation. If the hole is localized on the COOH moiety of glycine, valine, and lactic acid, a hydrogen transfer is favored from the COOH to the alpha-substituent. If the hole is localized on the alpha-hydroxy or -amine substituent and the singly occupied molecular orbital (SOMO, where the hole resides) is parallel to the C(alpha)-C(carboxylic) bond, C(alpha)-C(carboxylic) bond dissociation occurs through charge transfer from the alpha-substituent to the C(alpha)-C(carboxylic) bond. The present study reveals that the specific conformations of alpha-substituted carboxylic acids govern their radical cationic reactivity. The radical cation of pyruvic acid exhibits a special stability due to enolization of the alpha-keto form on the cationic surface.

Real-time χ^(2) evolution in twin-hole fiber during thermal poling and repoling

We report the real-time evolution of the second-order nonlinearity χ(2) induced in a twin-hole fiber during its first thermal poling and the subsequent repoling after thermal erasure. The considerable difference in the χ(2) evolution in the two poling processes indicates that the first poling has a strong history effect on the repoling. We use a two-charge dynamics model involving a fast charge carrier Na+ and a slow charge carrier H+ to model the χ(2) evolution in both poling processes. We quantitatively show that the difference in χ(2) evolution is due to the different initial charge distributions before each poling. The extra H+ ions injected into the fiber during the first poling change the initial charge distribution in the second poling, which leads to a different χ(2) dynamic.

Time-dependent charge distributions in polymer films under electron beam irradiation

The time-dependent charge distribution in polymer film under electron beam irradiation is studied by both experiment and numerical simulation. In the experiment, the distribution is measured with the piezoinduced pressure wave propagation method. In the simulation, the initial charge distribution is obtained by the Monte Carlo method of electron scattering, and the charge drift in the specimen is simulated by taking into account the Poisson equation, the charge continuity equation, Ohm’s law, and the radiation-induced conductivity. The results obtained show that the negative charge deposited in the polymer film, whose top and bottom surfaces are grounded, drifts toward both grounded electrodes and that twin peaks appear in the charge distribution. The radiation-induced conductivity plays an important role in determining the charge distribution in the polymer films under electron beam irradiation.

Effect of an external electric field on the charge distribution of electrostatic coagulation

This article analyzes the effect of an external electric field on the charge distribution of bipolar and unipolar charged particles numerically by solving the coagulation equation for charged particles, based on the analytical expression for the coagulation coefficient [Wang, L. Z., Zhang, X. R., & Zhu, K. Q. (2005). An analytical expression for the coagulation coefficient of bipolarly charged particles by an external electric field with the effect of Coulomb force. Journal of Aerosol Science, 36, 1050–1055]. For symmetric bipolar charged particles, the external electric field does not change the symmetry of the initial charge distribution as the coagulation time increases. In addition, the particle number concentration decays monotonically during coagulation regardless of the magnitude of the particle charge. For asymmetric bipolar charged and unipolar charged particles, however, the particle number concentration does not decay monotonically for each value of charge during coagulation, due to the effect of an external electric field.

Comprehensive simulation of the response of a silicon strip detector for position-sensitive measurements of X-rays

The paper describes a comprehensive simulation method to evaluate X-ray imaging response of a silicon strip detector with particular emphasis on the charge-sharing effects. The simulation steps include: generation of the initial charge distribution in the detector volume, transport of generated charge in the detector volume, calculation of charges induced in the readout strips, discrimination of noisy electronic signals, and finally determination of the count efficiency vs. photon position as a function of the discrimination threshold. The developed simulation tools are useful for optimising the designs and operating parameters of silicon strip detectors used as 1-D position sensitive devices in experimental techniques like X-ray powder diffraction, X-ray high-resolution diffraction and small angle X-ray scattering, using laboratory X-ray sources. The response of the detector as a function of the detector bias and discrimination threshold has been investigated for two measurement configurations: irradiation from the strip-side and from the back-side.

Deformation, nuclear motion and fragmentation of core-excited CO 2 probed by multiple-ion coincidence momentum imaging

The nuclear motion and geometry in core-excited CO 2 are probed using a multiple-ion-coincidence imaging technique. We demonstrate that CO 2 has a linear stable geometry in the C/O 1s −1 core-ionized state and a bent geometry in the C/O 1s −1π* core-excited state. The molecules in the C/O 1s −1π* A 1 and B 1 Renner–Teller states are probed to be bent in the A 1 state and linear in the B 1 state. The O O correlation angle distributions are well reproduced using a Coulomb explosion model which takes account of the zero point bending motion in the ground state, the classical bending motion along the potential energy curve of the core-excited state within the core-hole lifetime and the initial inhomogeneous charge distribution in the multiply charged molecular ion just before the dissociation. When the photon energy is tuned to be higher (lower) energy than the 1s −1π* resonance center, the events for the 1s −1π* A 1 states that result in the low O O correlation angle distribution are suppressed (enhanced).

A Vlasov–Poisson plasma model with non‐L1 data

AbstractIt is considered the Vlasov–Poisson equation for a plasma confined in an unbounded cylinder and it is proven an existence and uniqueness result for non‐L1 (but almost L1) initial charge distribution. Copyright © 2004 John Wiley & Sons, Ltd.

Bloch oscillations in a homogeneous nucleotide chain

It is shown that, at zero temperature, a hole placed in a homogeneous synthetic nucleotide chain with applied electric field demonstrates Bloch oscillations. The oscillations of the hole placed initially on one of the base pairs arise in response to disruption of the initial charge distribution caused by nucleotide vibrations. The finite temperature fluctuations result in degradation of coherent oscillations. The maximum permissible temperature for DNA “Bloch oscillator” occurrence is estimated.

Mixing selectivity in bicomponent, bipolar aggregation

The selectivity of aggregation in mixtures of two charged aerosols containing chemically dissimilar nanoparticles is studied by means of a newly developed direct simulation Monte Carlo method. This method allows to trace changes in complex multidimensional systems, in this case describing particle size, charge and aggregate composition. A new procedure was developed for estimating the effective collision diameter of an aggregate composed of primary particles of any size. Three model systems were studied: polydisperse aerosols with initially bipolar charge distribution, unipolarly charged polydisperse aerosols and quasi-monodisperse oppositely charged aerosols. The study is focused on the aggregate composition's dependence on the initial size and charge distribution. It was found that the use of bipolarly charged aerosols does not increase the selectivity of mixing whereas unipolarly, oppositely charged aerosols reach more rapidly a more homogeneous distribution of components within the aggregates. In the last case, the addition of one more elementary charge to the particles roughly doubles the fraction of bicomponent, 1 : 1 mixed nanoaggregates and accelerates the process.

Charge dynamics in electron-irradiated polymers

Recent studies of charge trapping and charge transport in spolymer films irradiated and charged with monoenergetic electrons of range smaller than the sample thickness and thereafter stored or annealed under various conditions are discussed. An analytical model used to describe the phenomena takes the following parameters into consideration: Charge and energy deposition profiles, charge drift due to a shallow-trap-modulated mobility, deep trapping without release, trap filling due to a finite trap density, and ohmic relaxation due to a radiation-induced conductivity during irradiation and its delayed component after irradiation. The model calculations show the effect of various parameters on the shape of the initial charge distribution and on its evolution with annealing time. - Experiments with the laser-induced pressure-pulse (LIPP) method on 12 and 25<FONT FACE="Symbol">m</font>m thick fluorocarbon and polymide films, charged with 10 or keV electron beams, respectively, yield the charge distributions after irradiation and the changes of the distributions due to annealing of the samples at 120oC for various times. A comparison of experimental and analytical results reveals the trapping kinetics and permits to estimate the deep-trap density and the <FONT FACE="Symbol">mt</font>-product, where <FONT FACE="Symbol">m</font> is the trap-modulated mobilit and <FONT FACE="Symbol">t</font> is the trapping time.

COAGULATION OF CIGARETTE SMOKE PARTICLES

Experimental measurements on the deposition of cigarette smoke particles (CSP) in the human airways have produced results that are inconsistent with typical deposition data based on particle size. Previous work relating to hygroscopic growth indicates that hygroscopicity alone can not account for this discrepancy. The present study investigates coagulation of CSP modeled as a polydisperse-charged aerosol as a possible explanation. The results of the model more accurately predict the experimental coagulation data for mainstream CSP than models that treat CSP as a monodisperse or polydisperse-uncharged aerosol. An aerosol with an initial charge distribution based on Boltzmann equilibrium yields slightly larger coagulation rates than the mainstream CSP polydisperse-charged model. The numerical results indicate that the size and charge distribution of sidestream CSP, with a concentration of 10 6 particles cm -3, remain stable. In 2 s, the size distribution of mainstream CSP, with a concentration of 10 9 particles cm -3, shifts to a larger size while becoming flatter and wider. The diameter of average mass increases from 0.29 to 0.5 μm. Numerical results confirm experimental reports for mainstream CSP, which indicate that the total number of charged particles increases with time and, in the early stages of coagulation, the amount of charge per particle cannot be estimated based on the particle size. This study shows that polydisperse-charged CSP, allowed to coagulate for 2 s in the mouth, will not produce size distributions that yield the observed deposition of CSP. However, additional coagulation will take place as the CSP travels through the respiratory tract, which will be investigated in future work.

Transient dynamics of Josephson-coupled multilayers

Abstract A novel model is proposed to allow the exploration of the short-time (transient) dynamics of a superconductor–insulator multilayer with Josephson coupling between the layers. This model treats the charge on the layer interface planes as a dynamic variable, whose evolution is determined via the interlayer charge-current continuity equations. The high frequency current responses of both the superconducting and the insulating layers are included in the proper time domain form, derived for the general case of nonuniform layers from standard BCS theory. We present the model equations for the response of the system to an initial charge distribution, with the focus of determining the Josephson switching properties of an overdamped, nonuniform multilayer. Such structures may have potential applications in superconducting flux quantum electronics.

Surface Coulomb explosions: The influence of initial charge distributions

Using molecular dynamics simulations, we have studied the Coulomb explosion processes on silicon surfaces. Three different initial shapes of ion distributions are used to model the possible localized charge distributions generated in highly charged ion (HCI)-surface impact. The three distinct distributions include a hemispherical, a flat disk, and a long and thin cylindrical geometrical. At t = 0, 100 singly-charged Si ions are embedded in the Si(111) surfaces. In about 100 fs, the strong repulsive electrostatic forces in these three systems cause Coulomb explosions and thus create three different shapes of craters on the surfaces. All simulations are carried to 1.6 ps, at which point the size and shape of the craters are nearly stabilized. The detailed analysis of the ejected ions, atoms, and the substrates reveals the dynamical consequences of the different initial conditions. For these 100 ions, the differences in the total number of the ejected particles, ranging from 245 to 317 particles, appear to be determined by the initial shape of the ionized region and not by the initial repulsive energy restored in the charged region. Contrary to intuition, a long and thin cylindrical distribution is the most efficient pattern for ejecting particles. The underlying mechanism is that ions with this initial configuration transfer more energy to the surrounding atoms. In all three cases, the number of ejected neutral particles are much greater then the number of ions (6–10 times as many atoms as ions). Among the ejected particles, a small percent of particles are found to return the surface at a later time. The angular distribution of ejected particles are also analyzed. While the differences in the distributions of polar angle of the Si atoms of the three configurations is small, the differences in distributions of the ions portray a strong shape dependence in the polar angle.

Calculation of field distortion by a large-gained avalanche in a gas counter

Simulation performed by using the continuity equation of electrons and positive ions is applied to examine the three-dimensional formation and the time evolution of an electron avalanche in a cylindrical gas counter. The field made by space charge is calculated exactly and concurrently by the relaxation method in the evolution. The gas multiplication for photon detection is consistently estimated for an Ar-based mixture by not using any adjustable parameter for geometrical dependency. Field distortion at the time of the finish of the evolution is studied systematically. The distortion is concentrated in a small region of angle in case of a large diameter of the anode. The shape of the distortion is calculated for several distributions of the initial charge. The distortion is enhanced by the distribution that occupies small area in the transverse plain in the avalanche. Change of the field distortion depending on the anode diameter and the initial charge distribution is shown quantitatively by these calculations. The transition to a streamer by the continuous supply of electrons is suggested to depend strongly on the charge distribution in an avalanche.

Electrostatic sterility

However after frequent ophthalmologic operations, the implantation of intraocular lenses (IOLs) mild or rarely serious cellular reactions were observed around the IOLs. These postoperative reactions called the author's attention to the electrostatic attitudes o intraocular lenses and other instruments. In order to determine the procedure how a non-toxic implant made of highly insulating materials carry harmful contamination to the eye on their surface the electrostatic properties of the implant materials were examined. After previous studies of our laboratory, a problem oriented finite-difference numerical model was developed in order to examine the influence of changing dielectric properties of the IOL's environment and the transient effect of the accidental initial charge distribution.

X-Ray Photoelectron Spectroscopic (XPS) Studies of Clean and Ion Beam Bombarded Sb2Te2Se and (Bi0.7Sb0.3)2Se3.0(111) Surfaces

The first detailed study of photoelectron spectra of Sb2Te2Se and (Bi0.7Sb0.3)2Se3 (111) clean and sputtered surfaces is presented as part of an XPS examination of the surface chemistry of these and related materials. The core level binding energies and surface chemical composition have been determined from the XPS data. On substitution of Te by Se in Sb2Te3 leading to Sb2Te2Se the core level binding energies in Sb and Te increase by 0.3 eV while in Bi2Se3 the binding energy of core electrons does not change on replacement of Bi by Sb. The measured core level shifts are caused by changes of the initial state charge distribution and result in increase of average ionicity of bonding in the Sb2Te2Se crystal. The surface composition of Sb2Te2Se sample calculated from intensities of photoelectron spectra agrees well with the bulk composition of the crystal while (Bi0.7Sb0.3)2Se3 sample shows enrichment in Bi. The effect of argon ion bombardment on surface composition for various impact conditions has been investigated. The surface enrichment in Sb and Bi for Sb2Te2Se and (Bi0.7Sb0.3)2Se3 sample due to different atomic sputtering yields is observed. It follows from the relative intensities of photoelectron spectra measured at different detection angles that the ordered arrangement of the superficial layers sampled by the XPS method is damaged by sputtering at ion energies as low as 200 eV and doses I &gt; 2 . 1015 ion/cm2.

Influence of Carrier Diffusion on Transient One-Carrier Injection Current in an Insulator with Traps

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Additivity of Proton affinities: Theoretical studies of fluorine‐ and methyl‐substituted benzenes

AbstractRing proton affinities (PAs) in fluorobenzene and toluene were examined by the MP2(fc)/6–31G**//HF/6–31G* + ZPE(HF/6–31G*) model. The calculated PAs are in good accordance with the available experimental evidence, their order being PA(p) &gt; PA(o) &gt; PA(m) &gt; PA(i), where p, o, m and i stand for para, ortho, meta and ipso positions, respectively. The relative values of the proton affinities can be interpreted in terms of the ground‐state charge distribution (initial state effect) and the characteristic π‐bond fixation produced by protonation (final state effect). The influence of the latter is either concerted with the initial charge distribution leading to higher PAs (ortho and para positions) or disconcerted as in meta protonation, which has a lower PA value. Finally, it is shown that PAs in difluorobenzenes and fluorotoluenes are additive and can be reduced to the characteristic PAs of fluorobenzene and toluene with good accuracy.