Analysis of Flexural Vibrations of a Piezoelectric Semiconductor Nanoplate Driven by a Time-Harmonic Force

The performance of devices fabricated from piezoelectric semiconductors, such as sensors and actuators in microelectromechanical systems, is superior; furthermore, plate structures are the core components of these smart devices. It is thus important to analyze the electromechanical coupling properties of piezoelectric semiconductor nanoplates. We established a nanoplate model for the piezoelectric semiconductor plate structure by extending the first-order shear deformation theory. The flexural vibrations of nanoplates subjected to a transversely time-harmonic force were investigated. The vibrational modes and natural frequencies were obtained by using the matrix eigenvalue solver in COMSOL Multiphysics 5.3a, and the convergence analysis was carried out to guarantee accurate results. In numerical cases, the tuning effect of the initial electron concentration on mechanics and electric properties is deeply discussed. The numerical results show that the initial electron concentration greatly affects the natural frequency and electromechanical fields of piezoelectric semiconductors, and a high initial electron concentration can reduce the electromechanical fields and the stiffness of piezoelectric semiconductors due to the electron screening effect. We analyzed the flexural vibration of typical piezoelectric semiconductor plate structures, which provide theoretical guidance for the development of new piezotronic devices.

Probing anomalous quartic $$\gamma \gamma \gamma \gamma $$ couplings in light-by-light collisions at the CLIC

The anomalous quartic neutral couplings of the $$\gamma \gamma \gamma \gamma $$ γ γ γ γ vertex in a polarized light-by-light scattering of the Compton backscattered photons at the CLIC are examined. Both differential and total cross sections are calculated for $$e^+e^-$$ e + e - collision energies 1500 GeV and 3000 GeV. The helicity of the initial electron beams is taken to be $$\pm \,0.8$$ ± 0.8 . The unpolarized and SM cross sections for the same values of helicities are also estimated. The 95% C.L. exclusion limits on two anomalous photon couplings $$\zeta _1$$ ζ 1 and $$\zeta _2$$ ζ 2 are calculated. The best bounds on these couplings are found to be $$6.85 \times 10^{-16} \text { GeV}^{-4}$$ 6.85 × 10 - 16 GeV - 4 and $$1.43 \times 10^{-15} \text { GeV}^{-4}$$ 1.43 × 10 - 15 GeV - 4 , respectively. The results are compared with the exclusion bounds obtained previously for the LHC and HL-LHC. It is shown that the light-by-light scattering at the CLIC, especially the polarized, has a greater potential to search for the anomalous quartic neutral couplings of the $$\gamma \gamma \gamma \gamma $$ γ γ γ γ vertex.

Resonant Ultrarelativistic Electron–Positron Pair Production by High-Energy Electrons in the Field of an X-ray Pulsar

The process of resonant high-energy electron–positron pair production by an ultrarelativistic electron colliding with the field of an X-ray pulsar is theoretically investigated. Resonant kinematics of the process is studied in detail. Under the resonance condition, the intermediate virtual photon in the X-ray pulsar field becomes a real particle. As a result, the initial process of the second order in the fine structure constant effectively reduces into two successive processes of the first order: X-ray-stimulated Compton effect and X-ray-stimulated Breit–Wheeler process. For a high-energy initial electron all the final ultrarelativistic particles propagate in a narrow cone along the direction of the initial electron momentum. The presence of threshold energy for the initial electron which is of order of 100 MeV for 1-KeV-frequency field is shown. At the same time, the energy spectrum of the final particles (two electrons and a positron) highly depends on their exit angles and on the initial electron energy. This result significantly distinguishes the resonant process from the non-resonant one. It is shown that the resonant differential probability significantly exceeds the non-resonant one.

Electron contamination for 6 MV photon beams from an Elekta linac: Monte Carlo simulation

In external beam radiotherapy, the photons from a linear accelerator (linac) machine undergo multiple interactions, not only in the patient but also in the linac head and the air column between the linac head and the patient. Electrons are released from these interactions and contaminate the beams. The current study evaluates electron contamination for 6 MV photon beams from an Elekta linac using Monte Carlo simulation. The linac head was simulated by the BEAMnrc code and the absorbed dose in a phantom was calculated using the DOSXYZnrc code. The parameters of the initial electron beams on the target, such as mean energy and radial intensity distribution, were determined by matching the calculated dose distributions with the measured dose (at 10 x 10 cm2 field size and 90 cm source-skin distance). The central axis depth-dose curves of electron contamination were calculated for various field sizes from 5 x 5 cm2 to 40 x 40 cm2. We investigated the components that generated the electron contamination for a field size of 10 x 10 cm2. The optimal initial electron beam energy was 6.3 MeV with a full-width half maximum (FWHM) of the radial intensity distribution of 1.0 mm. These parameters were found to be in good agreement with the measured data. Electron contamination increased as the field size increased. At a depth of 1.0 mm and field sizes of 5 x 5, 10 x 10, 20 x 20, 30 x 30, and 40 x 40 cm2, the doses from electron contamination were 3.71, 5.19, 14.39, 18.97 and 20.89 %, respectively. Electron contamination decreased with increased depth. At a depth of 15 mm, the electron contamination was about 1 %. It was mainly generated in the air column between the linac head and the phantom (3.65 %), the mirror (0.99 %), and the flattening filter (0.59 %) (for the depth of 1.0 mm and the field size of 10 x 10 cm2).

Linac photon beam fine-tuning in PRIMO using the gamma-index analysis toolkit

Background In Monte Carlo simulations, the fine-tuning of linac beam parameters to produce a good match between simulated and measured dose profiles is a lengthy, time-consuming and resource-intensive process. The objective of this study is to utilize the results of the gamma-index analysis toolkit embedded inside the windows-based PRIMO software package to yield a truncated linac photon beam fine-tuning process. Methods Using PRIMO version 0.1.5.1307, a Varian Clinac 2100 is simulated at two nominal energy configurations of 6 MV and 10 MV for varying number of histories from 106 to more than 108. The dose is tallied on a homogeneous water phantom with dimensions 16.2 × 16.2 × 31.0 cm3 at a source-to-surface-distance of 100.0 cm. For each nominal energy setting, two initial electron beam energies are configured to reproduce the measured percent depth dose (PDD) distribution. Once the initial beam energy is fixed, several beam configurations are sequentially simulated to determine the parameters yielding good agreement with the measured lateral dose profiles. The simulated dose profiles are compared with the Varian Golden Beam Data Set (GBDS) using the gamma-index analysis method incorporating the dose-difference and distance-to-agreement criteria. The simulations are run on Pentium-type computers while the tuned 10 MV beam configuration is simulated at more than 108 histories using a virtual server in the Amazon.com Elastic Compute Cloud. Results The initial electron beam energy configuration that will likely reproduce the measured PDD is determined by comparing directly the gamma-index analysis results of two different beam configurations. The configuration is indicated to yield good agreement with data if the gamma-index passing rates using the 1%/1 mm criteria generally increase as the number of histories is increased. Additionally at the highest number of histories, the matching configuration gives a much higher passing rate at the 1%/1 mm acceptance criteria over the other competing configuration. With the matching initial electron beam energy known, this input to the subsequent simulations allows the fine-tuning of the lateral beam profiles to proceed at a fixed yet lower number of histories. In a three-stage serial optimization procedure, the first remaining beam parameter is varied and the highest passing rate at the 1%/1 mm criteria is determined. This optimum value is input to the second stage and the procedure is repeated until all the remaining beam parameters are optimized. The final tuned beam configuration is then simulated at much higher number of histories and the good agreement with the measured dose distributions is verified. Conclusions As physical nature is not stingy, it reveals at low statistics what is hidden at high statistics. In the matter of fine-tuning a linac to conform with measurements, this characteristic is exploited directly by the PRIMO software package. PRIMO is an automated, self-contained and full Monte Carlo linac simulator and dose calculator. It embeds the gamma-index analysis toolkit which can be used to determine all the parameters of the initial electron beam configuration at relatively lower number of histories before the full simulation is run at very high statistics. In running the full simulation, the Amazon.com compute cloud proves to be a very cost-effective and reliable platform. These results are significant because of the time required to run full-blown simulations especially for resource-deficient communities where there could just be one computer as their sole workhorse.