光子计数探测器的响应函数标定及校正方法

建立了XCounter Flite X1型光子计数探测器的响应函数模型。利用X射线荧光作为射线源，通过蒙特卡罗方法，模拟了入射能谱与探测器材料中的能量沉积能谱。响应函数包括高斯函数与经验型的修正函数，修正函数由直线段构成，通过最小二乘的方法来确定其中的参数，探测器像素不一致性的校正包括阈值校正与响应函数的校正。把经过校正的模型结果与实验结果对比并计算平均误差，得到在各能量段下误差都小于2.5%的结果，利用该响应函数进行能谱恢复与材料分解，两者具有比较理想的结果，充分说明了该校正方法的有效性与准确性。     

Autocorrelation-based blind spreading-factor detection for CDMA

Spreading factor (SF) in code-division multiple-access (CDMA) systems depends on the data rate of the user, and is, therefore, unknown a priori for a communication receiver. In this letter, the blind SF-detection problem in an additive white Gaussian noise channel is studied, and a novel one-step autocorrelation-based SF detector is proposed. Three decision rules are derived to find simple and robust SF detectors for practical communication receivers. Performance of the detectors is studied and compared with the optimal detector via Monte Carlo computer simulations. The autocorrelation-based SF detector appears to give performance close to the optimal detector, assuming perfect knowledge of signal-to-noise ratio (SNR). It is also found to be significantly more robust to SNR estimation errors than the optimal detector.     

The limiting energy resolution of SiC detectors in ion spectrometry

The Monte Carlo method is used to simulate the complete stopping of α particles in SiC. A histogram of energy losses in nuclear-scattering events is obtained. The energy-loss spectrum has the characteristic asymmetric shape with the line full width at the half-maximum FWHM_nucl ≈ 4.22 keV. The final shape of the spectral line is obtained by a convolution with the Gaussian function that describes the contribution of the ionization and noise fluctuations (originated in the detector and instrumentation) to the signal. The resulting value of FWHM for the line is equal to 8.75 keV (at a noise variance of 1.7 keV). The experimental energy resolution of the detectors was found to be poorer than the calculated value by a factor of 2. It is established that the losses of charge during its transport in the detector bulk are insignificant, so that the discrepancy between the calculated and experimental values of the resolution should be attributed to the nonoptimal design of the detector window.     

Coupled Monte Carlo-drift diffusion analysis of hot-electroneffects in MOSFETs

A general method of calculating the substrate current for n-MOSFETs, using a two-dimensional conventional device simulator coupled with a full-band-structure Monte Carlo simulation, has been enhanced through the use of efficient estimators and statistical weighting of the high-energy electrons. The detailed physics of hot-electron effects in MOSFETs is explained on the basis of the energy distribution as a function of the spatial variables. Monte Carlo results show that the distribution function in the region of the device that makes the largest contribution to the substrate current does not fit a Maxwell-Boltzmann function. The effective cooling of the distribution, due in part to energy loss to impact ionization, is shown clearly. The results of the Monte Carlo calculation are used to evaluate the validity of the assumption of a constant mean path for inelastic scattering used in various analytic treatments. The calculated values of substrate current are compared to experimental results     

A new solid-conversion gas detector for high energy X-ray industrial computed tomography

A new type of solid-conversion gas detector is investigated for high energy X-ray industrial computed tomography (HECT). The conversion efficiency is calculated by using the EGSnrc Monte Carlo code on the Linux platform to simulate the transport process of photons and electrons in the detector. The simulation results show that the conversion efficiency could be more than 65%, if the X-ray beam width is less than about 0.2 mm, and a tungsten slab with 0.2 mm thickness and 30 mm length is employed as a radiation conversion medium. Meanwhile the results indicate that this new detector has higher conversion efficiency as well as less volume. Theoretically this new kind of detector could take place of the traditional scintillation detector for HECT.     

Near‐Infrared Light Propagation in an Adult Head Model with Refractive Index Mismatch

We investigate near‐infrared light (NIR) propagation in a model of an adult head using an extensive Monte Carlo (MC) simulation. The adult head model is a four‐layered slab which consists of a surface layer, a cerebrospinal fluid layer, a gray‐matter layer, and a white‐matter layer. We study the effects of a refractive index mismatch on the model, calculating the intensity of detected light, mean flight time, and partial mean flight time of each layer for various refractive indices of the cerebrospinal fluid layer as functions of source‐detector spacing. The Monte Carlo simulation shows that the refractive index mismatch presents very rich results including rapidly decaying intensity of detected light and a peak and cross‐over in the partial mean flight time with source‐detector spacing. We also investigate spatial sensitivity profiles at various source‐detector spacings, discussing the index mismatch effect on the model.     

Investigation of X-ray fluorescence computed tomography (XFCT) and K-edge imaging

This work provides a comprehensive Monte Carlo study of X-ray fluorescence computed tomography (XFCT) and K-edge imaging system, including the system design, the influence of various imaging components, the sensitivity and resolution under various conditions. We modified the widely used EGSnrc/DOSXYZnrc code to simulate XFCT images of two acrylic phantoms loaded with various concentrations of gold nanoparticles and Cisplatin for a number of XFCT geometries. In particular, reconstructed signal as a function of the width of the detector ring, its angular coverage and energy resolution were studied. We found that XFCT imaging sensitivity of the modeled systems consisting of a conventional X-ray tube and a full 2-cm-wide energy-resolving detector ring was 0.061% and 0.042% for gold nanoparticles and Cisplatin, respectively, for a dose of ～ 10 cGy. Contrast-to-noise ratio (CNR) of XFCT images of the simulated acrylic phantoms was higher than that of transmission K-edge images for contrast concentrations below 0.4%.     

Efficient Evaluation of System Reliability by Monte Carlo Method

This paper presents a new Monte Carlo method to estimate the reliability of a large complex system represented by a reliability block diagram or by a fault tree. Two binary functions are introduced; one dominates the system structure function and the other is dominated by the structure function. These functions can be constructed easily by using part of path sets and cut sets of the system. Through the use of these binary functions, two variance-reducing techniques (control variate and importance sampling) are applied to the Monte Carlo evaluation of the system reliability. We prove that the new Monte Carlo method gives a reliability estimate with a smaller variance than that of the crude Monte Carlo method.     

考虑各项异性散射的锗中电子输运的全能带蒙特卡洛模拟

在考虑各向异性散射的基础上,对锗中电子输运特性进行了全能带蒙特卡洛模拟.计算过程如下:锗的全能带由nonlocal empirical pseudopotential 方法求得;态密度的相对值通过不同能量的状态数得到;声子色散谱由adiabatic bond-charge模型求出;电子-声子散射率在低能量时采用费米黄金律得出的非抛物线散射率,高能量则通过态密度对其修正而得到;散射后的状态满足能量守恒和动量守恒.通过比较计算结果与实验报道,证实了该模型算法的正确性,由于该模型能正确反映锗中电子的速度与能量特性,同时又能大大降低散射率的计算成本,故可运用在器件模拟中.     

Effect of electron transport properties on unipolar CdZnTe radiation detectors: LUND, spectrumplus and coplanar grid

Device simulations of (1) the laterally contacted-unipolar-nuclear detector (LUND), (2) the SpectrumPlus, (3) and the coplanar grid made of Cd_0.9Zn_0.1Te (CZT) were performed for ¹³⁷Cs irradiation by 662.15 keV gamma-rays. Realistic and controlled simulations of the gamma-ray interactions with the CZT material were done using the MCNP4B2 Monte Carlo program, and the detector responses were simulated using the Sandia three-dimensional multi-electrode simulation program (SandTMSP). The simulations were done for the best and the worst expected carrier mobilities and lifetimes of currently commercially available CZT materials for radiation detector applications. For the simulated unipolar devices, the active device volumes were relatively large and the energy resolutions were fairly good, but these performance characteristics were found to be very sensitive to the materials properties. The internal electric fields, the weighting potentials, and the charge induced efficiency maps were calculated to give insights into the operation of these devices.     

Lookup tables-based detection of range spread targets in compound Gaussian environment with multiple-pulse non-coherent integration

The paper deals with detection of range spread targets embedded in K-distributed clutter with unknown parameters. A two-step detection algorithm based on multiple-pulse cell-averaging scheme and using lookup tables is proposed. First, the threshold factors that maintain a constant probability of false alarm for various clutter parameters are offline computed and stored. Then, at the detection stage, the most appropriate threshold factor is selected through the estimation of actual environment parameters. We assume that the target energy is spread over a finite number of cells, according to the multiple dominant scattering (MDS) centers model. Next, an expression for the overall reflected target energy, following the multiple-pulse integration, is derived. Finally, we proposed a specific binary hypothesis test by taking into account the number of primary cells, the target energy profile and the number of pulses. The performances analysis of the proposed detector is carried out using Monte Carlo simulations for different clutter parameters and various MDS models. The obtained results are then compared to those of the order statistics-based generalized likelihood ratio test (OS-GLRT). Simulations indicate that the performances of the proposed detector are closely related to the radar resolution, the target energy profile and the number of integrated pulses.     

Bayesian Monte Carlo multiuser receiver for space-time codedmulticarrier CDMA systems

We consider the design of optimal multiuser receivers for space-time block coded (STBC) multicarrier code-division multiple-access (MC-CDMA) systems in unknown frequency-selective fading channels. Under a Bayesian framework, the proposed multiuser receiver is based on the Gibbs sampler, a Markov chain Monte Carlo (MCMC) method for numerically computing the marginal a posteriori probabilities of different users' data symbols. By exploiting the orthogonality property of the STBC and the multicarrier modulation, the computational complexity of the receiver is significantly reduced. Furthermore, being a soft-input soft-output algorithm, the Bayesian Monte Carlo multiuser detector is capable of exchanging the so-called extrinsic information with the maximum a posteriori (MAP) outer channel code decoders of all users, and successively improving the overall receiver performance. Several practical issues, such as testing the convergence of the Gibbs sampler in fading channel applications, resolving the phase ambiguity as well as the antenna ambiguity, and adapting the proposed receiver to multirate MC-CDMA systems, are also discussed. Finally, the performance of the Bayesian Monte Carlo multiuser receiver is demonstrated through computer simulations     

Monte Carlo simulation of boron implantation into single-crystalsilicon

An improved Monte Carlo simulation model has been developed for boron implantation into single-crystal silicon. This model is based on the Marlowe Monte Carlo code and contains significant improvements for the modeling of ion implantation, including a newly developed local electron concentration-dependent electronic stopping model and a newly developed cumulative damage model. These improvements allow the model to reliably predict boron implant profiles not only as a function of energy, but also as a function of other important implant parameters such as tilt angle, rotation angle, and dose. In addition, profiles of implant generated point defects (silicon interstitials and vacancies) can be calculated     

表面照射下激光与生物组织的光热作用分析

建立了表面照射下激光与生物组织光热作用的二维变物性数学模型,采用Monte Carlo模拟方法数值求解了表面照射下组织内激光能量的分布,进而采用有限差分方法数值求解Pennes生物传热方程得到了组织内的温度分布。计算结果表明：假设组织热物性为常数,将高估组织的温升;假设激光能量在组织内按指数规律衰减,计算得到的光束范围内的组织温升高于采用Monte Carlo方法的模拟结果;在功率相同的情况下,光束半径越大,光束范围内组织的温升越小,径向温升区域越大。     

Resolution and noise properties of MAP reconstruction for fully 3-D PET

We derive approximate analytical expressions for the local impulse response and covariance of images reconstructed from fully three-dimensional (3-D) positron emission tomography (PET) data using maximum a posteriori (MAP) estimation. These expressions explicitly account for the spatially variant detector response and sensitivity of a 3-D tomograph. The resulting spatially variant impulse response and covariance are computed using 3-D Fourier transforms. A truncated Gaussian distribution is used to account for the effect on the variance of the nonnegativity constraint used in MAP reconstruction. Using Monte Carlo simulations and phantom data from the microPET small animal scanner, we show that the approximations provide reasonably accurate estimates of contrast recovery and covariance of MAP reconstruction for priors with quadratic energy functions. We also describe how these analytical results can be used to achieve near-uniform contrast recovery throughout the reconstructed volume.     

Modeling hot-electron gate current in Si MOSFET's using a coupleddrift-diffusion and Monte Carlo method

A coupled two-dimensional drift-diffusion and Monte Carlo analysis is developed to study the hot-electron-caused gate leakage current in Si n-MOSFETs. The electron energy distribution in a device is evaluated directly from a Monte Carlo model at low and intermediate electron energies. In the region of high electron energy, where the distribution function cannot be resolved by the Monte Carlo method due to limited computational resources, an extrapolation technique is adopted with an assumption of a Boltzmann tail distribution. An averaging method is employed to extract the effective electron temperature. Channel hot electron injection into a gate via quantum tunneling and thermionic emission is simulated, and electron scattering in the gate oxide is taken into account. The calculated values of gate current are in good agreement with experimental results. The simulation shows that the most serious hot electron injection occurs about 200-300 Å behind the peak of average electron energy due to a delayed heating effect     

Electron transport properties in GaAs at high electric fields

Electron distribution function, drift velocity, mean energy, valley population fractions and diffusion coefficient were calculated at high, up to 100 kV/cm, electric fields. Calculations were carried out by the Monte Carlo method. The three-valley model of GaAs conduction band was used. The obtained results were compared with the experimental data.     

Channeling of low-energy implanted ions through the poly-Si gate

Ion channeling through the poly-Si gate is investigated using Monte Carlo simulations. It is shown that even at very low energies, channeling may lead to dopant penetration through the gate oxide, resulting in large threshold voltage variations, in particular, of narrow-channel, submicron devices, Unlike thermally activated dopant diffusion through the gate dielectric that is severe only in the case of B, direct penetration due to channeling is a potential problem with all commonly used dopants. The maximum channeling range (minimum poly-Si thickness to prevent dopant penetration) is calculated as a function of implant energy for B, P, and As ions     

A Detailed Monte Carlo Study of Multiple Scattering Contamination in Compton Tomography at 90 degrees

A low dose technique has been recently proposed for tomographic studies of the lung, which makes use of a gamma camera to detect 90 degrees Compton-scattered photons from external planar gamma source. In this paper, we present a detailed Monte Carlo study of this technique. A 20 x 20 x 20 cm3 water phantom was simulated as a target and a large gamma camera equipped with an imaging collimator as a detector. The multiple scattering contamination of the single scattered signal was studied as a function of the source-detector geometry and of the incident energy in the range 100-500 keV. The multiple to single scatter ratio has an approximate 1/E0.7 dependence and increases almost linearly with the phantom depth and the transversal thickness at 90 degrees . Simulation has been also performed with a 16 x 16 x 10 cm3 sawdust phantom of 0.3 g/cm3 density; the Monte Carlo results agree to within a few percent with experimental data.     

扫描电子显微学中二次电子发射过程的蒙特卡洛模拟

利用蒙特卡洛模拟固体中电子散射轨迹的计算方法，系统地研究了扫描电镜中二次电子信号的发射过程。该模拟电子与固体相互作用的蒙特卡洛模型包含了级联二次电子产生的过程，并且采用光学介电函数方法描述电子的能量损失和相伴的二次电子激发。由于模拟计算可以给出背散射电子和二次电子的绝对产额，以及它们随加速电压和样品的原子序数的变化关系，因此可以用于模拟元素衬度和形貌衬度像。还计算得到了关于二次电子产生和发射的其它分布，并与实验结果作了比较。