塑料闪烁光纤在γ光子下的康普顿能谱能量分辨率模拟分析

利用Geant4软件模拟了塑料闪烁光纤在中能γ光子(0.8–6 MeV)入射情况下的康普顿能谱,讨论了能量分辨率与半径的变化关系,分析了γ光子在闪烁光纤间串扰对能量分辨率的影响。结果表明,选择合适的光纤半径,能得到较好的康普顿能谱效应,半径为15–35 mm时,最佳的能量分辨率可达8.7%。     

闪烁体-光纤组合式辐射探测器γ相对灵敏度研究

根据光纤光导的耦合理论,推导了闪烁体-光纤组合式探测器γ灵敏度理论公式,计算了LSO、 ST401两种闪烁体-光纤探测器的相对灵敏度,在平均能量为1.25 MeV的60Co射线源上测得10 mm厚LSO闪烁体-光纤组合式探测器的灵敏度是相同厚度ST401的19倍,理论计算和实验结果在不确定度范围内一致.     

快中子照相样品散射本底及图像特性的Monte Carlo模拟

模拟了14MeV中子在穿透样品后与闪烁体光纤的作用。对每根光纤中的能量沉积进行了计算,并转换成可见光(496nm)光子数在模拟实验中,分析了影响图像质量的因素。首先计算了散射中子本底与闪烁体和样品(聚乙烯)间距的关系。当间距为厘米量级时散射中子本底对图像的影响很小。其次,计算表明系统对样品的甄别厚度与入射中子总数N有关,在一定范围内近似与logN成线性关系。最后,通过模拟结果给出了理想平行中子束入射情况下系统的平面分辨率。     

方形闪烁光纤阵列出射荧光特性数值模拟

为对方形塑料闪烁光纤阵列的出射荧光特性进行分析,利用Geant4蒙特卡罗程序包将中子在闪烁光纤阵列中的能量沉积弥散过程和荧光（E≤100eV）在光纤内部的传播过程进行耦合输运模拟。分析14MeV中子入射点的位置对荧光强度分布的影响,找出计算闪烁光纤阵列调制传递函数的1个等效点。讨论了荧光强度分布、荧光收集效率与角分布之间的关系。以5cm厚闪烁光纤阵列为例,讨论了闪烁光纤阵列的调制传递函数和平板闪烁体的能量沉积分布函数、抽样函数之间的关系。结果表明,闪烁光纤阵列的调制传递函数近似可用平板闪烁体的能量沉积分布函数经函数rect(x,y)抽样后的结果作二维傅里叶变换得到。     

改善CeF_3闪烁探测器输出特性的方法研究

常用的CeF3闪烁探测器,由于闪烁体发光与光电器件光谱响应不完全匹配,使得探测器的主要输出特性——输出幅度和脉冲波形形状发生明显变化,较难依据相似性通过探测器输出复原得到辐射源的真实特征。本工作针对这种不完全匹配情况,分析比较了几种改善闪烁探测器输出的方法,提出了在CeF3无机闪烁体与光电器件光阴极之间加移波膜片耦合的方法。原理分析和实验测量结果表明:采取移波膜片耦合方法可使传统CeF3闪烁探测器输出特性得到明显改善,输出幅度增加50%以上,脉冲时间响应前沿由传统方式的12ns变化到移波方式的约3ns。     

GdI3:Ce闪烁体中子探测性能测试与改进研究

为改善GdI₃:Ce闪烁体在探测中子过程中的γ抑制能力,使用Geant4和XCOM计算了其γ线性吸收系数,并通过模拟计算与实验测量研究了铅屏蔽法抑制γ的有效性。结果表明：GdI₃:Ce闪烁体在探测中子过程中易受低能γ射线的干扰;随着铅层厚度的增加,100 keV~1 MeV的γ射线对中子探测的干扰减小,而3~10 MeV的γ射线的干扰呈先增加后减小的趋势。对²⁵²Cf中子源的实验测试发现,在碘化钆闪烁体外围添加铅层后,中子峰得以显现;随着铅层厚度的增加,中子峰内净计数减小,而净计数与本底计数的比值上升。模拟和实验结果均表明,在使用GdI3:Ce闪烁体探测中子时,应根据中子探测效率和信噪比的优化确定γ屏蔽铅层的厚度。     

光纤传输型便携式大面积表面污染监测仪实验样机的性能测试与分析

研究了基于大面积塑料闪烁体(灵敏面积高达1 200 cm2),利用波长转换光纤(WLS)开展β表面污染测量的可行性。初步采用了波长转换光纤阵列与塑料闪烁体直接耦合的设计方式。基于该设计,编制了计算程序,开展了系统的光传输模拟计算,对塑料闪烁体尺寸、波长转换光纤的数量和排布方式进行了优化设计。同时,结合分析结果完成了测量系统的整体设计,加工制作了专门的实验样机,并对该装置进行了性能测试。结果表明:该实验样机的灵敏探测面积为1 200 cm2、探测效率约为15.4%、不同灵敏区域的不均匀度约为9.7%、探测下限约为0.05 Bq/cm2,可以满足正常运行和事故情况下的β表面污染监测需求。     

闪烁光纤在快中子辐照下部分特性的蒙特卡罗模拟

采用蒙特卡罗方法对闪烁光纤在快中子辐照下的特性进行了研究。利用Geant4模拟计算得到了快中子照射下,闪烁光纤中的能量沉积效率与光纤长度、半径及入射中子能量的变化曲线,同时对量子效率随上述参数的变化关系进行了模拟分析。研究结果表明选择合适尺寸的闪烁光纤和入射中子能量对于快中子成像技术具有重要的意义,并为实际测量工作中的参数选择提供了的理论指导。     

α、β复合闪烁体厚度与探测效率的关系

通过对不同质量厚度的ZnS(Ag)涂层和不同厚度的塑料闪烁体相结合的复合闪烁体探测效率的实验研究,找出了其最佳结合比,使复合闪烁体对α、β粒子的探测效率达到最优化.     

~6LiI/~(nat)LiI闪烁体探测器效率的模拟计算及实验测量

碘化锂晶体闪烁体具有中子测量效率高、探测器尺寸小等优点,它是目前相当成熟和用途广泛的探测器,在中子测量及环境监测等各方面均有广泛应用,它由闪烁体、光导、光电倍增管、高压四部分组成。本工作利用Monte Carlo程序研究了6LiI/natLiI闪烁体探测器尺寸与探测效率的关系,并通过GEANT4及MCNP程序模拟计算了高富集度的6LiI晶体和天然LiI晶体及纯7LiI晶体的探测效率。从理论模拟结果可以看出10 mm厚度的两种探测器对热中子探测效率都很高,6LiI达到98%,而natLiI也能达到65%。利用反应堆热中子刻度了6LiI/natLiI闪烁体探测器的热中子探测效率,考虑了铅砖及高密度聚乙烯的散射,以及环境中子的贡献后,实际测量6LiI探测器探测效率在90%左右,natLiI探测器探测效率在70%左右。     

Optimization of a LSO-Based Detector Module for Time-of-Flight PET

We have explored methods for optimizing the timing resolution of an LSO-based detector module for a single-ring, "demonstration" time-of-flight PET camera. By maximizing the area that couples the scintillator to the PMT and minimizing the average path length that the scintillation photons travel, a single detector timing resolution of 218 ps fwhm is measured, which is considerably better than the ~385 ps fwhm obtained by commercial LSO or LYSO TOF detector modules. We explored different surface treatments (saw-cut, mechanically polished, and chemically etched) and reflector materials (Teflon tape, ESR, Lumirror, Melinex, white epoxy, and white paint), and found that for our geometry, a chemically etched surface had 5% better timing resolution than the saw-cut or mechanically polished surfaces, and while there was little dependence on the timing resolution between the various reflectors, white paint and white epoxy were a few percent better. Adding co-dopants to LSO shortened the decay time from 40 ns to ~30 ns but maintained the same or higher total light output. This increased the initial photoelectron rate and so improved the timing resolution by 15%. Using photomultiplier tubes with higher quantum efficiency (blue sensitivity index of 13.5 rather than 12) improved the timing resolution by an additional 5%. By choosing the optimum surface treatment (chemically etched), reflector (white paint), LSO composition (co-doped), and PMT (13.5 blue sensitivity index), the coincidence timing resolution of our detector module was reduced from 309 ps to 220 ps fwhm.     

PET detector using waveshifting optical fibers and microchannelplate PMT with delay line readout

We are developing a positron emission tomography (PET) detector with depth of interaction (DOI) sensitivity for breast and small animal imaging. The detector uses layers of high density scintillators sandwiched between orthogonal ribbons of wavelength shifting optical fibers. The fibers are coupled to microchannel plate (MCP)-based position sensitive photomultiplier tubes (PSPMTs) containing a crossed delay line readout anode. Preliminary measurements have been made using blue-green waveshifting fiber ribbons coupled to Lu₂(SiO₄)O:Ce (LSO). The measured photoelectron yield due to wavelength shifted light from the fibers is sufficient to ensure a Poisson probability of only 1.0% that no photoelectron is generated in a given MCP PSPMT. Measurement of internally reflected scintillation light permits energy resolution of better than 25% at 511 keV. Optical fiber readout of high density inorganic crystals provides a significantly larger detective area per unit phototube area than do one-to-one coupling schemes using PSPMTs. With its ability to accurately image single photons, the MCP PSPMT solves problems that have prevented other types of PSPMTs from being used in such a configuration, namely poor resolution at the low light output level of the optical fibers     

A model for MeV Cn track damage in YIG

An experiment by groups at Ecole Polytechnique and Institute de Physique Nucléaire (Orsay) used high resolution TEM to study the morphology of MeV C₆₀ and C₂₀ tracks in Y₃Fe₅O₁₂ (YIG). Their data consist of measurements of latent track radii, total track lengths, and the length over which the radius remains constant. The one parameter in the model presented here is determined separately from the tracks of MeV/amu heavy ions in YIG; thus, the model uses no free parameters for fitting the C_n data. The predicted track radii, the total track lengths, and the lengths with constant radius agree with the data to within ±10%. The extension of this model to other details of track morphology will also be discussed.     

Cerium-doped lutetium oxyorthosilicate: a fast, efficient newscintillator

The authors discuss a single-crystal inorganic scintillator, cerium-doped lutetium oxyorthosilicate (Lu_2(1-x)Ce_2x (SiO₄) or LSO). It has a scintillation emission intensity which is ~75% of NaI(Tl) with a decay time of ~40 ns. The peak emission wavelength is 420 nm. It has a very high gamma-ray detection efficiency due to its density of 7.4 g/cm³ and its effective atomic number of 66. Its radiation length of 1.14 cm is only slightly longer than bismuth germanate (BGO). The scintillation properties of Ce-doped LSO are compared to NaI(Tl), BGO, and cerium-doped gadolinium oxyorthosilicate (GSO). In addition to desirable physical properties such as high density and high atomic number, LSO also processes a combination of high emission intensity and fast decay which together are superior to any other known single crystal scintillator     

Analysis and Optimization Laminar Viscous Flow Through a Channel

Optimizing of laminar viscous flow through a pipe by two dimensionless values is investigated analytically. Dimensionless entropy generation and pumping power to heat transfer rate ratio are used as basis for constant viscous and the temperature dependence on the viscosity. For this matter we calculate entropy generation and pumping power for a fully developed in a pipe subjected to constant wall temperature for either constant viscosity and the variable viscosity. The variation entropy generation increase along the pipe length for viscous fluid is drawn, either the variation summation dimensionless entropy generation and the pumping power to heat transfer rate ratio are varying the fluid inlet temperature for fixed pipe length and are varying pipe length for fixed fluid inlet temperature are drawn. For low heat transfer conditions the entropy generation due to viscosity friction becomes dominant and the dependence of viscosity with the temperature becomes essentially important to be considered.     

小型自然循环钠冷堆堆芯初步设计研究

提出了一种适用于分布式发电系统的小型自然循环钠冷堆AMTEC系统。通过对堆芯的临界计算和热工水力分析,研究了堆芯燃料装载量不变情况下,芯块半径、燃料棒长度和圈数对堆芯有效增殖因数k_eff、堆芯压降和传热的影响。同时分析了不同额外停堆裕量下,B₄C吸收层厚度和堆芯初始剩余反应性随燃料棒圈数的变化关系。计算结果表明：保持堆芯当量直径和冷却剂通道总截面积不变的情况下,减少燃料棒圈数和活性区长度不仅可增加k_eff,且能降低堆芯压降;为提高额外停堆裕量需增加吸收层厚度,但降低了堆芯初始剩余反应性,不利于电厂的经济性。     

Core Design and Operating Strategies for Better Daily Load Following Performance in BWR Cores

New core design and operating strategies have been proposed for daily load following of an improved BWR core with large power swing.

The core concepts were based on the WNS core which uses an axially two-zoned enrichment fuel. One principal design strategy utilized was to reduce power in the lower portion of the core by adjusting a division point of the axially two-zoned enrichment fuel. One operating strategy introduced is for controlling Xe distributions. This method, coupled with a direct power distribution control by control rods, could decrease the xenon induced power change in the lower part of core.

The BWR core designed and operated under the new strategies was shown to meet the daily load demand with large power swing: 1-h reduction in power from 100 to 50%; 8-h hold at 50% power; 1-h increase in power from 50 to 100%; and 14-h hold at 100% power.     

遗传算法在AC-600堆芯换料优化中的应用研究

对遗传算法在核电厂堆芯换料优化问题中的应用进行了初步研究,提出了新的编码和译码方法.并以AC-600反应堆为例,分别对低泄漏和外-内布置进行了功率峰因子最小和寿期最长的优化方案计算,得到了满意的优化结果.     

Investigation of E–H mode transition in magnetic-pole-enhanced inductively coupled neon–argon mixture plasma

In this paper, E–H mode transition in magnetic-pole-enhanced inductively coupled neon–argon mixture plasma is investigated in terms of fundamental plasma parameters as a function of argon fraction(0%–100%), operating pressure(1 Pa, 5 Pa, 10 Pa and 50 Pa), and radio frequency(RF) power(5–100 W). An RF compensated Langmuir probe and optical emission spectroscopy are used for the diagnostics of the plasma under study. Owing to the lower ionization potential and higher collision cross-section of argon, when its fraction in the discharge is increased, the mode transition occurs at lower RF power; i.e. for 0% argon and1 Pa pressure, the threshold power of the E–H mode transition is 65 W, which reduces to 20 W when the argon fraction is increased. The electron density increases with the argon fraction at afixed pressure, whereas the temperature decreases with the argon fraction. The relaxation length of the low-energy electrons increases, and decreases for high-energy electrons with argon fraction, due to the Ramseur effect. However, the relaxation length of both groups of electrons decreases with pressure due to reduction in the mean free path. The electron energy probability function(EEPF) profiles are non-Maxwellian in E-mode, attributable to the nonlocal electron kinetics in this mode; however, they evolve to Maxwellian distribution when the discharge transforms to H-mode due to lower electron temperature and higher electron density in H-mode. The tail of the measured EEPFs is found to deplete in both E-and H-modes when the argon fraction in the discharge is increased, because argon has a much lower excitation potential(11.5 eV) than neon(16.6 eV).