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In this work, we investigated the effects of 63 MeV proton irradiation on the electrical performance of long-wave infrared InAs/GaSb type-II superlattice barrier detectors. The dark current density increase due to displacement dose effects does not exceed a factor of 2.6, regardless of the absorber thickness, when measured after a proton fluence of 8×1011 H+/cm2 at 100 K. Subsequent analysis showed that the dark current changes behavior after irradiation, and the dominant current is no longer diffusion but generation–recombination and trap-assisted tunneling. Device simulation also allowed us to extract a minority carrier lifetime before and after irradiation, which decreases with increasing fluence as a result of the 63 MeV proton-induced mild material degradation.

A model on magnetic drug targeting is developed to determine the capture conditions on the efficient dispersion of drug-coated nanoparticles in a tumor environment. These particles consist of a non-magnetic core material containing embedded magnetic nanoparticles and a therapeutic agent (such as a photodynamic sensitizer). In the present problem, we have studied the solute dispersion during magnetic drug targeting through a microvessel. The particles are injected into the microvascular system upstream of the cancerous tissue and captured in the tumor using an applied magnetic field. The fluid velocity and the particle velocity due to the magnetic field are calculated analytically, while the solute transport equation is solved numerically by using the finite difference method. The fluid force is assumed on the capillary and the carrier undergoing magnetic force due to an external magnetic field. Solving the equations of fluid flow and solute transport simultaneously, the influence of the model biological parameters, such as volume fraction of the nanoparticles, magnetization, magnet–tumor distance, permeability of the capillaries, Peclet number, drug elimination, source term, and radius of the nanoparticle in the dispersion of drug-coated nanoparticles in microvessels, is also discussed. The results show that higher values of volume fraction of the magnetic particles, magnetization of the magnet, drug elimination, and source term associated with longer times are taken by drug-coated magnetic nanoparticles to reach the tumor position. Moreover, an increase in the tumor–magnet distance, permeability of the microvessel, Peclet number, and radius of the nanoparticle slows the rate at which the drug-coated magnetic nanoparticles reach the tumor position.

This study develops an exact analysis of various bio-heat models, namely, classical, single-phase-lag, and dual-phase-lag models, for heat transfer in a single layer of living tissue. A new hybrid scheme coupled with the separation of variables and finite Fourier transform method has been employed for the closed-form solution. The skin surface of living tissues is heated by constant or unique oscillating heat sources to facilitate non-invasive therapeutic treatment. The existing literature survey highlights that most of the research works were associated with a constant initial temperature of tissues, whereas the present work reveals an implementation of a variable initial temperature to justify the prediction of thermal flow in living tissues before the therapy process to be initiated. Two unique oscillating (time-dependent) heat sources at the skin surface are implemented for the comparative aspect to find out an effective therapeutic condition. The results conclude that a sinusoidal heat source is a better option for the longer duration of the therapeutic heating to reach the thermal equilibrium, while a cosine heat source is more applicable for the short time span of treatment with a range of time-lags in the dual-phase-lag bioheat model. Finally, for the accuracy aspect of the model, the present work was validated with the published work by considering a design case. For the accurately analytical establishment, the present work can predict an actual thermal response which may help to decide a proper treatment protocol for thermal therapies.

The problem of radiation scattering in integral reconstruction algorithms for single-photon emission computed tomography (SPECT) has not been completely solved. This is primarily due to the fact that the radiation transport equation (RTE) does not have an exact analytical solution in general form for a scattering indicatrix corresponding to a real medium. A special form of the scattering indicatrix is proposed here that corresponds to a scattering medium with a property named “scattering straight back.” With this scattering indicatrix, it is possible to obtain an exact analytical solution of the RTE with exact boundary conditions for an arbitrary distribution of radiation sources. From this exact solution, a new expression can be obtained for the measured data, named as generalized attenuated exponential Radon transform. A new definition of projections is proposed to obtain an exact solution of the inverse tomographic problem. This solution has the same level of rigor as the exponential Radon transform for a purely absorbing medium. The proposed algorithm is compared with the traditional SPECT algorithm (the inverse exponential Radon transform) using Monte Carlo simulation based on the nuclear physics software package Geant4. It is demonstrated that the new method is superior to the traditional method in terms of both the standard deviation criterion and better visual distinction of the details in the reconstructed tomogram. The results obtained can be used to improve the existing algorithms for image reconstruction in SPECT, as well as to aid in developing new designs of emission tomographs.

This article reports our simulation results on a period of surface micro-protrusions, which are generated on titanium and stainless steel 304 target surfaces by femtosecond pulsed laser irradiation. The period of the generated micro-protrusions for varying laser fluence level has been estimated using an approach derived from the linear hydrodynamic Kuramoto Sivashinsky model. Some of the parameters, needed for calculating the period of surface micro-protrusions, have been estimated by numerically solving one-dimensional heat equations. Temperature evolution inside the target upon irradiation with a femtosecond laser pulse has been simulated using the two temperature model until the time electron and lattice subsystems attain thermal equilibrium. Thereafter, temperature evolution in the target has been simulated by defining a single temperature of the target at every position and time. We have validated our theoretical model by comparing simulated variation of period of surface micro-protrusions with incident laser fluence, ablation depth per pulse, and time required for thermalization between electrons and lattice subsystems with the reported experimental data for titanium target. Subsequently, the validated model has been used to simulate the period of surface micro-protrusions which are generated on the stainless steel 304 target via femtosecond laser irradiation.

Graphite ablation by an electric arc or a laser/solar flux is widely used for the synthesis of carbon nanomaterials. Previously, it was observed in multiple arc experiments that the ablation rate is a strong nonlinear function of the arc current and it drastically increases at some threshold current. We developed an analytical model explaining this transition in the rate of ablation by an electric arc or a laser/solar flux. The model not only explains the observations but can also accurately predict the experimentally observed ablation rates. The model takes into account redeposition of carbon back to the ablated surface, which is the key process responsible for the observed effects.

We present a heat transfer model to obtain the temperature distribution within aligned nanorod arrays. The model includes thermal conduction along each nanorod in addition to a phenomenological treatment of near-field radiative energy exchange among the nanorods and allows us to investigate the interplay between these effects. The contribution of the near-field radiative energy exchange to overall heating is found to increase significantly with a decrease in inter-nanorod spacing and an increase in array size; the effect is also more prominent for nanorods with lower thermal conductivity and higher emissivity.

The diffusion of nanoparticles in a polymer matrix is an area of current interest. However, a complete understanding is still limited as it is often difficult to quantify the much slower motion of nanoparticles in a polymer matrix. To combat this problem, we have developed a protocol to measure the diffusion coefficient of soft nanoparticles in a linear polymer matrix. Recently developed synthetic control over soft nanoparticle structures combined with this protocol provides a pathway to separately elucidate the effects of the molecular weight and nanoparticle softness on its diffusive behavior. These results indicate that the nanoparticle softness and deformability dictate its motion. Increasing the cross-linking density of the nanoparticle for all molecular weights increases its hardness and suppresses its motion in the linear matrix. Additionally, the nanoparticle molecular weight dependence deviates from the exponential dependence for star polymers suggesting that these nanoparticles benefit from the cooperative motion of the matrix to open pathways for the nanoparticle. Finally, comparison of these experimentally determined values to the Stokes–Einstein theory demonstrates that the nanoparticles diffuse much slower than a hard sphere. This is interpreted to indicate that there exist additional interactions between the nanoparticle and polymer matrix that are not captured by Stokes–Einstein, including threading or entanglement of the linear chain with the nanoparticle.

A nonlinear photocarrier radiometry (PCR) based quantitative defect characterization method is applied to determine the electronic transport parameters of the implantation layer of B+ ion-implanted silicon wafers with different implantation doses. A rigorous two-layer nonlinear PCR model is employed to fit the experimental modulation frequency dependences of PCR amplitude and phase to determine the transport parameters, that is, the carrier lifetime, carrier diffusion coefficient, and front surface recombination velocity of the implantation layer via multiparameter fitting. In the multiparameter fitting, the effects of the implantation layer thickness determination on the extraction of the electronic transport properties of the implantation layer are discussed via setting the thickness as a free parameter in the multiparameter fitting and fixed parameters determined by Monte Carlo based TRIM calculation. The fitted implantation layer thicknesses are in good agreement with that determined via TRIM calculation with a modified electronic damage threshold. Monotonic dependences of the transport properties of the implantation layers on the implantation dose are observed, and the effects of impurity density on the transport properties of the implantation layers are discussed. Good agreements between the experimental implantation dose dependence of the nonlinearity coefficient and corresponding theoretical calculations with the determined transport parameters are obtained. These results show that the two-layer nonlinear PCR model is accurate for quantitatively characterizing the transport properties and thickness of the ion-implantation layers of silicon wafers, and the nonlinear PCR technique is appropriate for precise defect characterization in the semiconductor manufacturing processes.

The thermoacoustic resonance effect induced by the interaction of multiburst modulated microwave pulses with biological tissue can be used to study the absorption and mechanical properties of tumorous tissues. Here, the mechanism of the thermoacoustic resonance response of the tumor is simulated using the finite element method, and the tumor size dependence on the resonance frequencies is investigated. The simulation results show that the maximum acoustic signal amplification depends on the microwave pulse modulation frequency and consequently the tumor radius. The results also indicate that the resonance frequencies decrease with an increase in tumor size. Utilizing the modulation frequency corresponding to the maximum amplification facilitates the use of microwave pulses with lower power and increases the safety of thermoacoustic imaging, resulting in the reduction of cost and risk of thermoacoustic tomography systems.

Solenoid coils with grounded ends positioned on the dielectric window were proposed and numerically studied to reduce the capacitive coupling in conventional inductively coupled plasma (ICP) sources. The capacitive coupling between the plasma and the coils was subsequently suppressed, leading to a significant reduction of the window erosion. To understand the plasma characteristics and the advantages of this configuration, the discharges by the solenoid coil were modeled and compared with a conventional planar coil ICP discharge. The solenoid coil could generate a plasma with higher density than the planar coil under the same absorbed power. The ratio of inductive to capacitive heating power of the solenoid coil was more than one magnitude higher than that of the planar ICP source. The voltage drop on the dielectric window under the solenoid coil was significantly reduced, which was attributed to a potential shielding effect of the grounded end of the radio frequency coil.

The temperature analysis in magnetic hyperthermia focuses on the main parameters that have a fundamental role in thermal damage of malignant tissue. This paper studies the influence of the parameters as (i) ferrofluid infusion rates, (ii) particle zeta potential, and (iii) magnetic nanoparticle (MNP) size on the thermal damage of tissues produced by the heating of the magnetic systems injected within the malignant tissues when an external alternating magnetic field is applied. The spatial and temporal MNP distribution, the temperature field, and thermal damage were computed considering the convection–diffusion–deposition of the particles within tissues, the bioheat transport equation, and the Arrhenius formulation for thermal tissue damage. A ferrofluid injection modeled by Darcy's equation influences strongly the bioheat transport within a malignant tissue. This extended study focuses on the essential role of these parameters to accurately predict the optimum MNP dosage which induces a hyperthermic temperature field and thermal damage of the malignant tissues.

We study the impact of the heater-induced longitudinal field on the vector rubidium atomic magnetometers detecting low-frequency transverse magnetic field, which enlightens the parameter optimization of the applied heaters. Our results, both theoretical simulation and experimental measurement included, indicate that by carefully adjusting the frequency and amplitude, the impact of the heater-induced longitudinal field can be efficiently eliminated; meanwhile, the signal amplitude can be potentially enhanced. The numerical simulation agrees well with the experimental measurement in various cases, demonstrating that our proposed model is well suitable for predicting the actual performance of atomic magnetometers by taking the heater-induced longitudinal field into account.

Intense single-frequency ultrasonic waves generate cavitation clouds in liquids, while at the same time, the pulsating cavitation bubbles emit broadband noise. In gas-containing liquids such as soda and beer, an ultrasonic source with a driving frequency of 20 kHz is shown to easily stimulate the formation of dense cavitation clouds and large cavitation bubbles in experiments. The acoustic signal is acquired at the boundary (opposite the ultrasonic source) just outside the cavitation cloud. Analysis of this signal using the fast Fourier transform reveals a gap in the acoustic spectrum at low frequencies, which means that the low-frequency acoustic waves cannot penetrate the cavitation area. However, these low-frequency waves can be detected in the cavitation cloud, which means that they are localized there. Through a linear analysis of a nonlinear equation for soundwave propagation in bubbly liquids, we can interpret the localization observed in the experiments. When soundwave frequencies are close to the...

Proton irradiation-induced effects on AlGaN/GaN high electron mobility transistors (HEMTs) were studied by emulating a certain space radiation environment (upstream of the earth's bow shock) using a relatively low energy (100 keV) proton beam with fluences of 1 × 1010, 1 × 1012, and 1 × 1014 protons/cm2. In order to isolate radiation-induced effects produced by the modification of the epi-layer from the effects produced by the change in the device structure (such as contacts), the epi-layers were irradiated prior to device fabrication, followed by material/device characterization. Proton irradiation-induced sub-gap traps were detected by spectroscopic photo current-voltage measurement. Raman study revealed that the proton irradiation had induced strain relaxation on the AlGaN/GaN HEMTs epi-layer. No substantial change in the crystal quality of the epi-layer was indicated by Raman and PL studies. With increasing proton fluences, increasing charge carrier density was observed, which was estimated via Raman spectroscopy and the charge-control model analysis. The magnitude and direction of the transistor threshold voltage shift were also dependent on proton fluence. Overall, degradation of transistor output characteristics of the fabricated HEMTs was observed with increasing proton fluence. However, based on the observed performance and the level of influence on material/device characteristics by 100 keV protons, it can be suggested that the AlGaN/GaN HEMTs have high endurance for exposure to relatively high fluences of the low-energy proton beam.

Saturable absorption is a key technology for shaping the waveform of light such as in passive mode-locking. The combination of high-Q optical microcavities with a saturable absorber allows stable lasing and soliton formation. This work describes saturable absorption by carbon nanotubes (CNTs) on silica microtoroids. CNTs, which are saturable absorbers capable of a fast response time and broadband absorption, were grown on silica microtoroids by chemical vapor deposition (CVD). Raman spectroscopy revealed that the CNTs are in good quality (G/D ratio ∼ 7) and about 1.0 nm in diameter, thus confirming that a sample for use in the telecommunication band can be prepared by CVD. A counter-propagating pump-probe experiment enabled us to investigate the characteristics of CNTs as saturable absorbers while suppressing thermo-optic bistability in a microcavity system. The results revealed a saturable absorption coefficient of 0.042 cm−1, a saturable intensity of 25.9 MW/cm2, and a modulation depth of 28%. This is the first step toward the demonstration of the robust mode-locking in a silica microtoroid consisting of CNTs.Saturable absorption is a key technology for shaping the waveform of light such as in passive mode-locking. The combination of high-Q optical microcavities with a saturable absorber allows stable lasing and soliton formation. This work describes saturable absorption by carbon nanotubes (CNTs) on silica microtoroids. CNTs, which are saturable absorbers capable of a fast response time and broadband absorption, were grown on silica microtoroids by chemical vapor deposition (CVD). Raman spectroscopy revealed that the CNTs are in good quality (G/D ratio ∼ 7) and about 1.0 nm in diameter, thus confirming that a sample for use in the telecommunication band can be prepared by CVD. A counter-propagating pump-probe experiment enabled us to investigate the characteristics of CNTs as saturable absorbers while suppressing thermo-optic bistability in a microcavity system. The results revealed a saturable absorption coefficient of 0.042 cm−1, a saturable intensity of 25.9 MW/cm2, and a modulation depth of 28%. This is t...

In this work, swift heavy ion irradiation induced effects on the electrical properties of single layer graphene are reported. The modulation in minimum conductivity point in graphene with in-situ electrical measurement during ion irradiation was studied. It is found that the resistance of graphene layer decreases at lower fluences up to 3 × 1011 ions/cm2, which is accompanied by the five-fold increase in electron and hole mobilities. The ion irradiation induced increase in electron and hole mobilities at lower fluence up to 1 × 1011 ions/cm2 is verified by separate Hall measurements on another irradiated graphene sample at the selected fluence. In contrast to the adverse effects of irradiation on the electrical properties of materials, we have found improvement in electrical mobility after irradiation. The increment in mobility is explained by considering the defect annealing in graphene after irradiation at a lower fluence regime. The modification in carrier density after irradiation is also observed. Based on findings of the present work, we suggest ion beam irradiation as a useful tool for tuning of the electrical properties of graphene.

Ferrofluids are colloids of superparamagnetic nanoparticles that are envisaged for use in hyperthermia, which is based on nonradiative relaxation after interaction with a high-frequency magnetic field or light. For such applications, an important parameter is the thermal diffusivity. In this communication, we present an experimental study of the dependence of thermal diffusivity of ferrofluids on the size of the magnetite nanoparticles by employing the mode-mismatched thermal lens technique. The results show a huge enhancement of the thermal diffusivity by increasing the average size of the nanoparticles, while the number density of the nanoparticles is maintained as constant.

Microvessels are generally not simple straight tubes, but rather they continually bifurcate (namely, diverging bifurcation) and merge with other microvessels (namely, converging bifurcation). This paper presents a simulation study on the three-dimensional motion and deformation of a red blood cell (RBC) in a bifurcated microvessel with both diverging and converging bifurcations. The motion of the fluids inside and outside of the RBC is modeled by smooth dissipative particle dynamics. The RBC membrane is modeled as a triangular network, having the ability to not only resist the stretching and bending deformations, but also to conserve the RBC volume and surface area. The bifurcation configurations have been studied, including the bifurcated angle and the branch diameter, as well as the RBC properties, including the initial shape, shear modulus, and bending modulus. The simulation results show that the RBC deformation can be divided into five stages, when the RBC flows through a diverging-converging bifurca...

The electrohydrodynamic (EHD) effect (“ion wind”) associated with corona discharges in air has been extensively investigated and modeled. We present a simple experiment that shows how both the magnitude and direction of EHD forces can change in such a way as to impart energy continuously to an oscillating electrostatic pendulum. The amplitude of oscillations of an electrostatic pendulum subject to EHD forces can grow approximately exponentially over a period of minutes, and we describe a qualitative theory to account for this effect, along with implications of these experiments for theories of ball lightning.

We report on the refractive index change in Au3+-doped silicate glasses irradiated by a femtosecond laser and successive heat treatment. The refractive index of the irradiated area increased after the femtosecond laser irradiation and decreased with increasing annealing temperature up to 450°C and then increased again with increasing annealing temperature. Absorption spectra of the glass samples before and after femtosecond laser irradiation and after further annealing manifested the formation of color centers after laser irradiation, disappearance of color centers after annealing at 300°C, and precipitation of gold nanoparticles after annealing at high temperature. The mechanisms of the observed phenomena are discussed.

We report on impedance magnetocardiography studies using high-Tc superconducting quantum interference device (SQUID) magnetometers in a partially shielded environment. The SQUID detects magnetic-field variations due to impedance changes in the thorax at 15kHz. SQUID measurements are compared to impedance magnetocardiograms recorded at 100kHz using a copper pickup coil. An electromagnetic finite element model is used to simulate the distribution of ac during the cardiac cycle.

We have studied the basic conductive characteristics in carbon nanotubes (CNTs) for the purpose of application to a high-frequency device. In the analysis, the current flow in the CNTs is viewed as the quantum transport of electronic waves. First, we analyzed the dispersion relations of electronic waves in the CNTs based on the linear combination of atomic orbitals expansion method. In addition, we investigated the current-density distributions around the circumference of the CNTs and the current-voltage characteristics by using the transfer-matrix method. As a result, the current distributions were found to be significantly controlled by both the chirality of the CNTs and the position of the current sources around the circumference. Based on these results, we propose herein a ferrite device that acts as a filter in the terahertz frequency domain. In this device, the high-frequency current flowing on the CNTs may excite and receive directly the spin waves in the ferrite film beneath them, and the operatin...

A purely optical method to generate and to steer an ultrasonic beam is presented. The generation of the ultrasonic beam in the kilohertz to megahertz regime is achieved by using the optoacoustic effect in a layer attached to the surface of a water tank, which absorbs time-modulated laser pulses. In order to tailor the emitted acoustic wave, diffractive optics concepts are applied to acoustics. To this end, the laser light intensity distribution at the absorbing surface is controlled by sending a computer-calculated pattern to a transmissive high-resolution liquid-crystal display in the beam path. This results in an acoustic analog to the “diffractive optical elements” in laser optics, with the emitted ultrasonic wave emerging in a predetermined way. The proof of principle is demonstrated for examples such as phase gratings and binary Fresnel lenses. The optoacoustic ultrasound generation with holographic diffractive steering is a flexible method with the potential of new applications in ultrasound diagnos...

A study of the synthesis of Co nanoparticles by ion implantation and the effects of postimplantation annealing was carried out. Silica was implanted with 35keV Co+ ion beams to doses ranging from 8×1015to1×1017atoms∕cm2. Nanoparticle size, distribution, and structure were ascertained via transmission electron microscopy measurements, which reveal the presence of spherical nanoparticles in both as-implanted and annealed samples. X-ray photoelectron spectroscopy show the chemical state of the nanoparticles in both as-implanted and annealed samples to be metallic. Temperature-dependent field-cooled and zero-field-cooled, susceptibility measurements indicated superparamagnetic behavior, which is analyzed by accounting for the size dispersion of the nanoparticles. Results showed that the magnetic properties of the specimen can be tailored by implantation conditions. Annealing in vacuum at 900°C for up to 10h leads to an increase of the average nanoparticle size and a broader size distribution. Mie scattering a...

3.0MeV Ni2+ in the beam doses from 1×1013to9×1014ions∕cm2 are implanted into LiNbO3 single crystals at room temperature. After 300°C annealing for 30min in air ambient, dark mode measurement is done by the prism-coupling technique. Waveguides from both raised extraordinary index layer and barrier-confined are formed by low and high beam dose implantation, respectively. In the samples implanted by mediate beam doses, a phenomenon of “missing mode” is observed. The experimental results are analyzed and compared with the simulated results from a theoretical model, which is based on the assumption that the change of index induced by implantation is mainly governed by degradation of polarization and reduction of material density. With a fiber probe, the waveguide loss from single transverse magnetic mode is measured, which is about 3dB∕cm.

The effect of N+ ion irradiation on the magnetic anisotropy of ultrathin Co films in the Pt/Co(1 nm)/Pt (111) system is investigated. The magneto-optical measurements indicate that the as-deposited trilayers display a perpendicular magnetic anisotropy whose properties drastically change with the fluence in the range of 1012 to 1016 ions/cm2. The coercive field progressively decreases as the fluence increases. The superconducting quantum interference device measurements evidence that, for the highest fluences, the paramagnetic limit is not reached moreover an in-plane switching of the anisotropy is observed. Simulations are performed to evaluate damage energy induced by N+ irradiation.

Tungsten nanoparticles were generated by photolytical (UV) laser-activated chemical vapor deposition from WF6/H2/Ar gas mixture. Emission spectroscopy of thermal radiation allowed temperature determination of the nanoparticles while varying the laser fluence. A model including known cooling mechanisms was used to calculate the laser-induced temperature as a function of time and laser fluence, where the only fitting parameter was the absorption efficiency of the particles, obtained from measured temperatures. Size decrease of the particles due to evaporation was modeled at different laser fluences, and connected to size-distribution measurements from transmission electron microscopy micrographs, where a maximum geometric mean diameter (for the experimental conditions used) of 10 nm was obseved at a laser fluence of ∼120 mJ/cm2. Measurements and the model calculations showed that the laser-excited particles reached the melting temperature of tungsten at ∼95 mJ/cm2. Above ∼130 mJ/cm2, very high rates of evap...

We have studied the effects of ion irradiation at low doses (

The effect of laser energy on formation of a plasma inside a cavity was investigated. The temperature and electron number density of laser-induced plasmas in a fused silica cavity were determined using spectroscopic methods, and compared with laser ablation on a flat surface. Plasma temperature and electron number density during laser ablation in a cavity with aspect ratio of 4 increased faster with irradiance after the laser irradiance reached a threshold of 5 GW/cm2. The threshold irradiance of particulate ejection was lower for laser ablation in a cavity compared with on a flat surface; the greater the cavity aspect ratio, the lower the threshold irradiance. The ionization of silicon becomes saturated and the crater depths were increased approximately by 1 order of magnitude after the irradiance reached the threshold. Phase explosion was discussed to explain the large change of both plasma characteristics and mass removal when irradiance increased beyond a threshold value. Self-focusing of the laser be...

A computational method for optimizing both single- and two-layer surface coils is presented that is relatively easy to implement in practice using either a variational approach or through standard numerical matrix diagonalization techniques. The technique is applied, in particular, to develop a self-shielded gradiometer that is relatively immune to radiofrequency (rf) interference from distant sources with a minimal compromise to its ability to sense rf from nearby sources, properties necessary for nuclear quadrupole resonance detection of buried land mines. Results of the optimization procedure are evaluated using finite-element numerical techniques and measurements on prototype coils for a number of configurations. These results show that practical coils can be constructed that approximate the theoretically optimized coils quite well. In addition, the trade off between the surface coil sensitivity and noise immunity is presented for the self-shielded gradiometer configuration.

In electroencephalography (EEG) and event related potentials (ERP), localizing the electrical sources at the origin of scalp potentials (inverse problem) imposes, in a first step, the computation of scalp potential distribution from the simulation of sources (forward problem). This article proposes an alternative method for mimicing both the electrical and geometrical properties of the head, including brain, skull, and scalp tissue with resistors. Two resistor mesh models have been designed to reproduce the three-sphere reference model (analytical model). The first one (spherical resistor mesh) closely mimics the geometrical and electrical properties of the analytical model. The second one (cubic resistor mesh) is designed to conveniently handle anatomical data from magnetic resonance imaging. Both models have been validated, in reference to the analytical solution calculated on the three-sphere model, by computing the magnification factor and the relative difference measure. Results suggest that the mesh...

We examined the classical electrostatic effects due to geometric surface structures on conductive field emission needles numerically using the finite element method and compared our results to several commonly applied analytic relations. Analysis of the morphology of electrochemically prepared Mo needles by high-resolution transmission electron microscopy was incorporated in the numerical analysis in the form of small surface protrusions and gross needle shape. We found that the error between the electrostatic potential defined by popular analytic equations and both analytic equations derived in prolate spheroidal coordinates and finite element method results was significant for ellipsoidal needles with and without surface protrusions. The morphology of the surface protrusion was found to introduce a significant nonlinear potential barrier near the needle surface. Finally we numerically analyzed a nonsymmetric, nonhomogeneous experimental needle indicating that even larger errors in the electrostatic potential can be expected relative to analytic equations.

The sensitivity of positron annihilation spectroscopy (PAS) to process parameters associated with the formation of buried oxide layers in silicon (SIMOX, Separation by IMplantation of OXygen, structures) has been investigated and evaluated. Ion beam current density, implantation temperature, and ion dose and energy were varied over ranges used in standard and low-dose SIMOX formation. Whereas the positron (Doppler broadening) parameter for as-implanted samples was found to have no measurable sensitivity to low-energy ion current density, the results suggest that there is a small dependence on implantation temperature. More significantly, PAS is highly sensitive to ion dose (i.e., to changes in dose of a few percent), and to ion energy. An empirical parameter is proposed as a measure of the former. The results for as-implanted samples suggest that beam-based PAS is an ideal method for improved dose control, particularly during the formation of low-dose SIMOX. Measurements on fully-formed SIMOX samples are ...

We report here synthesis and optical absorption study of energetic ion-irradiated hydroxyl-free ZnO semiconductor quantum dots. Quantum dot samples were synthesized by a quenching technique and 100-MeV chlorine ion was selected for the irradiation experiment with doses 1×1011, 5×1011, 5×1012, and 1013 ions/cm2. With an increase in fluence, the optical absorption spectra of irradiated quantum dots reveal redshift of the energy-gap parameter with respect to unirradiated (virgin) quantum dots. The narrowing of the energy gap of nanoparticles indicate particle growth under ion irradiation which is confirmed from transmission electron microscope images. The possible reason for such variations was encountered using an effective-mass approximation model which fits well for small effective mass and high-dielectric constant (e0).

A planar gradiometric device for estimating current vectors in specimens, was fabricated as two orthogonal magnetic field gradiometers integrated on a 14.5-mm-square substrate. The gradiometers consist of superconducting quantum interference devices (SQUIDs) and pickup coils made from a low-critical-temperature superconductor. The two gradiometers (one for the x direction and one for the y direction) have exactly the same structure, and their gradiometric centers coincide at the center of the substrate. The gradiometric balance of each gradiometer is 1/2000–1/3700, the cross-talk ratio between the two gradiometers is less than 1% below a few hundred hertz, and their intrinsic noise is 2–3 pT/(m√Hz). The gradiometric configurations of both the pickup coils and the SQUIDs provide good gradiometric balance. The parallel-type gradiometric SQUIDs produced a simple yet effective structure, although the superconducting loops of such SQUIDs have conventionally been thought to lead to adverse effects in the presence of a changing magnetic field. Magnetic fields up to 1 mT, i.e., 20 times the geomagnetic field, applied to a zero-field cooled SQUID reduced the critical current, but did not affect the intrinsic noise. This result shows that the configuration of the SQUIDs kept the circulating current in the superconducting loop away from the Josephson junctions.

During electrochemical atomic force microscopy (AFM) measurements that involve metal deposition or dissolution, the photodiode voltage changes when the electrode potential changes. These changes in photodiode voltage are caused by deflection of the laser beam due to concentration gradients near the electrode. In this work, the changes in photodiode voltage versus polarization time were studied using a commercial AFM. For an anodically polarized electrode, the concentration increases near the electrode due to metal dissolution. This causes a steady decrease in photodiode voltage. For a cathodically polarized electrode, oscillations in the photodiode voltage are observed. These oscillations are caused by Rayleigh–Benard instabilities due to the fact that the concentration, and hence the density of the electrolyte near the electrode, is lower than the bulk electrolyte. A mathematical model was constructed that calculates the changes in photodiode voltage with polarization time.

We have investigated the temporal decay of luminescence due to electron–hole droplet (EHD) in silicon-on-insulator (SOI) wafers. The formation of EHD is realized under ultraviolet light excitation because of its shallow penetration depth and the presence of the buried oxide which acts as a diffusion barrier to carriers. The decay mechanism of the EHD luminescence is discussed on the basis of its dependence on the excitation intensity, surface/interface condition, temperature, and thickness of the superficial Si film as well as of time-resolved spectroscopic analysis. The dependence can be explained by assuming that the decay is determined by the concentration of free excitons and the intrinsic lifetime of the spatially localized EHD. We demonstrate that the formation of EHD plays an important role in the luminescence decay observed in a superficial thin film. Comparison of the EHD luminescence decay among SOI wafers fabricated by various techniques shows the great potential of this method for characterization of SOI wafers.

We report on the dependence on field and temperature of the critical current of micro superconducting quantum interference devices (SQUIDs): SQUIDs with diameters as small as 1 μm using Dayem bridges as weak links. We model these SQUIDs by solving the Ginzburg–Landau equations with appropriate boundary conditions to obtain the supercurrent–phase relationships. These solutions show that the phase drops and depression of the order parameter produced by supercurrent flow are often distributed throughout the micro-SQUID structure, rather than being localized in the bridge area, for typical micro-SQUID geometries and coherence lengths. The resultant highly nonsinusoidal current–phase relationships Ic(φ) lead to reduced modulation depths and triangular dependences of the micro-SQUID critical currents on applied magnetic flux Ic(Φ). Our modeling agrees well with our measurements on both Al and Nb micro-SQUIDs.

A new process of shock wave consolidation of nanoparticles into a nanocrystalline coating is theoretically considered. In the proposed scheme, the nanoparticle layers, which are attached to the substrate surface by adhesion, are compacted by plane ultra-short shock waves coming from the substrate. The initial adhesion is self-arisen at any contact between the nanoparticles without a pre-compression. The absence of the nanoparticle ejections due to the shock wave action is connected with the strong adhesive forces, which allow nanoparticles to be attached to each other and to substrate while they are being compacted; this should be valid for small enough nanoparticles. Severe plastic deformation of the nanoparticles and the increased temperature due to collapse of voids between them facilitate their compaction into the monolithic nanocrystalline layer. We consider the examples of Cu and Ni nanoparticles on Al substrate using molecular dynamic simulations. We show the efficiency of the action of multiple sh...

This paper is to study ramping turn-to-turn loss and magnetization loss of a no-insulation (NI) hightemperature superconductor (HTS) pancake coil wound with (RE)Ba2Cu3Ox (REBCO) conduc-tors. For insulated (INS) HTS coils, a magnetization loss occurs on superconducting layers during aramping operation. For the NI HTS coil, additional loss is generated by the “bypassing” current onthe turn-to-turn metallic contacts, which is called “turn-to-turn loss” in this study. Therefore, theNI coil’s ramping loss is much different from that of the INS coil, but few studies have beenreported on this aspect. To analyze the ramping losses of NI coils, a numerical method is developedby coupling an equivalent circuit network model and a H-formulation finite element method model.The former model is to calculate NI coil’s current distribution and turn-to-turn loss, and the lattermodel is to calculate the magnetization loss. A test NI pancake coil is wound with REBCO tapesand the reliability of this model is validated by experiments. Then the characteristics of the NIcoil’s ramping losses are studied using this coupling model. Results show that the turn-to-turn lossis much higher than the magnetization loss. The NI coil’s total ramping loss is much higher thanthat of its insulated counterpart, which has to be considered carefully in the design and operation ofNI applications. This paper also discusses the possibility to reduce NI coil’s ramping loss bydecreasing the ramping rate of power supply or increasing the coil’s turn-to-turn resistivity.

In magnetic hyperthermia, the temperature control within the malignant tissues is an important step to increase the efficiency of the therapy. A temperature analysis is a good method to improve the heating process of the magnetic particles injected within tissues. This paper analyzes the thermal effects induced within malignant tissues by the magnetic systems like: magnetite and Fe-Cr-Nb-B when an external time-dependent magnetic field is applied. The heat generation by Neel and Brown relaxations was modeled using the thermal and magnetic properties of the Fe-Cr-Nb-B particles experimentally determined. A lognormal particle size distribution was considered for these magnetic systems with dimensions from 5 nm to 30 nm. After their injection at the center of the tumor, according to the solution of the transient convection-diffusion equation in a porous medium, the mass concentration of the particles within ferrofluid has a spatial and temporal distribution. The ferrofluid injection process was modeled using...

Topographical evolution of 100 MeV Au8+-ion-irradiated InP surfaces was studied using atomic-force microscopy (AFM). The surfaces were roughened under dense electronic excitations. Root-mean-square roughness measured from AFM studies showed an exponential saturation behavior with fluence. Sample temperature during irradiation was found as a parameter to control the amount of roughness on the surface and the evolution of irradiated surface topography is discussed in terms of thermal spike model. © 2001 American Institute of Physics.

H+ was implanted into single-crystal silicon with a dose of 1×1016/cm2 and an energy of 30 KeV, and then He+ was implanted into the same sample with the same dose and an energy of 33 KeV. Both of the implantations were performed at room temperature. Subsequently, the samples were annealed in a temperature range from 200 to 450 °C for 1 h. Cross-sectional transmission electron microscopy, Rutherford backscattering spectrometry/channeling, elastic recoil detection, and high resolution x-ray diffraction were employed to characterize the strain, defects, and the distribution of H and He in the samples. The results showed that co-implantation of H and He decreases the total implantation dose, with which the surface could exfoliate during annealing. During annealing, the distribution of hydrogen did not change, but helium moved deeper and its distribution became sharper. At the same time, the maximum of the strain in the samples decreased a lot and also moved deeper. Furthermore, the defects introduced by ion i...

Atomic force microscopy, transmission electron microscopy, optical, and magneto-optical microscopy have been used to study how structural and magnetic properties are changed when a Co/Pt multilayer is quasihomogeneously irradiated with Ga ions. Under low irradiation fluence, both grain size and texture in the multilayer increase. These effects continue for fluences in excess of 1×1015 Ga ions/cm2, but beyond this dose significant thinning of the multilayer is also observed. Three distinct irradiation-induced magnetic regimes with sharp transitions between each were identified. For Ga fluences less than 5×1012 ions/cm2, the irradiated region retains perpendicular uniaxial anisotropy but with coercivity lower than that of the as-grown film. For fluences between 5×1012 and 1×1013 Ga ions/cm2, a transition from perpendicular to in-plane magnetization was experienced. Very little change of the in-plane magnetic properties of irradiated multilayers is then observed until the sample experiences a ferromagnetic t...

The poly(phenylcarbyne) polymer films were coated on silicon substrates and then irradiated by a pulsed ultraviolet laser (λ=248 nm) with various fluences (1–60 mJ/cm2) at an atmospheric pressure of nitrogen. The structures of the resulted films were investigated by Raman spectroscopy. The morphologies of the films were examined by scanning electron microscopy (SEM). The electron field emission properties of the films as cathodes were studied. Raman spectrum analysis and SEM results indicate that the polymer film is converted to nanoparticle carbon film with the laser fluence from 10 to 60 mJ/cm2. The conversion mechanism from the polymer to nanoparticle carbon and electron field emission mechanism from the converted carbon film is discussed. The converted carbon film showed better field emission properties, i.e., lower turn-on threshold emission field, higher emission current density, and higher emission light spot density with increasing laser fluence from 10 to 50 mJ/cm2. By increasing the laser fluence from 50 to 60 mJ/cm2, the emission property showed a slight degradation. A turn-on threshold emission field of 1.8 V/μm (at 1 nA/cm2) and emission current density of 30 mA/cm2 with an emission light spot density of 104/cm2 (at 14 V/μm) were observed for the polymer film irradiated with a fluence of 50 mJ/cm2. Circular and multiple-line field emission patterns are demonstrated using the laser irradiation of the poly(phenylcarbyne) polymer.

The decrease in hardness induced by gamma irradiation in poly(methyl methacrylate) (PMMA) has been investigated. The hardness is assumed to decrease linearly with the concentration of radiation-induced defects. Annealing at high temperatures induces defect annihilation as tracked by an increase in hardness. The annihilation follows first-order kinetics during isothermal annealing. The dependence of hardness on the reciprocal of the time constant satisfies the Arrhenius equation, and the corresponding activation energy of the kinetic process decreases with increasing dose. The hardness of postannealed PMMA decreases linearly with increasing dose.

We investigated the influence of grinding as a pretreatment prior to the emitter film formation of single-walled carbon nanotubes (SWNTs) on the field emission and the geometric properties of SWNTs. Field emission from emitter films composed of ground SWNTs was found to be dramatically enhanced as well as to be homogeneous and stable, compared with that from emitter films composed of unground SWNTs. Transmission electron microscopy observations showed that several SWNTs protrude from the ends of ground SWNT bundles. The improved emission current is attributed to the large field enhancement at the sharpened ends of the SWNT bundles.

We report on the effects of photon (γ) and electron (β) irradiation in a dose range extending from 100 to 5×109 Gy in a variety of silica samples studied by electron paramagnetic resonance. The E′ centers and a weak intensity satellite signal of their resonance line were generated both in γ- and in β-irradiated samples. We investigated the dependence of their intensity on the irradiation dose. Evidence of the existence of a common generation mechanism for the related paramagnetic point defects is found. These defects are induced mainly through the conversion of precursors except at very high doses, where the direct activation from the unperturbed matrix is concurrent. Our data support the model attributing the satellite signal to the weak hyperfine structure of the E′ center arising from interaction with a second nearest neighbor nuclear spin.

Boron exhibits anomalous diffusion during the initial phases of ion implant annealing. Boron transient enhanced diffusion is characterized by enhanced tail diffusion coupled with an electrically inactive immobile peak. The immobile peak is due to clustering of boron in the presence of excess interstitials which also enhance boron diffusion in the tail region. We present a simple model for the formation of immobile boron interstitial clusters and associated point defect interactions derived based on atomistic calculations.