基于LINAC慢正电子源的EGS4模拟

介绍基于ＢＥＰＣ ＬＩＮＡＣ和ＢＦＥＬ ＬＩＮＡＣＥ慢正电子束设备中正电子源部分的Ｍｏｎｔｅ－Ｃａｒｌｏ模拟过程。结果表明,从Ｔａ靶出射的快正电子产额在ＢＥＰＣ ＬＩＮＡＣ和ＢＦＥＬ ＬＩＮＡＣ运行兼容模式时分别为１．８４＊１０＾１０／ｓ和３．９＊１０＾１１／Ｓ。     

慢正电子的获得及其应用

本文是关于慢正电子束的获得及其应用的文献综述。 (1)能量为1eV左右慢正电子束的获得,通常是用~(22)Na、~(58)Co、~(68)Ga、~(64)Cu作为正电子源;还有用电子直线加速器的电子轰击Ta靶得到脉冲正电子源,由正电子源发射的正电子经慢化器(Cu、W叶片、复式MgO)就可以得到1eV—几百eV的正电子慢化效率极限为10~(-3),一般可达10~(-5)左右。为了使慢正电子单色化还要经一系列电子光学透镜才行。     

用单能慢正电子束测量Si中的正电子迁移率

用单能慢正电子束流作为探针,测量了Ｐ型Ｓｉ中不同Ｂ杂质浓度的正电子湮没Ｓ参数和正电子能量的函数关系。在实验上系统地研究了正电子扩散长度和迁移率随半导体中杂质浓度的变化规律,观察到杂质的掺入不影响缺陷的开体积,但正电子在硅中的迁移率随杂质浓度的增加而减小。     

A positron moderator using porous metal

Two types of porous metal moderators (i.e. porous nickel layer and multi-wire tungsten layer) are proposed and tested on a slow positron beam line. A moderation efficiency of about 2×10-4 has been achieved, which is higher than that for W vane geometry moderator by a factor of 4.     

正电子发射断层成像探测单元光电倍增管的增益调整

保持各个光电倍增管的增益一致性对正电子发射断层成像（PET）的质量至为重要。事件的响应输出取决于光电倍增管对于事件发生位置的光收集效率和事件的能量。光电倍增管对于某位置的光收集效率与光电倍增管对此位置所张的立体角成正比。当光电倍增管增益漂移后,通过计算出的各个管子的输出信号比值的变化完成对增益的自动调整,结果证明方法是有效的。     

钨发射慢正电子能谱测量

＾２２Ｎａ发射的正电子照射在经过２２００℃，１５ｍｉｎ退火的多晶钨箔上慢化并收集成慢正电子束，用平面减速栅能量分析器和通道电子倍增器测量能谱，测得在 １３３μＰａ的环境中，钨发射的慢正电子的能量峰位在１．７ｅＶ，自然宽度〉１．５ｅＶ。     

用于正电子发射断层成像的闪烁晶体

正电子发射断层成像要求闪烁晶体材料具有高光输出、短衰减时间和强阻止本领,且价格低廉。本文综述了闪烁晶体材料最新的发展状况和其特征对正电子发射断层成像系统的性能影响。     

成像板扫描仪的灵敏度曲线标定

开展了通用公司Typhoon FLA 7000型成像板扫描仪的灵敏度曲线标定实验。利用超短脉冲激光-固体靶相互作用产生的超热电子源同时辐照了成像板与已标定的LiF热释光探测器（TLD）,通过相对标定方法得到该扫描仪在光电倍增管不同倍增电压下的灵敏度曲线,并对测得的灵敏度曲线进行了数值拟合。拟合结果表明,灵敏度曲线随电压增大呈双指数规律减小,且与已有文献报道的利用标准放射源标定获得的结果一致。该方法为在无标准放射源情况下成像板扫描仪灵敏度曲线标定提供了一种有效途径。     

GEANT4 simulation of slow positron beam implantation profiles

The paper presents the positron implantation profiles, which are important for proper interpretation of data produced in slow-positron depth defect spectroscopy (VEPAS). In the paper, we compared the profiles reported in other publications and those obtained using the GEANT4 codes, which are used for the simulation of interaction of energetic particles with matter. The comparison shows that the GEANT4 codes produce profiles which match fairly well with those generated by other codes, which take into account more accurately processes at low energies when positrons interact with core electrons, valence electrons, plasmons etc. The profiles in different materials simulated for different implant energies were parameterized using two analytical formulas: the Makhovian profile and the profile proposed by Ghosh et al. [V.J. Ghosh, D.O. Welch, K.G. Lynn, in: E. Ottewite, A.H. WeissSlow (Eds.), Positron Beam Techniques for Solids and Surfaces, Jackson Hole, Wyoming, AIP Conference Proceedings, Vol. 303, New York, 1994, p. 37]. The adjustable parameters obtained are presented in Table 1 and Table 2. The total backscattering probability obtained from the GEANT4 simulations is in agreement with experimental data reported.     

用于束流剖面探测的位置灵敏探测器

介绍了用于剩余气体束流剖面探测系统的位置灵敏探测器的原理和结构。测试了探测器的性能，得到了位置分辨、位置线性、探测效率及探测器本底。     

Design and performance of the pulsed positron beam at Chalmers University of Technology

A slow monoenergetic pulsed positron beam at Chalmers University of Technology has been built. The system consists mainly of chopper, buncher and accelerator. The achieved positron energy range is in range between 230 eV and 15 keV. The FWHM of the beam resolution function is around 700 ps. The beam intensity is around 10³ cps.     

双维位置灵敏探测器性能测试

对用于放射性次级束测量的大面积双维位置灵敏探测器（PSD）的性能进行了测试。通过选择实验中电子线路的脉冲成形时间,并改进数据处理方法,即主放大器的成形时间常数要大于6μs,而位置路的时间常数约为0．5－1μs,且在位置公式中使用能量信号作为公母,测试得到了较好的位置分辨和能量分辨及线性。结果表明,这一类型的PSD探测器可以用于低能放射性束流的测量。     

基于微通道板的电子探测器的性能研究

设计了一个基于微通道板的电子探测器，从实验上详细研究了在一种新的高压分配方式下各个分压对探测效率的影响，其中包括两片微通道板之间所加电压对探测效率的影响。     

A practical image reconstruction and processing method for symmetrically off-center detector CBCT system

CBCT scanners have been widely used in angiography, radiotherapy guidance, mammography and oral maxillofacial imaging. To cut detector size, reduce manufacturing costs and radiation dose while keeping a reasonable FOV, the flat panel detector can be placed off-center horizontally. This scanning configuration extends the FOV effectively. However, each projection is transversely truncated, bringing errors and artifacts in reconstruction. In this paper, a simple but practical method is proposed for this scanning geometry based on truncation compensation and the modified FDK algorithm. Numerical simulations with jaw phantom were conducted to evaluate the accuracy and practicability of the proposed method. A novel CBCT system for maxillofacial imaging is used for clinical test, which is equipped with an off-center small size flat panel detector. Results show that reconstruction accuracy is acceptable for clinical use, and the image quality appears sufficient for specific diagnostic requirements. It provides a novel solution for clinical CBCT system, in order to reduce radiation dose and manufacturing cost.     

GH903合金的离子注入表面改性研究

用离子注入、氧化和慢正电子束分析研究了ＧＨ９０３合金的氧化性能的改善与微观作用机理。注入的Ｃｒ＋．Ｙ＋的能量均为６０ｋｅＶ，注入的剂量分别由１ｘ１０１７．ｃｍ－２（Ｃｒ＋）、１ｘ１０１５Ｃｍ－２（Ｙ＋）和［１ｘ１０１５．ｃｍ－２（Ｙ＋）＋１ｘ１０１７·ｃｍ－２（Ｃｒ＋）］。结束显示，注入样品与未样品相比。氧化增重分别减少４．８％（注Ｃｒ＋）、２４．２％（注Ｙ＋）和３２．３％（注Ｙ＋＋Ｃｒ＋）。这表明合金氧化性能改善的作用机理主要是注入离子对样品浅表层内缺陷的填充与退火。同时，注入元素的化学性能和使样品表面更致密也起了重要作用。