【作者】 周雷；

【导师】 周永钊； 梁昊；

【作者基本信息】 中国科学技术大学 ， 物理电子学， 2009， 博士

【摘要】 慢正电子束技术(Slow Positron Technique)已经成为凝聚态物理学、化学和材料科学研究的主要研究工具之一,对材料表面微观缺陷的分析有明显的优越性。慢正电子湮没寿命测量主要利用慢正电子束流的单色性及能量连续可调等特点,研究缺陷在材料表面或近表面不同深度的分布信息。获得慢正电子湮没寿命测量起始时间的方法主要有两种:(1)利用慢正电子入射到样品时产生的二次电子获取时间起始信号;(2)利用对慢正电子束流进行脉冲化的脉冲调制信号作为起始信号。对慢正电子寿命谱仪而言,前者由于二次电子的能量和角度的分散性导致时间分辨率较低,目前最好可达350ps(FWHM);而后者的时间分辨率可达135-250ps(FWHM)。为打破国内材料科学的研究限制,追赶国际先进正电子研究手段的步伐,根据低能脉冲调制正电子束技术,中国科学技术大学的核固体物理实验室设计了一套脉冲式慢正电子寿命谱仪装置,其脉冲调制系统包括斩波器(Chopper)、预集束腔(Pre-Buncher)、主集束腔(Main-Buncher)和脉冲调制电子学系统。谱仪装置的设计目标是使连续的慢正电子束流经过三路信号的调制后,在样品处得到宽度为150-200ps的正电子窄脉冲,并实现慢正电子谱仪装置的寿命测量达到好于200ps的时间分辨。根据谱仪装置的设计原理并在参考赫尔辛基大学和北京高能所同类装置电子学系统的基础上,将谱仪电子学系统分为两个子系统:其一为脉冲调制电子学系统,该系统必须产生三路同步的高精度调制信号,来实现对慢正电子束的“筛选”和“聚焦”,以便在样品处得到宽度为150-200ps的正电子窄脉冲;同时,该系统必须提供与斩波器信号同步的时间起始信号。其二为高精度时间测量电子学系统,该系统必须对脉冲调制电子学系统给出的时间起始信号和探测器给出的时间停止信号进行高精度的时间间隔测量,以达到谱仪装置好于200ps的时间分辨。为了能够达到谱仪装置的设计目标,对电子学系统提出以下要求:1.斩波器信号为50MHz的脉冲信号,且具有大于5V的幅度、小于2ns的上升沿和下降沿、约7ns的脉冲宽度:预聚束腔和主聚束腔信号分别为频率50MHz和200MHz的正弦信号,且幅度都大于2V;2.三路信号间的相位晃动在60ps范围内;3.三路信号间的相位关系能够以小于50ps的步长进行调节;4.在20ns内测量精度好于64ps的时间测量。从上述要求可知,如何获得快边沿脉冲信号、保证各路信号间的低晃动及实现高精度的时间测量将是电子学系统设计的难点所在。针对设计要求及难点,在电子学系统设计中主要采用如下的技术路线:以一路时钟信号作为“源”信号,运用直接数字频率合成技术由此“源”信号产生一个频率可进行微调的高频信号,并通过对此高频信号进行分配分频得到三路信号,这是保证信号间相位稳定的关键;运用脉冲宽带放大技术获得大幅度快边沿信号,在获得快边沿信号的同时保证了引入较小的相位晃动;运用高精度的门延时技术实现信号间的相位调整;使用专用的TDC器件实现高精度的时间测量。本论文设计有如下几个主要特点:1.由一路信号通过变化得到三路信号的方法保证了在每次系统上电时信号间都能够有确定的相位关系;2.脉冲宽带放大技术获得大幅度快边沿信号,同时保证了对信号附加较小的晃动;3.精密门延时技术实现信号间相位的调整;4.专用的高精度时间测量器件TDC-GPX的使用,保证了系统的测量精度;5.内部集成DSP硬核大容量可编程逻辑器件(FPGA)的使用,为在线核信号数字处理提供研究平台,同时,为电荷修正的前沿定时方式时间测量作进一步的研究提供了测试平台,便于与恒比定时方式时间测量进行对比性实验;6.国内首台自行设计、以集成电路芯片为基础的脉冲式慢正电子寿命谱仪电子学系统原型机。系统的主要功能集成在NIM插件电路上实现,避免了使用大量的分立器件而存在的不兼容风险,同时,避免了由国外商业模块搭建的集成系统不容易根据实验条件改造和调节的缺点,增加了系统的灵活性。目前,本论文所设计的电子学系统原型机已经完成,并经过了一系列电子学测试,测试结果表明电子学系统设计满足设计指标要求,即将与慢正电子寿命谱仪一起进行联合实验测试。更多还原

【Abstract】 Positron annihilation lifetime spectroscopy(PALS) is a sensitive probe of vacancy type defects in materials.Low-energy positron beam is accepted to be a useful tool for surface studies,such as nondestructive depth profiling of defects in surfaces and interfaces.There are two practical methods to obtain the start signal from a slow positron beam:(1) detecting the secondary electrons generated on the implantation of the positron into the sample as the start signal;(2) pulsing the positron beam by means of an appropriate radio-frequency fields and using the pulsing signal of the beam pulsing system as the start signal.The former method,although easier to implement,suffers usually from poor resolution due to the spread in secondary electron velocities.The best time resolution was 350 ps.The former technique gives good results(high counting efficiency and time resolution) but its complexity of installation and operation make it available in only a few laboratories.The time resolution was 135～350 ps.In order to break the limitations of the internal research for materials science and reach international advanced level,an apparatus of low-energy positron lifetime spectroscopy(LEPLS) based on beam pulsing method has recently been established in the University of Science and Technology of China(USTC),and the beam pulsing system consists of a reflection type chopper,a pre-buncher,a main-buncher,and a beam-pulsing electronics system.The design goal is to compress the DC beam into pulse duration of 150～200ps(FWHM) at the target and obtain time resolution with better than 200ps for lifetime measurement of LEPLS.According to the principle of LEPLS and referring to the similar apparatuses in the University of Helsinki and the Beijing High Energy Institute,the prototype electronics system is in two electronics subsystems:beam pulsing subsystem and time measurement subsystem.The beam pulsing subsystem generates three beam pulsing signals to chop and buncher the beam,and one "start" signal for lifetime measurement.The time measurement subsystem measures the time interval of the "start" and the "stop" signal from PMT.In order to achieve the design goals of LEPLS,the requirements for electronics system are as follows:1.Chopper signal is a 50 MHz pulse signal with amplitude greater than 5 V, edge time less than 2ns and pulse width about 7ns;pre-buncher signal is a 50MHz sine signal with amplitude greater than 2 V;main-buncher signal is a 200MHz sine signal with amplitude greater than 2 V.2.Phase jitter between three signals is less than 60ps.3.The step of phase adjust between three signals is less than 50ps.4.RMS of time measurement in range of 20ns is better than 64ps.According to the requirements above,the difficult points of the electronics system design are how to obtain the signal with fast edge-time,maintain the low jitter between three signals,and achieve good resolution of time interval measurement.Aiming at the design requirements and difficult points,the main technical features of this system are:generating three required signals from one source signal and using techniques of frequency synthesis,clock multi phase,precise delay and broadband pulse amplification to meet requirements of the waveform,frequency and phase relationship for the three beam pulsing signals,and using dedicated TDC IC for time interval measurement.Most key characters in this thesis are listed as follows:1.The method of generating three required signals from one source signal ensures to obtain certain phase between three signals at the electronics system power-up every time.2.Using the technique of broadband pulse amplification to obtain signal with fast edge-time,and add small additional jitter to the signal meantime.3.The precise phase adjust is achieved by using the technique of precise gate delay.4.A TDC-GPX chip with high precision for time interval measurement is used to maintain the good time resolution.5.A large capacity and high performance FPGA(Field Programmable Gate Array) device,by taking the advantages of its embedded DSP cores and other abundant resources,provides a platform to implement digital nuclear signals processing in hardware,and implement time measurement in leading edge timing with charge correction which is used to compare with time measurement in constant fraction timing.6.The first prototype of electronics system for LEPLS internally,based on IC,is self-designed and achieved.The core function of the electronics system is integrated in two NIM modules,which avoids using a large number of discrete devices with the risk for compatibility,and avoids the shortcoming that the integrated system constructed by the foreign commercial modules is not easy to transform in accordance with the experimental conditions.Presently,the prototype of electronics system has been produced.After a series of electronics test,the electronics system prototype is proved to meet the electronics requirements.Afterwards the in-filed test with LEPLS will be implemented.更多还原