【作者】 熊涛；

【导师】 叶邦角；

【作者基本信息】 中国科学技术大学 ， 粒子物理与原子核物理， 2010， 博士

【摘要】 慢正电子脉冲束装置是目前国际上最先进的正电子湮没实验技术之一,和常规慢正电子束相比,它能够从脉冲化系统中得到一个时间信号,因而可以用来研究薄膜材料的寿命以及进行材料表面、表层研究,并且以它为平台,可以建立二维寿命谱仪(2D—PALS),时间一动量关联谱仪(AMOCS)和电子偶素飞行时间谱仪(Ps—TOF)等一些新型的正电子湮没实验技术。我们设计的慢正电子脉冲束装置是我国第一台完全基于放射源的慢正电子脉冲束装置,它的建立对实验室今后的研究工作具有非常重要的意义。本文首先介绍了常用正电子湮没实验技术,以及慢正电子束和慢正电子脉冲束的发展历史。接着详细介绍了装置的设计。本装置主要由放射源,慢化体,输运管,斩波器,预集束腔,主集束腔,加速环电极,法拉第筒和样品室等部件组成。其中放射源是50mCi22Na源；慢化体设计为多层钨网透射式结构；脉冲化系统由斩波器、预集束腔和主集束腔组成：斩波器设计为反射式结构,由三层栅网组成,第一层接地,第二层加斩波信号,第三层加负高压；预集束腔设计为电容加载同轴谐振腔结构,并设计了一个调节活塞,便于对它的本征频率进行调节,用示波器测量得到其本征频率在48.947744～51.927436MHz范围内可调,与用MAFIA程序模拟得到的结果基本一致；主集束腔同样设计为电容加载同轴谐振腔结构,测量得到其本征频率为201.402581MHz,与模拟结果一致。最后得到的正电子的入射能量在0.5～30keV范围内连续可调,到达样品处的束流脉冲宽度≤200ps。接着介绍了真空系统的设计。利用机械泵和分子泵作为前级泵进行预抽真空,当系统真空度达到5×10-3Pa左右,再用两台离子泵继续抽真空使系统达到更高的真空度,最后得到系统真空度约为5×10-6Pa。本装置的电子学控制系统设计为信号产生系统和时间测量系统。其中信号产生系统会产生三路高精度的高频信号,从而实现对慢正电子束的脉冲化和集束；同时提供一路与斩波器信号同步的时间信号作为时间测量系统的终止时间信号；而时间测量系统则对光电倍增管给出的起始时间信号和信号产生系统给出的终止时间信号进行时间间隔的测量。并且对它们进行了一系列电子学测试。本文还介绍了地线系统的设计,它采用的是两地线设计方案。最后介绍了慢正电子束流空间聚焦和时间聚焦的模拟计算以及相应的装置调试。发现束流的空间聚焦调试结果较好,但时间聚焦的调试由于存在较大的噪声信号,因而很难进行。更多还原

【Abstract】 The pulsed slow-positron beam device is one of the most advanced experimental technology of positron annihilation. Comparing to the normal slow positron beam device, a time signal can also be obtained from the pulsed system, so, the pulsed slow-positron beam apparatus can be used to studying the positron lifetime of thin-film materials and to research the surface of many other materials. Besides, it can be served as a platform, because based on the system, the two-dimensional positron annihilation lifetime spectrometer (2D-PALS), age-momentum correlation spectroscopy (AMOCS), positronium time-of-flight measurement (Ps-TOF) and some other new type positron annihilation experimental apparatus can be installed. Our designation of the pulsed slow-positron beams is the first model in our nation. It has a serious significant for future researching work.In this article, we first introduce the normal positron annihilation experimental technologies and the history of the slow-positron beam and the pulsed slow-positron beam.Next, we introduce the details of the design of this apparatus. This apparatus is mainly composed of the radioactive source, moderator, chopper, pre-buncher, main-buncher, accelerator rings and Faraday cup. The radioactive source is 50mCi22Na, moderator is designed as a transmission structure with W meshes. The pulsed system consists of a chopper, a pre-buncher and a main-buncher. The chopper is designed as a reflection type structure, which consists of three W meshes, the chopper signal is applied to the second mesh. The pre-buncher is designed as a coaxial resonator with a frequency tuner which can tune frequency in a small range. The main-buncher is also designed as a coaxial resonator. We measure their resonance frequency with Network Analyzer, and the results, which pre-buncher’s resonance frequency can be tuned in the range of 48.947744MHz-51.927436MHz and the main-buncher’s resonance frequency is 201.402581MHz, approximately accord with the simulation result by MAFIA. Finally, the incident positron energy is variable from 0.5 to 30keV, and the positron pulse is compressed to≤200ps at the sample position.Following above, we introduce the designation of the vacuum system. Using the mechanical pump and the molecular pump as the fore pump to pre-pump vacuum, after the degree of vacuum is come up to about 5×10-3Pa, another two ion pumps are used so that we can obtain a better degree of vacuum, a degree of about 5×10-6 Pa could be achieved at last.The electronic control system is designed to have signal generation subsystem and time-measurement subsystem. The beam pulsing subsystem generates three beam pulsing signals to chop and buncher the beam; at the same time, the signal generation subsystem also supply a synchronous time signal of the chopper as the ending time signal for the time-measurement subsystem, while the time-measurement subsystem measures the time interval of the "start" time signal which comes from the photomultiplier tube(PMT) and "end" time signal which comes from the signal generation subsystem. In addition, we take many electronic tests for those subsystems.Besides, the designation of the ground-wire subsystem is also introduced in details, it adopts the so called two-ground-wire design proposal.At last, we introduce the simulation calculation about the spatial focusing and the time focusing of the slow positron beams and the debug of the corresponding subsystems. After many debugs, we find the result of the spatial focusing is good, but, because of the large noisy signals, the time focusing process is hard to debug.更多还原