对TGS装置平板准直距离影响探测效率的探究

TGS系统的平板准直对探测效率及最后的计算有很重要的影响,通过MCNP5程序建立TGS模型,考虑了核退役中几种常见的并且密度相差较大的物质(玻璃棉、木屑、水、水泥、铝合金等),放射源使用常用投射源60Co,以0.5 cm的步进距离进行模拟试验,最后根据结果得出准直器距离在3～3.5 cm时可获得相对较好的效果。     

层析γ扫描测量（TGS）技术研究进展报告

为了能够满足实际应用的要求,层析γ扫描测量（TGS）技术采用75Se做透射源,利用75Se的121．5、264．4、279．8和400．7keV能峰对铀和钚核材料进行分析,结合TGS测量装置及其探测器的结构,利用蒙特卡罗模拟计算与实验相结合的方法建立TGS效率刻度矩阵。在数据分析软件中,编写了效率刻度数据库。     

层析γ扫描透射测量的计算机模拟研究

层析γ扫描方法是非破坏性评价中的一种重要方法,在核保障技术、核废物处理方面具有重要的应用。本工作结合实际问题,构造了层析γ扫描（TGS）透射测量仿真模型,建立了计算机模拟仿真平台,为TGS图像重建软件的计算机模拟研究奠定了基础。     

蒙特卡罗方法选择TGS准进器的最佳形状

采用蒙特卡罗方法和技术，模拟了ＴＧＳ连续扫描模式，对不同形状的准直器，计算了被测量废物桶中各个像素内的纵向效率的最大百分差，从中选取了棱形准直器和调整后的棱形准直器作为ＴＧＳ装置的扫描直器。     

蒙特卡罗方法选择 TGS 准直器的最佳形状

采用蒙特卡罗方法和技术，模拟ＴＧＳ连续扫描模式，对不同形状的准直器，计算了被测量废物桶中各个像素内的纵向效率的最大百分差，从中选取了棱形准直器和调整后的棱形准直器作为ＴＧＳ装置的扫描准直器。     

层析γ扫描技术中吸收衰减校正计算机软件研制

层析γ扫描(TGS)技术是非破坏性分析(NDA)中的一项重要技术。发射扫描测量中吸收衰减校正因子计算是TGS的关键技术之一。本工作采用Monte-Carlo模拟发射扫描测量方法,根据Monte-Carlo粒子输运理论,推导出吸收衰减校正因子数学公式。开发了TGS发射测量计算软件,用60Co标准源进行     

γ射线扫描装置新型准直器设计

准直器是γ射线扫描装置的重要组成部分,为了拓宽其量程,设计了一种新型准直器。利用MC法模拟计算了填充介质分别为空气、水、混凝土时,不同能量γ射线在新型准直器调节下的探测效率。通过与传统方法的对比研究,发现新型准直器对不同能量γ射线的调节效果基本一致,解决了低能γ射线难以穿透屏蔽块造成某些低能核素的活度难以被准确测量的问题。研究结果表明,采用新型准直器可以将γ射线扫描装置的量程扩大4倍左右。     

基于神经网络的废物桶活度测量方法研究

针对废物桶活度传统分段γ扫描（Segmented Gamma Scanning,SGS）测量精度低、层析γ扫描（Tomographic Gamma Scanning,TGS）测量时间长的问题,提出了基于神经网络的新型活度测量方法（New Gamma Scanning,NGS）。在该方法中,对介质均匀分布的废物桶进行测量时,将三个不同位置的探测器的计数率输入神经网络,可以直接输出等效环源半径,最终实现废物桶内核素总活度的准确重建。对400 L均匀水泥废物桶进行了多组模拟测量,利用不同方法分别进行了活度重建。结果表明：对于单个源,新方法的平均相对误差为4.26%,远小于SGS的误差（68.15%）,与60网格的TGS的误差接近（3.97%）;对于多个源,新方法的平均相对误差为24.27%,而SGS为48.02%,TGS为28.61%。新方法的精度高于SGS,达到了TGS的水平,而测量时间缩短到了TGS的1/20。新方法在保证了高精度的前提下大大缩短了测量时间,为低、中水平放射性固体废物的测量提供了技术支撑。     

层析γ扫描透射图像重建方法

研究了层析γ扫描(TGS)透射图像，并用两种TGS线性衰减系数重建方法进行TGS透射图像的计算机模拟仿真。结果表明：与国外文献报道的算法相比较，本文采用的两种重建方法的误差小、适应范围广，并能满足实际TGS装置的要求。     

Monte-Carlo统计迭代图像重建算法

线性衰减系数图像重建是层析γ扫描(TGS)的一个核心问题。本文从粒子输运方程出发，应用Monte-Carlo方法，提出了一种基于Monte-Carlo方法的统计迭代图像重建算法。模拟结果表明，与一般TGS图像重建算法相比，该重建算法的重建图像误差大为减小，能够满足TGS装置的要求。     

层析γ扫描中的探测效率刻度

探测效率刻度技术是层析γ扫描测量中最重要的技术之一。本工作研究用蒙特卡罗方法刻度层析γ扫描系统探测效率的方法。对3×3×3体素组成的样品模型,用蒙特卡罗软件计算了层析γ扫描测量装置的探测效率矩阵。在实验室层析γ扫描原型装置上,实验研究了层析γ扫描测量装置的探测效率。对两者进行了比较,相对偏差绝对值小于5％。研究结果表明了蒙特卡罗方法刻度层析γ扫描测量装置探测效率的可行性。     

Characteristics of parallel-hole and pinhole collimators for nuclear medicine imaging

In this paper,the spatial resolution and geometry sensitivity of parallel- and pin-hole collimators in compact gamma camera are presented,and quantitatively compared by GEANT4 code in Monte-Carlo library.The results show that the geometry sensitivity for pinhole collimator rapidly drops with increasing the gamma source-to- collimator distance(SCD);and for parallel-hole collimator,mildly.Meanwhile,the spatial resolution for pin-hole collimator mildly deteriorates;and for parallel-hole collimator,severely.The pin-hole collimator for close imaging objects has higher geometry sensitivity than parallel-hole collimator.Our findings are helpful for setting compact gamma camera collimators in nuclear medicine imaging.     

Pinhole collimator design for nuclear survey system

A conventional knife-edge collimator, which is widely used in gamma camera for medical diagnosis, is not suitable for nuclear imaging system because many scattering radiations near the pinhole aperture happen and blur image. A new pinhole collimator, which shapes a channeled aperture for reducing image degradation induced by the scattering radiations, is introduced and its characteristics are analyzed by Monte Carlo simulation. Resolutions defined as the full-width at half-maximum (FWHM) of point spread function and efficiencies are calculated about several pinhole diameters from 4 to 8 mm and channel heights from 2 to 10 mm. For this calculation, we assumed that ¹³⁷Cs radiation sources with 662 keV mono-energies enter into our designed collimator at the 1 m distance from the detector plane. The efficiencies and resolutions of the channeled collimator are compared with those of the conventional collimator. By comparison results, it is verified that the new collimator takes advantage more than the conventional collimator. The optimum channel height and diameter of the pinhole collimator from simulation results are also proposed and designed. We finally acquired nuclear image mounting this collimator in the nuclear survey system.     

Surface contamination of the charge－coupled device

An experimental method to study the influence of surface contamination of a thinned.backside illuminated charge-coupled device(CCD) upon its quantum efficiency in soft X-ray region is suggested.A transmission grating spectrometer(TGS),in which the transmission grating is coupled to a thinned.backside illuminated charge coupled device,is used to measure the continuum X-ray emission from the end of cylindrical target irradiated by laser.In the meausred spectra,only the carbon K absorption edge at wavelength of 4.4nm due to condensation of the vacuum oil on the CCD surface is clearly seen.The surface contamination is considered as an effective “carbon filter” and the filter absorption to correct the quantum efficiency of the CCD camera is taken into account.The effective thickness of the carbon filter is determined by comparing the jump height of the measured spectra at 4.4nm with those of the carbon absorption coefficient curves obtained from various carbon thickness.The accuracy of this method is tested by comparing the X-ray spectrum measured by the TGS with that obtained by a soft X-ray spectrometer.     

基于LaBr3(Ce)晶体耦合SiPM阵列的可变角分辨率伽玛相机的设计与研制

编码孔径成像技术由于探测效率高、信噪比高、角分辨率好、成像质量稳定可靠等优点而广泛应用于核安全、核设施的去污及退役的测量、核医学等领域。建立通过改变编码准直器和探测器之间距离进而实现可变角分辨的伽玛成像系统。整个成像系统主要由编码准直器、位置灵敏探测(position sensitive detector, PSD)、数据采集卡以及图像重建系统组成。该成像系统的编码准直器采用修正均匀冗余阵列(modified uniformly redundant array, MURA)编码方式,为了保障对较高能量射线的探测能力,编码准直器的材料采用含钨量90%的钨铜合金,PSD通过LaBr₃(Ce)晶体耦合SiPM阵列组成,重建算法采用的是直接互卷积算法,快速高效。测试结果显示,整个位置灵敏探测器的平均能量分辨率为4.96%(662 keV);该辐射成像系统可以准确地对Am-241、Cs-137、Co-60进行清晰成像,并通过改变编码准直器和探测器之间的距离成功分辨出两个Cs-137点源的位置。     

Enhancement of proton collimation and acceleration by an ultra-intense laser interacting with a cone target followed by a beam collimator

A special method is proposed of a laser-induced cavity pressure acceleration scheme for collimating, accelerating and guiding protons, using a single-cone target with a beam collimator through a target normal sheath acceleration mechanism. In addition, the problems involved are studied by using two-dimensional particle-in-cell simulations. The results show that the proton beam can be collimated, accelerated and guided effectively through this type of target. Theoretically, a formula is derived for the combined electric field of accelerating protons. Compared with a proton beam without a beam collimator, the proton beam density and cut-off energy of protons in the type II are increased by 3.3 times and 10% respectively. Detailed analysis shows that the enhancement is mainly due to the compact and strong sheath electrostatic field, and that the beam collimator plays a role in focusing energy. In addition, the simulation results show that the divergence angle of the proton beam in type II is less than 1.67 times that of type I. The more prominent point is that the proton number of type II is 2.2 times higher than that of type I. This kind of target has important applications in many fields, such as fast ion ignition in inertial fusion, high energy physics and proton therapy.     

高分辨micro-SPECT成像系统针孔准直器的优化设计

为了最大限度地减少γ射线在针孔准直器上的透射和散射效应,采用蒙特卡罗方法对单光子发射计算机断层(SPECT)成像系统使用的针孔准直器进行优化.模拟计算结果与实验测试得到的系统灵敏度和空间分辨率曲线相符,达到了高分辨小动物成像的要求.