Simulation of depth–dose distributions for various ions in polyethylene medium

Study of depth–dose distributions for intermediate energy ion beams in tissue-like media such as polyethylene (CH₂)_n provides a good platform for further improvements in the fields of hadrontherapy and space radiation shielding. The depth–dose distributions for ¹²C ions at various energies and for light and intermediate ion beams (³He, ¹⁶O, ²⁰Ne and ²⁸Si) as well as for heavy ions ⁵⁶Fe in polyethylene were estimated by using simulation toolkit: Geant4. Calculations were performed mainly by considering two different combinations of standard electromagnetic (EM), binary cascade (BIC), statistical multifragmentation (SMF) and Fermi breakup (FB) models. The energies of the ion beams were selected to achieve the Bragg peaks at predefined position (∼60 mm) and as per their availability. Variations of peak-to-entrance ratio (from 7.44 ± 0.05 to 8.87 ± 0.05), entrance dose (from 2.89 ± 0.01 to 203.71 ± 0.63 MeV/mm) and entrance stopping power (from 3.608 to 208.858 MeV/mm, calculated by SRIM) with atomic number (Z) were presented in a systematic manner. The better peak-to-entrance ratio and less entrance dose in the region Z = 2 to 8 (i.e. ³He to ¹⁶O) may provide the suitability of the ion beams for hadrontherapy.     

Nuclear fragmentation database for GCR transport code development

A critical need for NASA is the ability to accurately model the transport of heavy ions in the Galactic Cosmic Rays (GCR) through matter, including spacecraft walls, equipment racks, etc. Nuclear interactions are of great importance in the GCR transport problem, as they can cause fragmentation of the incoming ion into lighter ions. Since the radiation dose delivered by a particle is proportional to the square of (charge/velocity), fragmentation reduces the dose delivered by incident ions. The other mechanism by which dose can be reduced is ionization energy loss, which can lead to some particles stopping in the shielding. This is the conventional notion of shielding, but it is not applicable to human spaceflight since the particles in the GCR tend to be too energetic to be stopped in the relatively thin shielding that is possible within payload mass constraints. Our group has measured a large number of fragmentation cross sections, intended to be used as input to, or for validation of, NASA’s radiation transport models. A database containing over 200 charge-changing cross sections and over 2000 fragment production cross sections has been compiled. In this report, we examine in detail the contrast between fragment measurements at large acceptance and small acceptance. We use output from the PHITS Monte Carlo code to test our assumptions using as an example ⁴⁰Ar data (and simulated data) at a beam energy of 650 MeV/nucleon. We also present preliminary analysis in which isotopic resolution was attained for beryllium fragments produced by beams of ¹⁰B and ¹¹B. Future work on the experimental data set will focus on extracting and interpreting production cross sections for light fragments.     

The influence of in situ pitch-angle cosine coverage on the derivation of solar energetic particle injection and interplanetary transport conditions

Modelization of solar energetic particle (SEP) events aims at revealing the general scenario of SEP injection and interplanetary propagation and relies on in situ measurements of SEP distributions. In this paper, we study to what extent the LEFS60 and LEMS30 electron telescopes of the Electron Proton Alpha Monitor (EPAM) on board the Advanced Composition Explorer are able to scan pitch-angle distributions during near-relativistic electron events. We estimate the percentage of the pitch-angle cosine range scanned by both telescopes for a given magnetic field configuration. We obtain that the pitch-angle coverage is always higher for LEFS60 than for LEMS30. Therefore, LEFS60 provides more information of the directional distribution of the observed particles. The aim of the paper is to study the relevance of the coverage when fitting LEFS60 particle measurements in order to infer the solar injection and the interplanetary transport conditions. By studying synthetic electron events, we obtain that at least 70% of the pitch-angle cosine range needs to be scanned by the telescope. Otherwise, multiple scenarios can explain the data.     

数字仿真在航空可靠性工程中的应用

回顾近10年来蒙特卡罗数字仿真在我国航空可靠性工程领域的应用和发展,它展示出可靠性仿真在该领域的作用和地位。最后,对今后可靠性仿真研究提出了几点看法。     

镜反射凹面及相互可视表面的轨道外热流计算

通过对空间表面所受外热流以及镜反射表面所具有的特性的分析，提出了采用蒙特卡罗法计算镜反射凹面及镜反射相互可视表面的轨道热流的方法，并对半球凹面和两相互垂直可视平板的轨道热流进行了计算。     

IMPROVED STRATIFIED SAMPLING MONTE CARLO METHOD TO ANALYZE RELIABILITY OF STRUCTURAL SYSTEMS

ＩＭＰＲＯＶＥＤＳＴＲＡＴＩＦＩＥＤＳＡＭＰＬＩＮＧＭＯＮＴＥＣＡＲＬＯＭＥＴＨＯＤＴＯＡＮＡＬＹＺＥＲＥＬＩＡＢＩＬＩＴＹＯＦＳＴＲＵＣＴＵＲＡＬＳＹＳＴＥＭＳＬｉＱｉａｎｇ；ＦｅｎｇＹｕａｎｓｈｅｎｇ（ＤｅｐａｒｔｍｅｎｔＯｆＡｉｒｃｒａｆｔＥｎ...     

脉冲雷达频率稳定度问题的仿真研究

本文用蒙特卡罗计算机仿真法研究振荡信号频率起伏对脉冲雷达性能的影响。由于现代脉冲雷达存在明显的混叠效应,和由放大链引入附加噪声等情况,使其仿真方法要比连续波时复杂得多。本文提出的五阶ARMA滤波以及间隔取数据的办法可较好地解放混叠效应的仿真问题。文末举脉冲多普勒雷达为例,介绍仿习具体步骤和仿真结果。     

应用于DSMC方法的二维Voronoi网格技术

研究了Voronoi网格技术并将其应用于直接模拟蒙特卡洛DSMC(Direct Simulation Monte Carlo)计算.基于Dirichlet镶嵌与Voronoi图理论,Voronoi网格利用特征点表征网格单元,具有建立粒子与网格单元之间映射关系的独特算法,适合于DSMC方法的统计特点.在剔除过于靠近边界的特征点以及必要情况下边界细化的基础上,通过区分由边界节点表征的非完整Voronoi网格单元以及由计算区域内镶嵌点表征的完整Voronoi网格单元,解决了Voronoi网格的二维边界匹配问题.Voronoi网格技术支持自适应DSMC计算.映射效率对比表明,Voronoi网格的DSMC计算效率高于三角形网格,低于多级直角网格.通过MEMS微喷管流动数值模拟,验证了Voronoi网格技术在DSMC方法中的有效性.      

星载红外探测阵列对导弹跟踪定位的研究

通过分析研究建立了前视红外探测阵列 (FL IR)对导弹进行精确跟踪、定位的数学模型 ,其中包括导弹的运动模型、大气干扰模型和探测阵列的观测模型。根据探测阵列的原始观测数据 ,利用扩展卡尔曼滤波器 (EKF)精确跟踪导弹目标。由于导弹与探测器的距离较远 ,故可视为点目标。导弹在探测阵列上投影的位置由两部分组成 :导弹真实运动位置和由于大气干扰造成的偏移。滤波器分别估计了这两种位移在探测阵列上的变化。最后用蒙特卡罗方法分析了滤波器的性能。