基于PCA和BM3D的噪声估计方法及其在中子图像去噪中的应用

针对中子图像的去噪问题，本文提出了一种弱化残差图像特征值的噪声估计方法。该方法将残差图像作为噪声图像，利用主成分分析（PCA）分析了残差图像特征值，估计了噪声强度。根据噪声强度，应用三维块匹配方法（BM3D）去除了噪声，逐步弱化残差图像特征值，得到了最终噪声估计值。实验结果表明，该方法能估计模拟噪声图像和实际含噪中子图像的高斯噪声，计算效率高，可实现较好的去噪复原效果。     

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。     

基于调制核回归和正则化Richardson-Lucy算法的中子图像复原方法

由于中子成像系统中的诸多物理因素影响,使其得到的中子图像通常会发生严重的图像降质。本文将调制核回归(SK)引入到TV-RL算法中,提出了一种中子图像去模糊去噪方法 SK-TV-RL,即基于SK的TV-RL算法。该方法能解决以往算法噪声放大的问题,并能复原降质图像中的细节信息。对比实验结果表明,无论基于客观评价还是主观评价,该方法都能有效地提高图像复原质量。     

基于改进Richardson–Lucy算法的中子照相图像几何不锐度修正方法研究

中子照相系统因其特定的成像结构,使得成像后的中子照相图像不可避免地存在几何不锐度。为提升中子照相图像的质量,本文提出一种基于改进Richardson–Lucy算法的中子照相图像几何不锐度修正方法,该方法首先根据几何不锐度的形成原理构建点扩散函数,然后通过拉普拉斯算子和中值滤波方法去除图像中的γ白斑噪声,最后采用Richardson–Lucy算法对图像进行复原。采用线对模板样品进行测试,并与4种现有的中子照相图像几何不锐度修正算法进行对比,验证了本文算法可将衡量中子照相图像几何不锐度修正效果的平均梯度、空间频率指标分别提升60.23%和29.90%,能够实现几何不锐度的修正,同时解决图像复原过程中γ白斑噪声的放大问题,为实现高分辨率的中子照相提供重要技术支撑。     

基于剪切波变换的辐射图像泊松噪声降噪技术研究

为了降低由统计涨落引起的辐射图像噪声，提出了一种基于剪切波变换的降噪方法。该方法以低剂量射线或质量厚度大的物体的辐射图像为研究对象，对此类辐射图像进行了噪声分析，利用Anscombe变换将统计涨落引起的泊松噪声转换为高斯噪声，再运用剪切波分解、阈值去噪、剪切波重构和Anscombe逆变换得到降噪图像。结果表明，当剪切波分解层数为5，采用改进阈值函数及极小极大原理阈值时可达到最优降噪效果，该方法能较好地去除辐射图像中的泊松噪声并保留边缘、细节信息，在视觉和量化指标上均优于传统降噪方法。     

邻域小波自适应降噪在X射线图像处理中的应用

针对使用低剂量的X射线成像时,会带来泊松噪声和高斯白噪声,影响成像质量.目前对X射线图像的降噪方法主要是针对高斯自噪声模型,而泊松噪声和高斯白噪声是两种不同性质的噪声,因而传统的降噪方法不能有效地去除泊松噪声.邻域小波自适应降噪法,就是在不同的小波高频子代根据最大的噪声方差自适应的调整阈值,然后使用贝叶斯收缩算法,达到...     

定量中子数字成像系统的散射校正迭代算法研究

针对中子散射将直接影响在进行无损检测某种成分时的定量分析,研究分析了中子散射对定量中子数字成像系统图像降质原理,采用点扩展函数的叠加表征成像系统中子散射对图像降质的过程,并在构建中子成像点扩展函数模型的基础上,提出了对中子图像进行散射校正的空域迭代算法。研究结果表明,对于成分相同但几何形状不同的H2O样品,在样品到探测器之间距离不同情况下所得到的中子数字图像,在利用该算法进行散射校正后,影响成像质量的散射成分能较好得到抑制,所恢复出的样品厚度趋近于实际厚度。     

基于主成分变换的核径迹图像噪声分析及消除

由于径迹片在生产、贮存和蚀刻过程中易形成杂质、气泡及划痕,导致所获取的核径迹图像常常伴随着各种类型的噪声,因此如何有效地去除噪声的影响,为后续核径迹图像识别、分类与计数奠定基础,是图像预处理阶段的一个重要任务.本文选择一组聚焦性能好的核径迹图像,基于主成分变换和傅立叶变换的噪声分析,使用高斯低通滤波器进行去噪处理.实验结果表明了该方法的有效性和可操作性.     

基于小波变换的Wiener滤波投影预处理算法在图像重建中的应用

研究了一种将小波变换与Wiener滤波相结合用于投影数据预处理的算法。由于小波变换具有多分辨率分析特性,通过小波多尺度分析和小波系数控制,并结合Wiener滤波具有MMSE上最优性能的特点提出一种二者相结合的方法用于投影数据降噪预处理。对该算法进行实验仿真,结果表明,降噪后重建的图像主观质量得到明显改善,重建图像的噪声得到有效抑制。峰值信噪比方面可比一般的降噪方法提高5-12dB。     

基于CBS人体安检图像的噪声滤除方法

为滤除处于开放环境下的CBS人体安检图像上的噪声干扰,针对CBS人体安检图像噪声数据类型,对比研究了不同噪声滤除方法在CBS人体安检图像的应用效果,提出了一种面向CBS图像的混合中值高斯噪声滤除方法,基于PSNR和MAE的量化实验结果证明了该方法在CBS人体安检成像上的有效性。     

Comparison of 3D OS-EM and 4D MAP-RBI-EM reconstruction algorithms for cardiac motion abnormality classification using a motion observer

Using a heart motion observer, we compared the performance of two image reconstruction techniques, a 3D OS-EM algorithm with post Butterworth spatial filtering and a 4D MAP-RBI-EM algorithm. The task was to classify gated myocardial perfusion (GMP) SPECT images of beating hearts with or without regional motion abnormalities. Noise-free simulated GMP SPECT projection data was generated from two 4D NCAT beating heart phantom models, one with normal motion and the other with a 50% motion defect in a pie-shaped wedge region-of-interest (ROI) in the anterior-lateral left ventricular wall. The projection data were scaled to clinical GMP SPECT count level before Poisson noise was simulated to generate 40 noise realizations. The noise-free and noisy projection data were reconstructed using the two reconstruction algorithms, parameters chosen to optimize the tradeoff between image bias and noise. As a motion observer, a 3D motion estimation method previously developed was applied to estimate the radial motion on the ROI from two adjacent gates. The receiver operating characteristic (ROC) curves were computed for radial motion magnitudes corresponding to each reconstruction technique. The area under the ROC curve (AUC) was calculated as an index for classification of regional motion. The reconstructed images with better bias and noise tradeoff were found to offer better classification for hearts with or without regional motion defects. The 3D cardiac motion estimation algorithm, serving as a heart motion observer, was better able to distinguish the abnormal from the normal regional motion in GMP SPECT images obtained from the 4D MAP-RBI-EM algorithm than from the 3D OS-EM algorithm with post Butterworth spatial filtering.     

形态滤波在快中子图像降噪处理中的应用

在基于CCD耦合闪烁光纤阵列的快中子照相系统中,快中子图像受噪声污染较为严重.针对快中子图像中的椒盐噪声和泊松噪声,研究了形态滤波技术在降噪方面的应用.结果表明在对快中子图像处理过程中,基于二维多方向结构元素的形态滤波在滤除噪声和保持图像细节等方面效果较佳.     

A nodal SN transport method for three-dimensional triangular-z geometry

A new nodal S_N transport method has been developed to perform accurate transport calculation in three-dimensional triangular-z geometry, where arbitrary triangles are transformed into regular triangles via a coordinate transformation. The transverse integration procedure is applied to treat the neutron transport equation in the regular triangle. The neutron angular distributions of intra-node fluxes are represented using the S_N quadrature set, and the spatial distributions of neutron fluxes and sources are approximated by a quadratic polynomial. The nodal-equivalent finite difference algorithm for 3D triangular geometry is applied to establish a stable and efficient iterative scheme. The present method was tested on four 3D Takeda benchmark problems published by the nuclear data agency (NEACRP), in which the first three problems are in XYZ geometry and the last one is in hexagonal-z geometry. The results of the present method agree well with those of the reference Monte-Carlo calculation method, the difference in k_eff being less than 0.1%. This shows that multi-group reactor core/criticality problems can be accurately and effectively solved using the present method.     

YOSHIO NISHINA CENTENNIAL SYMPOSIUM on EVOLUTIONAL TRENDS OF PHYSICAL SCIENCE

Axially simplified method of characteristics in three-dimensional geometry (ASMOC3D) has been proposed, and the 3D neutron transport calculation code based on this method, SHIKOKU, has been developed. Since ASMOC3D handles 3D problems by a two-dimensional (2D) neutron track set and simplified treatment in axial direction, the required memory and the computational time are expected to be less than those required by a direct 3D characteristics calculation scheme. SHIKOKU is applied to two problems of 3D geometry and the results of these problems show good agreements with the reference solutions obtained by a Monte-Carlo code. SHIKOKU is also applied to an actual three-loop-type PWR core. The prediction error of the radial power distribution is satisfactory and it is shown that the computational time and the required memory for a whole-core calculation by ASMOC3D are not prohibitive for presently available PCs.     

基于可移动紧凑加速器D-T中子源的热中子层析实验研究

中子照相是十分重要的无损检测方法之一，尤其是针对含氢材料、同位素等的无损检测，中子照相技术具有其他射线成像不可比拟的优势。中国工程物理研究院核物理与化学研究所基于紧凑型D-T中子源，研发了可移动中子成像检测仪，成功实现了热中子照相和快中子照相实验检测。为确定基于该装置开展热中子层析检测的可行性，本文进行了数值模拟计算，利用该仪器开展了针对轻重材料模拟件的热中子层析成像实验，利用采集的181幅投影图像，在图像信噪较低和采集幅数较少条件下，成功重建了铝和聚乙烯材料包裹下的0.2 mm直径的钆丝。     

Visualization and measurement of two-phase flow by using neutron radiography

To apply neutron radiography (NR) technique to fluid research, high frame-rate NR with a steady thermal neutron beam has been developed in the present research program by assembling up-to-date technologies for neutron source, scintillator, high-speed video and image intensifier. This imaging system has many advantages such as a long recording time (up to 21 min), high-frame-rate (up to 1000 frames s⁻¹) imaging and no need for triggering signal. Visualization of air-water two-phase flow in a metallic duct was performed at the recording speeds of 250, 500 and 1000 frames s⁻¹. The qualities of those consecutive images were good enough to observe and measure the flow structure and the characteristics. It was demonstrated also that some characteristics of two-phase flow could be measured by using the present imaging system. Image processing technique enabled measurements of various flow characteristics in two ways. By utilizing geometrical information extracted from NR images, data on flow regime, rising velocity of bubbles, and wave height and interfacial area in annular flow were obtained. By utilizing attenuation characteristics of neutrons in materials, measurements of void profile and average void fraction were performed. It was confirmed that this new technique may have significant advantages both in visualizing and measuring high-speed fluid phenomena when the other methods such as an optical method and X-ray radiography cannot be applicable.     

Noise analysis performed at the sodium cooled power reactor KNK I for malfunction diagnosis and plant surveillance

In order to use neutron noise analysis as an effective tool for early malfunction detection it is necessary to identify the driving forces and to calculate their contributions to the power fluctuations. In this paper the influence of a considerable number of measured noise sources on neutron noise within a large frequency range (10⁻³ Hz to 10³ Hz) is investigated for the sodium cooled power reactor KNK I (thermal core, 58 MW_th).

The experimental basis for the analysis is numerous records of the following signals at various power levels: neutron noise which has been measured with an in-core fission chamber and 3 ex-core ionisation chambers; the sodium inlet temperature and the coolant flow in both primary coolant loops and the movement of the control rods. In addition signals from acoustic-, seismic- and pressure transducers and the coolant outlet temperature were collected.

The influence of the thermohydraulic- and of the control system on neutron noise has also been calculated by means of the relations for linear and multiple-input systems. Important for this analysis is the reactivity-power transfer function. Calculations of this function could be confirmed by measurements using a pseudo-random binary signal as reactivity input.

The following results were obtained from the analysis of the auto-power spectral densities of the neutron flux: Fluctuations of the coolant inlet temperature and the coolant flow are relatively small sources for neutron noise. However, reactivity adjustments resulting from the automatic control system because of the inherent instability of the reactor turned out to be an important driving force.

The influence of still unknown driving forces increased considerably with the reactor power. Since the coolant flow was proportional to the reactor power in order to keep the coolant temperature constant, this result indicates that turbulent flow must have induced stochastical movements of core components. These movements are considered to have mainly caused the unknown reactivity driving forces. Their magnitude could be determined reliably only in the frequency range, in which external feedback mechanisms through the primary coolant system were negligible. For 30 to 50 % reactor power the contribution was about 30 % (for f > 5·10⁻³ Hz) and for full power it increased to about 80 % (for f > 5·10⁻² Hz) of the measured neutron noise. For frequencies > 5 Hz the white detection noise prevails. Single peaks in this frequency region could be explained by coherence function investigations between in-core and ex-core neutron detector signals and by correlation of these signals with displacement- and pressure fluctuations.

Though the measured neutron noise could not be unambiguously related to driving forces, the combination of analytical and empirical methods makes the results also applicable for the design of surveillance techniques for other sodium cooled reactors (e.g. LMFBRs). Examples for possible applications are given.     

Study of kinetic parameters in the RA-4 reactor by means of computational modeling and neutron noise measurements

It was carried out a study of the kinetic parameters of the reactor RA-4 (SUR-100 type reactor) by means of a simulation with calculation codes and a measurement with the neutron noise technique. For the simulation the WIMS (cell calculation) and PUMA (3D diffusion code to model a complete reactor) codes were used to estimate the ‘effective delayed neutron fraction’ (β_eff), the ‘prompt neutron generation time’ (Λ), and the ‘prompt neutron decay constant’ (α). In addition, a sensitivity analysis was made to determine the best nuclear data evaluations for this model and the influence of variables such as the mean speed of neutrons, the nuclear delayed neutron fraction and the delayed neutron fission spectrum. The neutron noise technique was used to determine α. The comparison between the results for the value of α obtained by the simulation and the experience shows a good agreement. In this way the validity of a diffusion calculation for a small reactor with important heterogeneities is verified, making certain modifications to avoid the limitations of this theory.