源强对Rossi-α噪声分析的影响

通过数值模拟手段研究了源强对Rossi-测量的影响。为了定量研究中子源强对Rossi-测量的影响,基于MCNP软件;开发了用于计算Rossi-分布的数值模拟工具。反应堆模型和加速器驱动次临界系统的示意模型的数值模拟结果被用于展示瞬发中子衰减常数与源强之间的关联。数值模拟结果表明,入射源强源中子强度对Rossi-测量结果有显著影响。对于处于次临界状态的反应堆和加速器驱动次临界系统模型,在入射源强较小时,Rossi-方法可以正确给出反应堆中子衰减常数,但当入射源强较大时,测量不能给出正确的中子衰减常数。通过研究源强和测量结果的关系,可以找到能够给出正确测量结果的最大可用源强。通过选择一个可用源强范围内的较强中子源,可以减少测量所需时间。     

源强对Rossi-α噪声分析的影响

通过数值模拟手段研究了源强对Rossi-测量的影响。为了定量研究中子源强对Rossi-测量的影响,基于MCNP软件;开发了用于计算Rossi-分布的数值模拟工具。反应堆模型和加速器驱动次临界系统的示意模型的数值模拟结果被用于展示瞬发中子衰减常数与源强之间的关联。数值模拟结果表明,入射源强源中子强度对Rossi-测量结果有显著影响。对于处于次临界状态的反应堆和加速器驱动次临界系统模型,在入射源强较小时,Rossi-方法可以正确给出反应堆中子衰减常数,但当入射源强较大时,测量不能给出正确的中子衰减常数。通过研究源强和测量结果的关系,可以找到能够给出正确测量结果的最大可用源强。通过选择一个可用源强范围内的较强中子源,可以减少测量所需时间。     

可移动式中子监测隐性爆炸物系统的初步探索与研究

瞬发γ中子活化分析技术具有快速、原位、不需取样、准确、灵敏度高且能够实时多核素在线分析的特点,因此该技术是监测隐性爆炸物,尤其是非金属类爆炸物的最有效手段之一.在国内外有不少科技工作者对中子法监测隐性爆炸物技术进行了大量的研究,并取得了一定的成绩.主要对中子技术探测地雷和隐性爆炸物的各种方法和技术路线进行讨论,对同位素镅铍中子源和14 MeV脉冲中子管活化分析方法进行了初步探索研究,并对可移动式系统的源探的几何布置进行了探讨.Because it can on line analyze many elements quickly and precisely without sampling and movement, prompt gamma neutron activation analysis is one of the most effective methods to monitor latent dynamite especially nonmetal. Many researchers studied the neutron detecting latent dynamite technique and get some achievements. This paper mainly discussed each method and technology route of neutron detecting landmine and latent dynamite, studied the Am Be isotope neutron source and 14 MeV pulse neutron tube activation analysis, and analyzed the geometrical layout of movable system.     

核查中的中子探测技术研究

介绍了通过探测中子辐射,用被动方法研究了包括模拟核弹头装置在内的各钚组合系统的可探测性和中子能谱等内容．用主动方法研究了核材料铀的裂变缓发中子测量技术和区分核与非核材料的方法．讨论了进一步开展的核查技术实验.In this paper,by neutron radiation detection with the passive method, the study on detectability, neutron spectra, etc. for the plutonium combined systems including the stimulated nuclear warhead assembly are introduced. The sutdy on technique of measuring fission delayed neutrons from uranium materials and the method of distinguished nuclear from non nuclear materials with the active method are described. Further experiments on verifying technique are also discussed.     

基于瞬发伽马中子活化分析技术的金属样品检测研究

在合金材料的生产过程中,不同金属元素的含量改变对产品的性能以及可靠性有重要的影响,通过在线检测技术能够对产品中不同元素含量进行实时分析,从而指导其工业生产过程,提高产品质量。目前常规的无损检测方法受限于分析深度,无法对样品内部成分进行分析,从而影响测量结果的准确性。瞬发伽马中子活化分析（PGNAA）技术是一种高灵敏度、多元素同时分析的无损检测技术,可以对大块样品进行快速分析。针对PGNAA技术在大体积金属样品检测的可行性进行研究,通过测量快中子与样品发生非弹性散射反应激发的伽马射线对样品成分进行分析。搭建了一套测量系统,包括D-T中子发生器,中子反射体,中子准直体,高纯锗（HPGe）探测器及探测器屏蔽防护。首先,对Fe,Ti,Cr,Ni和Cu等5种金属元素进行了分析研究。对不同质量下的样品进行测量,通过伽马能谱处理软件GAMMAFIT对HPGe探测器测量到的特征伽马峰进行拟合,获得全能峰的净面积。分析全能峰净面积与样品质量之间的响应,对探测器的探测效率变化造成的非线性响应进行修正,得到不同元素的校准曲线,结果显示各元素修正后的校准曲线均具有良好的线型关系。对不同元素的质量检测限进行分析,不同金属元素的质量检测限分别为Fe（44 g）,Ti（25 g）,Cr（33 g）,Ni（108 g）和Cu（72 g）。利用测量系统对不锈钢合金样品中的Fe和Cr元素含量测量开展了研究,通过测量标准样品建立了Fe和Cr元素的定标曲线,并对未知样品进行了测量分析。同时与X射线荧光光谱（XRF）测量结果进行了对比分析,结果表明两种方法的Fe元素和Cr元素测量值偏差分别为4.08%和2.97%。研究结果表明,利用PGNAA技术可以对多种金属元素和合金样品进行测量分析,为后续其他金属样品的检测奠定了研究基础。     

散射中子对超瞬发临界系统动态行为的影响

在点反应堆理论的基础上,建立了研究散射中子影响超瞬发临界系统动态行为的模拟计算方法, 利用CFBR-Ⅱ堆的脉冲实验验证了该方法的正确性。通过分析计算表明, 在超瞬发临界状态, 散射中子的影响包括增加反应性、使脉冲波形后沿展宽和增加脉冲裂变产额。建立的模拟计算方法为预测脉冲堆外各种物体产生的散射中子的动力学效应提供了途径, 对提高脉冲堆的核安全有应用价值。     

散射中子对超瞬发临界系统动态行为的影响北大核心CSCD

在点反应堆理论的基础上,建立了研究散射中子影响超瞬发临界系统动态行为的模拟计算方法,利用CFBR-Ⅱ堆的脉冲实验验证了该方法的正确性。通过分析计算表明,在超瞬发临界状态,散射中子的影响包括增加反应性、使脉冲波形后沿展宽和增加脉冲裂变产额。建立的模拟计算方法为预测脉冲堆外各种物体产生的散射中子的动力学效应提供了途径,对提高脉冲堆的核安全有应用价值。     

2.8MeV中子诱发238U裂变的瞬发中子谱高能段实验测量

在中国原子能科学研究院的高压倍加器上利用中子飞行时间方法测量了2.8 MeV中子引起238U裂变的瞬发中子能谱,通过增大样品量和设计合适的屏蔽体提高了效应/本底比,使得测量数据的不确定度满足预期目标（在5.5～14 MeV能区内,能量间隔为0.5 MeV条件下能谱的不确定度小于10%）。将实验几何、中子源能量分布及角分布、探测效率、束流的时间结构等实验条件输入到MCNP程序里,模拟了出射的中子谱,模拟结果与测量结果在不确定度范围内一致,验证了在入射中子能量较低时238>U的裂变瞬发中子谱评价数据的可靠性。The Prompt Fission Neutron Spectrum (PFNS) of 238U induced by 2.8 MeV neutron was measured using the Cockcroft-Walton accelerator in China Institute of Atomic Energy (CIAE). The signal/background ratio was improved by increasing the amount of sample mass and using an appropriate shielding system. The final uncertainty of neutron energy spectrum in 5.5～14 MeV region is less than 10% with a bin size of 0.5 MeV which has reached this project's anticipation. The experimental geometry, the angular distribution and energy distribution of neutron source, the detection efficiency and time structure of deuteron beam were inputted into the MCNP code to simulate the outgoing neutron spectrum. The simulated results agree with the experimental ones within the uncertainty. The result indicates that the evaluated PFNS for 238U at low neutron energy is reliable.     

临界堆中子增殖系数测量统计涨落数值模拟

建立了临界堆中子增殖系数(参数)计算模型,采用闪烁体加光电倍增管组成的电流型探测器对参数进行测量。采用构造理想函数加统计涨落偏差的方式,利用数值模拟方法,研究了统计涨落对参数测量结果的影响。结果表明:测量信号统计涨落会影响参数测量精度;考虑统计涨落影响,为使参数满足要求,需要使探测器入射粒子数达到一定数量;针对不同测量时间,入射粒子起始强度需要满足不同要求;当起始强度不能满足要求时,可以采用增加测量时间;指数信号带随机涨落时,较大的求值区间能有效减弱统计涨落的影响。     

临界堆中子增殖系数测量统计涨落数值模拟

建立了临界堆中子增殖系数(参数)计算模型,采用闪烁体加光电倍增管组成的电流型探测器对参数进行测量。采用构造理想函数加统计涨落偏差的方式,利用数值模拟方法,研究了统计涨落对参数测量结果的影响。结果表明：测量信号统计涨落会影响参数测量精度;考虑统计涨落影响,为使参数满足要求,需要使探测器入射粒子数达到一定数量;针对不同测量时间,入射粒子起始强度需要满足不同要求;当起始强度不能满足要求时,可以采用增加测量时间;指数信号带随机涨落时,较大的求值区间能有效减弱统计涨落的影响。     

CSR的辐射防护

CSR(cooling storage ring)按计划将于2005年底建成调束,届时从¹²C到²³⁸U的重离子将可以分别被加速到900和400MeV的能量. HIRFL(兰州重离子加速器Heavy Ion Research Facility in Lanzhou)将 用作CSR的注入器. 为了CSR的屏蔽设计,本文利用现有的实验数据计算了由于束流损失产生的中子及其能谱、角分布,同时也估算了屏蔽体外表面的中子剂量、环境中子剂量及天空返照中子剂量. 在源项计算中使用了400MeV/u ¹²C+Cu反应的中子产额、能谱、角分布的实验数据. 计算表明, CSR对环境剂量影响最大的是天空返照中子.     

用于强流中子发生器的低能束流传输线的设计模拟

兰州大学设计研发的ZF-400强流中子发生器设计D-T中子产额6×1012 n/s,主要由ECR离子源、低能束流传输线（LEBT）、加速管、旋转靶等部分组成。LEBT负责将从离子源引出的束流进行分析聚焦并注入到加速管中。LEBT对束流的聚焦及分析的好坏程度决定了加速管中束流的损失程度、中子的产额以及靶的寿命。本工作就该强流中子发生器所需的低能传输线进行了设计。使用螺线管、分析磁铁和四极透镜组合的方案。利用TRACK软件对此方案进行模拟,得到符合要求的束流线及元件的参数。用TraceWin进行了验证模拟,验证结果符合要求。另外,通过软件模拟确定了杂质离子损失的位置,据此设计了束流管道冷却方案。通过模拟发现,这种透镜的组合方式可以让整个LEBT以较低的功率获得低损失、高纯度的打靶束流。The ZF-400 Intense Neutron Generator, which is designed by Lanzhou University with an expected neutron yield of 6×1012 n/s, is consist of ECR ion source, low energy beam transport (LEBT) line, accelerating tube and rotating target. The beam extracted from ECR source is analyzed and focused through LEBT, then, the beam is introduced into the accelerating tube. The focus and analysis ability of LEBT is very important for the beam loss, neutron yield and target's life. A LEBT line for intense neutron generator is designed in this paper, the project consisted of a solenoid, a bend magnet and three quadrupole lens. The qualified LEBT and its parameters have been got through the simulation with TRACK code. Then TraceWin program was employed to check this simulation and the result satisfied our requirement. Besides, the loss position of impurity ions was known through the simulation, and a project of beam line's cooling system was designed according to this result. Through the simulation, we find the optimal lens combination plan which can obtain lower beam loss and higher beam purity at low running power of LEBT.     

On the Effective Metric for an Electromagnetic Wave Propagating in the Field of an Intense Laser Radiation

It is shown that in the nonlinear electrodynamics of vacuum, which is a corollary of quantum electrodynamics, a weak electromagnetic wave propagates in the field of an intense laser radiation as if this took place in some effective space-time whose metric tensor depends on the electric field of the laser radiation. The components of the metric vector of the effective space-time along the geodesic lines of which the rays of the weak electromagnetic wave propagate in the given case have been found. It is pointed out that this property may serve as a basis for observing the manifestations of the nonlinearity of the electrodynamics of vacuum.     

冷中子三轴谱仪屏蔽体的蒙特卡罗模拟

冷中子三轴谱仪( CTAS ) 的屏蔽体对于保障工作人员安全、降低散射大厅本底及提高信噪比具有重要的意义。采用蒙特卡罗程序MCNP5 对谱仪各部分屏蔽体进行了计算,并结合Mcstas 程序确定了CTAS 入口处的中子源,大大提高了计算效率。经过模拟计算和优化表明:单色器后端使用厚350mm、密度4.6 g/cm3 的重混凝土,衔接屏蔽体使用厚300 mm、密度3.6 g/cm3的重混凝土,生物屏蔽采用厚150 mm、密度3.6 g/cm3 的重混凝土可保证屏蔽体外表面的剂量率满足散射大厅的剂量要求。The shielding of Cold neutron Triple-Axis Spectrometer( CTAS ) is important for radiation safety of workers, and reduce the background of scattering hall as well as enhancing the ratio of signal-to-noise. In this study,Monte-Carlo simulation was performed to conduct the calculation on the shielding of CTAS. To increase the calculation efficiency, neutron source was obtained by using Mcstas code. The results indicate that, in the case of heavy concrete ( density 4.6 g/cm3 ) with thickness of 350 mm for the shielding behind the monochromater, and heavy concrete ( density 3.6 g/cm3 ) with thickness of 300 mm for the other monochromater shielding, as well as the heavy concrete ( density 3.6 g/cm3 ) with thickness of 150 mm for biological shielding, the dose rate outside shielding may meet the requirement of national standard of China.     

Performance of an Electrostatic-Energy Analyzer in the Presence of Intense Ultraviolet Radiation

Significant distortion in the characteristics of an electrostatic-energy analyzer has been observed when used in an intense ultraviolet-radiation environment. The observed effects are attributed to ultraviolet photons striking the electrode and/or resistive divider structures in the analyzer, resulting in voltage changes on the guard rings. The distortion can be evaluated by matching experimental characteristics with model-based calculations, thereby reducing the error in the measurement of ion energy.     

反应堆235U裂变源辐射防护材料的优化设计

以热中子反应堆²³⁵U裂变源为辐射源,利用MCNP程序对其能谱进行模拟并研究其辐射防护,结果表明对²³⁵U裂变源所发射的能量高于3MeV的瞬发中子,重金属具有良好的屏蔽效果,而对于能量低于1MeV的中子,轻氢材料的防护效果更好;W/LiH,W/B₄C,TiH₂/W三种复合材料当质量比满足：W：LiH=19：1,W：B₄C=9：1,W：TiH₂=3：1时材料的屏蔽效果最佳;通过遗传算法结合MCNP模拟,得到W/TiH₂/B₄C,TiH₂/Cu/Gd,TiH₂/B₄C/Gd三种复合材料的最优组分配比,源每次裂变产生的粒子穿过这三种材料后在等效组织中造成的剂量当量率（10^-11Sv·h^-1）与材料厚度呈指数下降关系,三种材料分别可近似为1.071exp（-0.187 8x）,1.077exp（-0.166 2x）和1.608exp（-0.171 9x）,x为材料厚度（cm）.     

Nonlinear Transport of Three-Dimensional Electron Gases Under the Influenceo f an Intense Terahertz Radiation

We present a detailed theoretical and numerical investigation on nonlinear transport of a model three-dimensional electron gas driven by an intense terahertz (THz) radiation at lattice temperature T = 10,77,300 K using the conventional and recently developed balanceequation approach. Ionized-impurity, acoustic-phonon and polar optical-phonon scatterings were taken into account for electrons in a single parabolic band. The heating of electrons and the suppression of the dc electron mobility by the irradiation of the intense THz field are predicted. We find that the dc average mobility of electrons peaks around a certain value of the amplitude of the ac field at low lattice temperature.     

Dielectric Detector of Intense X-Ray Radiation Fluxes

Physics of Atomic Nuclei - The high sensitivity of traditional diagnostic methods hinders their application in inertial confinement fusion with the pulsed radiation power of target plasma being as...     

Diffusion Studies with Synchrotron Radiation and with Neutron Scattering

With scattering methods we are able to detect the elementary diffusion jump. This is a report on investigations with methods working in the time domain, i.e., nuclear resonant scattering of synchrotron radiation and neutron spin echo. The accent of this paper is on diffusion in ordered alloys. We finish with an outlook on what will be possible with the upcoming potential of future synchrotron sources. This revised version was published online in August 2006 with corrections to the Cover Date.