一种圆轨迹锥束CT中截断投影数据的高效重建算法

本文基于数据重排方法, 提出了T-BPF (Tent-BPF)算法, 该算法先将锥束投影数据重排成平行投影数据, 然后使用一种推导的BPF型算法重建重排后的平行投影数据. T-BPF算法将原BPF算法反投影中变化的角度积分限变成固定的, 反投影中各层循环之间没有了相关性, 这意味着T-BPF算法较原BPF算法具有更好的可并行性. 实验结果显示: 使用GPU对256³的Shepp-Logan体模的图像重建进行并行加速, T-BPF算法在保证重建质量的前提下, 加速比达到了1036, 较原BPF算法有很大提升. T-BPF算法为截断投影数据的3D图像快速重建提供了方法. 关键词： X射线光学 CT 图像重建 GPU     

14MeV中子照相中散射中子对成像影响的Monte Carlo模拟

依据实验参数，建立了14MeV快中子照相的物理模型，并利用Monte Carlo方法对照相过程进行了模拟. 分析了经聚乙烯样品散射的中子对快中子图像的影响随样品与探测器间距及样品参数的变化. 计算结果表明，样品与探测器的距离d<5cm时，样品中的散射中子对图像的影响强烈依赖于d，而当d>20cm时，样品散射中子对图像的影响可忽略；当样品密度为3—5g/cm³时散射中子对图像的影响相对最大；样品宽度越大，图像中的散射成分越多，当宽度在3cm以上时散射成分的强度趋于饱和. 关键词： 14MeV中子 快中子照相 散射中子 Monte Carlo模拟     

中子与α相互作用的两体等效可分离势

本文提供了一种n与α相互作用的等效两体可分离势,并给出了n-α散射的S_1/2,P_1/2,P_3/2分波相移对入射中子动能的解析表达式.用它计算各种动能的中子微分截面和极化都比较方便,计算结果与20MeV以下的中子散射实验数据作了比较,它们符合得很好.     

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.     

EAST运动斯塔克效应诊断数据处理中GPU并行化加速算法的研究

在EAST装置单道运动斯塔克效应(MSE)诊断系统数据处理中，采用CPU(中央处理器)+GPU(图形处理器)异构化模型，实现了数字谐波分析(DHA)算法的并行化加速计算。由CPU完成数据的加载及简单的数学计算，由GPU实现DHA算法的傅里叶正、逆变换及滤波等并行化计算，与串行算法相比，获得了2000倍以上的加速，可以满足MSE诊断实验期间及时数据处理的要求。     

中子照相数值模拟与实验的对比

 介绍了热中子照相的MCNP数值模拟方法，模拟了300＃反应堆Maxwell谱热中子束穿透样品射到转换屏的过程，给出了热中子对铝、铅、铁、铜的穿透能力，分析了引起模拟与实验结果不同的原因。通过模拟得到了清晰的图像，对比数值模拟与在300#反应堆中子照相装置上的实验结果，数值模拟结果图像与实验结果图像非常相似；散射中子对图像的影响也符合相同的规律，随着样品与转换屏之间的距离的增大，散射中子对结果图像的影响越来越小，当样品与转换屏之间的距离为样品尺寸的2倍时可以忽略散射中子的影响。     

BESⅢ主漂移室基于模式匹配的径迹重建

BESⅢ主漂移室径迹重建算法MdcPatRec, 是采用面向对象技术用C++开发的软件. 该算法使用模式匹配的方式寻找径迹段, 连接径迹段为径迹并进行最小二乘拟合. 使用模拟数据对算法进行了检验, 结果表明该径迹重建算法能够提供高的重建效率和好的动量分辨, 各项指标均达到设计要求. 同时具有好的抗噪声能力, 可以满足BESⅢ的使用要求.     

中子输运方程基于界面修正的源迭代并行计算方法

中子输运方程的计算量非常大。在现有的计算机条件下，进行精密物理的数值模拟所需要的中子计算仍是非常的费时间和费内存的，不采用并行计算是难以承受的。并且，由于中子输运隐式离散纵标方法引起的数据强相关，以及计算过程必须严格沿中子运动方向进行(否则会出现计算不稳定)，因此会出现相当严重的算法同步的问题，使得隐式格式在大型并行计算机上实施时所能得到的并行度十分有限，严格限制了其实现具有高并行度的迭代计算的可能性。因此，对中子输运方程隐式差分格式进行并行改造是十分必要的。     

中子准直实验装置设计

设计了以中子发生器为源的中子准直实验装置,分别做了不同材料准直器的厚度、准直孔大小改变时,从准直孔和孔周围不同位置射出中子的实验,并将实验中得出的数据与理论值相比较,从而确定该实验装置的使用可行性.     

考虑旋转中心标定误差的中子偏置计算机断层扫描成像方法

中子偏置CT(computed tomography)扫描是一种有效的大尺寸样品层析检测方法,但投影数据截断会导致较大的CT系统转台旋转中心标定误差,严重影响成像质量。基于投影数据对称性原理,提出了一种计算旋转中心左侧和右侧投影数据和之间方差的偏置CT扫描旋转中心精确标定算法。设计了对称补数据重建算法和投影数据预处理重建算法,验证得到,对称补数据重建算法对旋转中心标定误差更为敏感,较小的误差值会导致补齐后投影数据出现拼接缝以及拼接错位问题。提出了一种中子投影数据噪声仿真方法,设计的三维仿真模体验证了所提标定算法与投影数据预处理重建算法在不同旋转中心偏置大小以及不同强度投影噪声条件下的性能优势。基于反应堆中子源开展了中子偏置CT扫描成像验证实验,获得了样品清晰的内外部结构细节,中子CT成像系统的成像视野扩大了31.4%。     

Nano-metrology of porous structures — I Comparison of measured neutron scattering with calculated scattering to access pore lattice,diameter, and wall parameters,using models of extended arrays of regular or randomised pores

Neutron scattering offers a length-scale-independent method of probing structured matter on an atomic scale through nano-scale to meso-scale. A protocol is presented that provides a versatile method of determining structure, by comparison of measured and calculated neutron scattering, for any structural distribution that can be described algebraically or numerically, requiring no particular model other than the model of the structure, and needing no adjustable parameters other than the scale and other parameters describing the physical model. The method enables the direct comparison of measured and calculated scattering from structured matter: from simple finite and infinite bodies, from extended regular array of pores, or from extended arrays of pores with a partially randomised character. Examples are given for the radial distributions of a range of regular bodies, of large arrays of highly ordered porous materials such as templated SBA-15 and MCM-41 silicas, as well as for more disordered materials such as sol–gel silicas. Monte Carlo integration of the calculated scattering for ensembles of up to about 100,000 pores has been studied using these techniques. The method enables the calculation of the solid–solid density correlation function G(r)$G\left(r\right)$ for model systems, and hence, by Fourier transformation, the expected scattering. Example measured scattering is compared with the calculated scattering, with further data presented in a related paper. The technique allows the direct calculation and comparison with measurement of all three of the main pore structural parameters: lattice spacing, pore diameter, and pore-wall thickness. Example SBA-15 wide and small angle neutron scattering (SANS) data, measured on NIMROD (the Near and InterMediate Range Order Diffractometer at ISIS), is used as an initial evaluation of the applicability of the techniques. The method is also applicable to determining structure by comparing calculating with measured diffraction broadening, and an example is given using SBA-15 diffraction data, measured on D20, at the Institut Laue-Langevin (ILL), Grenoble.     

HL-2A中子相机屏蔽系统模拟

利用蒙特卡罗模拟程序,建立了HL-2A中子相机蒙特卡罗粒子输运(MCNP)物理模型,对D-D聚变中子和γ射线的屏蔽进行了模拟计算。对石蜡碳酸锂混合物、聚乙烯、铅和316L不锈钢4种常用中子慢化吸,收剂组成的屏蔽层材料的屏蔽效果进行了对比。计算结果表明,石蜡碳酸锂混合物和铅组合是中子相机的最佳屏蔽层材料,其中石蜡碳酸锂混合物用于慢化吸收中子,铅用于屏蔽中子和γ射线。此外,利用MCNP模拟计算得到了屏蔽中子和γ射线所需的屏蔽厚度,以及准直管的中子散射率。     

Evaluating the accuracy performance of Lucas-Kanade algorithm in the circumstance of PIV application

Lucas-Kanade(LK) algorithm, usually used in optical flow filed, has recently received increasing attention from PIV community due to its advanced calculation efficiency by GPU acceleration. Although applications of this algorithm are continuously emerging,a systematic performance evaluation is still lacking. This forms the primary aim of the present work. Three warping schemes in the family of LK algorithm: forward/inverse/symmetric warping, are evaluated in a prototype flow of a hierarchy of multiple two-dimensional vortices. Second-order Newton descent is also considered here. The accuracy efficiency of all these LK variants are investigated under a large domain of various influential parameters. It is found that the constant displacement constraint, which is a necessary building block for GPU acceleration, is the most critical issue in affecting LK algorithm's accuracy, which can be somehow ameliorated by using second-order Newton descent. Moreover, symmetric warping outbids the other two warping schemes in accuracy level, robustness to noise, convergence speed and tolerance to displacement gradient, and might be the first choice when applying LK algorithm to PIV measurement.     

Soliton microdynamics of the generation of new-type nonlinear surface vibrations,dissociation, and surfing diffusion in diatomic crystals of the uranium nitride type

The microdynamics of large-amplitude nonlinear vibrations of uranium nitride diatomic lattices has been investigated using the computer simulation and neutron scattering methods at temperatures T = 600–2500°C near the thresholds of the dissociation and destruction of the reactor fuel materials. It has been found using the computer simulation that, in the spectral gap between the frequency bands of acoustic and optical phonons in crystals with an open surface, there are resonances of new-type harmonic surface vibrations and a gap-filling band of their genetic successors, i.e., nonlinear surface vibrations. Experimental measurements of the slow neutron scattering spectra of uranium nitride on the DIN-2PI neutron spectrometer have revealed resonances and bands of these surface vibrations in the spectral gap, as well as higher optical vibration overtones. It has been shown that the solitons and bisolitons initiate the formation and collapse of dynamic pores with the generation of surface vibrations at the boundaries of the cavities, evaporation of atoms and atomic clusters, formation of cracks, and destruction of the material. It has been demonstrated that the mass transfer of nitrogen in cracks and along grain boundaries can occur through the revealed microdynamics mechanism of the surfing diffusion of light nitrogen atoms at large-amplitude soliton waves propagating in the stabilizing sublattice of heavy uranium atoms and in the nitrogen sublattice.     

GPU-based single-cluster algorithm for the simulation of the Ising model

We present the GPU calculation with the common unified device architecture (CUDA) for the Wolff single-cluster algorithm of the Ising model. Proposing an algorithm for a quasi-block synchronization, we realize the Wolff single-cluster Monte Carlo simulation with CUDA. We perform parallel computations for the newly added spins in the growing cluster. As a result, the GPU calculation speed for the two-dimensional Ising model at the critical temperature with the linear size L = 4096 is 5.60 times as fast as the calculation speed on a current CPU core. For the three-dimensional Ising model with the linear size L = 256, the GPU calculation speed is 7.90 times as fast as the CPU calculation speed. The idea of quasi-block synchronization can be used not only in the cluster algorithm but also in many fields where the synchronization of all threads is required.     

Instrumentation with polarized neutrons

Neutron scattering with polarization analysis is an indispensable tool for the investigation of novel materials exhibiting electronic, magnetic, and orbital degrees of freedom. In addition, polarized neutrons are necessary for neutron spin precession techniques that path the way to obtain extremely high resolution in space and time. Last but not least, polarized neutrons are being used for fundamental studies as well as very recently for neutron imaging. Many years ago, neutron beam lines were simply adapted for polarized beam applications by adding polarizing elements leading usually to unacceptable losses in neutron intensity. Recently, an increasing number of beam lines are designed such that an optimum use of polarized neutrons is facilitated. In addition, marked progress has been obtained in the technology of ³He polarizers and the reflectivity of large-m supermirrors. Therefore, if properly designed, only factors of approximately 2–3 in neutron intensity are lost. It is shown that S-benders provide neutron beams with an almost wavelength independent polarization. Using twin cavities, polarized beams with a homogeneous phase space and P>0.99 can be produced without significantly sacrificing intensity. It is argued that elliptic guides, which are coated with large m polarizing supermirrors, provide the highest flux.     

VITESS polarized neutron suite: allows for the simulation of performance of any existing polarized neutron scattering instrument

As neutron simulations packages are used for analysis of the expected performance for practically all newly built neutron instruments, possibilities for simulations with polarized neutrons have been relatively underdeveloped.During the last years we developed a new approach for the representation of time-dependent magnetic fields (both in magnitude and direction) for the VITESS simulation package. This allowed us to simulate the neutron spin dynamics in practically all polarized neutron devices (RF neutron flipper, adiabatic gradient RF flipper, the Drabkin resonator, etc.). In this article the above-mentioned VITESS instrument components (modules) will be presented and the simulated performance of a number of polarized neutron scattering instruments (NRSE, MIEZE, SESANS, etc.) will be demonstrated.Thus, we practically complete the polarized neutron suite of the VITESS, which seems sufficient for the simulation of performance of any existing polarized neutron scattering instrument. Future work will be concentrated on developments of dedicated sample modules (kernels) to allow for virtual experiments with VITESS.     

中国散裂中子源多功能反射谱仪屏蔽设计

本文介绍了利用蒙特卡罗粒子输运程序MCNPX2.5.0进行中国散裂中子源多功能反射谱仪屏蔽设计的屏蔽需求、辐射源项、计算方法和设计结果等内容.在计算中考虑慢化器泄漏源项、中子导管损失源项等不同辐射源项,使用分步计算和源项角度偏移、源项能量偏移、几何分裂等多种减方差方法,在保证计算结果精度的同时提高计算速度.在谱仪束线传输段、第二中子开关、散射室等的屏蔽计算中,通过比较了不同条件下的所需屏蔽确定最终屏蔽设计,确保谱仪屏蔽外人员可到达区域的剂量低于安全限值2.5μSv/h.     

Monte Carlo calculation of CR-39 efficiency for fast neutron detection using a combination of MCNP and SRIM codes,and comparison with experimental results

The elastic scattering of light nuclei, especially hydrogen, is widely used for detection of fast neutrons. Fast neutrons have a few interactions in solid state nuclear track detectors (SSNTDs), such as CR-39. It has been shown that to increase the efficiency of detector, a radiator layer, e.g. polyethylene may be used. In this work, the optimum thickness of the radiator layer has been determined. Some different thicknesses of aluminium layers have been used as discriminators for which the efficiency of detector to Am–Be neutron source has been calculated. A program FTRAC has been developed for the calculation of the detection efficiency. This software determines the specifications of recoil protons (using PTRAC card of MCNP code) and also the kinematics of interactions. Transport of recoil protons has been carried out using SRIM Monte Carlo code. A good agreement have been shown between measurement and simulation results.