SiC电子输运的蒙特卡罗模拟

采用了Monte Carlo方法研究了2H-、4H-和6H-SiC的电子输运特性.在模拟中,采用动量弛豫率近似的方法确定散射角,显著压缩散射次数,并用高效的新查表法确定自由飞行时间,相对于阶梯值的自散射方法,完全消除了自散射,大量节省cpu时间.     

Monte Carlo simulation of beta radiation response function for semiconductor Si detector

This paper describes the modeling of the Passivated Implanted Planar Silicon (PIPS) detector for the beta particles response function simulations with the MCNP-5 code. The simulated and measured energy response functions were compared and a good agreement was found in the entire range of energies. The verified model of a PIPS detector was applied in a non-destructive method that determines the activity of beta emitters in the sample with a known geometry and atomic number densities. The procedure for the identification of beta emitters in the samples was also described. Finally, the application of the proposed method for the determination of ¹³⁷Cs and ⁹⁰Sr activity in water samples taken from the RA reactor spent fuel storage pools and from the paper filter taken from an air monitor operated during repackaging of spent fuel elements, was presented.     

Coupled electron/photon Monte Carlo calculations of X-ray scattering with application to industrial radiography

Monte Carlo transport methods are used to simulate the scattering of X-rays in polystyrene and iron slabs. The calculations are made with monoenergetic X-ray sources in the energy region from 30 keV (100 keV for iron) to 20 MeV. This energy range includes the energy regions for diagnostic radiology (0.03–0.15 MeV), nuclear medicine (0.1–2.0 MeV) and industrial radiography (0.2–20 MeV). slab thicknesses for polystyrene were 53, 100, and 210 mm and for iron 7 and 14 mm. The present calculations include the effects of secondary electron/positron radiation which become quite important at high energies. As a function of the incident photon energy, the ratio of the scattered to the total radiation (scatter fraction) was found to have a characteristic ‘N’ shape. Increasing the atomic number of the scattering media has the effect of ‘squeezing the N’.     

Monte Carlo simulation of electron heating in a one-dimensional GaAs-quantum conductor

The process of transport of one-dimensional electrons in a GaAs quantum conductor is investigated by the Monte Carlo method. It is shown that in addition to scattering by polar optical phonons, the heating of charge carriers is also appreciably affected, in the case of electric field intensities higher than 5.10⁵V/m, by ionized-impurity scattering and the transverse dimensions of the conductor.     

Ballistic electron emission spectroscopy on Ag/Si devices

In this work we report on ballistic electron emission spectroscopy (BEES) studies on epitaxial layers of silver grown on silicon surfaces, with either a Si(111)-(7 ×?7) or Si(100)-(2 ×?1) surface reconstruction. The experiments were done at low temperature and in ultra-high vacuum (UHV). In addition, BEES measurements on polycrystalline Ag films grown on hydrogen-terminated H:Si(111)-(1 ×?1) and H:Si(100)-(2 ×?1) surfaces were performed. The Schottky barrier heights were evaluated by BEES. The results are compared to the values for the barrier height reported for macroscopic Schottky diodes. We show that the barrier heights for the epitaxial films substantially differ from the values measured on polycrystalline Ag films, suggesting a strong effect of the interface on the barrier height.     

Monte Carlo simulation of steps behavior on Si(111) surface during sublimation

Investigation of steps behavior on vicinal (111) surface during sublimation was carried out using 3D-model of diamond-like crystal. Step width periodic variations predicted by Schwoebel were found by modeling. Diamond-like crystal structure results in the asymmetry in atom fluxes to the upper or lower terraces, with the sign of asymmetry depending on the step structure. The essential effect of vacancies created on terraces on migration length of atoms was revealed.     

Expected performance of a hard X-ray polarimeter (POLAR) by Monte Carlo simulation

Polarization measurements of the prompt emission in gamma-ray bursts (GRBs) can provide diagnostic information for understanding the nature of the central engine. POLAR is a compact polarimeter dedicated to the polarization measurement of GRBs between 50 and 300 keV and is scheduled to be launched aboard the Chinese Space Laboratory around the year 2012. A preliminary Monte Carlo simulation has been accomplished to model the expected performance of POLAR while a prototype of POLAR is being constructed. The modulation factor, efficiency and effective area, background rates and minimum detectable polarization (MDP) were calculated for different detector configurations and trigger strategies. With the optimized detector configuration and trigger strategy and the total weight constraint of less than 30 kg, the primary science goal to determine whether most GRBs are strongly polarized can be achieved, and about 9 GRBs/yr can be detected with an MDP<10% for the conservative detector configuration.     

A Monte Carlo investigation of electron backscattering

Kearsley's, Haider's and Frujinoiu's cavity expressions depend upon the existence of reliable data on electron backscattering in different media. These data are not available in the literature. By using MCNP, backscatter distributions and their saturation values were obtained for electron beams with energies between 0.1 MeV and 9 MeV traversing different materials such as polyethylene, solid water, lithium fluoride, aluminium, copper and lead. The data obtained using MCNP show that electron backscatter probability most strongly depends on the scatterer's effective atomic number and electron energy. Backscatter probability becomes less dependent on energy and is mostly a function of the effective atomic number of the scatterer for electron energies below 0.5 MeV. For low atomic number materials MCNP data suggest that the backscatter saturation values are distributed linearly with the effective atomic number of the scatterer for all energies investigated.     

Monte Carlo simulation of a control for cathodic sputtering

Subject is a set-up for gas pressure and discharge power regulation of a cathodic sputtering plant. It comprises two film thickness monitors and a control unit. The film thickness monitors measure the deposition rate at target and substrate level. By means of a computer the control unit calculates a quotient from the monitor data and compares it with a set value. Any change in gas pressure or discharge power leads to a deviation of the quotient from the set value. The control unit eliminates the deviation by a regulation of gas pressure or discharge power. The Monte Carlo method makes it possible to calculate the pressure dependence of the set-sup. It was not possible to determine a relationship between discharge power and the output signal of the control unit by the used theoretical model.     

Application of variance reduction techniques in Monte Carlo simulation of clinical electron linear accelerator

Computation time constitutes an important and a problematic parameter in Monte Carlo simulations, which is inversely proportional to the statistical errors so there comes the idea to use the variance reduction techniques. These techniques play an important role in reducing uncertainties and improving the statistical results. Several variance reduction techniques have been developed. The most known are Transport cutoffs, Interaction forcing, Bremsstrahlung splitting and Russian roulette. Also, the use of a phase space seems to be appropriate to reduce enormously the computing time. In this work, we applied these techniques on a linear accelerator (LINAC) using the MCNPX computer Monte Carlo code. This code gives a rich palette of variance reduction techniques. In this study we investigated various cards related to the variance reduction techniques provided by MCNPX. The parameters found in this study are warranted to be used efficiently in MCNPX code. Final calculations are performed in two steps that are related by a phase space. Results show that, comparatively to direct simulations (without neither variance-reduction nor phase space), the adopted method allows an improvement in the simulation efficiency by a factor greater than 700.     

Monte Carlo simulation of model spin systems

In this paper, applications of the Monte Carlo technique to estimate the static and dynamic properties of model spin systems are discussed. Finite-size effects and choice of boundary conditions in simulating different types of real systems are outlined. Various applications of the Monte Carlo simulations to one-, two- and three-dimensional Ising models and Heisenberg models are dealt with in some detail. Recent applications of the Monte Carlo method to spin glass systems and to estimate renormalisation group critical exponents are reviewed. Communication No. 19 from the Solid State Structural Chemistry Unit.     

Monte Carlo simulation of the electron drift velocity in a one-dimensional GaAs quantum wire

Electron transport in a one-dimensional GaAs quantum wire is modeled in the electrical quantum limit with allowance for scattering of charge carriers by polar optical phonons, impurity ions, and surface inhomogeneities. Pis’ma Zh. Tekh. Fiz. 23, 22–27 (February 12, 1997)     

An approach to Monte Carlo simulation of low-energy electron and photon interactions in air

In this paper we present an approach to an energy deposition model for low energy (10 eV–10 keV) photons and electrons. The model is based on Monte Carlo simulations of the tracks and energy loss resulting from single electron scattering events with the constituent molecules of air, namely N₂ and O₂. The simulation code is based on the Geant4 toolkit. The original physics in Geant4 low-energy processes for photon interactions was used for the incoming photons and secondary X- and gamma-rays, from energies below 1–10 keV. Two new electron interaction processes were developed,and both of them were considered as discrete processes. The new implemented physics is based on experimental cross-sections for elastic and inelastic scattering, complemented with theoretical calculations. Ionization is treated as a subprocess of the inelastic process. The implementation of the new processes in the standard Geant4 frame is considered. Validation tests and comparison with other Monte Carlo results are discussed. The current limitations of this version of the code are also outlined. The code can be of interest in areas where energy deposition at microscopic level is crucial, such as microdosimetry.     

A Monte Carlo simulation model for stationary non-Gaussian processes

A class of stationary non-Gaussian processes, referred to as the class of mixtures of translation processes, is defined by their finite dimensional distributions consisting of mixtures of finite dimensional distributions of translation processes. The class of mixtures of translation processes includes translation processes and is useful for both Monte Carlo simulation and analytical studies. As for translation processes, the mixture of translation processes can have a wide range of marginal distributions and correlation functions. Moreover, these processes can match a broader range of second order correlation functions than translation processes. The paper also develops an algorithm for generating samples of any non-Gaussian process in the class of mixtures of translation processes. The algorithm is based on the sampling representation theorem for stochastic processes and properties of the conditional distributions. Examples are presented to illustrate the proposed Monte Carlo algorithm and compare features of translation processes and mixture of translation processes.     

A novel Monte Carlo simulation method for Ising systems

We propose a Monte Carlo method to obtain the thermodynamic functions of Ising systems. We perform a random sampling of spin configurations to determine the degeneracy of the energies of the system, from which an approximant to the partition function is determined. The main advantage of the method over conventional Metropolis lies in the fact that only a single Monte Carlo run is needed to obtain results valid for all temperatures, magnetic fields, and coupling parameters (FM or AFM). As an illustration of the method, we present results for the Ising model in a magnetic field on a 8x8 lattice. The method can be adapted to tackle the random field Ising model (RFIM), the dilute Ising model, and the Ising spin glass, in any spatial dimension.     

Monte Carlo simulation of tolerance synthesis equations

This paper presents results on ongoing research aimed at developing an integrated computer-aided tolerancing tool. Starting with explicit tolerance analysis equations used to model the relationship between a part's functional elements and an assembly's functional requirements, the reverse synthesis equations are obtained using a simple Jacobian inversion scheme. The explicit nature of the resulting equations lend themselves to conventional Monte Carlo simulation techniques to determine the percentage of rejects being produced given some statistical distributions of the tolerances appearing in these equations. An example is presented that will provide insight on the merits of the developed method.     

Monte Carlo simulation of low density flows

The necessity for adopting a kinetic-theoretical approach to obtain aerodynamic characteristics in low density flow past space vehicles is highlighted in this paper; it is shown how long-standing difficulties in theoretically handling such flows can be circumvented by adopting a Monte Carlo technique. The principles underlying the technique are briefly described, and are first illustrated by applying the technique to the evaluation of the drag of cylinders and cones in collisionless flow. The Markoff process underlying the Monte Carlo simulation of the full Boltzmann equation with collisions is then described in detail. Instead of the time-counter strategy of Bird, a theoretically sounder ‘Random Collision Number’ (RCN) strategy has been adopted in the present simulation. In this strategy the number of collisions in each time-step in the computation is a random number drawn from an appropriate distribution. Computer programs using this strategy have been developed for calculating aerodynamic characteristics like drag and heat transfer for a cone in the transition regime between free molecule and continuum flow. The results obtained from these programs show that both time-counter and RCN strategies require almost the same computer time.     

A spectral representation based model for Monte Carlo simulation

A new model is proposed for generating samples of real-valued stationary Gaussian processes. The model is based on the spectral representation theorem stating that a weakly stationary process can be viewed as a superposition of harmonics with random properties. The classical use of this theorem for Monte Carlo simulation is based on models consisting of a superposition of harmonics with fixed frequencies but random amplitude and phase. The resulting samples have the same period depending on the discretization of the frequency band. In contrast, the proposed model consists of a superposition of harmonics with random amplitude, phase, and frequency so that different samples have different periods depending on the particular sample values of the harmonic frequencies.

A band limited Gaussian white noise process is used to illustrate the proposed Monte Carlo simulation algorithm and demonstrate that the estimates of the covariance function based on the samples of the proposed model are not periodic.     

光子在闪烁晶体中传输的蒙特卡罗模拟

为了找到构筑闪烁晶体探测器的优化方法,使用蒙特卡罗方法对闪烁晶体BGO（Bi4Ge3O12,锗酸铋）的光收集效率进行了模拟研究。模拟结果表明：入射面为粗糙面,其余为抛光面,同时外层包装上高反射率的材料,可得到最大的光输出（约59．1％的光子被收集）;耦合剂的折射率的得到高的光输出也起着非常重要的作用。