Secondary electron emission yield calculation performed using two different Monte Carlo strategies

The secondary electron emission yield in Al₂O₃ and polymethylmethacrylate (PMMA) is calculated using two different Monte Carlo approaches, one based on the energy straggling strategy (ES), the other one on the continuous-slowing-down (CSD) approximation. This work is aimed at comparing the secondary electron emission yields calculated by these two Monte Carlo strategies with the available experimental data. The results of both methods are in good agreement with experimental data. The CSD strategy is about 10 times faster than the ES strategy.     

Gap application results for adjacent electron beams treatment

Nowadays, electron beams from high-energy linear accelerators (LINAC) are widely used in a variety of radiotherapy treatments being suitable especially for superficial tumors. Since this sort of ionizing radiation has stopping power higher than photons, deeper and healthier tissues can be preserved. On the other hand, when applying adjacent electron beams, “hot” spots can be observed, due to penumbra and/or scattering, contributing to the increase of the absorbed dose in the target volume. In this sense, the objective of this work was to investigate the effects of parallel adjacent electron beams using the chemical dosimeter Fricke Xylenol Gel (FXG) and compare the experimental results with ones acquired using Monte Carlo simulation. Thus, 10 × 10, 15 × 15 and 20 × 20 cm² fields were irradiated with 5, 8 and 10 MeV electron beams applying different gap widths. The experimental results and the simulations indicated overdose values up to 40% from the prescribed one for the specific tumor. This demonstrates that specific gaps are necessary in the case of treatments with parallel adjacent electron beams in order to prevent overdoses in the depth of interest.     

A novel double dielectric barrier discharge reactor with high field emission and secondary electron emission for toluene abatement

Dielectric barrier discharge (DBD) has been extensively investigated in the fields of environment and energy, whereas its practical implementation is still limited due to its unsatisfactory energy efficiency. In order to improve the energy efficiency of DBD, a novel double dielectric barrier discharge (NDDBD) reactor with high field emission and secondary electron emission was developed and compared with traditional DDBD (TDDBD) configuration. Firstly, the discharge characteristics of the two DDBD reactors were analyzed. Compared to TDDBD, the NDDBD reactor exhibited much stronger discharge intensity, higher transferred charge, dissipated power and gas temperature due to the effective utilization of cathode field emission and secondary electron emission. Subsequently, toluene abatement performance of the two reactors was evaluated. The toluene decomposition efficiency and mineralization rate of NDDBD were much higher than that of TDDBD, which were 86.44%–100% versus 28.17%–80.48% and 17.16%–43.42% versus 7.17%–16.44% at 2.17–15.12 W and 1.24–4.90 W respectively. NDDBD also exhibited higher energy yield than TDDBD, whereas the overall energy constant ${k}_{{\rm{overall}}}$ of the two reactors were similar. Finally, plausible toluene decomposition pathway in TDDBD and NDDBD was suggested based on organic intermediates that generated from toluene degradation. The finding of this study is expected to provide reference for the design and optimization of DBD reactor for volatile organic compounds control and other applications.     

组织等效介质中质子产生的次级电子沿其径迹径向空间分布的研究

为探讨质子在人体组织(或组织等效介质)中能量沉积的微观特性,在对人体组织材料进行分析的基础上,确定了水蒸汽作为组织等效材料的合理性,利用蒙特卡罗方法模拟了质子在介质中的微观行为.通过对计算所得数据进行系统分析,得出质子在组织等效材料中能量沉积微观特性的一些规律与特点.质子在介质中主要把其能量交给电子,最后由电子完成其能量沉积过程,所有这些小能量沉积事件构成了质子在介质中的几何径迹.     

Feedback model of secondary electron emission in DC gas discharge plasmas

Feedback is said to exist in any amplifier when the fraction of output power in fed back as an input.Similarly,in gaseous discharge ions that incident on the cathode act as a natural feedback element to stabilize and self sustain the discharge.The present investigation is intended to emphasize the feedback nature of ions that emits secondary electrons(SEs)from the cathode surface in DC gas discharges.The average number of SEs emitted per incident ion and non ionic species(energetic neutrals,metastables and photons)which results from ion is defined as effective secondary electronemission coefficient(ESEEC,_Eg).In this study,we derive an analytic expression that corroborates the relation between_Eg and power influx by ion to the cathode based on the feedback theory of an amplifier.In addition,experimentally,we confirmed the typical positive feedback nature of SEEfrom the cathode in argon DC glow discharges.The experiment is done for three different cathode material of same dimension(tungsten(W),copper(Cu)and brass)under identical discharge conditions(pressure:0.45 mbar,cathode bias:-600 V,discharge gab:15 cm and operating gas:argon).Further,we found that the_Eg value of these cathode material controls the amount of feedback power given by ions.The difference in feedback leads different final output i.e the power carried by ion at cathode(P_i _C￠∣).The experimentally obtained value of P_i _C￠∣is 4.28 W,6.87 W and9.26 W respectively for W,Cu and brass.In addition,the present investigation reveals that the amount of feedback power in a DC gas discharges not only affect the fraction of power fed back to the cathode but also the entire characteristics of the discharge.     

Formulae for secondary electron yield from insulators and semiconductors

The processes and characteristics of secondary electron emission in insulators and semiconductors were studied, and the formulae for the maximum yield(δ_m) at W_(pOm)≤ 800 eV and the secondary electron yield from insulators and semiconductors δ at the primary incident energy of 2 keV≤ W_(pO) 10 keV(δ_(2-10)) and10 keV ≤ W_(pO)≤100 keV(δ_(10-100)) were deduced. The calculation results were compared with their corresponding experimental data. It is concluded that the deduced formulae can be used to calculate δ_(2-100)at W_(pOm)≤ 800 eV.     

Plasmon excitation and electron promotion in the interaction of slow Na ions with Al surfaces

We studied the energy distributions of electrons emitted in the interaction of slow Na⁺ ions with polycrystalline Al surfaces. To study sub-threshold plasmon excitation we performed measurements as a function of emission angle which showed the excitation of bulk plasmons, confirming the kinetic nature of the excitation process.

Electron spectra show narrow transition lines due to Auger decay in vacuum of sodium projectiles excited in the 2p-shell by electron promotion in collisions with Al target atoms. Several previously unidentified transition lines could be attributed to the autoionization of doubly excited projectiles.     

Monte Carlo method for evaluation of surface emission rate measurement uncertainty

The aim of this study is to evaluate the uncertainty of 2πα and 2πβsurface emission rates using the windowless multiwire proportional counter method.This study used the Monte Carlo method(MCM)to validate the conventional Guide to the Expression of Uncertainty in Measurement(GUM)method.A dead time measurement model for the two-source method was established based on the characteristics of a single-channel measurement system,and the voltage threshold correction factor measurement function was indirectly obtained by fitting the threshold correction curve.The uncertainty in the surface emission rate was calculated using the GUM method and the law of propagation of uncertainty.The MCM provided clear definitions for each input quantity and its uncertainty distribution,and the simulation training was realized with a complete and complex mathematical model.The results of the surface emission rate uncertainty evaluation for four radioactive plane sources using both methods showed the uncertainty's consistency En＜0.070 for the comparison of each source,and the uncertainty results of the GUM were all lower than those of the MCM.However,the MCM has a more objective evaluation process and can serve as a validation tool for GUM results.     

Internal electron emission in metal-insulator-metal thin film tunnel devices bombarded with keV argon and gold-cluster projectiles

The kinetic excitation of a solid surface under bombardment with energetic ions is studied via internal electron emission in a metal-insulator-metal junction. Particular attention was focused on the dependence of the internal emission yield on the impact angle of the projectile ions, which was measured and compared to corresponding data for external emission. We find the internal and external yields to behave differently, therefore delivering complementary information regarding the depth distribution of the kinetic excitation profile. A first attempt to interpret the internal emission yield in terms of a simple electron emission model is described and shown not to be sufficient to explain the experimentally observed behavior. Besides atomic ions, measurements of the internal emission yield under bombardment with cluster projectiles were performed, which are shown to exhibit a nonlinear yield enhancement similar to that of the sputter yield. This finding constitutes a new case in the observation of “vicinage effects” in cluster-initiated kinetic excitation, which is attributed to the transition from a linear collision cascade to a collisional spike.     

Comparison between Monte Carlo and experimental aluminum and silicon electron energy loss spectra

A Monte Carlo (MC) simulation is described and used to calculate the energy distribution spectra of backscattered electrons from Al and Si. For the simulations, elastic scattering cross sections are calculated by numerically solving the Dirac equation in a central field. Inelastic scattering cross sections are computed within the dielectric response theory developed by Ritchie, and by Tung et al. Extension from the optical case to non-zero momentum transfer is done according to Ritchie and Howie. To evaluate surface and bulk contributions to the spectra, the Monte Carlo model treats the surface excitations according to the Werner differential surface and volume excitation probability theory. The Monte Carlo calculations are compared with the experimental reflection electron energy loss (REEL) spectra acquired in our laboratory.     

Comparison of secondary electron emission in helium ion microscope with gallium ion and electron microscopes

To evaluate secondary electron (SE) image characteristics in helium ion microscope, Si surfaces with a rod and step structures is scanned by 30 keV He and Ga ion beams and 1 keV electron beam. The topographic sensitivity of He ions is in principle higher than that for scanning electron microscope (SEM) because of the stronger dependency of SE yield versus incident angle for He ions. As shrinking to sub nm patterns, the pseudo-images constructed from line profile of SE intensity by the electron beam lose their sharpness, however, the images for the He and Ga ion beams keep clearness due to darkening the bottom corners of the pattern. Here, the sputter erosion for Ga ions must be considered. Furthermore, trajectories of emitted SEs are simulated for a rectangular Al surface scanned by the beams to study voltage contrast, where positive and negative voltages are applied to the small area of the sample. Both less high energy component in the energy distribution of SEs and dominant contribution of direct SE excitation by a projectile He ion keep a high voltage contrast down to a sub nm sized area positively biased against the zero-potential surroundings.     

Ion-induced electron emission – monitoring the structure transitions in graphite

The temperature dependence of ion-induced electron emission yield γ under 30 keV Ar⁺ ion impacts at incidence angles θ = 0−80° under dynamically steady-state conditions has been measured for polygranular graphite POCO-AXF-5Q. The fluencies were 10¹⁸–10¹⁹ ion/cm², the temperatures varied from the room temperature (RT) to 400 °C. The RHEED has shown that same diffraction patterns correspond to a high degree of disorder at RT. At high temperature (HT), some patterns have been found similar to those for the initial graphite surfaces. The dependence γ(T) has been found to be non-monotonic and for normal and near normal ion incidence manifests a step-like increase typical for a radiation induced phase transition. At oblique and grazing incidence (θ > 30°), a broad peak was found at T_p = 100 °C. An analysis based on the theory of kinetic ion-induced electron emission connects the behavior of γ(θ,T) to the dependence of both secondary electron path length λ and primary ion ionizing path length R_e on lattice structure that drastically changes due to damage annealing.     

离子束增强沉积碳膜及其在抑制电子发射中的应用

栅电子发射是影响行波管工作性能和制约其使用寿命的主要因素。国内研制的导弹雷达制导用行波管因存在严重的栅电子发射现象,寿命只有70h,不能付诸军事应用。采用离子束增强沉积的方法在栅网表面沉积一层碳膜后,可有效地抑制栅电子发射,使行波管寿命超过了1000h,在军事和航天领域获得了成功应用。报导了低能离子束增强沉积类金刚石薄膜和高能离子束增强沉积类石墨碳膜的制备技术、结构性能及其在抑制栅电子发射中的应用和机理研究的结果。     

MV级EPID成像系统中滤过板的应用

与kV级X射线成像相比,MV级电子射野影像系统(EPID)成像质量较差,图像模糊,影响患者图像定位和摆位验证的精确性。本文结合Monte Carlo模拟计算的方法,在不改变治疗靶的现有MV级EPID系统中,提出增加滤过板进行康普顿散射线的滤过,通过Monte Carlo模拟、Matlab作图、临床实验,得出在6 MV级EPID系统中,增加1层2 mm厚铜材料的滤过板,有助于成像质量的改善。     

Monte Carlo simulation of prompt neutron emission during acceleration in fission

The possibility of prompt neutron emission during acceleration of fission fragments (FFs) was examined by means of Monte Carlo method and statistical neutron emission model. Multimodal random neck-rapture model was used to describe the initial distribution of mass, charge, and total kinetic energy of the primary fragments. Statistical model was used to simulate the de-excitation process of the fragments from the moment of scission until full acceleration. By random number sampling, the fission process was simulated in order to obtain the basic physical quantities, and their correlations were analyzed to verify the adequacy of the model. It was found that, on the average, ～10% and ～16% of prompt neutrons for ²³⁵U(n _th,f) and ²⁵²Cf(sf), respectively, were emitted before reaching 90% of the final fragment kinetic energy.     

A molecular dynamics investigation of kinetic electron emission from silver surfaces under varying angle of projectile impact

We present a computer simulation study on the influence of the impact angle of the projectile on kinetic electron emission yields for 5-keV Ag → Ag bombardment. By means of a hybrid computer simulation model incorporating (i) the particle dynamics following the primary particle impact, (ii) the kinetically induced electronic substrate excitations via electronic friction and electron promotion and (iii) the transport of excitation energy away from the spot of generation, a full three-dimensional electron temperature profile within the volume affected by the atomic collision cascade is calculated. This profile is evaluated at the very surface of the target and taken as input for a thermionic model (‘hot-spot-model’) for kinetic electron emission. Averaging the results for different choices of the polar angle of incidence Θ over a large set of impact points, the obtained kinetic electron emission yields can be compared with experimental data and predictions from simple geometrical calculations. The presented simulation results appear to be reasonable in comparison with experimental data as well as with simple geometrical considerations of kinetic electron emission under oblique incidence.