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.     

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.     

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.     

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.     

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.     

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。     

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.     

Secondary electron emission from a thin carbon foil induced by equi-velocity protons and electrons of 0.5-3.5 MeV/u

The statistical distributions of the number of simultaneously forward-emitted secondary electrons (SEs) from a thin carbon foil have been measured with H⁰ and H⁺ projectiles of 0.5-3.5 MeV in coincidence with the foil-transmitted protons, irrespective of projectile incident charge state. From these results, the probabilities of the simultaneous emission of n electrons and the SE yields by loss electrons have been evaluated. At energies of 2.5-3.5 MeV, the obtained emission probabilities and the SE yields induced by loss electrons are larger than the corresponding ones by protons of the same velocity. Furthermore, the material parameter is found to be almost constant and is slightly larger than that for the protons. On the other hand, the forward SE yields by loss electrons is decreasing to zero with decreasing the incident energy. This behavior is consistent with the range-energy relation of electrons in carbon.     

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.     

蒙特卡罗法内照射剂量计算中不同物理模型的比较

在核医学内照射剂量计算中,Monte Carlo方法是一种十分重要的方法。但其计算结果的准确性直接取决于所使用的物理模型。分析了光子与物质作用过程的三种模型,比较了它们各自的特点,使用条件和对计算结果的影响,以便于科研工作中正确的选用。     

Axial ion–electron emission microscopy of IC radiation hardness

A new system for performing radiation effects microscopy (REM) has been developed at Sandia National Laboratory in Albuquerque. This system combines two entirely new concepts in accelerator physics and nuclear microscopy. A radio frequency quadrupole (RFQ) linac is used to boost the energy of ions accelerated by a conventional Tandem Van de Graaff–Pelletron to velocities of 1.9 MeV/amu. The electronic stopping power for heavy ions is near a maximum at this velocity, and their range is 20 μm in Si. These ions therefore represent the most ionizing form of radiation in nature, and are nearly ideal for performing single event effects testing of integrated circuits. Unfortunately, the energy definition of the RFQ-boosted ions is rather poor ( a few %), which makes problematic the focussing of such ions to the submicron spots required for REM. To circumvent this problem, we have invented ion electron emission microscopy (IEEM). One can perform REM with the IEEM system without focussing or scanning the ion beam. This is because the position on the sample where each ion strikes is determined by projecting ion-induced secondary electrons at high magnification onto a single electron position sensitive detector. This position signal is then correlated with each REM event. The IEEM system is now mounted along the beam line in an axial geometry so that the ions pass right through the electron detector (which is annular), and all of the electrostatic lenses used for projection. The beam then strikes the sample at normal incidence which results in maximum ion penetration and removes a parallax problem experienced in an earlier system. Details of both the RFQ-booster and the new axial IEEM system are given together with some of the initial results of performing REM on Sandia-manufactured radiation hardened integrated circuits.     

放射性核素电离空气电子离子对产生率计算

α衰变核素和β衰变核素发射的射线具有电离空气能力而被广泛应用于工业生产中。为研究α或β衰变核素对空气电离能力的大小,本研究采用蒙特卡洛方法模拟理想放射源发出带电粒子在空气中的能量沉积,并结合空气电离理论,计算放射源表面不同距离处电子离子对产生率。利用此计算方法,研究放射源形状、粒子能量、活度和粒子能量分布对电子离子对产生率的影响。结果表明,放射源表面空气中电子离子对产生率的大小主要受放射源活度的影响,而粒子能量及能谱分布等主要影响电离空气范围及电子离子对产生率衰减速率; 3.7×10⁶ Bq/cm²的α放射源最大电子离子对产生率可达10¹¹~10¹² cm^-3•s^-1量级, 3.7×10⁶ Bq/cm²的β放射源最大电子离子对产生率可达10⁹~10¹⁰ cm^-3•s^-1量级。研究结果可提供数据支持,为新的放射性同位素应用技术开发提供理论指导。