高能α粒子辐射屏蔽的蒙特卡罗模拟

本文应用蒙特卡罗方法模拟α粒子在不同材料(铝、聚乙烯、水、液氢)中的输运.根据得到的射程与能量关系及二次粒子种类和数量,评价不同材料对α粒子的屏蔽效果.得出如下结论:在空间辐射防护中,铝并不是最佳的防护材料;在含氢物质中,随着氢含量的增加,辐射防护能力有所提高;并且α粒子随入射深度的能量沉积曲线中出现了典型的布拉格峰.     

硼化钨材料中子屏蔽性能及次级γ剂量产生的模拟研究

对辐射防护材料硼化钨的中子吸收和次级γ射线屏蔽性能进行分析。采用Geant4程序,对材料厚度0～2 cm、能量为热中子～20 MeV的入射中子进行模拟分析。研究结果表明：(1)硼化钨材料主要作用于热中子～10^-2 MeV中子的吸收屏蔽。由不同材料对应的中子宏观分出截面和材料密度可知,厚度一定时,W₂B₅的中子吸收性能最优,质量一定时,WB₄中子吸收性能最优。以热中子为例,W₂B₅材料的中子宏观分出截面约为B203材料的8.67倍,是PB202屏蔽材料的40.59倍;(2)相比于传统中子吸收材料,W-B系化合物在低能中子吸收方面优势更为显著;(3)随着入射中子能量的增大,次级γ剂量对总剂量的贡献呈下降趋势;随着硼化钨材料厚度的增加,次级γ剂量对总剂量的贡献不断升高。为明确硼化钨应用场景及优势,实现中子源屏蔽装置的优化设计提供数据参考,具有实际的工程指导价值。     

BEPCII-LINAC试验束次级粒子的模拟分析及质子能谱测量

对BEPCII-LINAC试验束上1.89 GeV脉冲电子轰击铍靶产生的次级粒子进行蒙特卡罗模拟,得到次级粒子的能谱和角度分布谱.Geant4模拟显示混合粒子中质子能量大于35 MeV的通量较大,且质子产额达到3.97×108s-1,同时质子对器件的注量率φp可达6.32×105s-1·cm-2.用试验束41°磁谱仪和...     

B_4C俘获热中子时产生的次级γ光子所致有效剂量的研究

防护装具用含硼材料中10B俘获热中子时,产生的0.48 MeV次级γ光子经过材料自身衰减后对人体造成的有效剂量,与材料吸收热中子(被俘获)所减少剂量作比较,可评价所用屏蔽材料的合理性。通过建立保守的数学模型,在1.5 cm和2.0 cm两种厚度屏蔽材料情况下,分别计算了未经碰撞的γ光子和经康普顿散射的γ光子所致的总有效剂量。得到的屏蔽材料感生γ射线对人体造成的有效剂量,在材料厚度1.5 cm时为0.89 pSv,2.0 cm时为0.85 pSv,它们都不到防护材料所减少的热中子剂量(10.4 pSv)的8.7%,初步证明材料中使用B4C的合理性。     

离子电离与次级反应耦合的蒙特卡罗模拟方法研究与应用

氘或氚离子在靶物质中电离输运的同时会发生次级反应,为模拟这一过程,开发了耦合蒙特卡罗工具RSMC。程序用详细历史法和浓缩历史法模拟电离过程,调用ENDF或TENDL中D、T核数据计算次级反应,同时使用“强迫次级粒子产生”降方差技巧提高模拟效率。对中子深度分析问题、加速器单能中子源问题和热中子-聚变中子转换靶问题进行研究,验证了RSMC的正确性。     

SRAM单元中子单粒子翻转效应的Geant4模拟

应用Geant4工具,构造了不同特征尺寸的SRAM单元几何模型及单粒子翻转截面计算模型,分析了敏感体积和临界电荷对低能中子单粒子翻转效应的影响趋势,计算了反应堆裂变中子谱辐射环境下,不同特征尺寸SRAM的中子单粒子翻转截面,认为小尺寸SRAM器件的低能中子单粒子翻转效应更为严重。     

屏蔽材料γ射线积累因子的MCNP模拟

在γ射线的屏蔽设计中,选择合适的积累因子对屏蔽效果至关重要.目前,在设计屏蔽材料时,使用单层材料时考虑了轫致辐射的影响,而使用复合材料时并没有考虑.为了提高屏蔽厚度计算的准确性,在考虑轫致辐射的情况下,采用蒙特卡罗(Monte Carlo)方法对单层材料、多层组合材料的积累因子进行研究,模拟了不同γ射线能量和不同屏蔽材...     

PE/CNT复合材料对空间质子的屏蔽效能仿真分析

随着遥感卫星对观测精度需求的增长,国内外积极探索新型高效质子防护材料。为研究聚乙烯掺杂碳纳米管屏蔽质子效能,本文利用Geant4软件开展质子屏蔽仿真,探究碳纳米管掺杂浓度、管壁直径、排布方式和管壁层数对复合材料质子屏蔽效能的影响规律,并与纯聚乙烯进行对比分析。结果表明,质子屏蔽效能对掺杂浓度、管壁直径和排布方式较敏感,受管壁层数影响相对较小;在高掺杂浓度、大管壁直径和规则排布的情况下,复合材料表现出更优异的质子屏蔽效能;相同质量厚度下,碳纳米管掺杂浓度为10%和20%的复合材料质子屏蔽后的电离剂量分别比纯聚乙烯最大降低了7.40%和12.83%。本文研究结果为辐射防护材料设计提供了数据支撑。     

空间质子屏蔽材料优化选择的蒙特卡罗模拟

本文考虑在地球内辐射带、轨道高度为556 km、轨道与赤道倾角为90°的飞行器空间的质子环境,对不同的单质屏蔽材料(铝、钨、钽、铅等)和含钨复合涂层,利用SHIELD程序进行蒙特卡罗(M-C)模拟,分析了经过屏蔽材料层后在硅材料中产生的能量沉积、非电离能损( Non-Ionizing Energy Losses,NIEL)、透射质子能谱、二次粒子及其能量和核反应生成的核素等计算结果,并进行了屏蔽材料组分和排序的优化选择.     

常用防护用品对~(32)P,~(90)Sr+~(90)Y,~(147)Pm放射源屏蔽效果的测定

采用FJ358A型低量程β测量仪测量了P等3种β放射源32的定向剂量当量率,给出了3种源源正面空气中的定向剂量当量率与距离的关系曲线;给出了工作服、乳胶手套、防护眼镜等12种物品对3种源定向剂量当量率的屏蔽效果。提出了几点减少个人浅表剂量当量的建议。     

高能重离子辐照制备SiOC复合材料的研究

用120 kev碳离子注入非晶SiO2薄膜,再用高能Xe、Pb和U离子辐照.注碳剂量范围为2.0×1017-8.6×1017 cm-2,高能离子辐照剂量1.0×1010-3.8×1012 cm-2.辐照后的样品用傅里叶变换红外光谱仪进行系统分析.实验结果显示,高能离子辐照在注碳非晶SiO2薄膜中形成了大量的Si-O-C键和Si-C键.这些Si-O-C结构具有环链、开链和笼链等多种结构形式.随电子能损、辐照剂量或者沉积能量密度的增加,SiOC结构由类笼向环/开链结构演化.对高能重离子驱动产生SiOC结构的机理进行了简单的讨论.     

Secondary neutral and ionized particle measurements under MeV-energy ion bombardment

We have measured both secondary neutral and ionized particles from an InSb target under 3.0 MeV Si ion bombardment. Measurements of both ions and neutrals have not been carried out so far in the MeV-energy range. The mass spectra and axial emission energy distributions of secondary particles were investigated. Secondary ions were measured with a linear- and a reflective-type time-of-flight technique, whereas secondary neutral particles were photo-ionized by a UV pulsed laser (ArF: 193 nm) and measured with a reflective-type time-of-flight technique. Different results were obtained for neutral particles in comparison with ionized particles. The mean energy of neutral Sb atoms was much lower than that of neutral In atoms, whereas the mean energies of secondary In and Sb ions were nearly equal.     

Efficiency enhancements to Monte Carlo simulation of heavy ion elastic recoil detection analysis spectra

Monte Carlo (MC) simulation can be used to simulate heavy ion elastic recoil detection analysis spectra, including the broadening and tailing effects of multiple and plural scattering, although it is very costly in terms of computer time. In this work, kinematic relationships and experimental parameters are exploited to implement efficiency improvements in the MC modeling process. For thin films, incident ions that pass through the sample without undergoing a significant scattering event need not be tracked. Ions that might generate a detectable scattered or recoiled ion are predicted by generating, in advance, the impact parameters which will define its path. Light recoiled target atoms may be dealt with in the same way. For heavy atoms, however, the probability of large angle scattering events is so high that the paths of most recoil atoms are dominated by several scattering events with large angular deflections.     

A new apparatus for deep patterning of beam sensitive targets by means of high-energy ion beam

The paper reports on a high precision equipment designed to modify over 3-dimensions (3D) by means of high-energy heavy ions the local properties of thin and thick films. A target-moving system aimed at creating patterns across the volume is driven by an x–y writing protocol that allows one to modify beam sensitive samples over micrometer-size regions of whatever shape. The moving system has a mechanical resolution of 15 nm. The issue of the local fluence measurement has been particularly addressed. The setup has been checked by means of different geometries patterned on beam sensitive sheets as well as on superconducting materials. In the last case the 3D modification consists of amorphous nanostructures. The nanostructures create zones with different dissipative properties with respect to the virgin regions. The main analysis method consists of magneto-optical imaging that provides local information on the electrodynamics of the modified zones. Features typical of non-linear current flow hint at which pattern geometry is more functional to applications in the framework of confined nanostructuring of superconducting films.     

Three-dimensional particle simulation of ion thruster plume flows with EX-PWS

Ion thruster plumes from a multi-thruster array of different working configurations are simulated by a hybrid fluid-particle software. The particle in cell method is employed to model the transports of ions. The direct simulation Monte Carlo method is used to model momentum and charge exchange (CEX) collisions. The software is based on unstructured grids which make it easy to handle with complex geometry. The results of chamber simulation are compared with experimental data in ion current density and number density, which show good agreements. The maximum difference of current density along the thruster centerline is less than 9.30%. The interaction effects of plumes when multiple thrusters are operating in vacuum are predicted. Distributions of single charged xenon ions are significantly different in the near-field plume flow, however, merge into one in the far downstream region. Moreover, the interaction effect on the spatial distribution of CEX xenon ions is displayed as well.     

Validation of recent Geant4 physics models for application in carbon ion therapy

Cancer treatment with energetic carbon ions has distinct advantages over proton or photon irradiation. In this paper we present a simulation model integrated into the Geant4 Monte Carlo toolkit (version 9.3) which enables the use of ICRU 73 stopping powers for ion transport calculations. For a few materials, revised ICRU 73 stopping power tables recently published by ICRU (P. Sigmund, A. Schinner, H. Paul, Errata and Addenda: ICRU Report 73 (Stopping of Ions Heavier than Helium), International Commission on Radiation Units and Measurements, 2009) were incorporated into Geant4, also covering media like water which are of importance in radiotherapeutical applications. We examine, with particular attention paid to the recent developments, the accuracy of current Geant4 models for simulating Bragg peak profiles of ¹²C ions incident on water and polyethylene targets. Simulated dose distributions are validated against experimental data available in the literature, where the focus is on beam energies relevant to ion therapy applications (90-400 MeV/u). A quantitative analysis is performed which addresses the precision of the Bragg peak position and proportional features of the dose distribution. It is shown that experimental peak positions can be reproduced within 0.2% of the particle range in the case of water, and within 0.9% in the case of polyethylene. The comparisons also demonstrate that the simulations accurately render the full width at half maximum (FWHM) of the measured Bragg peaks in water. For polyethylene slight deviations from experimental peak widths are partly attributed to systematic effects due to a simplified geometry model adopted in the simulation setup.     

小泡径石榴石磁泡材料离子注入层的CEMS研究

近年来,用~(57)Fe的内转换电子穆斯堡尔谱学(CEMS)对离子注入层的磁性进行了一些研究。这些试样的泡径较大。随着磁泡技术向小泡径发展,而小泡径材料的垂直膜面的磁单轴各向异性较大,因此,它将给离子注入带来什么新的问题就为人们所关心。本文报道我们用CEMS对小泡径磁泡石榴石材料离子注入层所作的一些研究。     

重离子束均匀度的测定及治癌束流成形的模拟试验

采用５０μｍ厚聚碳酸酯薄膜，利用核径迹测技术测得φ４０ｍｍ的２５ＭｅＶ／ｕ＾４０Ａｒ＾１４＋离子束的均匀度在散焦情况下为３２．７％，在Ｄ“散焦＋样品旋转”情况下为５２．４％。还采用多层聚碳酸酯薄膜组件作为肿瘤块等效材料，进行了重离子束治癌束流成形模拟试验。     

A segmented conical electric lens for optimization of the beam spot of the low-energy muon facility at PSI: a Geant4 simulation analysis

The low-energy muon facility at PSI provides nearly fully polarized positive muons with tunable energies in the ke V range to carry out muon spin rotation(LE-μSR)experiments with nanometer depth resolution on thin films,heterostructures, and near-surface regions. The low-energy muon beam is focused and transported to the sample by electrostatic lenses. In order to achieve a minimum beam spot size at the sample position and to enable the steering of the beam in the horizontal and vertical direction, a special electrostatic device has been implemented close to the sample position. It consists of a cylinder at ground potential followed by four conically shaped electrodes,which can be operated at different electric potential. In LE-μSR experiments, an electric field at the sample along the beam direction can be applied to accelerate/decelerate muons to different energies(0.5–30 keV). Additionally, a horizontal or vertical magnetic field can be superimposed for transverse or longitudinal field μSR experiments. The focusing properties of the conical lens in the presence of these additional electric and magnetic fields have been investigated and optimized by Geant4 simulations. Some experimental tests were also performed and show that the simulation well describes the experimental setup.     

A segmented conical electric lens for optimization of the beam spot of the low-energy muon facility at PSI: a Geant4 simulation analysis

The low-energy muon facility at PSI provides nearly fully polarized positive muons with tunable energies in the keV range to carry out muon spin rotation (LE-μSR)experiments with nanometer depth resolution on thin films,heterostructures,and near-surface regions.The low-energy muon beam is focused and transported to the sample by electrostatic lenses.In order to achieve a minimum beam spot size at the sample position and to enable the steering of the beam in the horizontal and vertical direction,a special electrostatic device has been implemented close to the sample position.It consists of a cylinder at ground potential followed by four conically shaped electrodes,which can be operated at different electric potential.In LEμSR experiments,an electric field at the sample along the beam direction can be applied to accelerate/decelerate muons to different energies (0.5-30 keV).Additionally,a horizontal or vertical magnetic field can be superimposed for transverse or longitudinal field μSR experiments.The focusing properties of the conical lens in the presence of these additional electric and magnetic fields have been investigated and optimized by Geant4 simulations.Some experimental tests were also performed and show that the simulation well describes the experimental setup.