Dose estimation and shielding calculation for X-ray hazard at high intensity laser facilities

An ionizing radiation hazard produced from the interaction between high intensity lasers and solid targets has been observed. Laser-plasma interactions create "hot" electrons, which generate bremsstrahlung X-rays when they interact with ions in the target. However, up to now only limited studies have been conducted on this laser-induced radiological protection issue. In this paper, the physical process and characteristics of the interaction between high intensity lasers and solid targets are analyzed. The parameters of the radiation sources are discussed, including the energy conversion efficiency from laser to hot electrons, hot electron energy spectrum and electron temperature, and the bremsstrahlung X-ray energy spectrum produced by hot electrons. Based on this information, the X-ray dose generated with high-Z targets for laser intensities between 10¹⁴ and 10²⁰ W/cm² is estimated. The shielding effects of common shielding items such as the glass view port, aluminum chamber wall and concrete wall are also studied using the FLUKA Monte Carlo code. This study provides a reference for the dose estimation and the shielding design of high intensity laser facilities.     

Study of shielding design for SANS at CSNS

The small angle neutron scattering (SANS) instrument is presently being constructed at Chinese Spallation Neutron Source (CSNS) in China, and the biological shielding design is needed to prevent the instrument from causing excessive dose rates in accessible locations. In this paper, the study of shielding design for SANS that relies on Monte Carlo simulation is introduced. Beam line shielding calculations are performed considering both scenarios of closed versus open T0 chopper. The basic design scheme of the beam stop is discussed. The size of the T0 chopper rotor is also estimated.     

N_2-H_2容性耦合等离子体电非对称效应的particle-in-cell/Monte Carlo模拟

H_2-N_2混合气体电容性耦合射频放电在有机低介电系数材料刻蚀中具潜在研究意义.采用paxticle-incell/Monte Carlo模型模拟了双频(13.56 MHz/27.12 MHz)电压源分别接在结构对称的两个电极上的H_2-N_2容性耦合等离子体特征,研究了其电非对称效应.模拟结果表明,通过调节两谐波间的相位角θ,可以改变其电场、等离子体密度、离子流密度的轴向分布及离子轰击电极的能量分布.当相位角θ为0°时,低频电极(晶片)附近主要离子(H_3~+)的密度最小,离子(H_3~+,H_2~+,H~+)轰击低频电极的流密度及平均能量最高;当θ从0°变化90°时,低频电极的自偏压从-103V到106V近似线性增加,轰击电极的离子流密度变化约±18%,H~+离子轰击低频电极的最大能量约减小2.5倍,轰击电极的平均能量约变化2倍,表明氢离子能量和离子流几乎能独立控制.     

飞秒激光等离子体中电子热传导现象的数值模拟

采用相对论电磁粒子模拟程序研究了飞秒激光等离子体相互作用中产生的电流密度、电场和自生磁场的发展演化过程。介绍了电子的非局域热输运的基本特性以及激光加热过程中温度烧蚀前沿稠密等离子体子区的预热效应、临界面附近的限流效应,以及冕区的反扩散与限流效应,得到了经典Spitzer-Harm理论描述的电子热传导随自生磁场的演化情形。数值模拟表明:在线性强激光作用下,由于电子初始时刻的无规则热运动,在等离子体上激发电磁不稳定性,而不稳定性激发的强电磁场使电子束在非常短的距离内沉积能量,同时对在激光有质动力推开电子时形成的超热电子能量输运产生抑制作用。     

水下平台对卫星上行激光通信研究

在合理选择通信系统参量和通信信道参量的基础上,利用Monte Carlo方法模拟了卫星接收到的水下平台上行激光通信信号.分析了卫星接收信号的空间和时间分布特性,以及与望远镜接收视场角的关系,并计算了接收信号的信噪比,得出了3°望远镜视场角和15 μs信号积分时间的优化参量.基于计算结果,根据激光脉冲的PPM调制方式和最大似然检测方法,计算了系统的通信误码率,同时分析了海气界面、云等传输介质对通信的影响.研究结果表明:根据文中给定通信系统参量,在典型海水、海气界面、云等环境条件下,卫星与激光信号中心水平距离5 km范围内信号误码率<10－4.因此位于水下60 m的水下平台可能实现对卫星上行激光通信.     

复杂混合溶液成分高光谱分析的可行性

采用高光谱技术对复杂混合溶液进行检测分析,同时利用被测物质的吸光度和散射特性信息以提高光谱的信噪比。实验设计了高光谱采集装置,采集生物组织模拟液(Intralipid-10%)的漫反射高光谱图像,并用Monte Carlo方法和漫射近似理论对其进行了正向和反向推导,获得了632nm波长下,Intralipid-10%吸收系数为0.002 0cm-1,与标准参数相对误差为11.1%;约化散射系数为63.35cm-1,与标准参数相对误差为6.49%,基本符合标准参数的误差范围,验证了该高光谱检测系统的准确性。还利用该高光谱系统对不同厂家出品的牛奶、果汁等样本进行了高光谱采集,得到不同样本间差异较传统二维光谱更为明显的结果,充分证明了高光谱方法在复杂混合溶液成分分析中具有很强的可行性。     

外磁场中三维混合自旋Ising系统的磁学性质

用Monte Carlo方法研究了外磁场中自旋-1与自旋-3/2混合三维Ising系统的磁性质.结果表明,系统磁矩随晶场的增强阶梯式减小,由于能量简并的解除,外磁场存在时磁矩随晶场变化的台阶数目增多.当单离子晶场DA与DB在同一台阶区域取值时,系统磁矩随外磁场的增强平缓地趋于饱和值,而当二者在不同台阶区域取值时,系统磁矩随外磁场的变化则表现出了较明显的台阶结构.单离子晶场DA与DB对于系统无序相的影响是不同的.     

Monte Carlo方法模拟非直视紫外光散射覆盖范围

针对大气中紫外光散射通信的特点, 用Monte Carlo方法对紫外光非直视(NLOS) 通信三种工作方式的覆盖范围进行分析, 建立了基于Monte Carlo方法的NLOS紫外光传输模型.利用Monte Carlo模拟方法对三种NLOS散射方式的单次和多次散射路径损耗及覆盖范围进行模拟研究, 结果表明, 多次散射和单次散射的路径损耗基本一致, NLOS(a) 类全向发送全向接收通信方式覆盖范围最小但全方位性好, NLOS(b) 类定向发送全向接收通信方式的覆盖范围较大但有一定方向性, NLOS(c) 类定向发送定向接收通信方式的覆盖范围最大但有很强的方向性.     

Simulation study of neutrino nucleus cross section measurement in a segmented detector at a spallation neutron source

Knowledge of ν_e-Fe/Pb differential cross sections for ν_e energy below several tens of MeV scale is believed to be crucial in understanding supernova physics. In a segmented detector at a spallation neutron source, ν_e energy reconstructed from the electron range measurement is strongly affected because both multiple scattering and electromagnetic showers occur along the electron passage in target materials. In order to estimate these effects, a simulation study has been performed with a cube block model assuming perfect tracking precision. The energy spectrum distortion is observed to be proportional to the atomic number of the target material. Feasibility of unfolding the distorted ν_e energy spectrum is studied for both Fe and Pb. An evaluation of the statistical accuracy attainable is therefore provided for a segmented detector.     

Monte Carlo simulation of glandular dose in a dedicated breast CT system

A dedicated breast CT system (DBCT) is a new method for breast cancer detection proposed in recent years. In this paper, the glandular dose in the DBCT is simulated using the Monte Carlo method. The phantom shape is half ellipsoid, and a series of phantoms with different sizes, shapes and compositions were constructed. In order to optimize the spectra, monoenergy X-ray beams of 5-80 keV were used in simulation. The dose distribution of a breast phantom was studied: a higher energy beam generated more uniform distribution, and the outer parts got more dose than the inner parts. For polyenergtic spectra, four spectra of Al filters with different thicknesses were simulated, and the polyenergtic glandular dose was calculated as a spectral weighted combination of the monoenergetic dose.