Contrast sensitivity in 14 MeV fast neutron radiography

Fast neutron radiography(FNR) is an effective non-destructive testing technique.Due to the scattering effect and low detection efficiency,the detection limit of FNR under certain conditions cannot be determined.In order to obtain the minimum detectable thickness by FNR,we studied the contrast sensitivity of FNR lead samples,both theoretically and experimentally.We then clarified the relationship between pixel value and irradiation time,and sample materials and thickness.Our experiment,using a4-cm-thick lead sample,verified our theoretical expression of FNR contrast sensitivity.

Neutron imaging at Peking University

Neutron imaging techniques were investigated at Peking University based on a 4.5 MV Van de Graaff accelerator.The thermal neutron radiography,fast neutron radiography and fast neutron resonance radiography were tested.The low neutron flux limits the image quality.A new radio frequency quadrupole (RFQ) accelerator based on neutron source with a yield of 1 012 n/s is being set up.

基于BM3D算法的快中子图像降噪方法

在快中子照相过程中,由于受到各种因素影响,快中子图像严重降质,呈现大量噪声,不利于无损检测和后续的定量分析。通过噪声分析得出了快中子图像的噪声统计分布符合高斯噪声模型,探讨了将BM3D算法应用到快中子图像降噪处理的方法。实验结果表明,BM3D算法不仅能够有效滤除快中子图像的噪声,而且保护了图像的细节和边缘信息,可为快中子图像的降噪处理提供参考。

Corrections on energy spectrum and scatterings for fast neutron radiography at NECTAR facility

Distortions caused by the neutron spectrum and scattered neutrons are major problems in fast neutron radiography and should be considered for improving the image quality. This paper puts emphasis on the removal of these image distortions and deviations for fast neutron radiography performed at the NECTAR facility of the research reactor FRM-II in Technische Universit/it Mfinchen （TUM）, Germany. The NECTAR energy spectrum is analyzed and established to modify the influence caused by the neutron spectrum, and the Point Scattered Function （PSeF） simulated by the Monte-Carlo program MCNPX is used to evaluate scattering effects from the object and improve image quality. Good analysis results prove the sound effects of the above two corrections.

Numerical study of point spread function of a fast neutron radiography system based on scintillating-fiber array

For a scintillating-fiber array fast-neutron radiography system,a point-spread-function computing model was introduced,and the simulation code was developed. The results of calculation show that fast-neutron radiographs vary with the size of fast neutron sources,the size of fiber cross-section and the imaging geometry. The results suggest that the following qualifications are helpful for a good point spread function: The cross-section of scintillating fibers not greater than 200 μm×200 μm,the size of neutron source as small as a few millimeters,the distance between the source and the scintillating fiber array greater than 1 m,and inspected samples placed as close as possible to the array. The results give suggestions not only to experiment considerations but also to the estimation of spatial resolution for a specific system.

An empirical formula for scattered neutron components in fast neutron radiography

Scattering neutrons are one of the key factors that may affect the images of fast neutron radiography. In this paper, a mathematical model for scattered neutrons is developed on a cylinder sample, and an empirical formula for scattered neutrons is obtained. According to the results given by Monte Carlo methods, the parameters in the empirical formula are obtained with curve fitting, which confirms the logicality of the empirical formula. The curve-fitted parameters of common materials such as 6 LiD are given.

The pilot experimental study of 14 MeV fast neutron digital radiography

14 MeV Fast neutrons has good penetrability and the 14 MeV fast neutron radiography can meet the need of Non-Destructive Test of the structure and lacuna of heavy-massive sample, whose shell is made of heavy metal and in which there are some hydrogen materials, and the study of fast neutron digital radiography just begins in China. By the use of a D-T accelerator, a digital imaging system made up of a fast neutron scintillation screen made of ZnS(Ag) and polypropylene, lens and a scientific grade CCD, the experimental study of fast neutron radiography has been done between 4.3×1010-6.8×1010 n/s of neutron yield. Some 14 MeV fast neutron digital radiographs have been gotten. According to experimental radiographs and their data, the performance of the fast neutron scintillation screen and the basic characters of 14 MeV fast neutron radiography are analyzed, and it is helpful for the further research.

^9Be（d，n）加速器中子源中子照相的研究

加速器中子源比反应堆中子源更具灵活性,北京大学正在发展基于RFQ加速器的小型中子照相装置。为了更好地设计和优化此装置,实现高品质的中子照相,我们在北京大学4.5 MV静电加速器上建立了中子照相实验平台,包括科学级制冷、高灵敏度、低噪声的CCD数字成像系统,模拟基于厚铍靶9Be(d,n)反应RFQ中子源的条件,并利用此系统开展中子成像技术的研究。实验在像平面热中子注量率为5×103 cm-2·s-1或快中子注量率为3.7×104 cm-2·s-1的情况下获得了一定质量的热中子及快中子照片。当利用RFQ直线加速器强中子源时将可获得更高质量的图片,从而可以满足大多数的应用需要。

快中子照相的实验研究

快中子照相是一种优良的无损检测技术,它有着 X 射线、热中子照相等所不具有的独特优势。本文介绍了作者分别采用 Am-Be 同位素中子源和中子发生器进行的快中子照相研究,并利用自制的 Gd2O2S:Tb 快 241中子转换发光屏进行探测,成功实现照相。

快中子照相胶片成像法的初步实验

利用9Be(d, n)反应在北京大学 4. 5MV静电加速器上产生小于 7MeV能区的快中子, 采用胶片法进行了快中子照相的初步实验. 对快中子胶片成像法的基本性能进行了初步的测试和研究. 用国产的原材料研制了一种适用于快中子照相的关键部件———快中子转换屏. 实验结果表明研制的快中子转换屏能够满足快中子照相的需要.

用4.5 MV静电加速器开展快中子照相的研究

用国产原材料研制了一种适用于快中子照相的关键部件———快中子转换屏。利用9Be(d,n)反应在北京大学4.5 MV静电加速器上产生低于7 MeV能区的快中子,采用胶片成像法进行了快中子照相的初步实验。借助带不同孔径的Fe等不同材料制作的不同厚度的试验样品,对快中子胶片成像法的基本性能进行了初步测试和研究。实验结果表明,研制的快中子转换屏能够满足快中子照相的需要。

快中子共振成像实验研究

利用两次成像法对快中子共振成像进行研究。分别利用6.3 MeV和6.1 MeV快中子对石墨、铜、铝构成的样品进行成像,经图像处理后,消除铜和铝的影像而获得了碳元素的分布图像。

快中子照相中的点扩展函数计算

快中子照相中,基于反冲核原理探测快中子的有机闪烁体平板是普遍采用的快中子辐射转换体。模拟了D-T中子垂直入射BC400闪烁体平板,计算了不同厚度平板闪烁体的点扩展函数,对14.1 MeV快中子照相中闪烁体固有分辨率随厚度的变化进行了研究。计算结果表明,在不考虑二次中子与闪烁体作用及背景噪声等情况时,点扩展函数几乎不依赖于闪烁体厚度。同时,计算还表明在一定的分辨率范围内,由于荧光收集效率的限制,闪烁体厚度增加并不会改善图像对比度。

14 MeV快中子照相准直屏蔽系统的设计与优化

基于强流氘氚中子源科学装置HINEG设计了一套快中子照相准直屏蔽系统。采用中子输运设计与安全评价软件系统SuperMC和ENDF/B-Ⅶ.0数据库计算了准直中子束的中子能谱及注量率、γ射线能谱及注量率、直射中子注量率与γ射线注量率比值(φd/φγ)、直射与散射中子注量率比值(φd/φs)、准直束中子注量率的不均匀度等特性参数,并采用MCNP5程序进行了对比验证。研究了准直屏蔽系统的内衬材料、尺寸等对特性参数的影响规律,并通过优化获取了最优设计方案。计算结果显示,在同等计算条件下,SuperMC计算结果与MCNP计算结果相对偏差小于1%,准直屏蔽系统的φd/φγ为50.1,φd/φs为5.7,在Ф30cm视野范围内的中子注量率为4.80×10^7cm^-2·s^-1,其中直射中子注量率为4.09×10^7cm^-2·s^-1,中子注量率不均匀度为5.8%,满足快中子照相对准直束特性参数的要求。

高效快中子转换屏的研制及其性能测试

采用国产材料研制了基于移波光纤的快中子照相用转换屏。测试结果表明，新的转换屏与原有的2mm厚转换屏相比，发光强度可提高到原来的6～8倍，样品成像的统计性明显变好，光纤屏的饱和厚度约为25mm，空间分辨率好于2mm。

兰州大学的中子发生器研制及应用展望

1988年兰州大学成功研制了3×10^(12)s^(-1)的ZF-300强流中子发生器,主要用于核数据测量、材料辐照损伤等研究。为进一步开展活化法中子核数据测量、裂变物理等研究,兰州大学启动了基于倍压加速器的ZF-400强流中子发生器研制工程,该中子发生器的设计指标为D束流能量400 keV、D束流强度大于30 mA、D-D中子产额大于5×10^(10)s^(-1),D-T中子产额大于5×10^(12)s^(-1)。在裂变物理研究方面,已成功发展了描述裂变核断点裂变势的势驱动模型(potential-driving model),并开展了中子诱发典型锕系核素裂变发射中子前裂变产物的质量分布计算研究;将potential-driving model植入Geant4程序,发展了用于裂变发射中子后裂变产物质量分布、动能分布、裂变中子能谱等模拟的蒙特卡罗方法,并开展了可靠性评估研究;研制了一套用于裂变产物实验测量的双屏栅电离室(TFGIC),并完成了初步实验测试。在中子应用技术方面,为满足小型化中子应用技术系统的研发需求,兰州大学成功研制了长度984 mm、直径234 mm的紧凑型中子发生器,通过在引出加速电极和靶之间加电阻的方式产生偏置电场,实现对靶上二次电子的抑制。在自注入靶条件和150 keV氘束流能量下,D-D中子产额可大于5×10^(8)s^(-1),该中子发生器已具备产生D-T中子产额大于10^(10)s^(-1)量级的潜力。完成了基于紧凑型D-T中子发生器的快中子准直屏蔽体的设计,并研发了基于微通道板的快中子成像探测器,初步D-T快中成像测试显示,图像空间分辨率约为500μm。开展了基于紧凑型D-D中子发生器的核燃料棒^(235)U富集度及均匀性检测系统研发,仿真研究表明,在D-D中子产额5×10^(8)s^(-1)条件下,对核燃料棒中10%范围内的^(235)U富集度相对变化的检测置信度可达到99%。

改进型NL-means算法滤除快中子图像噪声

分析了快中子照相图像的主要噪声特点和传统NL-means滤波算法的不足,提出了一种改进的适用于快中子照相特点的Nl-means图像滤波算法。实验结果表明,改进之后的算法不仅可以有效滤除快中子照相图像中的大量白疵点,而且有效的保护了图像中的边缘与细节信息,可为快中子照相图像的进一步处理提供参考。

移动式加速器在快中子照相中的应用

介绍了移动式加速器的工作原理、结构和调试过程。该加速器具有体积小、重量轻、束流稳定度高、束斑直径小的特点。移动式加速器是首次在中国工程物理研究院应用于快中子照相。在调试过程中测试了小型移动式加速器的束流稳定性等参数,采用伴随粒子方法测量了直流中子产额。通过移动加速器和数字照相系统的组合,获得了初步图像。

14 MeV快中子照相用光纤转换屏研究

快中子照相技术因其超强的样品透视能力而成为射线无损检测技术近年来研究的热点,转换屏是中子照相装置的关键部件。光纤转换屏是一种新型的快中子照相转换屏,较大程度兼顾了光纤阵列的高探测效率和荧光屏的高成像质量,具有很好的应用前景。本文以D-T加速器为中子源,用ZnS和环氧树脂以及光纤研制了快中子照相光纤转换屏,耦合科学级CCD数字成像系统,进行了快中子数字照相技术研究,获取了不同光纤排列方式的光纤转换屏积分曝光图像,同时测量了快中子荧光屏和塑料闪烁体等其他快中子照相用转换屏的发光效率,实验结果表明,光纤转换屏的发光效率高于其他类型转换屏的。

中子照相技术及其应用

本文简述中子照相的原理、方法和特征。包括中子源 ,中子转换屏 ,中子成像技术等关键技术。给出在 4 5MV静电加速器上快中子照相的结果。